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black hole merger

The Laser Interferometer Gravitational-wave Observatory (LIGO) and the Virgo interferometer have been turning up more and more binary black hole mergers in their observing runs. Do the black holes involved in these mergers have anything in common or are they paired purely by chance?

Binary Black Holes and Where They Come From

The question of how binary black holes (BBHs) form is still wide open, further complicated by the fact that the masses of the black holes involved are higher than expected. Some astronomers have suggested that BBHs are the result of massive stars that were already in binaries, while others have proposed scenarios where black holes in dense stellar populations encounter each other and pair off. Another possibility is that the black holes in BBHs formed as they are in the early universe — skipping existence as a star — and ended up in binaries.

binary black hole merger

Artist’s illustration of the merger of two black holes in space. [LIGO/T Pyle]

BBH mergers are a good way to study BBHs themselves; properties of the merger components (like mass) are imprinted into the resulting gravitational waves. In their first two observing runs, LIGO and Virgo spotted ten BBH mergers, and the black holes involved appear to have masses ranging from 18 to 84 solar masses.

In a new study, Maya Fishbach and Daniel Holz (The University of Chicago) explored how BBHs pair off in terms of their masses. And they found something interesting — it turns out the black holes in binaries may have more in common with each other than we thought!

Underlying Distributions

Fishbach and Holz attempted to understand BBH pairing through different black hole mass distributions. Broadly speaking, they considered three scenarios:

stellar graveyard

This plot shows, in blue, the estimated masses without uncertainties for the black holes that LIGO detected in binaries during its first two observing runs. When uncertainties are included, all 10 of LIGO’s detected systems are consistent with having equal component masses. [LIGO-Virgo/Frank Elavsky/Northwestern U.]

  1. The black hole masses come from a distribution that is only constrained by minimum and maximum masses.
  2. The black hole masses come from a distribution that depends on minimum and maximum masses, and the ratio between the masses of the black holes in a BBH.
  3. The black hole masses come from a distribution that depends on minimum and maximum masses, the mass ratio between the BBH components, and the total mass of the BBH.

On modeling and applying these scenarios to the ten available BBH merger observations, Fishbach and Holz came away with two main findings: random pairings are thoroughly disfavored, and black holes in BBHs are five times more likely to be of similar mass than not. They also find that total system mass may not play a big role in BBH pairing.

The BBH formation models that end with black holes of similar mass are usually those that involve massive stellar binaries. This doesn’t rule out other formation mechanisms but Fishbach and Holz’s work suggests that future models may need to account for the mass ratio in BBHs.

Of course, this work is based on only ten observations. However, with more observations from LIGO/Virgo already on the way, astronomers will soon be able to further constrain and eventually solve this puzzle.

Citation

“Picky Partners: The Pairing of Component Masses in Binary Black Hole Mergers,” Maya Fishbach and Daniel E. Holz 2020 ApJL 891 L27. https://doi.org/10.3847/2041-8213/ab7247

AGN

Dramatic collisions of galaxies can provide fireworks shows in more ways than one. New observations have now confirmed a long-theorized link between galaxy mergers and the launch of powerful relativistic jets.

Feeding the Fire

We know that nearly every galaxy hosts a supermassive black hole of millions to tens of billions of solar masses. Some, like the one at the center of our own Milky Way, are quiet. But many actively accrete gas, flaring with bright emission across the electromagnetic spectrum. In addition to accreting material, some of these active galactic nuclei (AGN) also fling incoming material back out, forming powerful jets that zip along at velocities close to the speed of light.

Antennae Galaxies

When two galaxies collide, gas can be driven to their centers, feeding the supermassive black holes that lurk there. [NASA, ESA, and the Hubble Heritage Team (STScI/AURA)-ESA/Hubble Collaboration]

But how does the gas feeding this dramatic AGN activity arrive at the center of the galaxy in the first place? One theory is that violent collisions of galaxies deliver the necessary fuel. In this picture, when two galaxies merge, the turbulent collision feeds gas into the nuclei of the galaxies, causing the AGN to light up and triggering the launch of energetic jets.

One Moment in a Long History

How can we test this theory? Galaxy mergers take billions of years, so watching one in real time isn’t an option. But if we spot galaxies that are mid-merger and that also sport AGN activity with young jets, that would provide reasonably convincing evidence that the jet production is related to the merger.

With this goal in mind, a team of scientists led by Vaidehi Paliya (Deutsches Elektronen-Synchrotron DESY, Germany) went on the hunt for evidence of young AGNs that might also be in the process of colliding — and they found what they were looking for in TXS 2116–077.

A Merger Caught Red-Handed

TXS 2116–077 lies 4.3 billion light-years away. Notably, this young AGN hosts a speeding jet at its heart, pointed close to our line of sight. Because the jet power is relatively low for an AGN, we can observe the accretion environment around TXS 2116–077 without being blinded by the jet’s emission.

Paliya and collaborators used the 8.2-m Subaru Telescope in Hawaii to image TXS 2116–077, revealing that this galaxy is in the process of merging with a nearby companion. Their close separation of just 40,000 light-years indicates that the two galaxies are in a late stage of merger and approaching coalescence.

TXS 2116−077

This infrared image of TXS 2116−077, obtained with the Subaru telescope, reveals the presence of two galaxies interacting with one another. Overplotted contours show radio contours, revealing the compact jet. Click to enlarge. [Paliya et al. 2020]

The authors obtain follow-up data with the 10.4-m Gran Telescopio Canarias and the 4.2-m William Herschel Telescope in Spain, as well as with the Chandra X-ray Observatory. The combined observations show that gravitational interactions between the two galaxies have caused visible disturbances in their morphologies, and both galaxies boast active nuclei.

Old Collision, Young Jet

By modeling the observed stellar populations, Paliya and collaborators estimate that the merger of these galaxies began ~0.5–2.5 billion years ago. The jet, in contrast, is estimated to be only 15,000 years old, based on its approximate length and speed.

The fact that the jet clearly formed after these two galaxies began merging provides strong evidence in favor of mergers as a trigger for AGN accretion and the launch of relativistic jets. Targets like TXS 2116–077 therefore represent ideal sources for studying newly formed jets and their birth environments.

Citation

“TXS 2116−077: A Gamma-ray Emitting Relativistic Jet Hosted in a Galaxy Merger,” Vaidehi S. Paliya et al 2020 ApJ 892 133. doi:10.3847/1538-4357/ab754f

KELT-9b

As the ultra-hot Jupiter KELT-9b blazes across the face of its host star, we have an excellent opportunity to examine its scalding atmosphere. A new study now reports on what we’ve found.

A Passing Glance

In our efforts to learn more about worlds beyond our solar system, atmospheres provide a critical key. Characterizing the atmospheres of exoplanets can provide us with insight into the planets’ compositions and climates, their evolution, and even — with some potential caveats — their habitability.

transmission spectroscopy

As a star’s light filters through a planet’s atmosphere on its way to Earth, the atmosphere absorbs certain wavelengths depending on its composition. [European Southern Observatory]

In particular, transiting exoplanets provide us with a unique opportunity. As a planet passes in front of its host star, we briefly observe the star’s light filtering through the planet’s atmosphere. By exploring the spectrum of that light, not only can we identify the presence of specific atoms and molecules in the planet’s atmosphere, but we can also learn more about where they are and what the atmospheric properties are at those locations.

In a new study led by Jake Turner (Cornell University), a team of scientists digs deep into such a transmission spectrum for the exoplanet KELT-9b.

Not Exactly Temperate

KELT-9b is an extreme world. Clocking in with a dayside temperature of more than 4,500 K (~7,600 °F), it is the hottest planet known — hotter than many stars! This ultra-hot Jupiter orbits at a mere 0.035 AU from its scalding A- or B-type host star, whizzing around its host in just 1.5 days.

The intense radiation bombarding KELT-9b almost certainly takes a toll: this energetic light should dissociate molecules into their component atoms and ionize metals in the hot atmosphere, and it may inflate the envelope of hydrogen gas around the planet to the point where the hot gas escapes.

atmospheric absorption lines

Observed and modeled Hα (top) and Ca II (bottom three) spectral lines in the atmosphere of the ultra-hot Jupiter KELT-9b. [Adapted from Turner et al. 2020]

Turner and collaborators explore the extreme conditions in KELT-9b’s atmosphere with high-resolution transmission spectra taken with the CARMENES instrument on the Calar Alto 3.5-m telescope in Spain.

Detecting Atmospheric Thermometers

The authors find absorption lines indicating the presence of ionized calcium, Ca II, in KELT-9b’s atmospheric spectra; this is just the second time that Ca II has been observed in a hot Jupiter’s atmosphere. They also find prominent Hα absorption — evidence that confirms the existence of an extended envelope of hydrogen surrounding the irradiated planet.

By modeling the spectra they obtain for KELT-9b, Turner and collaborators are able to identify the pressures, altitudes, and temperatures at which these spectral lines form in the atmosphere. They find that the Ca II lines probe the atmosphere at an altitude of about 1.32–1.40 times the planet’s radius. The Hα line provides information from higher up, at 1.44 planetary radii.

Together, these absorption lines act as atmospheric thermometers, providing a picture of KELT-9b’s atmospheric temperature profile and yielding insight into the energy that enters and leaves the planet’s atmosphere.

These results demonstrate the power of this technique, revealing the remarkable wealth of information we can glean from some distant starlight filtered through the atmosphere of an extreme world.

Citation

“Detection of Ionized Calcium in the Atmosphere of the Ultra-hot Jupiter KELT-9b,” Jake D. Turner et al 2020 ApJL 888 L13. doi:10.3847/2041-8213/ab60a9

TDE

We’ve searched for decades for concrete evidence of intermediate-mass black holes, black holes with masses between 100 and 100,000 times that of the Sun. In spite of our best efforts, these monsters have remained elusive — but a new study provides some hope.

Searching for the Middle Sibling

two types of accreting black holes

Illustrations of two types of accreting black holes: a stellar-mass black hole accreting from a binary companion (top) and a supermassive black hole accreting gas in a galaxy’s center (bottom). [Top: ESA/NASA/Felix Mirabel; Bottom: ESO/M. Kornmesser]

Work over the past 25 years has well established the existence of supermassive black holes — heavyweights with millions to tens of billions of solar masses — lurking in the centers of galaxies. Similarly, dynamical measurements and gravitational-wave observations provide compelling evidence of stellar-mass black holes, black holes with ~10 solar masses.

But what about the range in between? Theory predicts that intermediate-mass black holes should be the building blocks for larger supermassive black holes, but we’ve yet to find concrete evidence for a black hole with a mass between 100 and 100,000 solar masses.

To Weigh a Black Hole

We have, however, found candidates! A number of observational clues have pointed to hidden middleweight black holes lurking both inside and outside of our own galaxy. Unfortunately, confirming these candidates is challenging, since there’s no simple means to weigh these black holes.

Our best bet for confirmation is to rule out alternative explanations. This is the approach taken by a team of scientists led by Dacheng Lin (University of New Hampshire) in the case of intermediate-mass black hole candidate 3XMM J215022.4−055108.

J2150–0551

New Hubble image of the environment around J2150–0551. The source is located in the outskirts of a distant lenticular galaxy; J2150–0551 is outlined here with a green box and a zoomed-in view is shown in the inset. [Lin et al. 2020]

A Promising Candidate

3XMM J215022.4−055108 (J2150–0551 for short) is a source visible in both X-rays and optical light. Recently, J2150–0551 exhibited a 12-year-long X-ray outburst — and the the leading explanation for this temper tantrum is that we’re seeing a normally invisible intermediate-mass black hole that’s indulging in a snack. In this picture, the X-ray outburst is due to the tidal disruption of a passing star and the subsequent accretion of the star’s material onto the black hole.

The catch? The outburst could also be explained by a source closer to home: the cooling crust of a galactic neutron star that was heated in a large outburst of accretion. So how can we rule out an ordinary, nearby neutron star and confirm the theory that J2150–0551 is instead a distant intermediate-mass black hole?

Evidence in Favor

Lin and collaborators look to new observations of J2150–0551, both in optical light with the Hubble Space Telescope and in X-rays with XMM-Newton. From these data, they confirm two important points:

  1. The optical light coincident with J2150–0551 is not a point source.
    If this were a galactic neutron star, we’d expect to see a point source; instead, the optical counterpart is an extended source consistent with a star cluster of about 10 million solar masses in the outskirts of a distant galaxy. Such a star cluster would be the perfect size to host an intermediate-mass black hole at its center.
  2. X-ray light curve

    The decay of the light curve for J2150–0551 is well fit by a simple tidal disruption event model over 12 years of data. Click to enlarge. [Adapted from Lin et al. 2020]

    The light curve has continued to decay over the past 12 years.
    This decay is beautifully fit by a simple tidal disruption event model.

Together, these observations and careful modeling strongly support a picture in which a 50,000 solar-mass black hole lit up after disrupting a small main-sequence star — providing some of the most compelling evidence yet for the existence of an elusive intermediate-mass black hole.

Citation

“Multiwavelength Follow-up of the Hyperluminous Intermediate-mass Black Hole Candidate 3XMM J215022.4−055108,” Dacheng Lin et al 2020 ApJL 892 L25. doi:10.3847/2041-8213/ab745b

M82

When did the first sources of light bombard the universe’s gas, tearing electrons from atoms in a period known as reionization? A new study uses the metal-filled gas surrounding galaxies to learn more about this important transition.

Drama in the Early Universe

epoch of reionization

In the schematic timeline of the universe, the epoch of reionization is when the first galaxies and quasars began to form and evolve. [NASA]

After the universe’s birth in the hot Big Bang, expansion and cooling allowed the soup of electrons and protons that pervaded space to recombine into neutral hydrogen atoms. But sometime within the first billion years after the Big Bang, these atoms were again ionized by high-energy radiation from the first sources of light in the universe.

How and when, exactly, did this period of reionization occur? One way we can seek to answer these questions is by studying the gas that lies both between and immediately around galaxies.

Clues from Distant Gas

While the broadly dispersed intergalactic medium (IGM) consists largely of hydrogen gas, the circumgalactic medium (CGM) immediately around galaxies is a little more complicated: it’s enriched with elements heavier than helium — “metals” — that have been produced by the galaxy’s stars and flung into the surrounding matter.

Because this distant gas is diffuse and dim, we can’t easily study its emission. Instead, we explore these clouds of gas by looking at how they absorb light from bright background sources.

quasar sightline sample

The authors’ sample. Horizontal lines show the redshift interval over which each line of sight was surveyed for neutral oxygen. Orange dots mark the locations of identified neutral oxygen absorbers. [Becker et al. 2019]

In a new study led by George Becker (University of California, Riverside), a team of scientists has examined the spectra of nearly 200 background quasars — bright, active galaxies — with redshifts up to z = 6.6, corresponding to a time when the universe was less than a billion years old. Their goal: to explore the absorption by clouds of metal-enriched CGM that lie between us and the quasars.

Oxygen Signature Surprise

Becker and collaborators looked for the signatures of several metals in these clouds, including neutral oxygen. From their sample, the authors were able to infer how absorbing clouds containing neutral oxygen are distributed over cosmic time between redshifts of 3.2 < z < 6.5.

If circumgalactic gas were gradually enriched, we would expect to see the number density of neutral oxygen absorbers increase with decreasing redshift (closer to us, or longer time since the Big Bang), as metal-enriched gas continues to accumulate around galaxies over time.

number density of metal absorbers

Number density of neutral oxygen (black circles), singly ionized magnesium (red squares), and triply ionized carbon (blue triangles) absorbers as a function of redshift. Instead of decreasing monotonically with increasing redshift (as do highly ionized species like the C IV), O I absorber number density dips for redshifts below z ~ 6. [Becker et al. 2019]

Instead, Becker and collaborators see a dip in the number of neutral oxygen absorbers around a redshift of z ~ 6 as the universe ages from high redshift toward today. The reason? The authors argue that it’s because the universe is becoming ionized at this time — so the CGM contains less neutral oxygen in the period right after z ~ 6 because more of the oxygen gas is ionized.

Locking Down a Timeframe

What does this mean? The ionization of the metal-enriched gas immediately surrounding galaxies is directly linked to the reionization of the hydrogen in the broader intergalactic medium: for high-energy background radiation to reach the dense gas around galaxies and ionize it, this means that the surrounding IGM must recently have become ionized.

Becker and collaborators’ observations of metals therefore help us to pinpoint the final stages of the epoch of reionization in the universe, shedding light on how the universe evolved to its current form.

Citation

“The Evolution of O i over 3.2 < z < 6.5: Reionization of the Circumgalactic Medium,” George D. Becker et al 2019 ApJ 883 163. doi:10.3847/1538-4357/ab3eb5

binary pulsar

Fast radio bursts (FRBs) are brief radio signals that last on the order of milliseconds. They appear to be extragalactic, coming from small, point-like areas on the sky. Some FRBs are one-off events, while others are periodic or “repeating.” The sources of FRBs are still unknown, but binary neutron star systems might be a piece of the puzzle.

Wanted: A Reliable Source of Repeating Fast Radio Bursts

Any proposed model for a repeating FRB must explain a number of observed behaviors. Among them are the following:
  1. Repeating bursts from a given FRB source are consistent in frequency and overall intensity on the timescale of years.
  2. Bursts exhibit small-scale variations in measures of the source’s magnetic environment.
  3. FRBs seem to be preferentially hosted in massive, Milky-Way-like galaxies.
Example of a Fast Radio Burst

Example of an FRB from a repeating source, showing the intensity and various frequencies contained in a single burst (darker means more intense, lighter means less intense). The red lines just below and above 550 MHz and those near 450 MHz and 650 MHz indicate frequencies that were unused due to other radio signals interfering [adapted from the CHIME/FRB Collaboration, Andersen et al. 2019].

Binary neutron stars (BNSs) have been considered as possible solutions to the repeating FRB puzzle. Specifically, binary neutron star mergers might produce FRBs, along with gamma-ray bursts and gravitational waves. But how could BNSs produce repeating, consistent FRBs?

In a recent study, Bing Zhang (University of Nevada Las Vegas; Kyoto University, Japan) attempts to explain repeating FRBs using BNSs in a novel way. Instead of considering the neutron-star merger itself, Zhang examined whether the years leading up to the merger could produce repeating FRBs.

A Magnetic Dance

Repeating FRBs put out an enormous amount of energy over a few milliseconds — at least as much energy as the Sun puts out over three days. To put constraints on the average FRB-producing BNS, Zhang used the double-pulsar system PSR J0737-3039A/B (pulsars are fast-rotating neutron stars with strong magnetic fields), which is very well characterized in terms of its component stars and overall structure.

Aside from having enormous amounts of rotational energy intrinsically and in their orbits, BNSs also have strong magnetic fields. These magnetic fields are key to the production of FRBs in Zhang’s scenario — as the neutron stars orbit each other, their magnetic fields interact, possibly triggering a flow of particles that would produce FRBs.

On the scale of centuries or even decades pre-merger, these triggers could occur repeatedly and consistently, satisfying a key requirement for repeating FRBs. This picture of interacting magnetic fields would also explain the small-scale variations in the magnetic environment measures, and there is an overlap between the sorts of galaxies that host FRBs and those that host the gamma-ray bursts that could be associated with BNS mergers.

By Way of Gravitational Waves

An observational test for this scenario is the detection of gravitational waves from an FRB source. Space-based gravitational wave detectors, such as the Laser Interferometer Space Antenna, would be well-suited for this. Ground-based detectors would also play a role, picking up waves from the BNSs actually merging.

And of course, the more FRBs we observe, the more we can narrow down their properties and sources. Fortunately, the Canadian Hydrogen Intensity Mapping Experiment (CHIME) is predicted to detect 2 to 50 FRBs per day, and other radio telescopes are hard at work as well. So maybe this FRB mystery will be solved sooner than we think!

Citation

“Fast Radio Bursts from Interacting Binary Neutron Star Systems,” Bing Zhang 2020 ApJL 890 L24. https://doi.org/10.3847/2041-8213/ab7244

The Planetary Science Journal

At the end of 2019, we announced the launch of a new publication in the American Astronomical Society’s journal family: The Planetary Science Journal (PSJ), a peer-reviewed journal that covers “all aspects of investigation of the solar system and other planetary systems.”

Now, PSJ has officially published its first issue. Read on for a look at the first articles included, and follow the links to the full open-access articles if you’d like to learn more!

lunar crust

Diagram of the fragmented lunar crust. [Richardson & Abramov 2020]

Origin of a Battered Lunar Layer

The surface of the Moon has been constantly bombarded by small, rocky bodies over its lifetime, fracturing its crust down to depths of perhaps 20 km. Planetary Science Institute scientists James Richardson and Oleg Abramov have now modeled this process to better understand how the upper megaregolith — the battered layer of lunar dirt 1–3 km deep that lies just below the Moon’s surface — formed.

“Modeling the Formation of the Lunar Upper Megaregolith Layer,” James E. Richardson and Oleg Abramov 2020 Planet. Sci. J 1 2. doi:10.3847/PSJ/ab7235

Will Asteroid Ejecta Arrive at Earth?

DART schematic

Schematic shows the planned impact of DART on Didymos B, while observatories on Earth watch. [NASA/Johns Hopkins Applied Physics Lab]

In 2022, the Double Asteroid Redirection Test (DART) spacecraft will fire a projectile into the binary asteroid (65803) Didymos to explore how we can deflect asteroids for planetary defense. Scientist Paul Wiegert (The University of Western Ontario, Canada) wonders whether some of the ejected material might escape the Didymos system and make its way to Earth — where we could examine it, but where it also might pose a risk to spacecraft. Wiegert finds that, based on the parameters of the mission, little DART-ejected material will reach our planet, and most will only arrive after thousands of years.

“On the Delivery of DART-ejected Material from Asteroid (65803) Didymos to Earth,” Paul Wiegert 2020 Planet. Sci. J 1 3. doi:10.3847/PSJ/ab75bf

Observing Mercury’s Exosphere at Twilight

Mercury

Mercury possesses only a very thin atmosphere. [NASA/Johns Hopkins University APL/Carnegie Institution of Washington]

Due to Mercury’s proximity to the Sun, ground-based observations of the planet’s exosphere — its tenuous outermost atmosphere — are best conducted at twilight. A team of scientists led by Carl Schmidt (Boston University and LATMOS, France) reports on how results from a new instrument, the Rapid Imaging Planetary Spectrograph, are mitigating some of the challenges of observing at twilight, like windshake, fluctuations in seeing and atmospheric transmission, and guiding problems.

“The Rapid Imaging Planetary Spectrograph: Observations of Mercury’s Sodium Exosphere in Twilight,” Carl A. Schmidt et al 2020 Planet. Sci. J 1 4. doi:10.3847/PSJ/ab76c9

NeoWISE

Artist’s illustration of the NeoWISE mission. [NASA]

Characterization of Nearby Asteroids and Comets

The Near-Earth Object Wide-field Infrared Survey Explorer (NEOWISE) spacecraft has been conducting an infrared survey to detect and characterize asteroids and comets since its reactivation in 2013 December. Led by Joseph Masiero (Jet Propulsion Laboratory/Caltech), a team of scientists now reports on the 374 near-Earth objects and 11,607 main-belt asteroids the mission detected in its fourth and fifth years.

“Asteroid Diameters and Albedos from NEOWISE Reactivation Mission Years 4 and 5,” Joseph R. Masiero et al 2020 Planet. Sci. J 1 5. doi:10.3847/PSJ/ab7820

Learning about Fluid Stability from Jupiter’s Jets

Jupiter global map

Jupiter exhibits complex fluid dynamics in its belts and zones. [ESA/Hubble]

Inviscid shear instability is a common type of fluid instability that governs the dynamics of everything from meandering jet streams in atmospheres and oceans to the formation of planets in protoplanetary disks. Scientist Timothy Dowling (University of Louisville) has used observations from the two Voyager flybys, the Galileo entry probe, the Cassini flyby, and the Juno orbiter to study this instability in Jupiter’s zonal jets — the atmospheric flows in the light bands that encircle the planet.

“Jupiter-style Jet Stability,” Timothy E. Dowling 2020 Planet. Sci. J 1 6. doi:10.3847/PSJ/ab789d

Giant impact

Could an early giant impact have stripped Mercury’s silicate mantle away? [NASA/JPL-Caltech]

Getting Rid of Mercury’s Mantle

Why does Mercury have such an unusually large iron core? One hypothesis is that Mercury formed with a silicate-to-iron ratio closer to that of Earth, but its silicate mantle was stripped off by a giant impact in its past, leaving behind the large fraction of iron. Scientists Christopher Spalding (Yale University) and Fred Adams (University of Michigan) show that the primordial solar wind — which was stronger than the solar wind of present day — could have produced enough drag to push the silicate ejecta away and prevent the material from reaccreting onto Mercury’s surface.

“The Solar Wind Prevents Reaccretion of Debris after Mercury’s Giant Impact,” Christopher Spalding and Fred C. Adams 2020 Planet. Sci. J 1 7. doi:10.3847/PSJ/ab781f

To keep tabs on more planetary science articles as they’re published in PSJ, you can visit the journal homepage and sign up for new issue notifications: https://iopscience.iop.org/journal/2632-3338

neutron star binary

In case you missed the news in January: the Laser Interferometer Gravitational-Wave Observatory (LIGO) has detected its second merger of two neutron stars — probably. In a recent publication, the collaboration details the interesting uncertainties and implications of this find.

GWB190425

Artist’s illustration of a binary neutron star merger. [National Science Foundation/LIGO/Sonoma State University/A. Simonnet]

What We Saw and Why It’s Weird

On April 25, 2019, the LIGO detector in Livingston, Louisiana, spotted a gravitational-wave signal from a merger roughly 520 million light-years away. This single-detector observation — LIGO Hanford was offline at the time, and the Virgo detector in Europe didn’t spot it — was nonetheless strong enough to qualify as a definite detection of a merger.

Analysis of the GW190425 signal indicates that we saw the collision of a binary with a total mass of 3.3–3.7 times the mass of the Sun. While the estimated masses of the merging objects — between 1.1 and 2.5 solar masses — are consistent with the expected masses of neutron stars, that total mass measurement is much larger than any neutron star binary we’ve observed in our galaxy. We know of 17 galactic neutron star pairs with measured total masses, and these masses range from just 2.5 to 2.9 times that of the Sun. Why is GW190425 so heavy?

What It Suggests For Formation Channels

BNS total masses

Blue and orange curves show the estimated total mass of GW190425 under different spin assumptions. In either case, the estimated mass is dramatically different from the total masses for the known galactic population of binary neutron stars, indicated with the grey histogram bars and the dashed line. [Abbott et al. 2020]

GW190425’s unusual mass may indicate that it formed differently from known galactic neutron star binaries.

Theory suggests that massive, fast-merging neutron-star pairs like GW190425 could potentially result from especially low-metallicity stars evolving in close binary systems. Under the right conditions, the energetic kicks caused by supernova explosions might be suppressed, allowing the objects to stay together in the close binary even after their evolution into neutron stars.

If this is the case, GW190425 could represent a population of binary neutron stars that we haven’t observed before. These binaries have remained invisible due to their ultra-tight orbits with sub-hour periods; the rapid accelerations of these objects would obscure their signals in pulsar surveys. The shortest-period neutron star binary we’ve detected with pulsar surveys has a period of 1.88 hours, and it won’t merge for another 46 million years. GW190425 could represent a very different binary neutron star population that’s just as common as the galactic population we know.

What If It’s Not Neutron Stars?

Unfortunately, the single-detector observation of GW190425 means we couldn’t pin down the gravitational-wave source’s location well — so follow-up observations haven’t yet spotted an electromagnetic counterpart like the one we had for GW170817, the first binary neutron star merger LIGO observed.

GW190425 localization

GW190425’s signal was localized to an unfortunately large area of ~16% of the sky, providing a challenge for electromagnetic and neutrino observatories hoping to discover counterparts. [Abbott et al. 2020]

This means we’re missing outside information confirming that this was a neutron star binary; it’s therefore possible that one or both of the merging objects was actually a black hole. If so, this would be smaller than any black holes we’ve detected so far, and we would need to significantly revamp our models of black hole binary formation.

There are clearly still a lot of open questions, but it’s early days yet! With the many recent upgrades to the LIGO and Virgo detectors, we can hope for more binary neutron star detections soon — and every new signal brings us a wealth of information in this rapidly developing field.

Citation

“GW190425: Observation of a Compact Binary Coalescence with Total Mass ~ 3.4 M⊙,” B. P. Abbott et al 2020 ApJL 892 L3. doi:10.3847/2041-8213/ab75f5

exoplanet K2-18b

One of our goals with the soon-to-launch James Webb Space Telescope (JWST) is to better characterize the atmospheres of exoplanets. But will clouds get in the way of our chances?

The Hunt for Water

transmission spectroscopy

As a star’s light filters through a planet’s atmosphere on its way to Earth, the atmosphere absorbs certain wavelengths depending on its composition. [European Southern Observatory]

When an exoplanet transits across the face of its host star, it presents us with a golden opportunity: with a sensitive enough telescope — like the upcoming JWST, scheduled to launch a year from now — we can explore the atmosphere of the planet as it filters the light from its host. Through this transmission spectroscopy, we can look for spectral features that indicate the presence of specific atoms and molecules in the planet’s atmospheric gas.

In the search for life beyond our solar system, surface liquid water is generally considered a necessary ingredient for a habitable world — so signatures of water vapor in planet atmospheres are a prime target for transmission spectroscopy. But any planet with abundant surface water is likely to have something else, too: clouds of liquid and ice condensing in its atmosphere.

A new study led by Thaddeus Komacek (The University of Chicago) explores whether these clouds will foil our chances of detecting water vapor in the atmospheres of terrestrial exoplanets.

transmission spectra

Transmission spectra for planets with clouds (open markers) and without clouds (closed markers), for rotation periods of 8 (orange) and 16 (blue) days. Clouds significantly mute the spectral features, especially for long-period planets. JWST’s expected noise floor is ~20 ppm. [Komacek et al. 2020]

An Obstructed View

Komacek and collaborators examine the results of three-dimensional general circulation models of tidally locked planets orbiting M-dwarf stars. The authors generate simulated transit spectra for planets with different rotation rates, incoming starlight, surface pressure, radius, and more. They then explore whether the presence of clouds in the atmospheres of these planets will impede JWST’s ability to detect the water vapor features that arise from lower in the atmosphere.

The result? Bad news. The authors find that the presence of clouds significantly mutes spectral features; when clouds are present, JWST would typically need to observe 10–100 times more transits of the planet to be able to detect the water vapor features in its atmosphere.

This impact is especially strong for slower-rotating planets. The climate models show that planets with periods longer than about 12 days form significantly more cloud cover on their daysides, due to more water vapor being carried to high altitudes. This leads to even stronger muting of these planets’ spectral features.

Hope for the Future

JWST

An artist’s illustration of the James Webb Space Telescope, set to launch in March 2021. [NASA/JWST]

The authors summarize with the disappointing conclusion that, with JWST, we’re going to have a tough time using atmospheric transmission spectroscopy to spot signs of surface water from the tantalizing targets of terrestrial planets orbiting in M-dwarf habitable zones.

There is still some hope, however. An extended mission lifetime for JWST, lowered signal-to-noise threshold for detection, or the discovery of a habitable-zone planet that JWST can monitor continuously could all push the spectral features above the telescope’s noise limit.

What’s more, since only the features from the atmosphere below the clouds will be affected, species that are well-mixed above the cloud deck may still be detectable. Even on cloudy worlds, we’re sure to have plenty to learn with JWST!

Citation

“Clouds will Likely Prevent the Detection of Water Vapor in JWST Transmission Spectra of Terrestrial Exoplanets,” Thaddeus D. Komacek et al 2020 ApJL 888 L20. doi:10.3847/2041-8213/ab6200

galaxy VCC 848

What happens when the large-scale drama of a violent galaxy merger plays out on small scales for a pair of dwarf galaxies? New observations document the scene of a recent dwarf-galaxy collision.

Dramatic Encounters

Antennae Galaxies

The Antennae Galaxies are an example of a starburst galaxy with rapid star-formation activity driven by a recent merger. [NASA, ESA, and the Hubble Heritage Team (STScI/AURA)-ESA/Hubble Collaboration]

When two galaxies merge, the collision can have dramatic consequences — particularly if the galaxies are rich in gas. The gravitational interaction of galaxies oscillating during a merger drives shock waves through their gas. This can trigger bursts of star formation, launch jets from active galactic nuclei, and result in the eventual formation of a new galaxy with drastically different morphology than the original merging pair.

We’ve seen this drama play out on large scales between giant galaxies, but we know a lot less about what happens when dwarf galaxies collide. Dwarf galaxies are the most abundant type of galaxy in the universe, but they’re also very small and faint. This poses a significant challenge to finding and studying dwarfs — which means there’s a lot we don’t know about how the mergers of dwarf galaxies impact overall star formation and the shape of the new galaxy that forms in the collision.

image of VCC 848

This g-band image of VCC 848 better shows the galaxy’s three extended shell-like structures (outlined with red arcs) surrounding the central body of stars. [Adapted from Zhang et al. 2020]

Fortunately, we may now have an opportunity to learn more. In a recent publication led by Hong-Xin Zhang (University of Science and Technology of China), a team of scientists reports on the discovery of a small, compact galaxy formed by the collision of two dwarfs.

Feeling Shell-Shocked

VCC 848 is what’s known as a blue compact dwarf galaxy — a small galaxy that’s actively undergoing a burst of star formation. Located in the outskirts of the Virgo Cluster some 65 million light-years away, this little dwarf shows telltale signs of a recent merger: careful analysis reveals a complex set of three extended shell-like structures of stars around the bright stellar main body.

Shell structures — which, previously, had only been detected in larger galaxies — are known to be a signature of a recent minor or major galaxy merger; they are formed as the merger sends ripples through the galaxy and disrupts its structure. The detection of these shells in such a small galaxy provides evidence that we’re looking at the recent merger of two dwarfs.

A Flurry of Activity

Zhang and collaborators use their observations of VCC 848 — made with the MegaCam instrument on the Canada–France–Hawaii Telescope in Hawaii — to analyze the stars of the galaxy and learn more about its history.

cluster formation rate

The average formation rate per billion years of star clusters above a certain birth mass, for the age intervals of 0–1 Gyr and 1–13 Gyr. This indicates that star formation was significantly more active in the last billion years. [Adapted from Zhang et al. 2020]

They determine that the two dwarfs that collided were likely similar in mass to within a factor of a few, and the merger triggered a burst of star formation over the past billion years that was ~7–10 times higher than normal. This enhancement in star formation peaked near the center of the galaxy a few hundred million years ago, and it’s since declined; current star formation activity is primarily in VCC 848’s outer regions.

VCC 848 is just one of several blue compact dwarfs with hints of tidal shells that the authors uncovered in their survey, so there’s more data on the way! We have a lot more to learn about what happens when tiny galaxies collide.

Citation

“The Blue Compact Dwarf Galaxy VCC 848 Formed by Dwarf–Dwarf Merging,” Hong-Xin Zhang et al 2020 ApJL 891 L23. doi:10.3847/2041-8213/ab7825

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