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accreting black hole

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: A Population of Bona Fide Intermediate Mass Black Holes Identified as Low Luminosity Active Galactic Nuclei
Author: Igor V. Chilingarian et al.
First Author’s Institution: Smithsonian Astrophysical Observatory and Sternberg Astronomical Institute
Status: Published in ApJ

Most, if not all, galaxies contain a supermassive black hole (SMBH) at their center. But where did these giants come from? Astronomers know that less massive black holes can form when a star collapses in on itself. These stellar-mass black holes can have the mass of a few dozen Suns, but they do not come close to the millions or billions of solar masses SMBHs have.

There are a few ideas for where SMBHs may have come from. One scenario is that the earliest stars left behind small baby black holes, which over time merged together to form more and more massive grown-up black holes that now reside at the center of galaxies. If this were the case, we’d expect to see some teenage, intermediate-mass black holes (IMBHs) — black holes larger than those born from stars but smaller than those we see in the center of galaxies.

Another possibility is that large gas clouds in the early universe collapsed to form massive black-hole “seeds” the size of hundreds of thousands of stellar masses. We then wouldn’t expect to see the adolescent phase of IMBHs, as SMBHs would start growing from these large seeds instead of small ones left behind by stars.

The authors of today’s paper wanted to see if they could find IMBHs, and thus evidence for the first scenario for SMBH formation.

Finding AGNsty Adolescents

While previous searches for IMBHs looked at only a few, pre-selected galaxies, Chilingarian et al. set up an automated search through 1 million catalogued galaxies to look for signs of SMBHs at their centers that are actively gaining mass — objects known as active galactic nuclei (AGN). AGN show distinct spectral lines, as seen in the bottom left of Figure 1. The authors used the width and strength of some of these lines to calculate the mass of the black hole for each galaxy.

Fig 1: The top row shows a diagram breaking down the components of an AGN. The black hole at the center (far right) has an accretion disk emitting light, while the clouds in the broad- and narrow-line regions (center) absorb and reemit the light, causing the peaks seen in the spectra at the lower left. The lower right plots show the curves used to fit the emission lines. [Chilingarian et al. 2019]

Of the million objects analyzed, 305 had black holes with masses less than that of 200,000 times that of the Sun, categorizing these as potential IMBHs.

By selecting a subset of these candidates for follow-up observations, Chilingarian et al. confirmed the AGN nature of ten of these candidates by detecting them in X-rays. This provided the authors with a sample of ten bona fide AGN with black-hole masses measured between 43,000 and 202,000 solar masses, five of which were previously known. Images of the ten galaxies that host the proposed IMBHs are shown in Figure 2.

Fig 2: Optical images from the Sloan Digital Sky Survey of the ten IMBH host galaxies. The white line shows the scale; five kiloparsecs (kpc) is approximately 1017 kilometers. The red circle is the location of their observations in X-ray wavelengths. The number at the top is their catalogue designation and the number at the bottom is their mass estimate in solar-mass units. The bottom row contains the galaxies with proposed IMBHs found in previous studies. [Chilingarian et al. 2019]

Results

These results are promising for the scenario where supermassive black holes are grown from stellar-mass seeds. If SMBHs grew from seeds larger than these observed intermediate-mass black holes, then we would not expect to see any IMBHs.

From their original sample of 305 possible IMBH host galaxies, 14 currently have enough observational evidence to test the most stringent observing criteria for IMBHs. Only six of those 14 (or 43%) successfully pass all the tests, suggesting that they host a real IMBH. Thus, the authors estimate a lower limit of 131 galaxies in their sample will have real IMBHs when further follow-up observations are performed.

The 305 possible IMBHs the authors explore in this paper are only those that are actively collecting more material. There may be many more non-accreting IMBHs out there that cannot be detected because they are too far away for our current instruments.

The accretion of matter from a host galaxy is not enough to explain the growth of stellar-mass black holes to IMBHs, so IMBHs must form from the merger of smaller black holes. The authors conclude that at least some of the SMBHs we observe must therefore ultimately be built from mergers of small, stellar-mass seed black holes.

About the author, Bryanne McDonough:

First year graduate student at Boston University where I am currently studying the distribution of dark matter and satellites around galaxies using data from the Illustris simulations. My primary research interests are galactic and extragalactic astrophysics using computational methods. Outside of grad school I enjoy reading and crafting.

WASP-12b

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: Obliquity Tides May Drive WASP-12b’s Rapid Orbital Decay
Author: Sarah Millholland, Gregory Laughlin
First Author’s Institution: Yale University
Status: Published in ApJ

It’s not an easy life being a hot Jupiter. Besides their eternal loss of privacy due to being on the hit list of astronomers since the very moment they are detected, theirs is a tale of intense drama. The extreme radiation and tidal flexing they experience due to their proximity to their host stars make them ideal targets for studying planetary science in extreme physical conditions, particularly since there are no hot-Jupiter analogs in our solar system. One of these weirdos, and also one of the most studied hot Jupiters in recent years, is WASP-12b. In addition to being one of the hottest hot Jupiters around (with equilibrium temperature of ~2500 K) and losing mass at an exceptionally high rate, it is also the only known hot Jupiter inspiralling so rapidly towards its star that we can observe this decay in real time. It has been speculated that sustained tidal interactions between the star and the planet could be responsible for the observed orbital decay of the planet. The authors of today’s paper investigate the case of WASP-12b in this context to understand what might be driving its orbital decay.

Turning with the Tide

Tidal interactions between a star and its planet act together to dissipate the orbital energy of the system in the interiors of the planet and the star. Think of the planet and the star acting like resistors that dissipate the gravitational energy of their orbits into heat due to tidal flexing in their interiors. This tidal dissipation is most effective when a planet has large orbital eccentricity and obliquity (the angle between the orbital plane and spin axes of the planet). However, for a lonely hot Jupiter with no other planetary companion in the system, tidal torques from the host star over time would eventually force the planet to reach an equilibrium state with low orbital eccentricity and obliquity, which would reduce the tidal dissipation the planet experiences. The fact that WASP-12b’s orbit is decaying rapidly indicates that there may be some other force at work helping to drive its sudden inspiral. The authors of today’s paper propose that the presence of another planet in the system (see Figure 1) could sustain WASP-12b’s obliquity, causing the planet to continue to experience high tidal dissipation and preventing its inspiral from stabilizing.

spin-orbit interactions

Figure 1: Schematic of the spin-orbit interactions between WASP-12b and another planet in the system. In the case of spin-orbit resonance (called a Cassini state), the spin angular momentum vector S1 and orbital angular momentum vector L1 precess about a common axis at the same rate. [Millholland & Laughlin 2018]

If there is another planet in the system, it’s likely that WASP-12b could have gotten locked in a state of spin–orbit resonance (known as a Cassini state, also observed in the case of Earth–moon system) which means that the rate of precession of its orbital axis is the same as the spin axis precession rate. This implies that as the orbital precession rate of WASP-12b changes due to dynamical interactions with the perturbing planet in the system, this will drive a change in its spin precession rate as well, increasing WASP-12b’s obliquity.

The authors simulate the evolution of the WASP-12 system using an obliquity tide model that couples two things: the relations governing the secular dynamical interactions between WASP-12b and another planet, and the tidal interaction between the star and WASP-12b. These interactions then combine to cause the inspiral. With very little fine-tuning of the properties of the perturbing planet and the efficiency of tidal dissipation (which is dependent on the unknown interior structure of WASP-12b), they are able to reproduce the inspiral rate that we have observed for the system (Figure 2). An interesting result of their analyses using the obliquity tide model is that even if WASP-12b starts with a very low obliquity (< 1°), it can easily get locked into a Cassini state and attain a high obliquity within the lifetime of the planetary system.

WASP-12b's obliquity evolution

Figure 2: Simulated evolution of WASP-12b’s obliquity (gray) and semi-major axis (purple) with respect to time from the obliquity tide model. Note that tidal dissipation due to obliquity tides is a runaway process: orbital decay (represented by the decreasing semi-major axis) due to tidal dissipation leads to increasing obliquity as the other planet forces WASP-12b to maintain the spin-orbit resonance. This further increases the efficiency of tidal dissipation. [Millholland & Laughlin 2018]

So obliquity tides can cause the observed orbital decay — but how do we confirm this? The good news is that, based on the predictions of their obliquity tide model, the authors conclude that the perturber planet is very likely within the limits of detection for extreme-precision radial velocity instruments coming up in the near future. Tighter constraints on transit duration variations or transit timing variations (a signature of orbital precession) from more precise long-term photometric monitoring of the system by TESS could also help in strengthening the obliquity tide hypothesis. If confirmed, obliquity tides might additionally be able to explain WASP-12b’s extreme radius inflation and unusual features in its thermal phase curves and will provide compelling evidence for in situ formation of hot Jupiters. Since tidal interactions are intimately tied to the interiors of the planets, they could also be an unprecedented tool for X-raying the interior structure, formation histories, and demographics of exoplanets.

About the author, Vatsal Panwar:

I am a PhD student at the Anton Pannekoek Institute for Astronomy, University of Amsterdam. I work on characterization of exoplanet atmospheres to understand the diversity and origins of planetary systems. I also enjoy yoga, Lindyhop, and pushing my culinary boundaries every weekend.

IGR J17062–6143

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: NICER Discovers the Ultracompact Orbit of the Accreting Millisecond Pulsar IGR J17062–6143
Author: T. E. Strohmayer et al.
First Author’s Institution: NASA Goddard Space Flight Center
Status: Published in ApJL

Dancing with the Stars

pulsar

Figure 1: Artist’s impression of a pulsar in a binary system, being fed by an accretion disk. [NASA/Dana Berry]

What does a ballerina doing a pirouette and a millisecond pulsar (MSP) have in common? Just as the ballerina will spin faster and faster as she brings her arms in toward her body, a MSP will also rotate more rapidly as the star’s radius is reduced by gravitational compression. These swiftly spinning stars are neutron stars — one of the possible end stages of stellar life. Neutron stars are born during a core-collapse supernova, which occurs once a massive star can no longer withstand its own gravity after fusion has ceased. These exotic compact objects are among the densest stars in the universe. They have 1.4 times the mass of our Sun crammed into a sphere that is roughly 20 km in diameter (the size of a large city!). As these neutron balls rotate, they can produce various types of electromagnetic radiation near their magnetic poles. If the magnetic axis and rotation axis of the neutron star are misaligned, we typically observe periodic pulses of radiation as the star rotates (if the radiation is beamed towards our telescopes) — hence the nickname “pulsar”! MSPs are particularly special compared to ordinary pulsars. They are typically spun-up by accretion of material from a nearby orbiting star via angular momentum transfer (see Figure 1), and they rotate hundreds of times per second (in some cases, even faster than your kitchen blender).

IGR J17062–6143: A Record-Breaking Orbit

In today’s astrobite, we cover the discovery of the orbit of an ultracompact X-ray binary, IGR J17062–6143 (depicted in the cover image above), which harbors an accreting MSP. Using the recently commissioned Neutron Star Interior Composition Explorer (NICER) X-ray instrument on board the International Space Station, Strohmayer et al. showed that the system consists of a very low-mass white dwarf in a record-setting 38-minute orbit with a pulsar that rotates at about 9,800 revolutions per minute.

The authors’ findings are based on an analysis of recurring X-ray pulses produced from hot spots near the pulsar’s magnetic poles (see Figure 2). In the case of IGR J17062–6143, these hot regions are formed from the white dwarf’s infalling material, which collects into an accretion disk around the pulsar. By calculating Fast Fourier Transform (FFT) power spectra of the NICER light curve, the authors found that the MSP’s orbital motion produced sidebands near the rotational frequency of the pulsar and its harmonics, which is a discerning feature of pulsars in binary systems.

Figure 2: Average power spectrum from NICER observations of IGR J17062–6143 obtained in 2017 August. The presence of sidebands indicates that the X-ray photons are affected by the orbital motion of the pulsar. [Strohmayer et al. 2018]

As the MSP whirls around its companion, the X-ray pulses are Doppler shifted by the orbital motion of the pulsar (see Figure 3). In other words, some pulses will arrive sooner when the MSP is moving toward us, and others will arrive later when the pulsar is moving away from us. This important property allowed the authors to show that IGR J17062–6143 has a nearly circular orbit, and the MSP and white dwarf companion are separated by only roughly 186,000 miles (300,000 km) — less than the distance between the Earth and the Moon! The authors also provide constraints on the viewing angle of the binary and the mass of the white dwarf, which they find is ~57–65 times less massive than the Sun assuming that the donor star is hydrogen-deficient. However, the precise chemical composition of the white dwarf is still under debate. This is interesting because it is thought that accretion of hydrogen-deficient material on short timescales is a feature unique to ultracompact binaries. Ultracompact binaries like IGR J17062–6143 therefore serve as laboratories for studying the physics of accretion in a unique environment. Further investigation is also needed to understand the nature of the X-ray outbursts observed from this object, which are believed to be powered by thermonuclear reactions on the surface of the neutron star.

Figure 3: Best-fit orbital solution (blue) and pulse phase residuals (black) as a function of mean pulsar longitude, derived from NICER observations of IGR J17062–6143. The orbit-predicted phase delay is added to the residuals to show the orbital variability. [Strohmayer et al. 2018]

A NICER Look Going Forward

Ultracompact X-ray binaries like IGR J17062–6143 are interesting for a variety of reasons. Since the donor star in these systems are sometimes helium rich, when one sees signatures of carbon and/or oxygen in their stellar spectra, we can learn about how the core of the star has evolved through helium burning. This is one of the topics Strohmayer et al. plan to investigate in their next work. Future NICER observations of IGR J17062–6143 will also allow astronomers to watch for enormous helium-powered thermonuclear explosions on the surface of the neutron star. Additionally, observations of X-ray pulsations from these systems will help astronomers probe the fundamental physics associated with neutron stars and their surrounding environments. With such an exciting discovery from the first set of NICER data, we can expect that there will be more remarkable things ahead!

About the author, Aaron Pearlman:

I am a Ph.D. candidate in Physics at Caltech. My research focuses on searching for new pulsars near the center of the Galaxy using JPL’s Deep Space Network radio dishes in the southern hemisphere. I am also interested in studies of magnetars, FRBs, gravitational-wave searches, and high-energy observations of compact objects. When I’m not hunting for pulsars, I can usually be found hanging out with dogs or trying the latest vegetarian cuisine Los Angeles has to offer!

reionization

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 neutral fraction of hydrogen in the IGM at redshift 7.5
Author: Austin Hoag, Maruša Bradač, Kuang-Han Huang, et al.
First Author’s Institution: University of California, Los Angeles
Status: Submitted to ApJ

The epoch of reionization (EoR) refers to a period in the universe’s history in which the element hydrogen, which to this day constitutes the majority of the baryonic matter in the universe, transitioned from being mostly neutral to mostly ionized (Figure 1). This transition occurred by a fairly intuitive process. Early on in the universe’s history, since there were no stars or galaxies to produce light, there were no energetic photons present to dislodge the electrons orbiting the nuclei of hydrogen atoms. However, once stars and galaxies began to form, there was an increase in the availability of such photons — and once the universe was roughly 1 billion years old, nearly 100% of hydrogen atoms had been ionized.

Figure 1: Timeline of the history of the universe, showing the EoR (click to enlarge). The term redshift in the x-axis label refers to the reddening of light from its rest color as a result of relative motion between the emitter and the observer. It is also written as “z“. [NAOJ]

Studying reionization presents an interesting puzzle for astronomers because we know the starting point (almost all hydrogen is neutral) and the ending point (almost all hydrogen is ionized), but have little idea of what path the universe took on its journey between the two. Did reionization start early or late in the universe’s history? Did it progress quickly or slowly? When did the various sources of ionizing photons (quasars, galaxy clusters, Population III stars, etc.) become abundant enough to have a significant effect on the transition?

One reason why these seemingly basic questions are still open is that astronomers mainly study light, and neutral hydrogen is notorious for absorbing lots of it. In fact, any photons with wavelengths of less than 912 Angstroms, the Lyman limit, will be absorbed by neutral hydrogen. Effectively, the neutral hydrogen that defines the EoR prevents it from being studied. This has forced astronomers to devise creative ways of studying this period of time.

Today’s paper covers one team’s strategy to measure the neutral hydrogen fraction, a value that is used to characterize the amount of remaining neutral hydrogen compared to the total. If this fraction equals one, then all the hydrogen in the universe is neutral. If it equals 0, then all the hydrogen in the universe is ionized. The larger the neutral hydrogen fraction, the deeper into the EoR measurements are probing.

Methods

The investigation detailed in today’s paper relied on a combination of observations and simulations of reionization. First, the authors observe the brightnesses of faint galaxies situated well within the EoR. Second, the authors use a simulation of reionization to model how the neutral fraction affects the observed brightness of galaxies. For the third and final step, they perform a statistical analysis comparing the observed and modeled brightnesses to find the neutral fraction at the redshift of their observed galaxies.

To obtain their sample of faint galaxies, the authors used MOSFIRE to observe several large galaxy clusters to target distant lensed background galaxies (Figure 2). A byproduct of general relativity, gravitational lenses are caused by massive foreground objects like galaxy clusters bending space. This bending effect focuses and amplifies the light of objects behind the lens, magnifying dim background galaxies and making them easier to detect. Singling out the lensed galaxy population in their data, the authors map out the distribution of brightnesses of these galaxies at a particular wavelength of light, called Lyman alpha (Lyα). Using Lyα is important both because it can be incredibly bright over large distances, and because the transmission of Lyα photons through space is strongly affected by the presence of neutral hydrogen.

Figure 2: One of the observations used in today’s paper, showing the contour of a modeled gravitational lens (orange line) and the locations of the authors’ targeted galaxies. [Hoag et al. 2019]

Using their observations and the modeled Lyα transmission from simulations, the authors use a Bayesian framework to infer the neutral hydrogen fraction contemporary with their observed Lyα-emitting galaxies. Inputting their observed Lyα measurements and corresponding galaxy ultraviolet luminosities into the model, they find that it is most likely that the neutral hydrogen fraction at a redshift of z = 7.6 is about 0.88. In other words, when the universe was about 700 million years old, its hydrogen was about 88% neutral. This result is suggestive of a late and rapid reionization scenario, compared with some theories and simulations that have reionization approaching this neutral fraction at much higher redshifts.

Figure 3: The average hydrogen neutral fraction obtained in different studies; the legend indicates the method used to find the values. The results from today’s paper are indicated by the red star, located markedly above most of the other measurements. [Hoag et al. 2019]

The Epoch Conclusion

This finding is an especially important result given that the neutral fraction is strongly biased towards the environment that sources the observations, and most observations from the EoR are of unusually luminous objects. For example, previous observations of quasar spectra at similar redshifts have been suggestive of much lower neutral fractions (see e.g. “QSO damping wings” in Figure 3). Discrepancies like these could arise because the quasar itself is contributing ionizing photons towards reionization, decreasing the local neutral fraction. The authors’ choice to target faint galaxies, which are much more common than quasars, may be preferentially targeting a more representative region of space, thus giving a better estimate of the global neutral fraction.

There are a multitude of methods being used to measure the neutral hydrogen fraction during reionization, and every last one of them is crucial to understanding the early history of the universe. Today’s paper demonstrates the application of a powerful tool that can be added to the high-redshift astronomer’s arsenal.

About the author, Caitlin Doughty:

I am a fourth year graduate student at New Mexico State University. I use cosmological simulations to study galaxy evolution during the epoch of reionization, with a focus on metal absorption in the circumgalactic medium.

trans-Neptunian object

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: Discovery and Dynamical Analysis of an Extreme Trans-Neptunian Object with a High Orbital Inclination
Author: J. C. Becker, T. Khain, S. J. Hamilton, et al. (DES Collaboration)
First Author’s Institution: University of Michigan
Status: Published in AJ

Three years ago, Mike Brown and Konstantin Batygin published their seminal paper predicting a massive, undiscovered Planet Nine. The publication was quickly picked up by the media and reinvigorated the study of objects beyond the orbit of Neptune, appropriately called trans-Neptunian objects (TNOs). Among the TNOs are Kuiper Belt objects, which have well-behaved orbits, and “extreme” TNOs (ETNOs), which have highly eccentric and inclined orbits.

There are many open questions about ETNOs. Were they born extreme or scattered into extreme orbits later in life? How long will their orbits be stable? How many ETNOs even exist? Does the existence of Planet Nine explain or complicate the population of ETNOs? Few researchers were asking or trying to answer these questions until recently, when the search for Planet Nine uncovered a batch of ETNOs that are more exciting than previously thought.

The Search for Planet Nine Continues

Many telescope surveys have tried without success to find Planet Nine in the three years since the theory was published, leading some astronomers and groups to question the hypothesis. Searching for Planet Nine requires special imaging techniques to identify slowly moving, cold, small objects from a trove of astronomical images, which has had the side effect of producing more discoveries of other types of TNOs.

This is where the Dark Energy Survey (DES) comes in. DES’s five years of high-resolution images have been put through a special pipeline designed to pick out only TNOs through a method called difference imaging. This technique is combined with machine learning to find the faintest TNOs possible and then “connect the dots” of each detection into complete orbits.

Extreme … In a Good Way

This paper reports on DES’s discovery of 2015 BP519, the “most extreme” TNO yet, because it has a highly elliptical orbit with an eccentricity of 0.92 and is inclined a whopping 54° out of the plane of the planets.

The high inclination of this ETNO is puzzling because the solar system formed from a disk, so something would have had to severely disturb the orbit of 2015 BP519. To test that hypothesis, the authors ran a simulation of 2015 BP519 forward and backward in time, showing how it would change orbit by interacting with the current solar-system objects.

Figure 1 shows many individual simulations (red lines) of how orbital inclination (i), eccentricity (e), semi-major axis (a), and perihelion (q) vary over billions of years. Looking backwards in time from today, the simulated inclination and eccentricity do not vary significantly. To the authors, this suggests whatever perturbed this ETNO is missing from the model.

Figure 1. Numerical simulations of 2015 BP519 forward and backward in time interacting with the gas-giant planets. Only the gas giants are simulated because the terrestrial planets contribute negligibly to the overall angular momentum of the solar system. Each red line represents one simulation. Inclination (i), which is the tilt of the orbit, and eccentricity (e), which is the measure of how elliptical the orbit is, do not vary much over billions of years. [Becker et al. 2018]

The authors then added Planet Nine into their simulation and ran it forward in time; results are shown in Figure 2. Inclination, eccentricity, semi-major axis and perihelion all smear out over time, which means that in some simulations, interactions with Planet Nine could bring 2015 BP519 back into the plane of the planets and into a more circular orbit. By the same logic, when 2015 BP519 was born in the plane of the solar system, interactions with Planet Nine over billions of years could be one potential method to scatter it into an extreme orbit. The authors only ran the simulations for Figure 2 forward because the history of Planet Nine is entirely uncertain. It is also possible, given the wide range of simulation outcomes, that Planet Nine had nothing to do with making 2015 BP519 so extreme; these results are merely consistent with the Planet Nine hypothesis.

Figure 2. Simulations of 2015 BP519 as Figure 1, but this time including Planet Nine. Inclination and eccentricity vary far more wildly here, showing how the presence of Planet Nine might explain how 2015 BP519 became so extreme. [Becker et al. 2018]

A Natural Fit

Another analysis of interest from this paper is provided in Figure 3, which plots all known TNOs, where bluer color indicates more “extremeness.” Semi-major axis is plotted on the x-axis versus the orbital elements on the y-axis. These quantities describe the orientation of the orbit in physical space. Figure 3 is quite detailed, but the important point is the higher density of dots in the shaded regions, which was the original impetus for the Planet Nine theory. 2015 BP519, marked by a star, adds another data point to support this theory.

Figure 3. Visualization of orbital elements on the y-axis versus semi-major axis on the x-axis for all known TNOs. Bluer dots represent more extreme orbits. BP519 coincides with the clustering of TNOs in the shaded regions that inspired the Planet Nine hypothesis. [Becker et al. 2018]

The bottom line is that these simulations and results are consistent with a massive ninth planet — but that is a far cry from requiring the existence of Planet Nine. Considering the difficulties in simulating the formation of planetary systems, the authors conclude that finding more TNOs and ETNOs would aid considerably in determining the conditions under which our solar system (and other planetary systems) formed. Regardless, 2015 BP519 is another in a growing set of bizarre objects that occupy the outer reaches of our solar system.

About the author, Will Saunders:

I am a first year Ph.D. student at Boston University, where I am trying to decide what to study. I received my Bachelors in Physics from the University of Pennsylvania. In my free time I enjoy listening to podcasts, visiting museums, and tasting new wines.

voids

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 Local Perspective on the Hubble Tension: Local Structure Does Not Impact Measurement of the Hubble Constant
Author: W. D’Arcy Kenworthy,  Dan Scolnic, and Adam Riess
First Author’s Institution: Johns Hopkins University
Status: Submitted to ApJ

Why so Tense?

Feeling a bit tense these days? So is the value of the Hubble constant. This parameter, written as H0, governs the rate of expansion of the universe, caused by some unknown dark energy. Despite cosmologists’ best efforts at massaging out the knots, H0 has long suffered from a tension between competing measurements. Today’s paper uses more precise data to reevaluate one possible cause of this tension.

The Hubble constant is the proportionality factor between the distance of a galaxy and the speed with which it moves away from us; galaxies that are further away move away increasingly faster, as expected from the expansion of space. We measure H0 in units of km/s/Mpc, which I like to think of as the clogs of the unit world: clunky-looking but really quite sensible. It means that for every megaparsec (Mpc) further away you look, the galaxies there appear to speed away faster by H0 kilometers per second.

There are two main independent ways H0 has been measured: from the cosmic microwave background (CMB) and from the cosmic distance ladder. The former measurement was most recently made by the Planck satellite; for details, check out this astrobite. This gives a value of H0 = 67.4 ± 0.5 km/s/Mpc.

Cosmic distance ladder

Figure 1: The cosmic distance ladder. We use stellar parallax, Cepheid variable stars and Type Ia supernovae (among others) as rungs along the ladder to measure distances to sources. [NASA/ESA]

For the latter measurement, we can just plot the speed of each galaxy as a function of its distance away from us. Fitting a line to this relation gives a value of H0 = 73.52 ± 1.62 km/s/Mpc. At first glance this isn’t too shabby, but looking closer, it disagrees with the CMB measurement by a quite statistically significant 3σ. Either there is some unknown physics at play, or at least one of our two methods is wrong.

A Void in the Distance Ladder

One place an issue could be hiding is in how we measure the distances to the galaxies, with what is known as the cosmic distance ladder, shown in Figure 1. The latest measurements are based on on Type Ia supernovae, which are fairly reliable standard candles. But to calibrate these distances, we need other more close-by candles, often Cepheid variable stars in local galaxies. You can guess where this is going: to calibrate the Cepheids, we have to keep stepping down the ladder. If any of these ladder rungs are “broken,” the more distant measurements will have a systematic offset.

Today’s paper considers what would happen if our distance measurements to nearby galaxies were off. This could be the case if the Milky Way were at the center of a local void (an under-density of galaxies), also known as a Hubble Bubble. This would cause the surrounding galaxies to be more strongly drawn towards higher-density regions, away from us. The extra pull would make the value for H0 that we measure locally higher than the true value; fixing this would bring it closer to the CMB measurement.

A Drop-Off in the Hubble Constant?

The authors use a sample of 1,295 supernovae covering a range of distances to probe the local structure with higher precision than has been done previously. They plot a modified Hubble diagram, known as a magnitude-redshift diagram (Figure 2), from which they calculate the value of H0.

supernova Hubble diagram

Figure 2: A Hubble diagram of the supernovae in today’s paper. Rather than the classic plot of galaxy velocity as a function of distance, with the slope of the points giving the value of the Hubble constant, this uses magnitude as a proxy for distance, and the x-axis is related to the velocity. The Hubble constant can be calculated from the x-intercept. [Kenworthy et al. 2019]

To investigate if there could be a void altering their measurement, they check how the Hubble constant changes with redshift. If there is a sharp drop-off in the value of H0, that could suggest a void with an edge at that redshift. The authors measure this by splitting the supernovae into two bins, above and below a given redshift zsplit, and calculating the difference in H0 between these samples.

Devoid of Voids

The results of this are shown in Figure 3. The authors find that the biggest change is at z = 0.023, but only to a significance of <2σ. This means that any inhomogeneities in the local density would have only a small effect on their measurement of the Hubble constant. All of the differences are much weaker than the offset needed to resolve the tension with CMB data, so voids clearly can’t explain the entire discrepancy.

Hubble constant as a function of void redshift

Figure 3: The change in the value of the Hubble constant as a function of the redshift of a potential void edge. The red crosses are voids predicted in previous works. The small changes disfavor the idea that a local void at any redshift has a significant effect on the Hubble constant. [Kenworthy et al. 2019]

To see if voids could still make some difference, the paper revisits two void models that have been predicted in previous works. They are at redshifts of z = 0.05 (KBC) and z = 0.07 (WS14), plotted as red crosses on Figure 3. They find that with their updated analysis, evidence for the voids evaporates: the changes in H0 at both of these redshifts disfavor the void models by 6.2σ and 4.5σ respectively.

While the paper only investigates a simple, sharp-edged void model, it makes a strong case that local voids aren’t significant to the measurement of the Hubble constant. Future supernova observations will allow us to probe other systematics that may be just the masseuse we need to relieve the Hubble tension.

About the author, Kate Storey-Fisher:

Kate is a PhD student in the Center for Cosmology and Particle Physics at New York University. She studies the large-scale structure of the universe using cosmological simulations and galaxy surveys. She is still waiting for the galaxies to respond to the SurveyMonkey she beamed to them.

NGC 1052

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: A Second Galaxy Missing Dark Matter in the NGC 1052 Group
Author: Pieter van Dokkum et al.
First Author’s Institution: Yale University
Status: Submitted to ApJL

Last March, a team of astronomers led by Pieter van Dokkum announced the discovery of a galaxy with almost no dark matter at all (for a great summary of the original discovery, check out this astrobite). The leading theories of galaxy formation rely on a significant dark-matter component, known as a dark matter halo, to create a galaxy, so this unexpected observational result naturally sparked a healthy amount of debate and discussion, both in the Twittersphere and in subsequent publications (and we have an astrobite for that too!). Since then, the original results have been scrutinized, criticized, and defended, and multiple follow-up studies have been conducted to try and provide additional evidence for or against the discovery. Today we discuss the latest exciting development in this ongoing saga: the discovery of a second galaxy that also appears to be severely deficient in dark matter.

A Quick Refresher on How We Got Here

Galaxies are a bit like icebergs: the part we can see is very small compared to the part that is hidden. For galaxies, it is possible to generate a rough estimate of the amount of visible, or baryonic, matter by adding up all of the light we receive and converting it into an equivalent stellar mass (there are many caveats to this estimate, as it depends strongly on which types of stars are responsible for the majority of the galaxy’s light, but all we need here is a rough estimate).

However, the stellar mass is just the tip of the galactic iceberg. It is possible to measure the total mass of the galaxy (including matter that does not emit light like a star), by measuring how fast things are moving in and around it due to the influence of gravity. For a spiral galaxy like the Milky Way, in which most objects are rotating in the same direction, estimating the mass is a simple as measuring the circular velocity of objects far from the center and applying the virial theorem to obtain the mass. In elliptical and dwarf galaxies, the stars are moving in random directions and the average velocity may be near zero, but it is possible to take advantage of the velocity dispersion to produce a similar estimate of the total mass. Because this mass estimate involves measuring how things move in galaxy, it is called the galaxy’s dynamical mass.

It turns out that a galaxy’s dynamical mass is typically at least 5–10 times — and in some cases up to several hundred times — larger than its stellar mass. The reason for this huge discrepancy, according to most astronomers, is the presence of a large, completely invisible component of the galaxy called a dark-matter halo. Our current understanding of galaxy formation and large-scale structure is rooted in the assumption that all galaxies form within dark-matter halos (alternatively, some prefer to do away with dark matter and instead tweak the fundamental physics with theories like Modified Newtonian Dynamics or Modified Gravity).

One way or another, the large dynamical mass-to-light ratio is currently viewed as a defining characteristic of a galaxy. It therefore came as quite a surprise last March when van Dokkum et al. announced the discovery of a galaxy named NGC1052-DF2 with almost no dark matter at all! By measuring the velocities of globular clusters orbiting DF2, the authors were able to show that the stellar mass and the dynamical mass had nearly the same value.

The unexpected discovery received significant amount of backlash; everything from the statistics in the original paper to the use of globular clusters as tracers was called into question, and even the hype the paper received drew some criticism. Between then and now, several important follow-up studies were conducted by the original team in an attempt to bolster their results. Particularly noteworthy is a paper published earlier this month that used observations from the Keck telescope to estimate the dynamical mass of DF2 in an independent manner, by measuring the velocity dispersion from diffuse stellar light instead of globular clusters. The final result from this paper was fully consistent with the original dynamical mass measured for DF2 using globular clusters, further strengthening the original conclusion.

But after the many follow-ups, one central concern still remained for van Dokkum’s group — what if this galaxy was just a statistical fluke? Perhaps DF2 is simply a satellite galaxy that has already undergone tidal stripping and lost most of its dark matter; if such a galaxy lay on a very specific orbit, our viewing angle might lead to its classification as an ultra-diffuse galaxy with very little dark matter. Drawing any serious conclusions about galaxy formation and evolution from just one galaxy made even the original authors uncomfortable. That is, until they found…

… Another Galaxy Lacking Dark Matter!

In today’s paper, van Dokkum et al. report the discovery of a second galaxy with little to no dark matter, named NGC1052-DF4 (see Figure 2). It resides in the same group as DF2, and was discovered by the same instrument that was used to study DF2: the Dragonfly Telephoto Array.

Figure 2. Zoomed-in image from the Hubble Space Telescope of NGC1052-DF4. The seven globular clusters whose velocities were used to estimate the dynamical mass of the galaxy have been tagged and are shown close-up in the small panels on the right-hand side. [van Dokkum et al. 2019]

To estimate the dynamical mass of DF4, the authors again measured the velocity dispersion of its globular clusters, just as they did in the case of DF2. Figure 2 also points out the locations of the seven globular clusters around DF4 whose velocities were measured in order to calculate the dynamical mass, and Figure 3 displays the constraints on the velocity dispersion (and therefore, by extension, on the dynamical mass itself) from these seven globular clusters.

As shown in Figure 3, the observed velocity dispersion, centered around ~4 km/s, is fully consistent with that of a galaxy consisting of only stars and no dark matter. Meanwhile, the expected velocity dispersion of a galaxy the size of DF4 that also has a typical dark-matter component is ~30 km/s, which is very strongly disfavored by these observations.

Figure 3.  Constraints on the velocity dispersion of NGC1052-DF4 from the seven globular clusters. The solid and the dashed lines represent two different approaches used by the authors to perform the statistical analysis, and both suggest a velocity dispersion of approximately 4 km/s. The vertical dotted line represents the expected velocity dispersion from DF4’s stellar mass alone (approximately 7 km/s), comfortably within the 1-sigma range for the observed velocity dispersion. The expected dispersion for a normal dark-matter halo with the observed stellar mass is approximately 30 km/s, which is highly disfavored in this analysis. [van Dokkum et al. 2019]

Many questions and concerns still remain, and the next item the authors plan to investigate is why both DF2 and DF4 seem to have such a uniquely numerous and bright population of globular clusters. But with these most recent observations, the authors can now boast two galaxies within the same galaxy group that seem to be severely dark-matter deficient. For both of them to be a statistical anomaly is significantly less likely than before DF4 was discovered. The authors conclude on the optimistic note that “one is an exception and two is a population.”

If true, the confirmation of a new population of galaxies like DF2 and DF4 could raise many interesting questions about our understanding of how these galaxies form: did a past encounter with another galaxy cause the stellar and the dark-matter components to separate? Is tidal stripping by a larger galaxy a possible mechanism for this? Could it have formed without any dark matter in the first place?

The discussion about galaxies that lack dark matter and their possible origins is still far from being resolved. While we wait for additional observations and papers scrutinizing these latest results, Twitter provides plenty of interesting discussions to keep us occupied (see here for a very recent example). Yet amid the new discoveries and the lingering concerns, one thing seems absolutely certain: the glorious process of scientific discourse will continue to unfold, and all we have to do is sit tight and enjoy the ride.

About the author, Tomer Yavetz:

I am a second year graduate student at Columbia University, where I currently focus on galactic dynamics, resonances and chaos, galaxy simulations, and the nature of dark matter. I grew up in Israel, received my bachelor’s degree from Princeton University, and spent three years solving earthly problems as a business management consultant before deciding to return to stargazing. When I’m not busy thinking about colliding galaxies and writing horoscopes, you can usually find me either cooking, eating, or exploring new running routes through the concrete jungle that is NYC.

TESS

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: A HOT SATURN ORBITING AN OSCILLATING LATE SUBGIANT DISCOVERED BY TESS
Author: Daniel Huber, William J Chaplin et al.
First Author’s Institution: Institute for Astronomy, University of Hawai’i
Status: Submitted to AAS Journals

NASA’s space mission TESS is currently hunting for new exoplanets in the southern hemisphere sky. But while its primary aim is to find 50 small (radii less than 4 Earth radii) planets with measurable mass, there is a lot of other interesting science to do. Today’s paper presents the discovery of a new exoplanet that is quite precisely characterised thanks to the complementary technique of asteroseismology used on the same data.

Meet TESS

TESS will survey stars over the entire sky, studying 26 strips for 27 days each. Data for selected bright stars is downloaded to provide data points every 2 minutes (i.e., a 2-minute cadence) and then processed through a pipeline to produce light curves. Another pipeline detects transit-like signals in these lightcurves — and it recently identified TOI-197.01 as a planet candidate (see Figure 1a). 

Is It an Exoplanet?

The authors used high-resolution imaging by the NIRC2 camera on the Keck telescope to rule out companion stars that could produce a similar light curve. An intense spectral monitoring campaign of 111 spectra from 5 different instruments in a seven-week period let them search for periodic Doppler shifts in the stellar spectrum caused by the mass of another object tugging on the star. The mass they calculated from these radial velocities (seen in Figure 3) confirmed that TOI-197.01 is an exoplanet.

Stellar Pulsations

Photometry from space is not only useful for finding exoplanets: Kepler could detect the periodic changes in stellar brightness caused by stellar pulsations or ‘star quakes’. Asteroseismology, the study of these pulsations, allows astronomers to investigate the inner structure of bright stars and calculate their key properties, including radius and mean density, very precisely. Astronomers expected they could also study stellar pulsations using TESS data.

After removing the transit signal from the TESS light curves (giving Figure 1b), the light curve is Fourier transformed from time (days) into frequency (µHz), giving the power spectrum seen in Figure 1c. Modeling the stellar pulsations along with the stellar granulation and white noise (see Figure 1c), the authors then ‘smoothed’ the power spectrum to identify the location of the tallest peak, i.e. the frequency of maximum power at 430 µHz, and its height, or power.  

TESS lightcurve of TOI-197

Figure 1: The TESS lightcurve of TOI-197. a) Raw TESS lightcurve showing two transits marked by grey triangles. b) Corrected TESS lightcurve with transits and instrumental effects removed. c) Power spectrum of the corrected lightcurve, where dashed red lines show the granulation and white noise. The solid red line is a fit to these as well as the stellar pulsations. [Huber et al. 2019]

The authors converted the ‘maximum’ power into amplitude and plotted this against the frequency of maximum power. By comparison against 1,500 stars from the Kepler mission they confirmed it had solar-like oscillations. Another important value is the large frequency separation, found by identifying the difference in frequency between the radial mode peaks. These are marked blue in Figure 2 and have a value of 29.84 µHz.

Power spectrum of TOI-197.01

Figure 2: a) Power spectrum of TOI-197.01 in the region of frequency space showing oscillations. Vertical lines mark identified individual frequencies, with blue showing the radial modes. b) Blue circles represent the radial modes that line up vertically when the difference between them is 28.94 µHz, illustrating the large frequency separation. Figure repeats in the x axis about 0. [Huber et al. 2019]

Modeling Stellar Properties

The authors then used stellar-evolution and oscillation codes to model the stellar properties. The luminosities for the model were calculated by combining the Gaia parallax with photometry from many different catalogues. They also input properties they modeled from the spectra — temperature, surface gravity (log g), and metallicity — and combined them with the individual frequencies and large frequency separation from asteroseismology. This resulted in two preferred models: i) a lower mass, older star (1.15 solar masses, ~6 Gyr old) or ii) a higher mass, younger star (1.3 solar masses, ~ 4 Gyr old). An independent constraint on surface gravity from an autocorrelation analysis of the light curve favours a higher mass model. Thanks to asteroseismology, the final estimates of stellar parameters have small uncertainties: radius (2%), mass (6%), mean density (1%), and age (22%).

Characterising the Planet

TOI-197 light curve

Figure 3: Data for TOI-197 folded on the best period of 14.3 days. Top: the TESS lightcurve. Bottom: radial velocity curve. [Huber et al. 2019]

Using the mean stellar density from asteroseismology, the authors jointly fit the photometric and radial-velocity data to obtain the planet properties, including period, radius, and mass. Figure 3 shows both sets of data folded on the best period of 14.3 days. The mass ratio is calculated from the maximum amplitude of the radial-velocity data. Combining this with the modeled stellar mass gives a minimum planet mass 35% lighter than Saturn. The transit depth gives the radius ratio, which combined with the modeled star radius means TOI-197.01 has the same radius as Saturn.

A Hot Saturn and a Bright Future!

The result is TOI-197.01 is a hot Saturn orbiting a late subgiant/early red giant star. The combination of spectra and the large frequency separation from asteroseismology shows the star has just started ascending the red giant branch. TOI-197.01 represents the starting point before gas giants reinflate due to the strong flux from their evolved stars. TOI-197.01 is significant as the first transiting planet orbiting a late subgiant/early red giant with detected oscillations measured by TESS, and only the 6th ever discovered (with the others found by Kepler). Indeed, fewer than 15 transiting planets are known around red giants in total.

This is an exciting result as it shows that even with only 27 days of data, TESS should allow us to study the oscillations of thousands of bright stars in the 2-minute cadence data. TOI-197.01 is also one of the most precisely characterised Saturn-sized planets, with density constrained to 15%, demonstrating what we can gain when we can ‘listen’ to exoplanet host stars.

About the author, Emma Foxell:

I am a PhD student at the University of Warwick. My project involves searching for transiting exoplanets around bright stars using telescopes on the ground. Outside of astronomy, I enjoy rock climbing and hiking.

Lucy

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: Light Curves of Lucy Targets: Leucus and Polymele
Author: Marc W. Buie, Amanda M. Zangari, Simone Marchi, Harold F. Levison, Stefano Mottola
First Author’s Institution: Southwest Research Institute
Status: Published in AJ

Asteroids, meteoroids, meteors, meteorites. Usually when we talk about these small chunks of debris and rock in the solar system, it’s about another possible apocalypse scenario. Studies of rocky objects that may pass near Earth’s orbit (near-Earth objects, or NEOs) are of obvious importance for the safety of humanity, but they are only one minor subset of the small bodies in our solar system. Most of the asteroids in our neighborhood live in the Asteroid Belt, a region between the orbits of Mars and Jupiter, and they’re referred to as “main-belt asteroids”. There are also large populations trailing Jupiter in its orbit (the Trojan asteroids) and floating out in the outer solar system near Neptune (the Centaur asteroids).

But apart from the potential threat posed by NEOs, why study these plentiful, seemingly uninteresting hunks of rock and metal that we will likely never encounter on Earth? It turns out that they actually serve as an important window into the formation of the solar system, providing us with information on how the planets formed and what our early solar system was made of. Since we have the chance to get up close and personal with the planets and asteroids nearest to us with rovers and other probes, scientists use this information to infer how other planetary systems form as well. In recent history, we’ve visited most of the major planets with satellites, such as Voyager or Cassini, or rovers, such as Curiosity on Mars; however, the smaller debris is still largely unexplored. The New Horizons mission provided a glimpse into icy debris in the outer solar system when it imaged Pluto and a Kuiper-Belt object (2014 MU69) in detail for the first time, and both NASA’s OSIRIS-REx and JAXA’s Hayabusa missions are working on returning samples from near-Earth asteroids.

A new asteroid mission has begun preparation as well, targeting multiple asteroids in the further-out Trojan group near Jupiter. The Lucy Discovery mission plans to visit multiple Trojans (actually, the largest number of independently orbiting objects ever visited by a single probe), including Leucus and Polymele, whose flybys are scheduled for September 2027 and April 2028 respectively. Until that date, though, astronomers are busy preparing for the mission and trying to gather all the data on these objects that we can from Earth. The authors specifically investigate Leucus and Polymele, using their light curves to tease out information about their color, composition, orbit, and reflectivity.

Lucy's orbital path

Figure 1: Lucy’s planned orbital path, illustrating the location of the Trojans in Jupiter’s orbit. [SwRI]

A light curve traces the variation in the light we receive from an object over time, and for asteroids, we’re mostly seeing light reflected from the Sun. One of the key parameters, then, is how reflective the asteroid’s surface is – its albedo. Tracing the variations in intensity and finding periodic patterns can also determine the rotation period of an asteroid; imperfections in the surface and any non-spherical shape can lead to bumps and wiggles in the light curve, and by tracking how long it takes to see the same imperfection again, we find how long the asteroid takes to rotate around its own axis. By collecting light in different wavelengths, astronomers can also derive colors, comparisons of how much light we get at one wavelength versus another.

For Leucus and Polymele, this research group used telescopes in the Las Cumbres Global Telescope Network (LCOGT), specifically two telescopes at Cerro Tololo in Chile and one at MacDonald Observatory in the United States. These images were taken in the red part of the spectrum of visible light, since asteroids tend to be brighter at longer wavelengths. After taking multiple data images over time, the results of the brightness of the asteroid vs. time are plotted into a light curve, as shown in Figure 2.

Leucus light curve

Figure 2: A light curve for Leucus, clearly showing its variability. [Buie et al. 2018]

Through their observations of Leucus, the team determined that it has a very long rotation period, and it may actually be a binary system. Future observations from the ground will be needed to determine if it is or not. If it is a binary, this can provide scientists with even more information about when it formed; in the early solar system, when more debris was flying around, it would have been easier to form a binary than it is now. From its colors, Leucus is also determined to be a “primitive” asteroid; this type of asteroid is very dark (even darker than coal!), and may be relevant to the question of how life began on Earth because they are thought to carry organic, carbon-rich material. Unfortunately, this paper reports that Polymele is still not well-understood — likely because it is so small and might be nearly spherical, providing very little variation in its light curve. It’s also extremely dark, though not as dark as Leucus.

Although these may seem like small steps at first — determining characteristics of one or two rocks in the vast solar system — they are actually stepping stones (literally!) to understanding the population of asteroids that surrounds us. Asteroids may be what brought life to Earth, and they are intact remnants of the story of planet formation. The key to a successful space mission is minimizing risk, and a large component of that is knowing what you’re getting into when you’re millions of miles away in space; studying these asteroids will prevent the spacecraft from encountering physical hazards (like collisions), and will inform what data we need to take in-situ to better understand them. By studying the future targets of the Lucy mission now, the lander, and the humans running the mission, will be prepared for its eventual encounter with them.

About the author, Briley Lewis:

Briley Lewis is a first-year graduate student and NSF Fellow at the University of California, Los Angeles studying Astronomy & Astrophysics. Her research interests are primarily in planetary systems — both exoplanets and objects in our own solar system, how they form, and how we can create instruments to learn more about them. She has previously pursued her research at the American Museum of Natural History in NYC, and also at Space Telescope Science Institute in Baltimore, MD. Outside of research, she is passionate about teaching and public outreach, and spends her free time bringing together her love of science with her loves of crafting and writing.

Fornax 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: The Binary Fraction of Stars in Dwarf Galaxies: the Cases of Draco and Ursa Minor
Author: Meghin Spencer et al.
First Author’s Institution: University of Michigan, Ann Arbor
Status: Published in AJ

Disclaimer: My advisor is an author on this paper, but somehow I didn’t realize that until after I’d finished writing the entire post. Hopefully you’ll forgive my compromised journalistic integrity! –Mia de los Reyes

Introduction

Stars aren’t usually only children. In fact, we think most stars are born in binary or multiple systems. But just how many binary systems are out there?

Understanding the fraction of binary stars is important in studying galaxies. For example, the number of binaries can affect some estimates of global galaxy parameters like star formation rates, which depend heavily on models of stellar populations. Binary stars can also lead to events like Type Ia supernovae (the thermonuclear explosions of some white dwarf stars with binary companions), so knowing the fraction of binary stars can help us figure out the rates of these events.

Segue 1

Two views of the ultra-faint dwarf galaxy Segue 1, a close neighbor and satellite of the Milky Way. Click to enlarge. [Sloan Digital Sky Survey (left) and M. Geha (right)]

Binary stars might be even more important in the smallest and faintest of galaxies, called ultra-faint dwarf galaxies (UFDs). UFDs are strange systems. They seem to be hybrids between globular clusters and dwarf galaxies, but they’re mostly classified as “galaxies” instead of stellar clusters. This classification is, in part, because UFDs (like other galaxies) appear to be dominated by dark matter based on observations of the velocities of their stars.

How does this work? The velocity dispersion (a measure of how much the stars’ velocities differ from the average motion of the galaxy) is high for a UFD. This suggests that there’s a lot of mass in the galaxy, making the stars orbit quickly around the galaxy’s center of mass. The velocity dispersions are even high enough to imply that there’s more matter in UFDs than just the visible matter: hence, dark matter! This could make UFDs promising targets to probe the physics of dark matter.

But binary stars could mess this all up. As the stars in binaries move around their companions, they can increase the velocity dispersion of a galaxy and make it seem like the galaxy has more mass than it really does. If UFDs have high fractions of binary stars, they might not have as much dark matter as we think!

Today’s Paper: Methods

To see if UFDs actually have lots of dark matter, we want to know if UFDs have lots of binaries. Unfortunately, there aren’t many measurements of the velocities of stars in UFDs. So today’s paper does the next best thing: the authors study the cousins of UFDs, called dwarf spheroidal (dSph) galaxies. These galaxies aren’t quite as puny as the ultra-faint dwarf galaxies, but they still have low masses compared to big systems like our Milky Way.

Lots of stellar velocities have been measured in dSph galaxies at different times, and Spencer et al. take advantage of these data. They come up with a model for the distribution of stellar velocities in a galaxy.  This model takes lots of inputs, including the fraction of binary stars, as well as various parameters that describe binary systems. Using Bayesian techniques, the authors fit the model to the observed velocity distributions of different dSph galaxies. The best-fitting models (shown in Figure 1) then provide estimates of the input parameters, including the fraction of binary stars in each galaxy.

Figure 1. The distribution of changes in velocity (β) for seven different dSph galaxies (different panels). Black line marks the observed distribution and blue shaded region is the best-fit model. For comparison, the red shaded region is a model without binary stars. Most of the seven galaxies appear to have a nonzero fraction of binary stars. [Spencer et al. 2018]

Today’s Paper: Results

The best-fit models give lots of information about the binaries in each dwarf galaxy, which the authors describe and compare to previous literature. For simplicity, we’ll just focus on the binary fraction.

Spencer et al. present the first measurements of the binary fractions of the Draco and Ursa Major dSphs, and they check that their binary fraction measurements for five other dSphs agree with literature values. They then compare the binary fractions for all seven dSphs, and they find that the chances of all dSphs having the same binary fraction are incredibly low! This suggests that we can’t just assume a constant binary fraction for all dwarf galaxies.

Next, the authors go a step further to try to figure out what properties in dSphs affect the binary fraction. They find that the dSphs with smaller velocity dispersions seem to have lower binary fractions (Figure 2)! If this trend extends to UFD galaxies (which have low velocity dispersions), this could mean that UFDs don’t have that many binary stars. That’s good news for dark matter lovers — it means that the velocity dispersions of UFDs might not be heavily contaminated by binary stars, so UFDs could indeed have lots of dark matter.

Figure 2. The stellar velocity dispersion σ of 7 dSph galaxies as a function of their binary fractions f. This suggests that dSph galaxies with higher velocity dispersions may have higher fractions of binary stars. The authors made lots of other plots like these, but this parameter had the most convincing correlation with binary fraction. [Spencer et al. 2018]

It’s hard to make any definitive claims based on only seven dSph galaxies, but these potential results open up lots of questions about binary stars. What physical mechanism causes dSphs to have different binary fractions? Do the trends that Spencer et al. presented for dSphs still hold for ultra-faint dwarf galaxies?

As usual, an interesting scientific result leads to more questions than answers.

About the author, Mia de los Reyes:

I’m a grad student at Caltech, where I study the chemical composition of stars in nearby dwarf galaxies. Before coming to sunny California, I spent a year as a postgrad at the University of Cambridge, studying star formation in galaxies. Now that I’ve escaped to warmer climates, my hobbies include rock climbing and finding free food on campus.

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