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mini-Neptunes

Less than 250 light-years from Earth lie two newly found planets orbiting a star not unlike our own. A new study introduces these discoveries and explores what we may learn from future observations of their puffy atmospheres.

Identifying Ideal Targets

TESS

Artist’s illustration of NASA’s TESS mission observing a system of transiting exoplanets. [MIT]

The Transiting Exoplanet Survey Satellite (TESS) mission was specifically designed to search for transiting planets smaller than 4 Earth radii around bright stars — and it’s already found more than 1,000 planet candidates, with 10,000 expected by mission end. These discoveries will help us to better understand the transition between rocky planets like Earth, which have compact atmospheres, and gaseous sub-Neptunes, which have extended, puffy atmospheres.

With the upcoming launch of the James Webb Space Telescope (JWST), we’d especially like to identify TESS discoveries that make ideal candidates for transit spectroscopy with JWST. Transit spectroscopy allows us to study the atmospheres of nearby planets as they orbit across the face of their bright host star.

TOI-421 light curves

TESS light curves showing the phase-folded transits for TOI-421 b (top) and c (bottom). [Adapted from Carleo et al. 2020]

In a new publication, a team of scientists led by Ilaria Carleo (Wesleyan University; INAF-Astronomical Observatory of Padua, Italy) detail TESS’s identification of a transiting planet candidate in the nearby system TOI-421. By conducting a comprehensive follow-up campaign with ground-based photometry, adaptive optics imaging, and spectroscopy, Carleo and collaborators not only confirmed the TESS candidate, but also discovered a second planet orbiting in the same system.

A Pair of Puffy Planets

We find planets all the time — so what makes TOI-421 b and c worth talking about? Carleo and collaborators’ detailed characterization of the planets shows intriguing properties that could help us learn more about the transition between rocky Earths and gaseous Neptunes.

The inner planet, TOI-421 b, has a low density that’s similar to that of Neptune — despite the fact that the planet’s mass is less than half of Neptune’s. Using atmospheric loss models, Carleo and collaborators demonstrate that this puzzling planet — which lies on a blistering 5-day orbit very close to its toasty host star — should have lost all of its hydrogen-dominated atmosphere early in its lifetime. In spite of this, the TOI-421 b’s low bulk density strongly points to the presence of a puffy hydrogen atmosphere. More study will clearly be needed to better understand what we’re missing about this mysterious planet.

S/N predictions

TOI-421 b and c lie among the 30 most favorable targets for atmospheric characterization, based on their predicted signal to noise with future observations. Click to enlarge. [Carleo et al. 2020]

As for TOI-421 c, this outer planet has roughly the same mass as Neptune, but its bulk density is extremely low — TOI-421 c’s density is less than half of Neptune’s. The authors show that the large radius of this planet and the quietness of its host star should make it an ideal target for followup atmospheric characterization.

Carleo and collaborators’ models suggest that these planets’ extended atmospheres can be probed with ultraviolet observations like those from Hubble; the authors also provide detailed predictions of what we expect to find in the transmission spectra of the two planets from JWST.

Comparison of these predictions to future observations of the TOI-421 system is sure to provide valuable insight from these intriguing, puffy planets.

Citation

“The Multiplanet System TOI-421,” Ilaria Carleo et al 2020 AJ 160 114. doi:10.3847/1538-3881/aba124

ASKAP Milky Way

The Earth, your body, and the electronic device you’re reading this on are all made up of ordinary, baryonic matter. A new study has now used bursts of radio emission to probe whether the outskirts of our galaxy are hiding vast quantities of “missing” baryonic matter.

Missing Matter

dark matter

The relative amounts of the different constituents of the universe. Ordinary baryonic matter makes up less than 5%. [ESA/Planck]

We’ve long known that only about 5% of the content of the universe is ordinary baryonic matter; the remainder is dark matter and dark energy. But when scientists have searched for this baryonic matter in the nearby universe, they found a puzzle: galaxies’ gas, dust, and stars only accounted for a small fraction of their expected baryonic matter.

Our own Milky Way is no exception — it also has a baryon fraction much lower than the overall baryon fraction in the universe. So where are its missing baryons? Were they expelled from our galaxy at some point in the past? Or did the Milky Way retain its baryons — but we haven’t detected them yet?

An Elusive Halo

If our galaxy’s baryons are still around, a likely hiding place is in the Milky Way’s outskirts, in the circumgalactic medium (CGM).

M104

The Sombrero galaxy, M104, provides an example of a galaxy and its halo — the diffuse gas that extends above and below the galaxy’s disk. [ESA/C. Carreau]

When our galaxy formed, gas was dragged inward with the collapsing dark-matter halo, shock heating and forming a surrounding bubble of hot, diffuse plasma — the CGM. This surrounding galactic halo may well contain our galaxy’s missing baryons today, but it’s very difficult to probe; since the gas is diffuse, we can’t measure it directly from within the Milky Way.

A new study led by Emma Platts (University of Cape Town, South Africa) has instead measured the galactic halo’s matter by observing how distant signals interact with the CGM as they travel to us.

Clues from Transients

Platts and collaborators use two types of radio transients to measure CGM distribution: pulsars, which are pulsating neutron stars that reside in our galaxy’s disk, and fast radio bursts, which are brief flashes of radio emission that originate far beyond our galaxy.

pulsar pulses

Pulsars, which typically lie in the galactic disk, emit radiation that sweeps over the Earth like a lighthouse, appearing as pulses. These pulses become dispersed as they travel through the galaxy to reach us. [Bill Saxton/NRAO/AUI/NSF]

Light from these sources travels across space to us, interacting with matter distributed along the way. The interactions slow down longer wavelengths of light more than shorter, causing the signal to spread out. The dispersion measure — the quantification of this spread — therefore tells us how much matter the signal traveled through to get to us.

Probing Our Surroundings

By statistically analyzing the distribution of pulsar and fast radio burst dispersion measures, Platts and collaborators placed bounds on the Milky Way halo’s dispersion measure: its minimum is set by the farthest pulsars, which lie interior to the halo, and its maximum is set by the closest fast radio bursts, which lie far beyond our halo in neighboring galaxies.

So are the Milky Way’s missing baryons hiding in the CGM? We can’t say for certain yet, but the results suggest no, if the baryons are distributed in the same way as the dark matter. The future should hold more certainty though! Our sample of fast radio bursts is rapidly growing, and the authors estimate that once we’ve cataloged several thousand, we’ll be able to bound the content of the Milky Way’s halo more definitively.

FRB dispersion

Schematic illustrating how transient radio signals travel to us. Pulsars (marked by sun symbols) lie in the galaxy, interior to the halo; their signals are dispersed only by the Milky Way’s interstellar matter. Fast radio bursts (marked by lightning symbol) lie in other galaxies; their signals are dispersed by the Milky Way’s interstellar matter, its halo, the intergalactic medium, the host galaxy’s halo, and the host itself. These two types of transients can therefore place upper and lower bounds on the matter in the Milky Way’s halo. [Platts et al. 2020]

Citation

“A Data-driven Technique Using Millisecond Transients to Measure the Milky Way Halo,” E. Platts et al 2020 ApJL 895 L49. doi:10.3847/2041-8213/ab930a

Gaia view

Hertzsprung–Russell (HR) diagrams have been an integral part of astronomy for over a hundred years. Stars at different stages of life occupy different parts of the diagram, which allows us to take in a population of stars at a glance. The Gaia mission provided us with an enormous sample of stars — what can we learn by putting them in a HR diagram?

Have You Heard About HR Diagrams?

Qualitative Hertzsprung-Russell Diagram

A qualitative HR diagram showing where different types of stars live. The colors are roughly true to life. Temperature is given on the x-axis with luminosity or brightness on the y-axis. [ESO]

HR diagrams are fairly simple — they’re just plots of stars’ brightness versus color. But it turns out that stars change drastically as they evolve, moving from one part of the HR diagram to another as they go through different stages of life. This means that if you wanted to learn the rough age of a star, you could just check where it lands on a HR diagram.

It’s also interesting to see where stars cluster on HR diagrams. Some parts of this plot will never be filled in because it’s physically impossible for stars to occupy those spaces. So, by plotting large samples of stars on HR diagrams, we can learn more about stellar evolution.

The Gaia mission is observing an enormous number of stars — 1.7 billion of them! — with very high precision, and a lot of great science has already been done with the first two Gaia data releases. In a recent paper, Wei-Chun Jao (Georgia State University) and Gregory Feiden (University of North Georgia) used Gaia data to study stars that cluster in a region on the HR diagram called the main sequence.

main-sequence gap

The subtracted HR diagram with different types of stars highlighted along with the main sequence gap. The x-axis is (blue – red) Gaia magnitude (i.e., color), and the y-axis is total Gaia magnitude. Click to enlarge. [Adapted from Jao & Feiden 2020]

Analyzing the Main Sequence

Main sequence stars have one thing in common: their fuel is hydrogen. However, they can fall on different parts of the main sequence based on characteristics like their mass, broader elemental composition, or magnetic activity. For instance, a high mass main sequence star will look bluer and brighter than a low mass main sequence star. The Sun (perhaps surprisingly) lands smack in the middle of the main sequence.

With the sheer volume of its data, the Gaia mission has added a third dimension to HR diagrams: density. It’s now clearer than ever which areas of the HR diagram stars avoid. In fact, a previous study led by Jao used Gaia data to identify a gap in the lower (redder, fainter) main sequence. In this new study, Jao and Feiden applied Fourier analysis to study the lower main sequence gap in more detail.

One way to understand Fourier analysis is that it breaks down a signal into component parts, allowing you to identify the most prominent component of the signal. When applied to an image, Fourier analysis can enhance real features and cut down on noise.

Mind the Gap!

fourier transformed residuals

The subtracted HR diagram after the Fourier analysis, with unusual features highlighted by dashed squares. The blue square is the main sequence gap; it’s shown in greater detail in the inset. The white squares labeled A and C show “stripes” across the main sequence. The white square labeled B shows a patchy feature near the main sequence gap. Click to enlarge. [Adapted from Jao & Feiden 2020]

To check for features like the lower main sequence gap, Jao and Feiden simulated HR diagrams that were based on the Gaia data but assumed that stars were distributed in such a way that no features existed. When this simulated HR diagram was subtracted from the real HR diagram, the differences between the two images revealed features like the main sequence gap. Fourier analysis was then used to determine the strongest components of this subtracted image.

Jao and Feiden found that there were more stars above the main sequence gap than below it. The gap was also more “empty” at the blue end than at the red end. They also found “stripes” running across the main sequence and a patchy feature near the main sequence gap.

What could be causing these features? The main sequence gap is likely due to atypical helium fusion, which causes the radius of stars to change. The gap might be less empty on the red side because it’s filled in by younger stars and indistinct binary systems. The other features, however, present an interesting puzzle. Could more detailed stellar models help? Stay tuned! 

Citation

“Fine Structures in the Main Sequence Revealed by Gaia Data Release 2,” Wei-Chun Jao and Gregory A. Feiden 2020 AJ 160 102. doi:10.3847/1538-3881/aba192

water world

Which habitable-zone planets can actually support life? A recent study uses a nearby planet — Proxima Centauri b — to examine how the presence and size of a land mass impacts the habitability of an ocean world.

A Target for Potential Life

In our galaxy, roughly 80% of stars are cool, dim M dwarfs — and one in six of these is thought to host an Earth-sized planet in its habitable zone. But being in a star’s habitable zone doesn’t guarantee a planet’s habitability! M-dwarf habitable-zone planets present valuable targets for observations and models to better understand which of these worlds can support life.

eyeball planet

Artist’s impression of a cold, tidally-locked planet. Ice covers much of the planet’s surface, but the point directly facing the planet’s host star remains ice-free. [NASA/JPL-Caltech]

Most habitable-zone planets around M dwarfs are likely tidally locked: one side of the planet experiences constant day; the other, constant night. Nominally, this would cause only one region of the planet to be heated — the point closest to the star — and the rest of the planet would be locked in darkness and ice. But if the planet is covered in a dynamic ocean, heat can be transported around the planet via ocean currents, affecting the potential habitability of the world. 

Do continents get in the way of this heat transport? And how do land masses affect the circulation of nutrients in the ocean, critical for sustaining ocean-based photosynthetic life? A new study explores the particular case of a tidally locked ocean planet with a continent — and it uses the nearby Proxima Centauri b as a model to do so.

Modeling a Nearby World

At just 4.2 light-years away, Proxima b is the closest known exoplanet and presents an excellent target for future follow-up observations. This habitable-zone M-dwarf planet is probably tidally locked, and estimates of its density have led to speculation that the planet is covered in a large ocean.

ocean transport

Ocean heat transport in the authors’ models for varying continent size; from top to bottom, continents (noted as the white rectangle in the figure) cover 0%, 4%, 22%, and 39% of the planet surface. Continents at the substellar point inhibit ocean heat transport. [Adapted from Salazar et al. 2020]

In a recent publication led by Andrea Salazar, a team of scientists from the University of Chicago has used a general circulation model to explore how heat and nutrients are transported on an ocean-covered, tidally locked Proxima b — both with and without the presence of a land mass in the ocean.

Salazar and collaborators placed a continent at the point on the planet closest to the star — because land masses are thought to migrate to the planet–star axis over time — and tested a range of continent sizes, covering from 0 to 40% of the total planet surface.

Promising Outcomes

The authors find that the presence of a continent decreases how efficiently heat and nutrients are transported from the dayside to the nightside of the planet — the larger the continent, the less efficient the transport. Nonetheless, in all cases, an ice-free ocean is maintained on the planetary dayside, and nutrients are circulated and delivered to the layer of the ocean where photosynthesis is viable, providing ideal conditions for photosynthetic marine life.

This work suggests that the presence of both a dynamic ocean and continents won’t decrease the habitability prospects of tidally locked planets like Proxima b. This is good news as we prepare for future observations with the James Webb Space Telescope, which may provide further insight into this nearby, potentially habitable world and others like it.

Citation

“The Effect of Substellar Continent Size on Ocean Dynamics of Proxima Centauri b,” Andrea M. Salazar et al 2020 ApJL 896 L16. doi:10.3847/2041-8213/ab94c1

Milky Way Center

What model of dark matter best describes our universe? A new study uses a unique region in our own galaxy to constrain one particular model: that of fuzzy dark matter.

A Matter of Modeling

dark matter

There are many models describing the composition and behavior of dark matter, and how its evolution has affected the structure of our universe. [AMNH]

Observations of our universe tell us that only 15% of the universe’s matter is the ordinary baryonic matter that we’re able to see. The remaining 85% is dark matter — mysterious material that has shaped the structure and evolution of our universe via its gravitational interactions, but that doesn’t give off any light.

Because we can’t directly observe it, dark matter is still a relative unknown — and there are many different hypothesized models that describe its nature. Is dark matter hot? Cold? Composed of subatomic particles? Or macroscopic objects like primordial black holes? There’s a model for all of these options, and the best way to test them is to compare their predictions to the actual structure that we observe.

nuclear bulge only

Plot of gas surface density from a simulation showing the formation of the CMZ — seen as the high-density gas ring at the heart of the plot — in the center of the Milky Way. This simulation included a nuclear bulge only, with no dark-matter core from the fuzzy dark matter model. [Li et al. 2020]

Constraints from an Odd Structure

One such constraining structure is a unique region in our own galaxy: the Central Molecular Zone, or CMZ. This extremely dense, rich collection of orbiting molecular gas lies in the very center of the Milky Way and spans just a few hundred light-years in diameter. Observations suggest that the molecular gas clouds orbit in a ring or a disk with a twisted 3D shape, but the thick dust that shrouds the galactic center limits what we can learn about the CMZ directly.

The CMZ’s shape is not its only mystery, however: we also don’t fully understand what caused this odd structure to develop. Past studies of the birth of our galaxy’s structure from a thin disk suggest that formation of the CMZ relies on a combination of the Milky Way’s barred gravitational potential and an especially dense nuclear region.

In a new publication led by Zhi Li (Shanghai Jiao Tong University, China), a team of scientists has now used this picture to constrain a dark matter model that relies on light dark-matter particles concentrated at the center of the galaxy.

Adding Fuzziness to the Milky Way

nuclear bulge + soliton core

Zoomed-in plot of gas surface density from a simulation showing the formation of the CMZ in the center of the Milky Way. This simulation included both a nuclear bulge and a dark-matter core from the fuzzy dark matter model. [Adapted from Li et al. 2020]

Li and collaborators conduct a series of cosmological simulations that model the formation of the Milky Way from a thin disk in a realistic gravitational potential. In some of these simulations, the authors include only a dense nuclear bulge at the center of the galaxy. In others, they also add a galaxy core consistent with the predictions of fuzzy dark matter, a model that describes the universe’s dark matter as very light bosons that exhibit wave behavior on some scales.

The authors show that the structure and dynamics of the CMZ can be reproduced well with only an exceedingly compact nuclear bulge. But the combination of a smaller nuclear bulge and a fuzzy-dark-matter core also neatly reproduces observations, leaving the door open for this dark-matter model.

So is our dark matter fuzzy or not? We can’t tell yet, but Li and collaborators outline some future observations — like pinning down the mass-to-light ratio in the galactic center — that will help us answer this question and better understand what’s going on with that invisible 85% of our universe’s matter.

Citation

“Testing the Prediction of Fuzzy Dark Matter Theory in the Milky Way Center,” Zhi Li et al 2020 ApJ 889 88. doi:10.3847/1538-4357/ab6598

TESS

This week, scientists are celebrating the recent conclusion of the primary mission for the Transiting Exoplanet Survey Satellite (TESS). On Monday, we talked about TESS’s ongoing contributions to exoplanet science. But what else has this spacecraft been up to? Today we’re exploring its addition to the mystery of a puzzling white dwarf.

Puzzling Pollution

G29-38 Debris disc

GD 394 was one of the earliest metal-rich white dwarfs discovered, though others with varying explanations have been detected more recently. In the illustration above, a disintegrating planetesimal accretes onto a white dwarf. [NASA/JPL-Caltech]

During TESS’s primary two-year mission, the spacecraft monitored 200,000 stars, discovering more than 2,000 planet candidates. But other targets also fell under the telescope’s scrutiny — including GD 394, an unusual white dwarf located less than 200 light-years away.

GD 394 has challenged our expectations for white dwarfs ever since its discovery in the 1960s. A white dwarf — a hot, dense stellar remnant — has such a strong gravitational pull that heavier elements are expected to rapidly sink to the white dwarf’s center, leaving only lightweight hydrogen, helium, and sometimes carbon and oxygen for us to identify in its atmosphere.

But early observations of GD 394 revealed an object unexpectedly polluted with heavier metals like silicon and iron in its atmosphere. To explain this, GD 394 would need to be actively and continuously accreting fresh, metal-rich material from some external source that we couldn’t detect.

EUV light curve

This phase-folded EUV light curve — constructed from observations of GD 394 in the mid-1990s made with the Extreme Ultraviolet Explorer satellite — show the strong dip in EUV flux over a 1.15-day period. [Adapted from Wilson et al. 2020]

An Unsteady Source

But GD 394’s metals were only part of its mystery. In the mid-1990s, observations showed that this white dwarf’s extreme ultraviolet (EUV) emission wasn’t steady — it varied in intensity by about 25% with a rapid period of about 1.15 days.

What could cause this variability? The answer remains unclear, as efforts to test various hypotheses have all come up short. A recent study of Hubble observations of GD 394, for instance, found the star’s far-ultraviolet light curve to be steady to within 1%. Had the variability’s source vanished in the last several decades? Or is the variability caused only in EUV light, and not at longer wavelengths?

In a recent publication, scientist David Wilson (McDonald Observatory, University of Texas at Austin) and collaborators now have presented new TESS observations at optical wavelengths that have further deepened the mystery of GD 394.

More Clues, But No Answers Yet

optical light curve

The TESS light curve for GD 394, folded onto the fitted period, shows the subtle sinusoidal variability at optical wavelengths. [Adapted from Wilson et al. 2020]

Wilson and collaborators monitored GD 394 for 52 days across two TESS sectors. From these observations, they identify clear evidence of a ~1.15-day period in the dwarf’s optical light curve — the first evidence of variability for GD 394 outside of EUV wavelengths. Compared to the EUV variability, however, the optical variability is tiny, at a level of just 0.12%! Only TESS’s high sensitivity allowed us to detect this subtle signal.

So what’s causing the long-lived variation of this star? Leading theories that could fit both sets of observations include metal spots caused by channeled accretion, occultation by an outflow from an orbiting (but not transiting) planet, and a magnetically induced hot spot.

Future simultaneous observing with an EUV telescope and a sensitive optical observatory like TESS could help us to differentiate between these models. In the meantime, TESS has cheerfully begun its extended mission — hunting for more planets, characterizing more stars, and exploring more cosmic mysteries.

Citation

“Optical Detection of the 1.1 day Variability at the White Dwarf GD 394 with TESS,” David J. Wilson et al 2020 ApJL 897 L31. doi:10.3847/2041-8213/ab9d7b

gas giant exoplanet

Over the past 25 years, we’ve found thousands of worlds beyond our solar system. Nonetheless, some categories of exoplanets remain elusive — for instance, planets that orbit their hosts on long, slow paths. A new study shows how we might hunt these worlds down.

Observational Limits

gas-giant transit

Artist’s impression of a hot Jupiter transiting across the face of its host star. [ESA/C. Carreau]

Since the first exoplanet discovery a quarter century ago, we’ve found more than 4,000 confirmed planets orbiting other stars. A large number of these discoveries are planets that transit across the face of their host star — most identified by the Kepler Space Telescope or, more recently, by the Transiting Exoplanet Survey Satellite (TESS). These exoplanets are valuable targets because we can use the transits to measure properties like their radii, densities, bulk compositions, and even their atmospheres.

Unfortunately, due to the nature of transit detections, our observations are inherently biased: it’s easier to detect and confirm short-period, large planets, which means we know a lot about hot Jupiters, but relatively little about wide-orbit, cooler planets.

Because the TESS spacecraft observes a typical region for less than a month, planets on wide orbits longer than 30 days will register — at most — a single transit in TESS data before the telescope moves on to the next section of sky. But can we somehow leverage these single transits to learn more about slow, wide-orbit planets?

Avoiding Overbooking

A planet candidate with only one transit detection could be confirmed with radial-velocity measurements of its host star. But high-precision radial-velocity instruments are in high demand! Without precise knowledge of a planet’s period, confirming that planet’s existence and measuring its properties would require a huge observational time investment from already overbooked radial-velocity instruments.

Fortunately, a team of scientists led by Samuel Gill (University of Warwick, UK) has now demonstrated a more efficient way of pinning down a planet after just one TESS transit.

NGTS-11 light curve

Light curve showing the best fit to the transit photometry of NGTS-11 from TESS and NGTS. [Gill et al. 2020]

Two Transits, Fewer Options

Gill and collaborators followed up on a planet candidate that had registered a single TESS transit in September 2018. The team conducted an intense photometric monitoring campaign of the candidate’s host star using the economical telescopes of the Next-Generation Transit Survey (NGTS) facility in Chile — and with 79 nights of observations, they detected a second transit of the candidate 390 days after the initial TESS transit.

From the combined photometric observations, the authors were able to narrow down the possible periods for the planet to just 13 options. Using these constraints, they could then wrap up with brief and efficient radial-velocity measurements to identify the properties of the planet.

A Cool Discovery

NGTS

The array of twelve 0.2-meter robotic telescopes of the NGTS facility in Chile. Only one of these economical telescopes was needed to identify a second transit of NGTS-11 b. [ESO/R. West]

The result? NGTS-11 b (or TOI-1847 b), as the confirmed planet is now named, is a Saturn-mass planet on a wide, ~35-day orbit. With an equilibrium temperature of just 435 K, NGTS-11 b is one of the coolest known transiting gas giants and a valuable target for future transmission spectroscopy to explore its atmosphere.

Gill and collaborators’ identification of NGTS-11 b from just a single TESS transit shows the power of using ground-based photometry to pin down wide-orbit planets. This approach could help us to dramatically expand our understanding of the slower, long-period worlds beyond our solar system.

Citation

“NGTS-11 b (TOI-1847 b): A Transiting Warm Saturn Recovered from a TESS Single-transit Event,” Samuel Gill et al 2020 ApJL 898 L11. doi:10.3847/2041-8213/ab9eb9

starburst galaxy MCG+07-33-027

The flow of gas into and out of a galaxy regulates many of its features, such as its rate of star formation and chemical content. Computer simulations have allowed us to probe these gas flows in greater detail, but there still remains much to uncover. For example, on what scales does gas flow out of actively star-forming galaxies?

Outlining Galactic Outflows

Currently, computer simulations model the flows of gas leaving a galaxy, called galactic outflows, very generally. They are typically informed either by observations or large-scale physics. However, galactic outflows need to be modeled with much finer detail to grasp their full impact on galaxies, which means that observations of outflows need to probe much smaller scales than they currently do.

galaxy J1157

A near-ultraviolet image of the galaxy J1157 taken by the Hubble Space Telescope’s Cosmic Origins Spectrograph in acquisition mode. The white circle indicates the extent of the spectrograph’s imaging capability. [Wang et al. 2020]

Typically, outflows are explored via absorption spectroscopy, which utilizes the fact that the spectra of light shining through a cloud of material will be imprinted with features unique to the cloud’s material. But this technique is only useful when we’re looking “down the barrel” of an outflow such that the outflow absorbs the light behind it — and this configuration prevents us from usefully probing the outflow size.

To study outflow sizes, we need the flip side of absorption: emission. The material spewed out in galactic outflows fluoresces — it absorbs light at specific wavelengths and reemits it at longer wavelengths. A group of researchers led by Bingjie Wang (Johns Hopkins University) traced this emitted radiation from outflows in nearby starburst galaxies using data taken by the Hubble Space Telescope (HST). They then used this emission to explore the sizes of the outflows.

How to Trace an Outflow

Starburst galaxies have much higher rates of star formation than the average galaxy (for example, the Milky Way). This means that their outflows will also be much more prominent, along with the emission used to trace the outflows.

outflow column density

The ratio between the column density of the material within the radius studied by the Cosmic Origins Spectrograph (COS) and the total column density, plotted versus 𝛂, which quantifies how the density of outflow material depends on radius. The color bar shows the ratio between the radius of COS’s observations and the galaxy radius. The galaxies studied are plotted as well, showing that for this sample, 𝛂 is between 1 and 1.5. This means that the outflow contains more material further away from the galaxy. [Wang et al. 2020]

By studying fluorescence emission lines, Wang and collaborators could focus on the material being swept out of the galaxy in an outflow. Comparing the strength of these fluorescence emission lines to the tracers typically used — resonance absorption lines — allowed them to then make inferences about the structure of the outflows and determine whether we’ve been interpreting them correctly.

Line, Please!

Wang and collaborators found that the fluorescence lines are systematically weaker than the resonance absorption lines. This suggests that the fluorescence lines produced within the HST’s view are more often associated with the galaxy’s stationary interstellar medium or the slow-moving central parts of outflows, rather than tracing the fast-moving material in the outer regions of the outflows.

So where are the strong fluorescence lines? Wang and collaborators suggest that since outflows contain more material the further they are from the galaxy, the fluorescence would also appear further away from a galaxy’s center. However, the HST instrument used for this study is not equipped to observe very large areas, meaning that the regions with strong fluorescence likely lie outside of the instrument’s field of view and are being excluded from the data.

This result suggests that outflow rates for distant galaxies are significantly underestimated with the technique we’ve been using to measure them, and they do not account for changing strength with distance from the galaxy. One thing’s for certain: we’re going to need a bigger telescope.

Citation:

“A Systematic Study of Galactic Outflows via Fluorescence Emission: Implications for Their Size and Structure,” Bingjie Wang et al 2020 ApJ 894 149. doi:10.3847/1538-4357/ab88b4

protoplanetary disk illustration

How did our solar system’s planets first form within the swirling disk of gas and dust that surrounded the newborn Sun? One of the best ways to answer this question is watch other solar systems as they form — and the Atacama Large Millimeter/submillimeter Array (ALMA) continues to help us do so.

A History of Large Targets

full DSHARP gallery

Gallery of 240 GHz (1.25 mm) continuum emission images for the disks in the DSHARP sample. The scale bars in the lower right of each image indicate 10 au. Click to enlarge. [Andrews et al. 2018]

When ALMA revealed early observations of the disks of gas and dust around young stars, we were stunned by the exquisitely detailed look this interferometer provided into newborn solar systems. Since then, again and again, ALMA has produced remarkable images of gaps, rings, and spiral arms in disks, all of which hint at how planets might be forming.

Thus far, however, we’ve mostly focused on imaging the especially large disks that give us the best look at disk substructure. As an example, the Disk Substructures at High Angular Resolution Project (DSHARP) survey used ALMA to image twenty large, bright disks with effective radii — the radius that encompasses 68% of the light from the dusty disk — of ~50 au on average.

But the vast majority of disks are faint, and their dusty disks are much more compact, with effective radii of less than 20 au. Do these more typical disks show the same wealth of substructures that we’ve spotted in larger disks? And what can this tell us about planet formation?

GQ Lup disk

ALMA continuum image of the GQ Lup disk. The scale bar at the lower right indicates 5 au. [Adapted from Long et al. 2020]

Stopping the Migration

In a new study led by Deryl Long (University of Michigan), a team of scientists has used ALMA to explore one of these more typical systems: a compact disk with effective radius of 19 au around the star GQ Lup A. The high angular resolution of ALMA’s observations allow the team to resolve the dust emission even in this small disk, revealing a wealth of substructures very similar to those spotted in larger disks.

What do these substructures tell us? One challenge to planet formation theories is that larger dust grains should migrate inward through the disk in a process called radial drift, accreting onto the star before they can clump together to form planetesimals.

Long and collaborators’ observations of the GQ Lup system suggest that pebble-sized dust grains can be trapped by variations in pressure in the disk, halting the grains’ drift and giving them a chance to clump. This means that small disks may have the same opportunity as large disks to form young planets.

The Birth of a Solar System

disk size v. luminosity

Size-luminosity relationship for millimeter continuum sources with disk properties. DSHARP disks are marked with green circles. The new observations of GQ Lup (red diamond) move us toward a discovery space of smaller disks. [Long et al. 2020]

Long and collaborators caution that there could be other explanations for the disk substructures we see in GQ Lup. But if they are indicators of planet formation, this disk provides us with a unique opportunity to learn more about how our own solar system formed.

One of the disk gaps identified by the authors lies at ~10 au, which is roughly the distance of Saturn from our Sun. Studying GQ Lup could therefore reveal how planets like Saturn develop from a dusty disk. What’s more, these observations of GQ Lup indicate that there may be a rich population of young solar-system analogs out there, just awaiting discovery and exploration.

Citation

“Hints of a Population of Solar System Analog Planets from ALMA,” Deryl E. Long et al 2020 ApJL 895 L46. doi:10.3847/2041-8213/ab94a8

quasar

Our universe is filled with distant supermassive black holes that feed on surrounding gas and dust, emitting bright gamma-ray radiation. A new study explores how many of these show periodic patterns in the variations of their high-energy light.

Putting on a Show

At the center of every galaxy lies a supermassive black hole of millions to billions of solar masses. Many of these — like the Milky Way’s own Sagittarius A* — are quiet, largely invisible lurkers. But some are considerably more attention-seeking, actively accreting material, spewing out winds and jets, and emitting radiation that spans the electromagnetic spectrum.

These active galactic nuclei (AGNs) show variability in their light curves on many different timescales — and a few have been caught flickering in a regular pattern.

Why the Patterns?

SMBH binary

Artist’s impression of supermassive black holes that have formed a binary as they’ve sunk to the center of their merged galaxies. [NAOJ]

There are lots of potential explanations for periodic variability in an AGN’s light curve. The black hole may host powerful jets that precess about an axis, driving a lighthouse effect in their emission. The flow of accreting material onto the black hole might wax and wane periodically. And in some cases, the emission we observe might even be coming from a pair of black holes: a supermassive black hole binary whose orbital motion causes periodic patterns.

To better understand the nature of these distant, dramatic sources, we need to study the AGN that show regular variability. But, though a number of studies have explored individual variable AGN, we haven’t yet conducted a large, systematic search for periodic patterns in high-energy AGN light curves. That is, until now.

A Sample of Regular Feeders

A team of scientists led by Pablo Peñil (Complutense University of Madrid, Spain) has leveraged a powerful tool: the Fermi Large Area Telescope (LAT), a space-based gamma-ray detector with a wide field of view that has been scanning the sky consistently for more than a decade. In that time, Fermi LAT has tracked the gamma-ray emission from thousands of AGN, providing Peñil and collaborators with a wealth of data they use to systematically hunt for patterns.

periodic AGN sky map

Sky map showing the locations of the 11 sources with periodic emission (filled symbols) and 13 sources with lower significance (open symbols). Click to enlarge. [Peñil et al. 2020]

By applying ten different algorithms that identify periodicity, the authors discover 11 AGN that robustly show patterns in their variability that repeat on roughly 1- to 5-year timescales, as well as another 13 AGN that show strong hints of periodicity but can’t be confirmed yet with the data we have. The authors’ 11 robust detections include two AGN with previously detected variability and nine new identifications.

Peñil and collaborators’ non-detections are also interesting. There were a number of AGN that, in previous individual studies, were reported to show periodic variability — yet they did not show statistically significant periodicity in the authors’ systematic analysis.

What’s Ahead

So now that we’ve identified periodically variable AGN, what can we do with them? The authors’ consistently identified sample can now be studied to better understand what causes the variability of these sources. We can also use the predictable patterns of these AGNs’ high-energy emission to efficiently schedule follow-up observations with telescopes that have more limited fields of view and observing duty cycles.

And this is just the start! Another few years of observation with Fermi LAT should greatly expand the sample of AGN identified with periodic variability, providing even more insight into the feeding behavior of these distant monsters.

Citation

“Systematic Search for γ-Ray Periodicity in Active Galactic Nuclei Detected by the Fermi Large Area Telescope,” P. Peñil et al 2020 ApJ 896 134. doi:10.3847/1538-4357/ab910d

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