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UGC 4459

Editor’s note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.

Title: Hidden AGN in dwarf galaxies revealed by MaNGA: light echoes, off-nuclear wanderers, and a new broad-line AGN
Authors: Mar Mezcua and Helena Domínguez Sánchez
First Author’s Institution: Autonomous University of Barcelona, Spain
Status: Accepted to ApJ

Beneath the radiant tapestry of massive galaxies that thread our universe, lesser-known cosmic entities lurk — dwarf galaxies. Weighing in with a stellar mass below 3 billion solar masses, these low luminosity celestial islands barely tip the scale (many individual supermassive black holes outweigh them!). Furthermore, despite being the most abundant type of galaxy in the universe, their formation and evolution are still not very well understood.

Small But Mighty!

Active Galactic Nucleus

Artist’s impression of a galaxy’s active nucleus shrouded in dust. [NASA/SOFIA/Lynette Cook]

Active galactic nuclei (AGN) glow vibrantly in the darkness of space. Fueled by the rapid accretion of matter onto compact black holes within galactic cores, these sources can outshine the collective starlight of their host galaxies. Additionally, their powerful outflows can heat and disperse cold molecular gas, which astronomers believe may quell star formation and regulate galactic growth.

Most AGN are suspected to feature supermassive black holes (SMBHs; black holes with masses greater than one million solar masses) at their centers; however, today’s authors present exciting evidence, in tandem with previous studies that have uncovered hundreds of AGN within dwarf galaxies that harbor lower mass black holes, that is compelling astronomers to return to the drawing board.

The Quest for Dwarf AGN

Previous studies of AGN in dwarf galaxies primarily relied upon single-fiber (3-arcsecond aperture) spectroscopic measurements taken at the galactic center (i.e. the Sloan Digital Sky Survey). Prominent emission lines were then identified in these spectra and their flux ratios plotted in a BPT diagram (see this astrobites article for more). Depending on a galaxy’s location on a BPT diagram, the primary emission source for each galaxy was then classified as star formation, AGN, Low-Ionization Nuclear Emission-Line Regions (LINERs; emission that can originate from AGN and hot old stars), or a composite of multiple ionization mechanisms.

However, these single-fiber measurements are often biased towards identifying central AGN, and they can fail at AGN identification if there is abundant star formation in the center of a galaxy. Moreover, strong host galaxy light can diminish AGN signatures.

Alternatively, spatially resolved spectroscopic measurements can provide more definitive evidence of AGN activity. In particular, integral field unit (IFU) spectroscopy traces emission line features from varying physical regions of a galaxy (Figure 1).

The SDSS/Mapping Nearby Galaxies at APO (MaNGA) survey is a critical step forward in this direction. This survey will provide IFU data for nearly 10,000 galaxies by the end of 2020, which will make it the largest such catalog. Today’s authors leverage MaNGA to conduct the largest dedicated study of dwarf galaxies that host AGN within the survey.

Filtering the Data

Of the 4,718 sources they investigate, the authors categorize 1,609 sources as dwarf galaxies after imposing an upper mass cutoff of 3 billion solar masses. Subsequently, they examine a spectrum of each spatial pixel, or spaxel, for each dwarf galaxy to conduct a spatially resolved BPT analysis. As shown in Figure 1, the BPT diagram plots the [OIII]λ5007/Hα flux ratio against the [NII]λ6583/Hβ flux ratio. The location of each spaxel on the diagram determines the primary emission mechanism (i.e. star-forming, AGN, LINER, or composite) at each galactic position.

sample dwarf galaxy (8456-3704)

Figure 1: Left: BPT Diagram showcasing emission line classifications (i.e. star-forming, AGN, LINER, or composite) for each spaxel for a sample dwarf galaxy (8456-3704). The black square represents the median BPT location of the spaxels that are classified as AGN/LINER. The gray square marks the SDSS single fiber measurement. Center: physical distribution of BPT spaxels. Empty squares trace the IFU coverage and gray squares indicate spaxels with a continuum signal-to-noise ratio greater than 1. Right: SDSS composite image. The pink hexagon shows the IFU coverage. [Mezcua & Sánchez 2020]

AGN/LINERs


Figure 2: Left: stacked spectrum (blue) of all galaxy spaxels (gray) that are located in the AGN/LINER region of the BPT diagram. The emission line component is shown in red. Right: zoom-in of the stacked spectrum in the spectral region of the emission lines used for the BPT diagram. [Mezcua & Sánchez 2020]

To resolve the fact that LINERs are not exclusively linked to AGN activity, the authors then utilize the WHAN diagram to further eliminate non-AGN from their sample (Figure 3). The diagram identifies sources with Hα equivalent widths of less than 3 Å, the threshold for AGN detection. However, the authors relax this requirement slightly to avoid eliminating any borderline AGN and evoke a cut-off of 2.8 Å. The final catalog that results from this step nets 37 dwarf galaxies that host an AGN within MaNGA.

WHAN diagram for the initial 102 dwarf galaxies

Figure 3: WHAN diagram for the initial 102 dwarf galaxies with their median BPT spaxels classified as AGN/LINER. Using an Hα equivalent width threshold of 2.8 Å, the final sample of AGN dwarf galaxies is reduced to 37. The color bar denotes the median specific star formation (star formation rate per unit stellar mass) of the AGN/LINER spaxels. [Mezcua & Sánchez 2020]

Dwarf AGN Unveiled

Of the 37 dwarf galaxies which host AGN, the authors investigate 35 with available SDSS single-fiber spectra. They report 12 AGN from the single-fiber spectra; the IFU measurements thus reveal 23 additional AGN that were either labeled as star-forming, composite, or quiescent (signal-to-noise ratio of BPT emission lines < 3) with the single-fiber method — a true testament to the utility of spatially resolved spectra.

So why did the single-fiber measurements fail? To address this, the researchers explore the photometric properties of the sample. Doing so, they find that the dwarf galaxies feature relatively low star formation, as determined by the B–V color index (the brightness profiles of the galaxies were redder than anticipated). In addition, the single-fiber measurements of the dwarf galaxies indicate that only six of the 37 IFU AGN are star-forming. These results collectively suggest that star formation suppressing the AGN signatures is unlikely to be the culprit for the unreported AGN detections. Rather, it is likely that the missed AGN are either off-nuclear or currently inactive.

SDSS

The Sloan Foundation 2.5-m telescope located at Apache Point Observatory, New Mexico was used to conduct the MaNGA survey. [SDSS]

Using the spatially resolved BPT diagrams, the authors analyze the non-central AGN spaxels and find diffuse, generally symmetric, elongated emission. These characteristics are consistent with light echoes — the ghostly remnants of previously active AGN. Yet, they cannot rule out the possibility that these are signatures of active off-nuclear AGN. To resolve this confidently, the authors express their intention to conduct follow up observations with high-resolution radio and X-ray wavelengths using FIRST and Chandra. These measurements can be coupled with models to better expose AGN activity.

Finally, the investigators compute the masses of the AGN black holes in their sample and initially determine 14 intermediate-mass black holes (IMBHs; black holes with masses between one hundred and a million solar masses) using the MBH–σ scaling relation with a modified low-mass dependency. If unmodified, they discover only three IMBHs in their sample. The remaining black holes in both cases are deemed SMBHs. These results suggest that not all dwarf galaxies contain universally massive black holes and that the fundamental nature of these galaxies requires further investigation.

Looking Forward

Today’s authors have exemplified the capabilities of IFU spectroscopy. Utilizing the MaNGA catalog, they have uncovered 23 AGN that would not have been detected with a single-fiber SDSS measurement. This suggests that IFU spectroscopy can be employed as a vital tool to study AGN in dwarf galaxies.

Ultimately, by analyzing AGN in dwarf galaxies, we may uncover how IMBHs and dwarf galaxies co-evolve. We may also determine if IMBHs play a role in seeding the growth of SMBHs!

About the author, James Negus:

James Negus is currently pursuing his Ph.D. in astrophysics at the University of Colorado Boulder. He earned his B.A. in physics, with a specialization in astrophysics, from the University of Chicago in 2013. At CU Boulder, he analyzes active galactic nuclei utilizing the Sloan Digital Sky Survey. In his spare time, he enjoys stargazing with his 8” Dobsonian Telescope in the Rockies and hosting outreach events at the Fiske Planetarium and the Sommers Bausch Observatory in Boulder, CO. He has also authored two books with Enslow Publishing: Black Holes Explained (Mysteries of Space) and Supernovas Explained (Mysteries of Space) .

IRAS 16547-4247

Editor’s note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.

Title: Salt, Hot Water, and Silicon Compounds Tracing Massive Twin Disks
Authors: Kei E. I. Tanaka et al.
First Author’s Institution: National Astronomical Observatory of Japan
Status: Submitted to ApJL

How Do Massive Stars Form?

massive star formation

This Hubble image shows N159, a nursery for massive star formation within one of the Milky Way’s satellite galaxies, the Large Magellanic Cloud. [ESA/Hubble & NASA]

Massive stars have an outsized impact on their local environments and throughout entire galaxies, as they are important sources of ultraviolet radiation, turbulent energy, and heavy elements. While the formation of their low-mass counterparts is largely understood, the process of forming high-mass stars is still unclear. It is unknown whether massive protostars accrete through disks — a scaled-up version of low-mass star formation — or form through an otherwise distinct mechanism.

While recent theoretical work and simulations support this disk accretion model, detecting the presence of such disks is not free from observational difficulties. To do so, observers seek to identify the signatures of rotating gas within these disks by using molecular emission lines at millimeter wavelengths. But high spatial resolutions are required to correctly disentangle emission from molecules in the inner disk versus those associated with surrounding gas structures, such as protostellar envelopes and outflows. The advent of interferometers, such as the Atacama Large Millimeter/Submillimeter Array (ALMA), has provided the necessary angular resolutions and led to the detections of an increasing number of disk-like structures around massive protostars. But, despite this progress, there is no consensus as to which molecular lines uniquely trace these massive circumstellar disks. Moreover, few studies have been conducted at sufficiently small spatial scales to directly probe the structure of these disks.

In today’s astrobite, we take a look at new high spatial resolution observations of massive protostellar object IRAS 16547-4247, which reveal the presence of two rotating massive disks and identify a potentially universal “hot-disk” chemistry found in the innermost disks around massive protostars.

 Massive Twin Disks in IRAS 16547-4247

Today’s authors used ALMA to observe the massive protostellar system IRAS 16547-4247, which is located over 9,000 light-years from Earth. Previous radio observations revealed the presence of jets and indicated that accretion is currently ongoing in the vicinity of the protostar. IRAS 16547 is also known to be a binary system, comprised of two compact dusty objects with a separation of 300 au, surrounded by a larger, rotating circumbinary disk. By observing IRAS 16547 at a resolution of only a few hundreds of au, today’s authors are able to investigate the gas dynamics of both massive protostellar disks in detail.

Figure 1 shows the continuum images of IRAS 16547 taken with ALMA. Emission from dust dominates the 1.3-mm continuum, highlighting the circumbinary disk and outflow cavities, while the 3-mm continuum reveals the jet structures. Three individual protostars are seen at both wavelengths: IRAS 16547-Ea (source A) and IRAS 16547-Eb (source B), which form the protobinary, and a weaker third source IRAS 16547-W. The protobinary comprised of sources A and B is surrounded by a circumbinary disk, while outflow cavities are located to the north and south.

image of IRAS 16547

Figure 1: An image of the 1.3-mm (color scale and grey contours) and 3-mm (cyan contours) continuum toward IRAS 16547. The cyan and grey circles in the lower left indicates the resolutions of the observations, while the scale bar shows a physical distance of 1,000 au, or about 20x the size of our solar system. [Adapted from Tanaka et al. 2020]

A wide variety of molecular lines are also detected in IRAS 16547. Figure 2 shows the integrated intensity maps of representative emission lines, which trace different components in the protobinary system from the circumbinary disk to the individual circumstellar disks. For instance, lines from molecules such as methyl cyanide (CH3CN) and sulfur dioxide (SO2), which are often assumed to trace disks, are instead found toward the circumbinary disk and outflow cavity. On the other hand, emission from super-heated water (H2O), silicon compounds such as SiO and SiS, and sodium chloride (NaCl) trace the individual protostellar disks. Notably, this is only the second detection of NaCl in a protostellar system, after the Orion Source I disk.

total emission plots

Figure 2: Map of total emission detected from various emission lines (color scale and black contours) overlaid with the 1.3-mm continuum emission (grey contours). Molecule names, transitions, and integrated velocity ranges are show in the upper left of each panel. Red crosses indicate the continuum peaks of sources A and B. The black circle in the lower left indicates the resolution of the radio observations. [Adapted from Tanaka et al. 2020]

Inner Disk Tracers: Hot Water and Salt

Figure 3 shows the velocity structure of selected lines that trace the rotation of the individual disks. In source B, the velocity gradients are close to parallel to disk A’s rotation, but lie in the opposite direction, suggesting that the circumstellar disk of source B is rotating in the opposite direction as disk A and the circumbinary disk.

velocity structure

Figure 3: Map of the velocity structure of selected molecular lines that trace the inner disks (blue and red contours) overlaid on the 1.3-mm continuum emission (grey scale and black contours). Molecule names, transitions, and integrated velocity ranges are shown in the upper left of each panel. Stars mark the continuum peaks of sources A and B. Cyan lines indicate the orientations of disk rotation (panels a, and c–e), and yellow lines show the outflows (panel b). The white circle in the lower left indicates the resolution of the radio observations. [Adapted from Tanaka et al. 2020]

schematic of massive protobinary

Figure 4: Schematic view of the massive protobinary in IRAS 16547. The central twin disks are seen in high-energy H2O transitions (“hot H2O”), as well as NaCl and silicon-compound lines that are produced by the destruction of dust grains. The circumbinary disk, dusty outflow cavity, and jet knots are indicated. The blue and red colors show the rotation of the gas in the disks. [Adapted from Tanaka et al. 2020]

As seen in both the overall integrated intensities in Figure 2 and the velocity structures in Figure 3, there are two classes of molecules that trace the innermost 100-au scale of the massive binary system: vibrationally excited “hot” lines, which is best illustrated by hot H2O; and refractory molecules, such as NaCl and silicon compounds SiO and SiS, which originate from the destruction of dust grains. A summary of the inferred physical structure of IRAS 16547 is shown in Figure 4.

Implications of “Hot-disk” Chemistry

These results suggest that hot water, silicon compounds, and salts may be common in hot massive protostellar sources and serve as valuable tracers of inner disk material. The presence of this “hot-disk” chemistry provides a promising path for future studies of massive star formation.

In addition, hot-disk chemistry has an important link to meteoritics in our solar system. The oldest materials contained in primitive meteorites are those associated with Ca-Al-rich inclusions (CAIs), which were either sublimated or molten at some point in our protosolar disk. This means that the presolar nebula had to be heated to at least 1500 K, which is in apparent contrast with the low temperatures of a few hundred Kelvin typically associated with protoplanetary disks. Thus, it is still unclear how and where CAIs formed. Further observations of hot-disk chemistry may provide important constraints on gas-phase conditions of refractory molecules and provide insight into the formation of high-temperature meteoritic components.

About the author, Charles Law:

Hi! I’m a third-year graduate student at Harvard/CfA. I study chemical complexity in protoplanetary disks and star-forming regions using telescopes such as the SMA, VLA, and ALMA. In my free time, I enjoy hiking, bicycling, and traveling.

Abell 3827

Editor’s note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.

Title: Geometric Support for Dark Matter by an Unaligned Einstein Ring in Abell 3827
Authors: M. Chen, T. Broadhurst, J. Lim, et al.
First Author’s Institution: The University of Chicago
Status: Accepted to ApJ

Perhaps the greatest and most pressing problem in modern astrophysics is the problem of dark matter. Dark matter is a purported physical substance that emits no electromagnetic radiation and appears to interact only with ordinary matter and itself through gravity. The existence of dark matter is hypothesized in order to explain numerous key observations throughout the universe, most notably:

Unfortunately, despite more than a decade of searching, there has yet to be a definitive detection of particle-like dark matter by any of the numerous laboratory experiments done on Earth (e.g., the XENON1T experiment). It is then natural to wonder, could a solution to the problem of dark matter reside in a new understanding of gravity that avoids invoking a mysterious undetected material?

Alternative theories of gravity must satisfy observational tests already met by general relativity, our current best understanding of gravity. One regime of such tests, as noted above, is in gravitational lensing by galaxy clusters. Without dark matter, all mass in the universe should be associated with visible sources (baryonic matter), and if dark matter did not exist, the mass of the baryonic matter should be sufficient to create the observed gravitational lenses we see throughout the universe. In today’s astrobite, we explore a work that uses a unique lensing system to put this possibility to the test.

A Unique Cosmic Telescope

The fundamental objective in this work is to determine whether a component of mass that is not associated with the visible matter in a galaxy cluster (a.k.a., dark matter) is needed to produce an observed configuration of gravitationally lensed images in a particular lens system. Light from all background sources is deflected and distorted by the presence of foreground objects (Figure 1). In the strong-gravitational lensing regime, these deflections produce multiple images magnified and altered in a manner entirely dependent on a) the distances between the observer, the source, and the lens, and b) the underlying foreground mass distribution. Since the quantities in (a) can be easily determined through a variety of methods, this means that the mass distribution of the foreground lensing system can be directly inferred from the positions and shapes of the lensed images!

Gravitational lensing

Figure 1: Schematic demonstrating gravitational lensing. Light rays of distant sources are bent around massive foreground objects, and when the two objects are neatly aligned, multiple images of the same object appear magnified in the plane of the sky as a ring. [ALMA (ESO/NRAO/NAOJ), L. Calçada (ESO), Y. Hezaveh et al.]

The galaxy cluster investigated in this work, Abell 3827, has been widely studied due to the unique multiple-image system created by chance alignment of a background galaxy and the foreground galaxy cluster mass distribution. This type of serendipitous arrangement, which results in a ring of lensed images (an “Einstein ring”; see Figures 1 and 2), is observed in many systems, though the unique feature of Abell 3827 is the level of detail revealed in the lensed galaxy. As we’ll see in the next sections, the detailed morphology of the lensed galaxy revealed in these magnified images is the key observable used to assess the necessity of dark matter in this lensing system.

Abell 3827

Figure 2: Left: Galaxy cluster Abell 3827 as seen in a Hubble Space Telescope color image. The dominant bright galaxies are highlighted (G1–5), as well two foreground stars. The white overlaid contours indicate intensity of X-ray emission. Right: The same color image but with the light from galaxies and stars subtracted. All of the images labeled A1–4 are the same background galaxy lensed into multiple images. The white lines here represent mathematical lines of infinite magnification. [Chen et al. 2020]

Reverse Engineering the Total Mass Distribution

The authors of this work use the principle described above to create different types of lens models for the galaxy cluster Abell 3827, where each model describes the deflection of light rays due to the underlying mass distribution; this is called the “deflection field.” Half of the models allow for contribution to the total mass distribution only from the baryonic components (cluster galaxies, intracluster stars, and/or cluster gas), while the second half of the models also include an invisible component of mass assumed to be dark matter. All of the models rely on detailed structures in the lensed galaxy as constraints. For later reference, models 1, 3 and 6 include a dark matter component, while models 2, 4 and 5 do not.

All particular lensed features (like the yellow-red core in the background galaxy seen in Figure 2) that are split into multiple images, in reality, originated from the same location in the source plane. This means that a good model of the mass distribution will produce a deflection field that, when used to reproject these lensed images back to the source plane, predicts each lensed image to originate from the same location (test 1). Alternatively, it is possible to test the accuracy of a model by using a single lensed image, reprojecting it to the source plane, projecting it forward again to the image plane, and comparing these predicted multiple images to those that are observed (test 2). In this case, the lens model that best describes reality is the one that best reproduces the observed features of the gravitationally lensed galaxy across all the multiple images. An example of test 2 is shown in Figure 3 for the six lens models used in this work alongside the observed data.

Abell 3827 Einstein ring

Figure 3: Left: Images of the lensed galaxy comprising the Einstein ring in Abell 3827 with the light from stars and galaxies in the cluster removed. Right: Predicted image configurations by different lens models tested in this work. Models 1, 3, 6 include different representations of a smooth dark matter component, while models 2, 4, and 5 only include contributions from baryonic components parameterized in different ways. Each model achieves some level of agreement with the observed data, though models 1, 3 and 6 with dark matter achieve the greatest. [Chen et al. 2020]

Is an Invisible Component of Mass Needed?

The requirement (or not) of an invisible component of matter to reproduce the lensed images in Abell 3827 would either favor theories of dark matter or alternative theories of gravity. If an invisible component of matter is not needed, a mass distribution associated with baryonic components of the cluster should be sufficient to both predict consistent locations in the source plane of multiple lensed images (test 1 above) and predict consistent morphologies and positions of multiple lensed images in the image plane (test 2 above). However, all models that include dark matter (models 1, 3, and 6) outperform those without dark matter in both tests. In fact, the dark matter models achieve an order of magnitude greater accuracy in predicting commensurate source locations of lensed images. Furthermore, as can be seen in Figure 3, these same models also predict lensed images in the plane of the sky far more similar to the observed data. Finally, the only component of baryonic matter that matches the alignment and orientation of the gravitational lensing mass is the one associated with intracluster stars. This can be understood if these stars were tidally stripped from galaxies due to the presence of a massive, cluster-scale halo of smooth dark matter that dominates the total mass of the system.

From the tests carried out in this work, the authors conclude that dark matter is indeed required to explain the lensing features in the cluster Abell 3827. Ultimately, mass distributions associated with only the baryonic components fail to produce reasonable predictions for the lensing system. Like the study here, further geometric tests of gravitational lensing will continue to provide new benchmarks for alternative theories of gravity. For the time being, it would seem that dark matter as a fundamental ingredient to the universe is here to stay.

About the author, Lukas Zalesky:

I am a PhD student at University of Hawaii’s Institute for Astronomy. I am interested in understanding the way galaxies form and evolve over billions of years, as well as gravitational lensing by galaxy clusters. Outside of research I spend my time playing music, video games, exercising, and exploring the beautiful island of Oahu.

massive star formation

Editor’s note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.

Title: The Role of Outflows, Radiation Pressure, and Magnetic Fields in Massive Star Formation
Authors: Anna Rosen and Mark Krumholz
First Author’s Institution: Center for Astrophysics | Harvard & Smithsonian
Status: Published in AJ

a bubble in the Orion Nebula

This newly formed star at the heart of the Orion Nebula is blowing a bubble that’s preventing further star formation around it. [NASA/SOFIA/Pabst et al.]

Stellar physics is a broad field that touches on a range of phenomena from magnetic fields to radiative processes and thermonuclear fusion to plasmas. Stars form through the gravitational collapse of cold, dense, dusty protostellar cores, themselves embedded in thick molecular clouds or filaments. Massive stars, defined as those with a mass greater than 8 solar masses, are of key interest in star formation. Although they are extremely rare, comprising less than 1% of the total stellar population, they make their presence known by dominating the surrounding interstellar medium (ISM) with their powerful stellar winds as well as shocks from their eventual supernovae. Their formation is known to be impeded by several feedback mechanisms, including outflows, radiation pressure and magnetic fields. Today’s paper uses a series of radiative magnetohydrodynamic simulations to understand the overall impact that these combined mechanisms have on star formation.

Pushing the Boundaries

The fact that massive stars are so rare is reflective of a more general problem with star formation: its inefficiency. Estimates of star formation efficiencies are as low as 33%. As massive stars begin to form, they launch powerful molecular outflows from their poles. These jets can interact with the surrounding molecular cloud and eject large quantities of material. This, in combination with other feedback mechanisms, limits the star’s ability to accrete material, ultimately limiting its final mass. Knowing the upper limit of just how massive a star can be is incredibly valuable, for it allows us to set the upper boundary of the initial mass function. This function models the initial distribution of stellar masses for a given population of stars, and it is impossible to simulate the evolution of a stellar population without one. This is where massive stars are important, for they are the dominant source of radiative feedback and energy injection into the ISM through supernovae. So, to help determine these upper mass limits, we must simulate the processes that inhibit star formation in as much detail as possible.

Massively Magnetic Outflows Radiation Pressure (Games)

What does an MMORPG like EVE Online have in common with a radiative magnetohydrodynamic simulation? An insane amount of calculations. As the name implies, such a simulation models radiative transfer in addition to magneto-fluid dynamics. The simulation models stellar radiation fields and collimated outflows (the flow is parallel everywhere) for every star, and also factors in the indirect radiative feedback from dust, magnetic fields, and supersonic turbulence. The authors ran three main simulations: TurbRad (radiative feedback only), TurbRad+OF (adds collimated outflows), and TurbRad+OFB (adds magnetic fields).

simulation density plots

Figure 1: Density plots for the authors’ three simulations, with the most massive star shown in the center of each panel. [Rosen & Krumholz 2020]

In Figure 1, after the stellar mass of the protostellar core exceeds 30 solar masses, we see several pressure-dominated bubbles expanding away from the star (this is most noticeable in the middle row TurbRad+OF simulation). This process is known as the “flashlight effect”, where thick material is beamed away from the poles, causing low-density bubbles to expand outwards.

Go With The Flow

Over time, strong entrained outflows begin to break through the protostellar core and eject large quantities of material, as can be seen in Figure 2.

simulated outflows

Figure 2: Projected y–z densities of the entrained outflows. [Rosen & Krumholz 2020]

The outflows in Figure 2 become steadier and more directed over time. Although the protostellar core is initially highly turbulent, as it accretes material its rotational axis stabilises over time. One of the key results of these simulations is that the momentum feedback from these outflows is the dominant feedback mechanism (compared to radiation pressure) and helps to eject significant fractions of material, reducing the star formation efficiency. Outflows also help to act as conduit through which radiation can escape, weakening the feedback effects from radiation pressure.

Don’t Forget the B Field

star formation efficiencies

Figure 3: The star formation efficiencies for the total stellar population (top) and the primary, most massive star (bottom) as functions of simulation time for the three different simulations. [Rosen & Krumholz 2020]

Magnetic fields are known to affect star formation. Indeed, Figure 3 shows that the star formation efficiency is further reduced by the presence of magnetic fields (compare the purple dashed line to the pink dashed line). Overall, the simulations that contained outflows resulted in lower efficiencies. So in order to reconcile observations that place overall star formation efficiency at around 33%, this work shows that it is necessary to account for the effects of outflows.

In such an involved phenomenon like star formation, there are many nuances. Magnetic fields slow the growth rate of stars by helping to prevent the core from fragmenting, however there are several non-ideal effects (such as the Hall effect) that could theoretically impact the star formation process. These non-ideal effects were not considered, although it is unknown whether such effects have any noticeable impact on star formation efficiencies.

A Joint Effort

This comprehensive series of simulations, one of the first to account for so many factors, demonstrates the role of outflows, magnetic fields and radiation pressure in limiting the formation of massive stars and reducing the overall star formation efficiency. This study shows that feedback from outflows dominates the feedback from radiation pressure, and that magnetic fields further inhibit star formation. Importantly, both outflows and magnetic fields are needed to reproduce the low efficiencies obtained from observations.

About the author, Mitchell Cavanagh:

Mitchell is a PhD student in astrophysics at the University of Western Australia. His research is focused on the applications of machine learning to the study of galaxy formation and evolution. Outside of research, he is an avid bookworm and enjoys gaming, languages, and code jams.

protoplanetary disk

Editor’s note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.

Title: An Unbiased ALMA Spectral Survey of the LkCa 15 and MWC 480 Protoplanetary Disks
Authors: Ryan A. Loomis et al.
First Author’s Institution: National Radio Astronomy Observatory
Status: Published in ApJ

Molecules in Protoplanetary Disks

Protoplanetary disks, which are comprised of gas and dust rotating around young stars, are the cradles of planet formation. Many efforts have focused on measuring the total amount of dust in these disks, as dust grains provide the raw materials to form terrestrial planets and the rocky cores of giant planets. However, the abundance and distribution of molecular gas within these disks also has an outsized impact on the composition of nascent comets and planetesimals. An understanding of the chemical complexity that is present at this early stage of planet formation is also directly relevant to questions of the origins of life. Moreover, molecules serve as valuable tracers of disk properties such as temperature, gas density, stellar mass, and ionization levels.

Observing this molecular gas is, however, not without its challenges. Protoplanetary disks are cold enough for the majority of molecules to freeze out onto the surfaces of dust grains and form ices, rendering them invisible to observations with radio telescopes. As only gas-phase molecules present detectable signatures from their rotational transitions, the amount of detectable emission in such disks is inherently limited. To compensate for this limitation, the majority of disk observations employ targeted studies that focus on a few specific molecules expected to be strongly-emitting. But as a result, molecular inventories toward disks remain incomplete and offer only a partial and potentially biased view of disk chemistry. In fact, only 23 different molecules have been detected in disks, which is a direct consequence of these previously narrow and inconsistent searches. To remedy this, today’s authors present an unbiased spectral line survey of two nearby protoplanetary disks, which provides us, for the first time, a comprehensive view of the chemical complexity within these disks, including the detection of five new molecules.

An Unbiased Spectral Line Survey

Spectral line surveys are conducted over a sufficiently broad frequency range to include the transitions of many different molecules and often lead to unexpected or serendipitous new molecular detections. Historically, they have been a powerful tool to probe the chemistry of cold clouds and star-forming regions, but with the advent of ALMA, sensitive line surveys can now also be efficiently performed at high spatial resolutions toward disks.

Today’s authors conducted such a spectral line survey using ALMA toward the protoplanetary disks around LkCa 15 and MWC 480. Both disks are relatively young (3–7 Myr), host large (>200 au) gas-rich disks, and reside in the nearby Taurus star-forming region (520 light-years). LkCa 15 is a T Tauri star, a type of young, low-mass star, while MWC 480 is a Herbig Ae star, its higher mass counterpart. Since MWC 480 is hotter, more luminous, and more massive than LkCa 15, this allows today’s authors to investigate the influence of disk parameters on disk chemistry and observed line inventories.

The observations spanned a frequency range of nearly 36 GHz from 275 to 317 GHz. An analysis technique known as matched filtering, which uses a filter template based on the known Keplerian rotation of the molecular gas, is used to amplify spectral signatures and results in the spectra shown in Figure 1. In total, 14 different molecules were detected at high signal-to-noise ratios with five of these species (C34S, 13CS, H2CS, DNC, and C2D) being detected for the first time in a protoplanetary disk. Eleven molecules were seen toward MWC 480, while only nine were observed in LkCa 15.

disk spectra

Figure 1: Spectra of the observed bandwidth. The spectrum of MWC 480 is shown in purple and that of LkCa 15 is in red and inverted for clarity. Regions contaminated by atmospheric absorption lines are transparent. Molecules with transitions detected at the >4σ level are labeled in the disk in which they were more strongly detected. [Loomis et al. 2020]

Comparing MWC 480 and LkCa 15

Integrated-intensity emission maps, which show the spatial distribution of flux received from each line, are shown in Figure 2. A broad range of emission patterns are observed. In particular, a double-ring structure is seen in LkCa15 in several molecules, including N2H+, H2CO, and DCO+. Interestingly, both rings show a close association with previously observed dust features, namely the inner ring lies at the edge of a known dust cavity and the outer ring is located near the edge of the millimeter dust disk. In the case of the outer ring, dust evolution may expose the outer disk to increased levels of irradiation, increasing temperatures and allowing CO to return to the gas phase (as it would otherwise be frozen onto grains at these large radii). If this is the case, it naturally explains why the chemically-linked and CO-sensitive molecules N2H+ and H2CO have double-ring profiles that closely resemble each other.

Integrated-intensity emission maps

Figure 2: Integrated-intensity emission maps of MWC 480 and LkCa 15, which show varied emission morphology. Emission for all species is normalized to the emission peak. The hatched ellipse in the lower left indicates the resolution of the observations. [Loomis et al. 2020]

From Figures 1 and 2, it is clear that the molecular inventories of MWC 480 and LkCa 15 differ dramatically, which is not unexpected given their different stellar and disk masses, radiation environments, and temperatures. To further illustrate this, Figure 3 compares the total flux, which is roughly proportional to the total amount of gas, for each molecule. From this comparison, for instance, we clearly see that, 13C18O is strongly detected in MWC 480 but is not observed in LkCa 15 (also see Figure 1). Thus, we can conclude that MWC 480 has a more massive gas-phase reservoir of CO and can be explained by the fact that in the colder LkCa 15 disk, much of the CO is likely frozen out and is not detectable in the gas phase.

Integrated flux density ratios

Figure 3: Integrated flux density ratios between MWC 480 and LkCa 15 for each molecule. If a molecule is not detected in one disk, ratio limits were determined using 2σ upper limits. Molecules are color coded by broad chemical similarity. [Loomis et al. 2020]

Missing Complex Organic Molecules?

Predictions from chemical models suggest that numerous larger, complex organic molecules, such as CH3OH, should have been detectable in this line survey, but as illustrated in Figure 4, they are not observed. In fact, CH3CN was the most complex molecule to be securely detected and even so, it was only seen in MWC 480. Thus, this spectral line survey indicates that emission from complex organic molecules in disks may be consistently suppressed.

CH3OH observations

Figure 4: CH3OH observations in MWC 480 (top) and LkCa 15 (bottom). Left: observed spectra with source systemic velocity shown as a dashed red line. Right: integrated-intensity maps showing the absence of detected CH3OH flux. The hatched ellipse in the lower left indicates the resolution of the observations. [Adapted from Loomis et al. 2020]

Despite the absence of larger molecules, these results show that spectral line surveys of disks are valuable tools not only to detect new molecules, but also to provide a more comprehensive view of disk chemistry. Additional line surveys taken at different frequencies can provide access to different lines and molecules, while a larger sample of disks will help to robustly confirm the chemical trends seen in this work.

About the author, Charles Law:

Hi! I’m a second-year graduate student at Harvard/CfA. I’m interested in observationally studying the chemical complexity found in space, including throughout high-mass star-forming regions and in protoplanetary disks. In my free time, I enjoy hiking, bicycling, and traveling.

NGC 5253

Editor’s note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.

Title: The Three Young Nuclear Super Star Clusters in NGC 5253
Authors: Linda J. Smith, Varun Bajaj, Jenna Ryon, and Elena Sabbi
First Author’s Institution: Space Telescope Science Institute and European Space Agency
Status: Published in ApJ

It’s well observed that stars form in clumps, known as star clusters. Around 70% of all galaxies are observed to contain star clusters in their centre (or nucleus), known as nuclear star clusters (NSCs), but it is not clear how they actually came to be there. In addition to how nuclear star clusters actually form, a heavily debated question is where they form. The two mostly likely formation scenarios are either the formation of clusters elsewhere in the galaxy that then migrate inwards, a process known as migration, or the in-situ formation of clusters in the galaxy centre triggered by infalling gas. This Astrobite gives an overview of these scenarios in more detail.

center of the Milky Way

The Milky Way hosts its own nuclear star cluster, seen here in this image of the galactic center. [ESO/S. Gillessen et al.]

Today’s paper, almost inadvertently, contributes a really valuable piece of observational insight into the formation of nuclear star clusters.

Just like graduate students who enjoy leaving things to the last minute, star cluster formation thrives under high pressure. In environments such as in the nuclei of galaxies, pressures become high enough to form massive star clusters (>105M⊙), known as super star clusters (SSCs). These high pressures are further increased by other factors such as galaxy mergers or starbursts.

Today’s authors look at the compact dwarf galaxy NGC 5253, which is undergoing a very young central starburst, hosting a rich population of star clusters that includes SSCs. The central starburst is likely triggered by an infalling stream of gas.

Using Multi-Wavelength Observations to Paint the Full Picture

The centre of this galaxy is well studied from X-rays all the way to radio wavelengths. For the 20 years prior to this study, much work had already been done in observations in radio, infrared, and optical, which is summarised below. First we’ll explore what each wavelength can tell us before summarising how today’s authors were able to use Gaia data to tie the whole picture together.

Radio

Radio observations conducted with ALMA found one dominant intense radio source from a region with a bright core termed the supernebula, as well as a secondary radio source just next to it. A number of dense, hot, and high pressure clouds are also identified in the central ~100 parsec (pc) starburst region, and one of these clouds is spatially coincident with the supernebula. The suggestion is that this cloud consists of hot molecular clumps or cores associated with the stars in the embedded supernebula cluster.

Infrared

Infrared (IR) observations using the NICMOS instrument on the Hubble Space Telescope (HST) identified a double central star cluster that initially appeared to be coincident with the two main radio sources. However, further studies from 2017 showed that these two IR sources were not associated with the supernebula, deepening the mystery of what was going on within the centre of NGC 5253.

Optical

Optical observations using the ACS instrument on HST identified the two extremely young clusters (also seen in the IR). They were also able to constrain the cluster ages to just 1±1 Myr, and their masses to 7.5±0.3 × 104M⊙ and 2.5±0.6 × 105M⊙. The two clusters lie close to the radio sources mentioned earlier, but it’s difficult to tell due to the uncertainty in HST astrometry.

Piecing Everything Together

Matching the observations from the radio emission with the IR and optical wavelengths is extremely difficult due to the varying precision of these measurements. However, with the publication of Gaia Data Release 2, today’s authors were able to not only add new measurements, but also accurately bring all previous observations together.

central region of NGC 5253

Figure 1: Optical view of the central region of NGC 5253. The SSCs previously identified at optical and IR wavelengths are shown as black circles labeled with 5 and 11 (hereafter SSC-5 and SSC-11), with the width of the circles indicating the uncertainty on the spatial measurement. The green ellipse indicates the radio emission from the supernebula and the mauve ellipse indicates the hot molecular cloud thought to be associated with the supernebula. The cyan circle indicates the secondary radio source. The white circle shows the spatial resolution of the image. [Smith et al. 2020]

Figure 1 shows the result of remapping the previous measurements into a Gaia reference frame, accurately mapping where the emission in each wavelength is coming from. It was previously speculated that the supernebula and its associated hot molecular cloud was spatially coincident with the SSC-11, however, from this remapping, we can see that this is not the case. In fact, neither of the radio sources is actually associated with the two central SSCs identified in the optical and IR. Instead, the radio sources are lying in a highly obscured region between the two clusters. The authors therefore conclude that, in fact, there are three SSCs in the centre of NGC 5253. This is further supported by looking at the system in the IR, shown in Figure 2.

IR center of NGC 5253

Figure 2: Same as Figure 1, but now shown in the IR. All over-plotted markers and contours are the same as Figure 1, noting that for this image 3 different filters were used, hence the 3 different spatial resolution indicators. [Smith et al. 2020]

In Figure 2, we can see two main peaks in the IR emission. The peak on the left is very close to SSC-5, suggesting that this emission is indeed coming from this star cluster. However, the bright emission peak on the right seems not to be associated with SSC-11, but instead with an obscured SSC just to its left.

Going further into these measurements, the authors are not able to resolve the size of both SSC-5 and SSC-11, which provides an upper limit on the size (half-light radius) of just 0.6 pc. With masses of over 104 M⊙, this is in agreement with the observed densities of other star clusters, which are the densest stellar systems in the universe. They also find that the obscured SSC appears to be a twin of SSC-11 in terms of mass, however SSC-11 is older than its obscured companion. We can determine the age difference of the two SSCs due to the fact that SSC-11 is visible in the UV and optical, indicating that it has cleared much of the surrounding dust from which the cluster was born. The obscured SSC is not seen in the optical or UV due to the fact it is still embedded in its birth dust cloud. The dense dust cloud surrounding the cluster absorbs the UV and optical light emitted by the stars and reemits in the IR, resulting in the bright peak observed in Figure 2.

The Fate of These Star Cluster Siblings

One reason why the result from today’s paper is so enlightening, aside from being an impressive compilation of 20 years of observations, is the fact that this gives an intriguing insight into how nuclear star clusters are formed. Recalling the beginning of this Astrobite, there were two main formation scenarios: migration and in-situ formation. Today’s study clearly supports the in-situ scenario. Due to their very young ages, we know that the three SSCs have formed in the centre of the galaxy. As they are at the deepest part of the potential well of the galaxy, they will likely merge, forming a nuclear star cluster.

Nuclear Star Clusters and Beyond

Finding galaxies such as NGC 5253 and achieving such high precision measurements over a wide range of wavelengths allows us to distinguish between these very subtle formation scenarios that were previously difficult to observe. In the hierarchical buildup of galaxies, dwarf galaxies are the building blocks of massive galaxies. It’s suggested nuclear star clusters could potentially host intermediate mass black holes, which are thought to be the building blocks of supermassive black holes. By studying the components that build massive galaxies, we can construct a fuller understanding of galaxy formation.

About the author, Jessica May Hislop:

Doctoral Student at the Max Planck Institute of Astrophysics in Munich, Germany. Studying the formation of nuclear star clusters and intermediate mass black holes in high resolution simulations of dwarf galaxies.

brown dwarf

Editor’s note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.

Title: Two intermediate-mass transiting brown dwarfs from the TESS mission
Authors: Theron W. Carmichael et al.
First Author’s Institution: Harvard University
Status: Accepted to AJ

Brown dwarfs are objects with masses 13–80 times the mass of Jupiter but roughly the same radius (0.7–1.4 Jupiter radii). The lower mass limit separates them from planets: unlike planets, brown dwarf cores are massive enough to fuse deuterium. On the other hand, if they get too massive (80 Jupiter masses) then their cores start to fuse hydrogen and they become a main sequence star.

Similarly to a planet, when a brown dwarf passes in front of its host star, it causes a dip in the star’s light curve. This allows us to detect brown dwarfs with missions like the Transiting Exoplanet Survey Satellite (TESS).

Brown Dwarf Desert

Because of their Jupiter-like radius and larger mass, brown dwarf transits should be as easy to detect as giant planets — yet there are only 23 known transiting brown dwarfs. The lack of known transiting brown dwarfs is known as the “brown dwarf desert”. The answer to this problem may lie in their formation mechanism — whether they form like stars or like planets. Accurate measurements of mass, radius and orbital parameters are necessary to understand the formation and evolution of brown dwarfs.

Today’s paper reports the discovery of two new transiting brown dwarfs with reliable measurements of mass, radius, and age. With ages greater than 3 Gyr, they are the oldest transiting brown dwarfs with well-constrained measurements.

Brown Dwarfs and Stellar Parameters

The authors observed two targets of interest (TOIs) with TESS and the Las Cumbres Observatory (LCOGT). Figure 1 shows the transit light curves for the two host stars: TOI-569 and TOI-1406.

brown dwarf transit light curves

Figure 1: Left: TESS and LCO light curve for TOI-569. Right: TESS and LCO lightcurve for TOI-1406. Both lightcurves include the EXOFASTv2 transit model in red. [Adapted from Carmichael et al. 2020]

Since the depth of the transit depends on the ratio of the radii of the brown dwarf and its host star, the authors are able to measure the radius of the brown dwarfs from these light curves. However, objects with a range of masses may have the same radius, so additional radial velocity (RV) measurements are needed in order to measure mass and verify that the transiting object is actually a brown dwarf.

Spectroscopic observations for TOI-569 were taken using the CHIRON echelle spectrograph, as well as the CORALIE and FEROS spectrographs. Spectra for TOI-1406 were obtained with CHIRON and the echelle spectrograph on the Australian National University (ANU) telescope. The resulting RV curves are shown in Figure 2.

radial velocity curves

Figure 2: Left: RV curve for TOI-569. Right: RV curve for TOI-1406. Both RV curves include the EXOFASTv2 model in red. [Adapted from Carmichael et al. 2020]

To obtain reliable masses and radii for the brown dwarfs, the authors simultaneously fit the light curves and RV curves using the MCMC fitting software EXOFASTv2. As an additional check, the authors use the pyaneti fitting software and find results consistent with the EXOFASTv2 models within 1 sigma. The measured masses and radii relative to Jupiter values are M = 64.1 MJ, R = 0.75 RJ for TOI-569b and M = 46.0 MJ, R = 0.86 RJ for TOI-1406b.

The authors also used EXOFASTv2 to fit the stellar parameters like age and mass using isochrone models. The brown dwarf is assumed to have the same age as the host star. TOI-569b and TOI-1406b have ages of 4.7 Gyr and 3.2 Gyr, respectively.

Mass–Radius Diagram

One way to test brown dwarf evolutionary models is using a diagram of mass vs. radius. Unlike a star, the deuterium-fusing core of a brown dwarf does not produce enough energy to fight gravitational collapse, causing the radius of the dwarf to decrease as it ages. This means that a brown dwarf of a certain age has an expected location in a mass–radius diagram. If you have an independent measurement of the brown dwarf age, then you can change the isochrone model parameters to match the observed position of the brown dwarf.

brown dwarf mass–radius diagram

Figure 3: Mass–radius diagram for the known transiting brown dwarfs. The different colored curves show substellar isochrone models for different ages. The newly discovered brown dwarfs are shown as the red and cyan points. [Carmichael et al. 2020]

Figure 3 above shows the mass–radius diagram for transiting brown dwarfs, focusing on those with accurate ages. The black points in Figure 3 have well-measured ages from star clusters. The addition of TOI-569b and TOI-1406b to the diagram allows the authors to test, for the first time, substellar isochrones for populations older than 2.5 Gyr.

TESS has so far allowed the discovery of 4 new transiting brown dwarfs and may find as many more in the future. However, many more discoveries are needed to fully understand the formation and evolution of brown dwarfs.

About the author, Gloria Fonseca Alvarez:

I’m a third year graduate student at the University of Connecticut. My current research focuses on the inner environments of supermassive black holes.

inner solar system

Editor’s note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.

Title: Constraining the Formation of the Four Terrestrial Planets in the Solar System
Authors: Patryk Sofia Lykawka and Takashi Ito
First Author’s Institution: School of Interdisciplinary Social and Human Sciences, Kindai University, Japan
Status: Published in ApJ

Our solar system is really unique. As far as we can tell after our first ~decade of exoplanet hunting and planet formation observations and theory, it’s quite the special snowflake. In order to answer one of the biggest questions in astronomy — “How did the Earth form?” — we first need to ask how our solar system formed. Our moon, Jupiter and Saturn, the asteroid belt, and our Sun’s solar activity all provide hints as to how one can possibly form this incredibly habitable planet on which we live. Inspired by this, today’s bite answers the question: How did the terrestrial planets form? The authors argue that in order to fully understand the formation of one planet, we really should be thinking of its formation as one member of a system.

planet formation models

Figure 1: The embryo (large circle) and planetesimal (small circle) mass and radial distribution derived to represent three different planet formation models: A normal distribution (top), Empty Asteroid Belt Model (middle), and pebble accretion (bottom). [Adapted from Lykawka & Ito 2019]

Simulating the Formation of Our Terrestrial Planets

The authors explore terrestrial planet formation through the medium of N-body simulations. They start with a number of embryos and planetesimals in a simulated solar environment. The embryos and planetesimals represent the large and small rocky bodies that are left over after gas dissipation of a protoplanetary disk, which is a remnant of stellar formation. We believe that our rocky/terrestrial planets are born out of collisions of these objects. The authors allow their simulated rocks to collide and interact with each other for a series of timesteps that equate to 400 million years. By that time, all of the planets will have formed and a stable system should be established. The authors create different initial distributions of the embryos and planetesimals that are inspired by different planet formation models, for example the Grand Tack model, Empty Asteroid Belt, and pebble accretion. They explore the mass ratios of embryos to planetesimals — i.e. which type of pre-solar body will carry the most mass. Is it mostly in the smaller pebble sized objects, or in the boulder sized objects?

The authors also have different configurations of the gas giant planets, which we believe should have mostly formed by the time terrestrial planets started to form. Their location and eccentricities would affect the movements of bodies within 2 AU. The authors run 540 different combinations of these initial contentions and let the rocks go wild. In addition to the final mass and location of the planets that form, they also keep track of the water mass fraction, which is the fraction of mass that water makes up on each planet. Water is transported to these planets via collisions. We know generally the water mass fraction of Mars and Venus, we know the value pretty darn well for Earth, and we really have no idea for Mercury. Finally, the authors also keep track of the giant impacts that each planet experiences. We believe our Moon came from a giant impact that happened early in the Earth’s formation. So for a simulated solar system to be an analog of our system, the planets should have similar water mass fractions and the correct number and timing of giant impacts.

Do You Have What It Takes to Be a Planet in the Solar System?

Out of these 540 different solar systems, it’s time to find the systems that are most like our own solar system. There are a few criteria to narrow down the options. The authors define the region within 2 AU of the star (where 1 AU is the distance of the Earth to the Sun, today) as the planetary region, and outside of that is the asteroid belt. Are there at least three planets in the planetary region? Yes? Good. Moving on. Are there at least two that are of Venus/Earth mass? Yeah? Fantastic. Next, identify which planet-analog is Venus and which is Earth. We can now define the Mercury and Mars regions inside the orbit of Venus and outside the orbit of Earth, respectively. Are there planets in these regions that correspond to Mercury’s and Mars’s masses? Maybe there are multiple! If so, we determine which is going to be the planet analog by using its radial location from the Sun. The authors used the above checklist to begin to determine which planet systems were most like ours. In the end, they found only seventeen systems that were similar to our terrestrial planet system.

solar-system analogs

Figure 2: The 17 terrestrial systems that were deemed solar-system analogs. The simulation system 0 is our solar system. Green dots are Mercury, pink is Venus, blue is Earth, red is Mars, and black is the dwarf planet Ceres. The size of each dot corresponds to a mass, and the arrow represents inclination. [Lykawka & Ito 2019]

So what does all this tell us about how our solar system formed? Right away, it’s clear that it is difficult to form Mercury. Only 38 total Mercury analogs were formed, and most of them were still over two times the mass of Mercury. The disk models that formed the most Mercury-analogs typically had an “inner region,” which meant there were embryos and planetesimals located very close to the Sun at the beginning of their model run. They also saw that Mercury always formed too close to Venus.  The models did a pretty good job creating Venus and Earth — yay! However, they did not get enough water to Earth. In most of their simulations, Earth had a giant impact within 10 Myr, which is consistent with when we think the Moon was formed. Go Moon! Mars analogs typically formed with a larger mass than the real Mars, and often there were multiple Mars-like planets that formed nearby.

The Ingredients to Make a Solar System

After synthesizing their results, the authors came down to five crucial criteria that are necessary to form the terrestrial planets:

  1. Disks need to start out with a concentration of total disk mass between ~0.7–1.2 AU — it helps form Venus and Earth.
  2. Disks need an inner region ~0.3–0.4 AU in order to form my favorite planet: Mercury.
  3. Beyond 1.2 AU up to the asteroid belt (~2 AU), there needs to be significantly less mass. This aids in the production of dwarf planets in the asteroid belt and helps achieve the correct mass ratio between Venus and Earth.
  4. The embryos (large boulder-y rocks) need to carry most of the mass of the disk, as opposed to planetesimals.
  5. Jupiter and Saturn need to be on eccentric orbits. This keeps the Mars-analog planet from getting too massive and chucks out any planets that meander over to the asteroid belt.

This paper aids our understanding of terrestrial planet formation and tests how well current planet formation theories can recreate Mercury, Venus, Earth, and Mars analogs. Some aspects, like the excess water we have on Earth, the orbital spacing, and inclination of the planets are still a mystery, but the authors do take us one step closer to understanding the uniqueness of our solar system.

About the author, Jenny Calahan:

Hi! I am a second year graduate student at the University of Michigan. I study protoplanetary disk environments and astrochemistry, which set the stage for planet formation. Outside of astronomy, I love to sing (I’m a soprano I), I enjoy crafting, and I love to travel and explore new places. Check out my website: https://sites.google.com/umich.edu/jcalahan

BlackLivesMatter logo of an upheld, closed fist, superposed on a photograph of the Milky Way.

Editor’s Note:

The American Astronomical Society endorses and is participating in the grassroots efforts to #ShutDownSTEM, #ShutDownAcademia, and #Strike4BlackLives on Wednesday, 10 June. We encourage everyone in our community to make a lifelong commitment to action to eradicate anti-Black racism in the astronomical sciences — and in academia and research more generally — beginning today.

Instead of our usual highlight, today we are sharing a post recently published on Astrobites that contains suggestions of ways to support Black astronomers and make astronomy more anti-racist. If you find this post useful, you can look for future posts in the #BlackInAstro series on Astrobites. For further reading, the AAS also has a list of discipline-wide resources on diversity, equity, and inclusion.

A Spanish translation of this post can be found here at Astrobitos.
Una traducción al español de este artículo está aquí en Astrobitos.

The U.S. is rising in protest in the wake of the murder of George Floyd by the Minneapolis Police Department. The murders of George Floyd, Breonna Taylor, Tony McDade, and Ahmaud Arbery are the most recent in a long history of extrajudicial murders of Black people in the U.S. We at Astrobites stand in solidarity with the protestors, and against the systemic anti-Blackness that continues to enact violence on Black people in this country. We recognize that these same systems pervade academia and our field, and contribute to the inequities present in astronomy.

Why are we discussing these issues on an astronomy website? First, our scientific research is stronger when it comes from a community grounded in respect and diversity. But most importantly, we believe that the people in our community should be prioritized over our science. In order to do so, astronomy must be explicitly anti-racist and actively work to support Black students and researchers.

As authors for a publication widely read by astronomers and students, we at Astrobites have a responsibility to address issues of inequity. We acknowledge that we can be and should be doing more to amplify Black voices and build a better community for our Black colleagues. As one effort, we are starting a new series called #BlackInAstro, aimed to highlight the work of Black astronomers and the ways in which racism affects their experiences.

In this first post, we’ll share things that astronomers (particularly non-Black astronomers) can do, both during this time and to address racism and anti-Blackness in the long term in our field. We’ve also put together a list of suggested resources — compilations, readings, and podcasts — for folks working towards becoming more anti-racist.

What Can Astronomers Do?

We have compiled some ways that astronomers — particularly non-Black astronomers — can support Black folks in the field, both immediately in response to events like this week’s and in the longer term. Some of these are specific to our positions in academia, while many are general things that everyone can do, and that Black folks have been asking everyone to do for a long time. Whether you are an undergraduate, a graduate student, a postdoc, or a professor, you can help make the field of astronomy more anti-racist!

  1. Listen to Black people and amplify their voices. Work to understand what Black people are telling you, and work to listen without getting defensive. This can be difficult, particularly because as astronomers we’re often trained to defend our scientific results — but it’s important to listen, and to reflect on and learn from our mistakes.
  2. Check in with Black colleagues, friends, and students. This is a time of immense grief, pain and trauma for Black folks. Check in with the Black folks in your professional and personal circles; share your support and offer to do anything you can.
  3. Talk to non-Black people about what’s happening. The burden to address issues of racism and anti-Blackness often falls on Black folks, which is labor-intensive and can be traumatic. Non-Black people have a responsibility to start these conversations.
  4. Advocate and don’t gatekeep. Direct effort into advocating for Black students and folks in the field, both by supporting institutional efforts to improve equity and by stepping up for individual Black colleagues. On the flip side, do not engage in gatekeeping behaviors that make the field more exclusionary, such as those that continue to occur in introductory STEM courses and graduate admissions.
  5. Educate ourselves. Don’t expect Black people to do the additional work of educating everyone else. Take time to read about the history of the oppression of Black people and the ways anti-Blackness manifests today. Focus on writings by Black folks. In an academic setting, one way to do this is to start an equity-focused journal club. If this is newer to you, this glossary is a good place to start. A list of further suggested reading is at the bottom of this post.
  6. Support #BlackInSTEM efforts. Signal-boost Black students and researchers in STEM and share their work. This is critical both for supporting these individuals and for showing others that Black folks can succeed in the field; representation matters. For one great list of Black junior astronomers, check out this Twitter list compiled by Ashley Walker.
  7. Speak up and push leadership to speak up. We should expect leaders in our spaces — department chairs, PIs, project/telescope/survey directors — to speak up and express support for Black folks under their leadership. Silence plays into the myth that our workplaces and identities can be separated.
  8. Don’t expect productivity. During this time, all of us should be focusing our energy on supporting Black colleagues and anti-racism efforts. Black folks especially are processing, grieving, and fighting. Expecting academic productivity in these times of crisis ignores the experiences and needs of Black people.
  9. Don’t take up space. During times of crisis for Black folks, focusing on unrelated issues can seem minimizing. This is especially true for academic-related matters which Black folks may not have time and energy for right now.
  10. Donate. Many of us are in the privileged position of still receiving paychecks during this pandemic. Donate to Black justice organizations and to organizations that promote Black folks in STEM. Here are some suggestions:

As a reminder, many of these suggestions aren’t just action items for the immediate crisis! We must continually work to support Black peers and colleagues.

Suggested Reading Lists

Note: Consider buying from Black-owned bookstores! Here is a list of some in the US.

Comprehensive Guides to Anti-Racist Reading (and Watching)
Some Suggested Reading on Race and Anti-Racism in the US and UK

Stamped from the Beginning: The Definitive History of Racist Ideas in America
Ibram X. Kendi
Award winning history of how racist ideas were created, spread and rooted in American society. A staple of US anti-racism literature.

How To Be an Antiracist
Ibram X. Kendi
Award winning guide through antiracist ideas, and how to work to oppose them in society and ourselves.

So You Want To Talk About Race
Ijeoma Oluo
A contemporary overview of race in the United States, including police brutality, the BLM movement, intersectionality and microaggressions in day-to-day life.

Me and White Supremacy
Layla Saad
A guide on understanding your own privilege, examine unconscious biases, and how to combat racism.

Why I’m No Longer Talking To White People About Race
Reni Eddo-Lodge
An incredible resource detailing untaught black history, relations between class and race, and how White people can improve allyship. A must-read if you think the UK doesn’t have a race problem.

White Privilege: The Myth of a Post Racial Society
Kalwant Bhopal
An academic look at why black and minority ethnic communities continue to be marginalised in modern times, and how this is linked to government policy in the United Kingdom.

The History of White People
Nell Irvin Painter
A book describing the origin of the concept of ‘whiteness’, and the building of race as a social construct over centuries.

White Fragility: Why It’s So Hard for White People to Talk about Racism
Robin DiAngelo
Examines how the behaviors of White people in response to challenges of racism – including anger, defensiveness, and argumentation – protects racial inequality, and discusses ways to engage more constructively.

Books on Race in Academia and STEM

Superior: The Return of Race Science
Angela Saini
An in-depth, cross-disciplinary look at the presence of race science in modern society, while thoroughly debunking it.
(Note that in some regions of the world, Amazon is offering free versions of this book for a limited time.)

Algorithms of Oppression: How Search Engines Reinforce Racism
Safiya Noble
Challenges the idea that search engines like Google offer an equal playing field. Describes data discrimination as a real social problem, in which a biased set of search algorithms privilege whiteness and discriminate against people of color, specifically women of color.

Racism without Racists: Color-Blind Racism and the Persistence of Racial Inequality in the United States
Eduardo Silva-Bonilla
Discussion of color-blind racism and the evolution of new racial stratification.

The Immortal Life of Henrietta Lacks
Rebecca Skloot
Discusses really important questions of bioethics of the HeLa cells that are used in almost every lab in the US today, but note that it is not by a black author!

Presumed Incompetent: The Intersections of Race and Class for Women in Academia
Edited by Gabriella Gutiérrez y Muhs, Yolanda Flores Niemann, Carmen G. González, Angela P. Harris
Documents and analyses lived experiences of women and women-presenting academics of color. A collection of essays about the intersectionality of race and gender.

African Cultural Astronomy
Jarita C. Holbrook, Johnson O. Urama, and R. Thebe Medupe
A collection of essays describing the current status of archaeoastronomy and ethnoastronomy research in Africa.

Podcasts

Cite Black Women Collective Podcast
This bi-weekly podcast features reflections and conversations about the politics and praxis of acknowledging and centering Black women’s ideas and intellectual contributions inside and outside of the academy through citation.

The Nod
The Nod uplifts Black experiences in the U.S. and abroad. For example, in episodes over the last couple years, hosts Brittany Luse and Eric Eddings have told stories about everything from Josephine Baker’s “rainbow tribe” to an oral history of the song “Knuck If You Buck.” Where else have you heard or read those stories? Probably nowhere.

The Stoop
Similar to The Nod, The Stoop highlights Blackness by digging deeper into stories that we don’t hear enough about. Hosts Leila Day and Hana Baba discuss what it means to be Black and how we talk about our Black experiences through conversations between the two, as well as experts and Black people across the diaspora. A recent episode examined the word “hotep,” its meaning, and how its use has changed over the years.

Code Switch (on NPR)
Code-switching is the practice of shifting between languages or forms of expression in different contexts. Hosted by journalists of color, Code Switch explores race and how it impacts every part of society. The most recent episode, “A Decade of Watching Black People Die,” is particularly relevant right now.

About the Author, the Astrobites Collaboration

This post was written collectively by multiple members of the Astrobites team. Meet the authors of Astrobites.

Milky Way

Editor’s note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.

Title: Evidence for an Intermediate-Mass Milky Way from Gaia DR2 Halo Globular Cluster Motions
Authors: Laura L. Watkins et al.
First Author’s Institution: University of Chicago
Status: Published in ApJ

We can’t put it on a digital scale, we can’t hang it on a balance and compare it against something else, so how does one measure the mass of our home galaxy? The authors of today’s paper use measurements of globular clusters in the halo of the galaxy taken from the Gaia satellite to estimate a mass for the Milky Way.

What Is Our Galaxy Made of and Why Should We Weigh It?

Our galaxy contains four major parts: the bulge, the disk (which contains the thin disk and the thick disk), the bar, and the halo (see Figure 1). The first three components are made up of baryons, particles that make up protons and neutrons and therefore most of the things around us. The halo, however, is dominated by dark matter, and the percentage of baryonic mass in the halo depends on how much dark matter there is. Dark matter is a mysterious substance that pervades the galaxy, interacting strongly with gravity and weakly with light. We know dark matter is there because of the rotation curve of the galaxy; if the mass was concentrated at the center, the velocity of the outer regions would be slower than the inner regions. In the case of the Milky Way, we see that the rotational velocity stays fairly constant all the way out, which points to some unseen matter being present (matter that we identify as dark matter). Because of its weak interactions with light, it can be really tough to measure the amount of dark matter, and thus how much it weighs. Overcoming this challenge to calculate a mass for the dark matter in our galaxy’s halo would be a big step in obtaining the mass of the Milky Way.

Measuring the mass of our galaxy is very useful for two reasons: first, because the mass of the galaxy and its distribution are linked to the formation and growth of our universe. Accurately determining the mass will help us understand where our galaxy sits on the scale of the cosmos. Second, it helps us learn about the dynamical history and future of the Local Group and the satellite population (specifically stellar streams).

Milky Way schematic

Figure 1: Left: where the Sun sits in the Milky Way, from a face-on perspective. Right: The different parts of the galaxy, from an edge-on perspective. [ESA]

How to Weigh a Galaxy

The estimate of the mass of a galaxy is dependent on many things, including which satellites are bound and how long they have been that way, the shape of the Milky Way, and the method used for analysis. Three techniques have been mainly used to measure the mass of the galaxy: the timing argument, abundance-matching studies, and dynamical methods. The timing argument measures the speed at which two galaxies are approaching each other and uses those dynamics to predict a mass. Abundance-matching studies uses the number of galaxies versus their circular velocity and the Tully-Fischer relation to obtain their luminosity, which can be used to estimate their mass. Finally, dynamical methods look at the velocity of tracer objects such as globular clusters; any mass distribution gives rise to a gravitational potential that causes objects to move, so by studying the motions of the objects, we can work backwards to recover the gravitational potential, and thus the mass. The authors of today’s paper use this dynamical method to measure the mass of the Milky Way.

Using Gaia to Map Motions

The team used data from the Gaia mission’s 2nd data release (DR2) to measure the proper motions of stars, or how they are moving across the sky. Gaia is a space-based instrument whose goal is to make a 3D map of the galaxy, and this data release contained measurements for billions of stars and 75 globular clusters. Gaia’s observations are so precise that it can measure a human hair’s width at 1,000 km, which is a resolution 1,000–2,000 times higher than that of the Hubble Space Telescope! (Check out this really cool video on Gaia to learn more about this amazing satellite.) Figure 2 shows just how many sources Gaia has measured. Out of the 75 globular cluster measurements released in DR2, the authors used 34 of them that spanned a range of distances from 2.0 to 21.1 kiloparsecs from the center of the galaxy — which allowed the authors to trace the Milky Way’s mass out to the outer halo.

Gaia data map

Figure 2: A map of the number of sources Gaia measures on a projection of the plane of the galaxy (centered on the galactic center). The lighter the color, the more sources. The two circles in the bottom right are two very small dwarf galaxies that orbit the Milky Way. This figure shows the billions of stars contained in DR2. [Brown et al. 2018]

In order to map the mass of the galaxy correctly, they need parameters like velocity anisotropy (which measures how the motions of stars vary in different directions), the density of the galaxy, and the potential of the galaxy. The team uses an NFW model, which is a model for how the density is distributed within the galaxy, to describe the potential of the galaxy. The authors then run simulations to determine the radius inside which particles are gravitationally bound to each other (the virial radius) and the mass contained inside the virial radius (the virial mass). By varying the virial parameters and sampling different models of the halo, the team was able to figure out the most probable mass of the galaxy. In addition, they use the velocities of the stars to map the circular velocity of the galaxy out to the radius of the farthest globular cluster. Figure 3 shows the potential of the different components of the galaxy and the results of varying the virial parameters of the halo.

galaxy potential vs distance

Figure 3: The potential of the galaxy versus distance. Each component of the galaxy is labeled. The authors vary the virial radius and concentration (which represents the density) of the halo, and the different values they sample over are shown by the shaded region around the halo curve. The combination of the components (i.e., the total potential of the galaxy) is the gray line. The authors map the potential of the entire galaxy, but the vertical dotted lines show the area in which they’re interested, which is the distance of the nearest and farthest globular cluster in their sample. The solid lines show the extent of the best-fitting power law to that region, and the dashed lines show the power-law fit outside the region of interest. [Watkins et al. 2019]

Evidence for an Intermediate Mass Milky Way

The authors find that the mass of the galaxy is 0.21 x 1012 solar masses, the circular velocity of the galaxy at the maximum radius they look at (21.1 kpc) is 206 km/s, and the virial radius is 1.28 x 1012 solar masses. This virial mass fits in most with intermediate values found by other studies. The circular velocity measurement the authors made indicates that the velocity is fairly constant in the outer regions, supporting the idea that dark matter is present in our galaxy. Some of the clusters the team used for measurements are on very radial or very tangential orbits, which could have been the result of galactic collisions. If they remove these clusters, the mass and velocity measurements are still within their error bars, showing that these estimates are robust even if there are substructures of globular clusters in the galaxy.

The amazing wealth of data from the Gaia mission has allowed the team to make one of the most precise estimates of the mass of the galaxy that has ever been achieved. As Gaia continues its mission over the next few years, it will obtain positions and velocities of even more clusters, paving the way for more robust studies of the mass of our galaxy.

About the author, Haley Wahl:

I’m a third year grad student at West Virginia University and my main research area is pulsars. I’m currently working with the NANOGrav collaboration (a collaboration which is part of a worldwide effort to detect gravitational waves with pulsars) on polarization calibration. In my set of 45 millisecond pulsars, I’m looking at how the rotation measure (how much the light from the star is rotated by the interstellar medium on its way to us) changes over time, which can tell us about the variation of the galactic magnetic field. I’m mainly interested in pulsar emission and the weird things we see pulsars do! In addition to doing research, I’m also a huge fan of running, baking, reading, watching movies, and I LOVE dogs!

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