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Milky Way

Weighing galaxies is a tricky business — especially when that galaxy is our own! In a recent study, scientists have tackled this problem by harnessing incredibly precise measurements of the motions of Milky-Way satellites.

A Challenging Measurement

satellites of the Milky Way

Locations of some of the ~50 satellite galaxies known around the Milky Way. [AndrewRT]

Our spot in the middle of our galaxy’s disk makes it difficult for us to assess the total mass of gas, dust, stars, and dark matter surrounding us; estimates for the Milky Way’s mass span from 700 billion to 2 trillion solar masses! Pinning down this number is critical for better understanding the structure and dynamics of our local universe.

So what’s the key to precisely weighing the Milky Way? A new study led by Ekta Patel (University of Arizona) — presented at the American Astronomical Society meeting two weeks ago — suggests it may be the barely preceptible motions of the small satellite galaxies that orbit around the Milky Way. Around 50 Milky-Way satellites are currently known, and simulations suggest that there may be up to 100–200 in total. By watching the motions of these satellites, we can trace the potential of their host — the Milky Way — and estimate its mass.

Illustris cosmological simulation

The Illustris-Dark simulation evolves our universe to the present day, providing a view of how dark matter organizes itself into galaxy halos over time. [Illustris Collaboration]

Tiny Motions of Tiny Galaxies

In this era of precision astronomy, remarkable measurements are becoming possible. In their study, Patel and collaborators use years of proper-motion observations from the Hubble Space Telescope for nine satellite galaxies of the Milky Way. The precision needed for measurements like these is insane: watching these satellites move is roughly like watching a human hair grow at the distance of the Moon.

Rather than using the instantaneous position and velocity measured for a satellite — which changes over time during the satellite’s orbit — Patel and collaborators demonstrate that the satellite’s specific angular momentum is a more useful parameter when attempting to estimate its host galaxy’s mass.

For each of the nine individual satellite galaxies, the authors compare its measured momentum to that of ~90,000 simulated satellite galaxies from the Illustris-Dark cosmological simulation. This matching is used to build a probability distribution for the mass of the host galaxy most likely to be orbited by such a satellite. The probability distributions for the nine satellite galaxies are then combined to find the best overall estimate for the Milky Way’s mass.

Tipping the Scale

Milky-Way mass estimates

Top: summary of the most likely Milky-Way mass estimated from each of the 9 satellite galaxies, using the instantaneous positions and velocities (left) and the momentum (right) of the satellites. The momentum method shows less scatter in the host masses. Bottom: probability distributions for the most likely Milky-Way mass for each of the satellites (colored curves) and combined (grey curve). Click for a better look. [Patel et al. 2018]

Using this technique, Patel and collaborators find a mass of 0.96 trillion solar masses for the Milky Way. The error bars for their measurement are around 30% — and while this is more confined than the broad range of past estimates, it’s not yet extremely precise. The beauty of Patel and collaborators’ method, however, is that it is both extendable and generalizable.

The authors only had access to precise proper motions for nine satellite galaxies when they conducted their study — but since then, the Gaia mission has provided measurements for 30 satellites, with more expected in the future. Including these additional satellites and using improved, higher-resolution cosmological simulations for comparison will continue to increase the precision of Patel and collaborators’ estimate in the future.

In addition, this approach can also be used to weigh our neighboring Andromeda galaxy, or any other galaxy for which we’re able to get precise proper-motion measurements for its satellites. Keep an eye out in the future, as techniques like this continue to reveal more properties of our local universe.

Citation

Ekta Patel et al 2018 ApJ 857 78. doi:10.3847/1538-4357/aab78f

KELT-9b

Move over, hot Jupiters — there’s an even stranger kind of giant planet in the universe! Ultra-hot Jupiters are so strongly irradiated that the molecules in their atmospheres split apart. What does this mean for heat transport on these planets?

Atmospheres of Exotic Planets

Ultra-hot Jupiter diagram

A diagram showing the orbit of an ultra-hot Jupiter and the longitudes at which dissociation and recombination occur. [Bell & Cowan 2018]

Similar to hot Jupiters, ultra-hot Jupiters are gas giants with atmospheres dominated by molecular hydrogen. What makes them interesting is that their dayside atmospheres are so hot that the molecules dissociate into individual hydrogen atoms — more like the atmospheres of stars than planets.

Because of the intense stellar irradiation, there is also an extreme temperature difference between the day and night sides of these planets — potentially more than 1,000 K! As the stellar irradiation increases, the dayside atmosphere becomes hotter and hotter and the temperature difference between the day and night sides increases.

When hot atomic hydrogen is transported into cooler regions (by winds, for instance), it recombines to form H2 molecules and heats the gas, effectively transporting heat from one location to another. This is similar to how the condensation of water redistributes heat in Earth’s atmosphere — but what effect does this phenomenon have on the atmospheres of ultra-hot Jupiters?

Planetary maps

Maps of atmospheric temperature of molecular hydrogen dissociation fraction for three wind speeds. Click to enlarge. [Bell & Cowan 2018]

Modeling Heat Redistribution

Taylor Bell and Nicolas Cowan (McGill University) used an energy-balance model to estimate the effects of H2 dissociation and recombination on heat transport in ultra-hot Jupiter atmospheres. In particular, they explored the redistribution of heat and how it affects the resultant phase curve — the curve that describes the combination of reflected and thermally emitted light from the planet, observed as a function of its phase angle.

For reasonable eastward wind speeds, Bell and Cowan found that the recombination of atomic hydrogen shifts the peak of the phase curve in the eastward direction, with the shift becoming more pronounced with increasing eastward wind speed. Additionally, because heat is distributed more evenly across the planet, including this process decreases the amplitude of the phase variations.

A Bright Future for Ultra-hot Jupiters

Modeled phase curves

Theoretical phase curves for three wind speeds. Transits and eclipses have been neglected. [Bell & Cowan 2018]

While this simple model doesn’t include potentially important effects such as the changing atmospheric opacity as a function of longitude or formation of clouds on the planet’s nightside, this result indicates that caution is required when interpreting phase curves of ultra-hot Jupiters. For example, neglecting recombination means assuming a lower heat transport efficiency, which will require artifically high wind speeds to match observed phase curves.

Only a few ultra-hot Jupiters are currently known, but that will soon change. The Transiting Exoplanet Survey Satellite (TESS) mission, which is set to begin its first science observations on June 17, 2018, will search for exoplanets around bright stars, including nearby cool stars and more distant hot stars. The hot stars may play host to these exotic exoplanets, and upcoming observations of ultra-hot Jupiters like KELT-9b will put this theory of heat redistribution to the test.

Citation

Taylor J. Bell & Nicolas B. Cowan 2018 ApJL 857 L20. doi:10.3847/2041-8213/aabcc8

nascent planets

Occasionally, science comes together beautifully for a discovery — and sometimes this happens for more than one team at once! Today we explore how two independent collaborations of scientists simultaneously found the very first kinematic evidence for young planets forming in a protoplanetary disk. Though they explored the same disk, the two teams in fact discovered different planets.

Evidence for Planets

HD 163296

ALMA’s view of the dust in the protoplanetary disk surrounding the young star HD 163296. Today’s studies explore not the dust, but the gas of this disk. [ALMA (ESO/NAOJ/NRAO); A. Isella; B. Saxton (NRAO/AUI/NSF)]

Over the past three decades, we’ve detected around 4,000 fully formed exoplanets. Much more elusive, however, are the young planets still in the early stages of formation; only a handful of these have been discovered. More observations of early-stage exoplanets are needed in order to understand how these worlds are born in dusty protoplanetary-disk environments, how they grow their atmospheres, and how they evolve.

Recent observations by the Atacama Large Millimeter/submillimeter Array (ALMA) have produced stunning images of protoplanetary disks. The unprecedented resolution of these images reveals substructure in the form of gaps and rings, hinting at the presence of planets that orbit within the disk and clear out their paths as they move. But there are also non-planet mechanisms that could produce such substructure, like grain growth around ice lines, or hydrodynamic instabilities in the disk.

How can we definitively determine whether there are nascent planets embedded in these disks? Direct direction of a point source in a dust gap would be a strong confirmation, but now we have the next best thing: kinematic evidence for planets, from the motion of a disk’s gas.

velocity kink

Observations of carbon monoxide line emission at +1km/s from the systemic velocity (left) vs. the outcome of a computer simulation (right) in the Pinte et al. study. A visible kink occurs in the flow, which can be reproduced by the presence of a 2-Jupiter-mass planet at 260 AU. [Pinte et al. 2018]

Watching Gas Move

In two papers published today in ApJL — one led by Richard Teague (University of Michigan) and the other led by Christophe Pinte (Monash University in Australia and Grenoble Alpes University in France) — astronomers have announced the detection of distinctive signs of planets in the gas motion of the disk surrounding HD 163296. This young star, located about 330 light-years away, is only ~4 million years old.

Unlike studies that hinge on observations of a disk’s dust — which only makes up ~1% of the disk’s mass! — both studies here took a new approach: they used detailed ALMA observations revealing the dynamics of the disk’s carbon monoxide gas. By studying the gas’s motion, the teams found deviations from the Keplerian velocity that would be expected if there were no planets present. The authors then ran simulations to demonstrate that the deviations are consistent with local pressure perturbations caused by the passage of giant planets.

velocity deviations caused by planets

Rotational velocity deviations due to changes in the local pressure, caused in this simulation by the presence of planets. [Teague et al. 2018]

Giants Found

What did they find? Teague and collaborators, whose technique to identify velocity variations is best suited to explore the inner regions of the disk, discovered evidence for two separate Jupiter-mass planets orbiting at distances of 83 AU and 137 AU in the disk. Pinte and collaborators, whose velocity-measurement technique better explores the outer regions of the disk, found evidence for a two-Jupiter-mass planet orbiting at 260 AU.

These results will rely on additional imaging in the coming years to confirm the presence of these newly born planets — and a detection of point sources at these radii remains a hopeful goal for the future. Nonetheless, the new techniques explored here by Teague, Pinte, and collaborators are a promising route for young exoplanet discovery and characterization in other disks imaged by ALMA and future instruments.

Citation

Richard Teague et al 2018 ApJL 860 L12. doi:10.3847/2041-8213/aac6d7
C. Pinte et al 2018 ApJL 860 L13. doi:10.3847/2041-8213/aac6dc

neutron-star merger

When two neutron stars merged in August of last year, leading to the first simultaneous detection of gravitational waves and electromagnetic signals, we knew this event was going to shed new light on compact-object mergers.

A team of scientists says we now have an answer to one of the biggest mysteries of GW170817: after the neutron stars collided, what object was formed?

black hole

Artist’s illustration of the black hole that resulted from GW170817. Some of the material accreting onto the black hole is flung out in a tightly collimated jet. [NASA/CXC/M.Weiss]

A Fuzzy Division

Based on gravitational-wave observations, we know that two neutron stars of about 1.48 and 1.26 solar masses merged in GW170817. But the result — an object of ~2.7 solar masses — doesn’t have a definitive identity; the remnant formed in the merger is either the most massive neutron star known or the least massive black hole known.

The theoretical mass division between neutron stars and black holes is fuzzy, depending strongly on what model you use to describe the physics of these objects. Observations fall short as well: the most massive neutron star known is perhaps 2.3 solar masses, and the least massive black hole is perhaps 4 or 5, leaving the location of the dividing line unclear. For this reason, determining the nature of GW170817’s remnant is an important target as we analyze past observations of the remnant and continue to make new ones.

Chandra GW170817

Chandra images of the field of GW170817 during three separate epochs. Each image is 30” x 30”. [Adapted from Pooley et al. 2018]

Luckily, we may not have long to wait! Led by David Pooley (Trinity University and Eureka Scientific, Inc.), a team of scientists has obtained new Chandra X-ray observations of the remnant of GW170817. By combining this new data with previous observations, the authors have drawn conclusions about what object was left behind after this fateful merger.

X-Rays Provide Answers

X-ray radiation is generated in a merger of two neutron stars when the merger’s shock wave expands and slams into the surrounding interstellar medium. The earliest X-ray detection from GW170817 — around 9 days after the merger — likely indicated the moment when that interaction began. GW170817’s X-ray emission continued to grow over the first ~100 days post-merger, expected as the shock continues to expand.

If the merger had produced a neutron star, however, there should be an additional source of X-ray radiation besides the shock: the neutron star itself. This emission should, by now, have started to dominate over the emission from the propagating shock. Instead, Pooley and collaborators find that the observed X-ray flux from GW170817 falls significantly short of what’s needed to justify the presence of a highly magnetized, spinning neutron star. For this reason, the authors conclude that GW170817 likely produced a black hole.

Future Confirmation

How can we be sure? Pooley and collaborators point out that we can confirm this theory just by observing GW170817 for another year. Around this time, energy released from the spin-down of a central neutron star would catch up to the decelerating shock front, causing a dramatic brightening in GW170817’s X-ray flux. 

If we don’t see this brightening, the authors argue that we can conclude with certainty that GW170817’s remnant is a black hole. Either way, continued observations of this remnant are sure to provide a wealth of information about the physics of mergers, shocks, and outflows that we can hope to mine for years to come.

Citation

David Pooley et al 2018 ApJL 859 L23. doi:10.3847/2041-8213/aac3d6

Mars atmosphere

In the search for life elsewhere in the universe, Mars has always represented an important target for exploration. What can this planet’s evolution — from a potentially habitable world to its current inhospitable state — tell us about the possibility of life on habitable exoplanets beyond our solar system?

A Past of Contrast

Mars ocean

Artist’s illustration of an ancient ocean on Mars. [NASA/GSFC]

Life on Mars has been a topic of speculation for well over a century. Today, scientists no longer consider it very likely that inhospitable Mars is inhabited by alien life — but the red planet wasn’t always so unwelcoming.

While today’s Mars is an arid desert, mounting evidence suggests that ancient Mars — the Mars of 4 billion years ago — not only had aqueous environments, but possibly even global oceans. Evidence for minerals, biogenic elements, and suitable energy sources for prebiotic chemistry increase the prospects for ancient Mars’s habitability.

So what caused the difference between this welcoming ancient Mars and the inhospitable Mars of the current epoch? Ancient Mars had one more thing: a thick protective atmosphere. Today’s Mars has only a thin, tenuous atmosphere remaining between its surface and the hostile conditions of space.

In a new theoretical study led by Chuanfei Dong (Princeton University), a team of scientists explore how Mars may have lost its atmosphere, causing the planet’s transformation to its current state.

atmospheric escape rates

Results from the authors’ simulations showing the ion (total ion shown in blue) and photochemical (heated atomic oxygen shown in magenta) escape rates over the Martian history. Ion escape rates were much higher 4 billion years ago. [Dong et al. 2018]

Loss of an Atmosphere

Dong and collaborators use sophisticated global 3D simulations of a Mars-like body to explore different types of atmospheric loss over time. By including the evolution of the ultraviolet radiation levels and solar wind strength, the authors can explore how atmospheric escape rates for ions and atoms have changed over Mars’s history.

Dong and collaborators find that Mars’s atmospheric ion escape rate, in particular, was more than two orders of magnitude higher 4 billion years ago compared to the present-day level. This is a result of the stronger solar wind and higher ultraviolet fluxes that came from the young Sun.

These high ion escape rates are more than enough to explain the rapid loss of Mars’s thick atmosphere very early in its lifetime — which leads to the depletion of any surface water Mars might have once had. Dong and collaborators argue that Mars might once have had a global ocean at least 2.6 m deep; according to the authors’ simulations, this water would have been depleted within 4 billion years, resulting in the arid Mars we observe today.

Beyond Our Solar System

exoplanet atmosphere

Artist’s impression of a rocky exoplanet with a thick water atmosphere. [MPIA]

What implications do these discoveries have on our search for life beyond our solar system? Early Mars may well be a prototype for small, rocky, potentially habitable planets orbiting solar-type stars. In this case, the authors’ results suggest that early atmospheric escape may be common among such planets, preventing life from easily persisting. The time-dependence of atmospheric loss is therefore an important element to keep in mind as we choose our targets and explore exoplanet candidates in the future.

Citation

Chuanfei Dong et al 2018 ApJL 859 L14. doi:10.1088/0004-637X/810/2/136

PSR J2215

How massive can a neutron star get? In a recent study, scientists may have identified the most massive neutron star yet — by leveraging observations of its highly irradiated companion.

Finding the Maximum

The maximum possible mass for a neutron star is a topic of heated debate; knowing this limit could put significant constraints on models of neutron-star interior structures and compositions, which are longstanding open questions in neutron-star studies.

Until now, the most massive known neutron star was pulsar PSR J0348+0432, which weighs in at 2.01 solar masses. For years, scientists have been on the hunt for other massive neutron stars to push this limit even higher — and now, J0348 may finally have been dethroned.

Led by Manuel Linares (Polytechnic University of Catalonia and the Canary Islands Institute of Astrophysics, Spain), a team of scientists has used a unique approach to measure a new, intriguing heavyweight: PSR J2215+5135.

A Tricky System

spectra of J2215

The spectra of J2215 look drastically different at different phases in its orbit: when we view its hot, irradiated side (bottom spectrum), it looks like an A5 star (2nd spectrum from bottom). When we view its dark, cool side (3rd spectrum from bottom), it looks like a G5 star (4th spectrum from bottom). [Linares et al. 2018]

PSR J2215+5135 is a so-called “redback” system consisting of a millisecond pulsar — a rapidly spinning, highly magnetized neutron star — closely orbiting an extremely low-mass companion star; the pulsar and its companion zip around each other in just 4.14 hours.

How can we measure PSR J2215’s mass? Ordinarily, we’d use the spectra of its companion star to identify Doppler shifts of absorption lines from the star’s atmosphere. This can reveal the star’s radial velocity, ultimately allowing us to model the system and obtain the masses of the neutron star and its companion. But PSR J2215 has thus far resisted such efforts, with different studies all finding significantly different radial-velocity measurements for the companion star. What’s going on with this tricky system?

New Champion Crowned

Linares and collaborators have an explanation: the companion star is being positively blasted with radiation by the nearby pulsar. As a result, the star effectively has two sides: the cool, dark side facing away from the neutron star, and the extremely hot, irradiated side facing toward it. The offset of the companion’s center of light from its center of mass complicates efforts to reliably measure its radial velocity from its spectrum.

light curves for J2215

Model fits to the orbital light curves for J2215 in three bands. [Linares et al. 2018]

Linares and collaborators circumvent this problem by using high-quality optical spectra from the Gran Telescopio Canarias and other telescopes to identify, for the first time, absorption lines from both the cool side and the hot side of the companion star. The authors use these lines from opposite sides of the star to bracket the center-of-mass velocity.

By jointly modeling both the radial-velocity data for the two star sides and the light curves in multiple bands, the authors are able to calculate the mass of the neutron star and its companion, respectively ~2.3 and ~0.33 solar masses. If verified, PSR J2215 would shatter the past record for maximum neutron-star mass, introducing new constraints to models of neutron-star equations of state.

What’s more, the authors’ novel technique for extracting the neutron star’s mass can be applied to many similar known systems, as well as the many we expect to discover in the future. With luck, we’ll be able to continue to push the limit of the maximum neutron-star mass, learning about these compact beasts in the process.

Citation

M. Linares et al 2018 ApJ 859 54. doi:10.3847/1538-4357/aabde6

NGC 1052-DF2

Does the galaxy NGC 1052-DF2 really lack dark matter, or is this ultra-faint dwarf just misunderstood? A recently published paper calls into question this recent, surprising discovery 65 million light-years from our home.

Scientific Process at Work

Scientists don’t always agree — and that’s a good thing! Believe it or not, what makes the process of science work is precisely this: researchers challenging each other and questioning their data, analysis, or results. As scientists work to defend and adapt their research, they improve the robustness of their models and outcomes, and the research field gradually progresses as a whole.

This past month, astronomy has had a healthy dose of scientific disagreement after an article was published in Nature suggesting the discovery of a galaxy lacking dark matter. In a recently published response, a team of scientists led by Nicolas Martin (University of Strasbourg, France; Max Planck Institute for Astronomy, Germany) suggests that uncertainties and small-number statistics have led to NGC 1052-DF2 being misunderstood.

A Quick Recap

globular clusters of NGC 1052-DF2

The 11 globular clusters associated with NGC 1052-DF2 (imaged by HST), and their spectra (taken with Keck/LRIS). [van Dokkum et al. 2018]

Last month, Pieter van Dokkum (Yale University) and collaborators announced new observations of the dwarf galaxy NGC 1052-DF2, a very low-surface-brightness satellite orbiting the elliptical galaxy NGC 1052. Van Dokkum et al. identified 10 objects that are likely globular clusters in NGC 1052-DF2 and measured their line-of-sight velocities.

Here’s where it gets tricky: the authors used the measured velocities of these 10 tracers to estimate a velocity dispersion for the sample. They then used this as a measure of the dynamical mass of the galaxy. Van Dokkum and collaborators reached the conclusion that the galaxy’s dynamical mass is very nearly the same as its stellar mass, suggesting that NGC 1052-DF2 has a very low mass-to-light ratio and is severely lacking in dark matter.

Scrutinizing with Statistics

So what’s the catch? Statistics — it’s a dangerous game to make strong statements from the dynamics of only 10 globular clusters. With small numbers such as these, the statistical result depends critically on the model assumptions and how the data is analyzed.

mass-to-light ratio limits

Inferred mass-to-light ratio for multiple models. Martin et al.’s inferred upper limits (black and red arrows) are much less restrictive than van Dokkum et al.’s (grey arrow). [Martin et al. 2018]

In their recent ApJL publication, Martin and collaborators demonstrate that the use of several different statistical models and analysis procedures produce widely different results for NGC 1052-DF2’s mass-to-light ratio relative to van Dokkum et al.’s analysis — underscoring the difficulty in extracting information from small data sets with large uncertainties.

Martin and collaborators argue that the constraints on the dwarf galaxy’s dark-matter content aren’t as strong as van Dokkum et al. suggest. Martin et al. find only weak constraints on the system’s mass-to-light radius — the authors agree that NCG 1052-DF2 is not massively dominated by dark matter, but Martin and collaborators find that its properties could still be consistent with other dwarf galaxies found in the Local Group.

Thus, instead of a definitive answer about NGC 1052-DF2’s dark-matter content, we are left with a tale of caution and a hope of future work on the subject. Frustrating? Perhaps — but that’s science! The dialog begun between these authors is likely only the beginning of an ongoing conversation about NGC 1052-DF2, dark matter, statistics, and analysis of observations — and it’s exciting to watch in real time as the field progresses.

*****

Note: See Mia de los Reyes’s post over at Astrobites for some excellent additional coverage of Martin et al.’s article and more on the topic of NGC 1052-DF2.

Citation

Nicolas F. Martin et al 2018 ApJL 859 L5. doi:10.3847/2041-8213/aac216

Titan's seas

The frigid seas of Titan, Saturn’s largest moon, host a secret: something is causing features to appear and disappear within them. Could these seas be bubbling? And if so, how could such bubbles form and grow?

Frigid Seas

Titan

Near-infrared mosaic showing sunlight reflecting off of Titan’s north polar hydrocarbon seas. [NASA/JPL-Caltech/University of Arizona/University of Idaho]

While observing Saturn’s rings in 1655 — that’s a whopping 363 years ago! — Dutch astronomer Christiaan Huygens spotted something else: Saturn’s largest moon, Titan. In 2007, more than three centuries later, we got our first up-close look at Titan when the Cassini/Huygens probe arrived at Saturn and discovered new surprises from this moon.

Titan is a hostile, chilly world, with a nitrogen atmosphere and an icy crust. Its distance from the Sun is roughly ten times the Earth–Sun distance, so Titan’s surface temperatures are only around 95 K (–290 °F). At this temperature, hydrocarbons methane and ethane take liquid form, pooling in pockets in Titan’s crust to form hundreds of hydrocarbon lakes and seas that dot Titan’s surface.

magic islands in Ligeia Mare

Evolution of transient bright features — “magic islands” — in Ligeia Mare, as seen by radar images. Click for a better look. [NASA/JPL-Caltech/ASI/Cornell]

Magic Islands

In the past few years, scientists have noticed something odd about one of these seas, the northern Ligeia Mare: radar observations of Titan’s surface have shown bright regions within the sea that appear and vanish from one flyby to the next. In acknowledgement of these features’ mysteriousness, scientists nicknamed them “magic islands”.

What could be causing these transient features? One of the more popular explanations is that streams of bubbles are being released as atmospheric nitrogen — originally dissolved in the liquid methane and ethane of the sea — separates out and rises to the surface.

Now, two scientists from the University of Reims Champagne-Ardenne in France, Daniel Cordier and Gérard Liger-Belair, want to understand whether such bubbles could even form in the unique environment of Titan’s seas.

Growing Bubbles from the Seafloor

Cordier and Liger-Belair start their study from what we know: if the “magic islands” are, in fact, bubbles, then they must be at least ~2cm in size near the surface of Ligeia Mare; if they were any smaller, they wouldn’t reflect the radar waves. The authors then use theoretical calculations and simulations to explore ways to form and grow bubbles to this size in a hydrocarbon sea.

bubble column schematic

Authors’ sketch of a column of bubbles within Titan’s liquid methane/ethane seas. Radar waves come in from above and reflect off of the bubbles. [Cordier & Liger-Belair 2018]

Cordier and Liger-Belair find that homogeneous nucleation, where bubbles form without a surface, can’t explain the observations under the conditions of Titan’s surface. But heterogeneous nucleation, in which bubbles form on surfaces like the seabed or suspended sediment particles, can work to produce bubbles that grow to reach ~2-cm sizes.

The authors show that the best explanation for large bubbles at Ligeia Mare’s surface is that bubbles form or are released tens of meters down at the seabed and then rise as a vertical column. As the bubbles bump into each other and combine within this plume, they arrive at the sea’s surface at a potential size perfect for reflecting radar waves.

Cordier and Liger-Belair’s work shows that nitrogen bubbles in a methane and ethane sea could indeed explain the “magic islands” we only discovered after three centuries of studying Titan. Who knows what we’ll learn in the next three centuries observing this mysterious moon!

Citation

Daniel Cordier and Gérard Liger-Belair 2018 ApJ 859 26. doi:10.3847/1538-4357/aabc10

Magellanic Clouds

The Magellanic Clouds — two nearby dwarf galaxies easily visible to the naked eye in the southern hemisphere — are key to understanding the dynamics and evolution of the Local Group of galaxies. Can an in-depth look at these galaxies’ outer regions help us make sense of their complicated interaction history?

A Closer Look at Our Galactic Neighbors

Magellanic Stream

A combined optical and radio view of the Milky Way and the Magellanic Stream, shown in pink. [David L. Nidever, et al., NRAO/AUI/NSF and Mellinger, Leiden/Argentine/Bonn Survey, Parkes Observatory, Westerbork Observatory, and Arecibo Observatory]

The Small and Large Magellanic Clouds (SMC and LMC) have been well studied, but these dwarf satellite galaxies continue to inspire new discoveries. Among them is the origin of the Magellanic Stream — a swath of neutral hydrogen trailing the Magellanic Clouds and spanning more than half a million light-years.

It was originally thought that the Magellanic Stream was the result of tidal interactions during close encounters with the Milky Way, but precise proper motion surveys revealed that the LMC and SMC are either passing near the Milky Way for the first time or are in a long (~4-billion-year) orbit around our galaxy — so the Magellanic Stream must result from interactions between the two galaxies themselves.

How long have the LMC and SMC been interacting, and how have these interactions shaped the two galaxies? A key to understanding the history of these dwarf galaxies is mapping the weakly gravitationally bound stars at their far edges that may be pulled into tidal streams or bulges as each galaxy is distorted by the presence of the other.

Stellar substructures

A map of the density of ancient stars surrounding the Magellanic Clouds reveals extended structures to the north and south of the LMC, while the western regions of the galaxy (to the right) are truncated. Click to enlarge. [Adapted from Mackey et al. 2018]

Mapping the Edges of Galaxies

Dougal Mackey (Australian National University) and collaborators used visible and near-infrared images taken by the Dark Energy Camera (DECam) — the workhorse instrument of the Dark Energy Survey — to map the faint outskirts of the LMC and SMC.

Though the purpose of the Dark Energy Survey is to better understand the nature of dark energy through observations of supernovae, weak gravitational lenses, and galaxy clusters, its sensitive imaging system and wide field of view (2.2 degrees in diameter) make it well-suited to exploring the faint fringes of nearby galaxies.

The DECam images of the Magellanic Clouds probed to a surface brightness of 32 magnitudes per square arcsecond, allowing Mackey and collaborators to investigate how different stellar populations are distributed in the outer regions of these galaxies.

Stellar population maps

Stellar density maps for young (<1 Gyr) and intermediate-age (1.5–4 Gyr) populations. The young stars trace a bridge between the galaxies, while the intermediate-age stars are offset from the ancient stars in the direction of the LMC. Click to enlarge. [Adapted from Mackey et al. 2018]

Structures Revealed in Faint Starlight

Mapping the density of stars revealed distinct stellar substructures on the outskirts of the LMC and SMC. While previous studies discovered isolated substructures on the outer limits of the LMC, the panoramic view from this study highlights the interconnected nature of the structures.

One important finding is that the intermediate-age (1.5–4 Gyr) stellar population of the SMC is distinctly offset from the ancient (~11 Gyr) stellar population. This result suggests that the Magellanic Clouds may have been gravitationally linked as far back as several billion years — hinting that these galaxies are on their first trip past the Milky Way. Future simulation work may provide a more cohesive picture of the LMC–SMC interaction, helping us better understand how our near neighbors have evolved.

Citation

Dougal Mackey et al 2018 ApJL 858 L21. doi:10.3847/2041-8213/aac175

protoplanetary disk

What’s one thing the interstellar medium, protoplanetary disks, stellar interiors, and the environments around black holes all have in common? They all contain dust grains moving within a fluid — and two scientists from the California Institute of Technology say we’ve been missing an important part of their behavior.

Pairing of Fluids and Dust

Crab Nebula

Hubble view of the Crab Nebula. Supernova ejecta are another instance of a coupled system of dust grains and fluid. [NASA/ESA/J. Hester and A. Loll (Arizona State University)]

Fluids — which can refer to liquids, gases, or plasmas — rarely exist in isolation in astrophysics. More often than not, fluids come laden with dust particles; examples of dusty fluids include the environments near star-forming regions, in planetary atmospheres, in the disks surrounding young stars, or even around active galactic nuclei. Since these fluid/dust systems are abundant across the universe and are fundamental to many key astrophysical processes, it’s important that we understand how they behave.

Caltech scientists Jonathan Squire and Philip Hopkins ask one particular question: what happens when dust particles move at a different speed than the fluid surrounding them?

Relative Motion

Relative motion of dust through fluid can arise naturally through many mechanisms. Radiation pressure, for instance, preferentially accelerates dust grains relative to gas in environments around active galactic nuclei or in the envelopes of stars, causing the dust to stream through the surrounding fluid. Or the fluid of a planetary atmosphere might be supported against gravity by thermal pressure, causing the heavier dust grains to settle downward through the fluid.

In a new study, Squire and Hopkins suggest that this relative streaming motion between dust grains and fluid can easily create instabilities — and this can have profound implications for our understanding of many fields of astrophysics.

Instabilities Found Everywhere

The authors used analytic calculations to show that coupled fluid/dust systems can develop “resonant drag instabilities” whenever the dust grains stream faster than any wave in the fluid.

planetary atmosphere

In a planetary atmosphere like the one shown in this artist’s impression, a fluid might be supported against gravity, whereas dust grains are not. This would create relative motion of the dust particles as they settle. [Max Planck Society]

These instabilities, it turns out, are quite easy to trigger, because astrophysical fluids host a variety of waves, any of which can form the basis for a resonant drag instability. Examples include sound waves, magnetosonic waves, Brunt–Väisälä waves, epicyclic oscillations, and others. The instabilities triggered by the streaming dust in the presence of these waves grow over time, causing spatial clumping of the dust and eventually seeding turbulence if they’re strong enough.

Squire and Hopkins present a way of calculating the growth rates and properties of these resonant drag instabilities in different fluids, and they demonstrate the behavior of the instabilities in three example fluid systems: hydrodynamic, magnetohydrodynamic, and stratified fluids.

The authors argue that the consequences of the resonant drag instability affect regions and processes like planetesimal formation, cool-star winds, active galactic nuclei torii and winds, starburst regions, H II regions, supernovae ejecta, and the circumgalactic medium. Their work toward understanding this instability is therefore broadly applicable across astronomical fields, providing critical insight into processes in our universe.

Citation

J. Squire and P. F. Hopkins 2018 ApJL 856 L15. doi:10.3847/2041-8213/aab54d

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