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Illustration of the warping of spacetime around two black holes

We’ve detected gravitational waves from mergers of compact objects like stellar-mass black holes, and we’ve found promising evidence for the spacetime disturbances from binary supermassive black holes. But what about when these two mass scales meet — could we detect the merger of a stellar-mass black hole with a supermassive black hole?

Stellar-Mass and Supermassive

Many galaxies host a central supermassive black hole, which may have the opportunity to consume stellar-mass black holes from its surroundings. Based on theoretical calculations, it’s likely fairly rare for a stellar-mass black hole to merge with a supermassive black hole, with each galaxy experiencing just a few dozen of these events every billion years.

infographic showing the relative frequencies of gravitational waves from different sources

Infographic showing the typical frequency ranges of the gravitational waves produced by different sources. Click to enlarge. [ESA]

Surprisingly, adding another supermassive black hole into the mix may greatly increase the odds of such an interaction. When stellar-mass black holes encounter a supermassive black hole binary, the likelihood of a merger is boosted up to hundreds of thousands of events per galaxy per billion years. The gravitational waves from this type of merger are too low frequency to be detected with our current observatories, but a recent research article has explored the possibility of detecting gravitational waves from these encounters in the not-too-distant future.

Simulating Gravitational Waves

Smadar Naoz (University of California, Los Angeles) and Zoltán Haiman (Columbia University) simulated the gravitational waves that would result from a stellar-mass black hole spiraling in to merge with one member of a supermassive black hole binary. This type of merger is called an extreme-mass-ratio inspiral. First, Naoz and Haiman estimated the number of extreme-mass-ratio inspirals as a function of the mass of the black holes in the binary system. Perhaps counterintuitively, stellar-mass black holes are much more likely to merge with the less massive black hole in a binary system, thanks to gravitational perturbations.

Plot of gravitational wave strain versus frequency

Predicted gravitational wave amplitude as a function of frequency for resolved (purple lines) and unresolved (grey lines) systems, compared to LISA’s estimated sensitivity. Click to enlarge. [Naoz & Haiman 2023]

The team then calculated the amplitude of the gravitational waves produced in each merger and found that future observatories should be able to detect these events. They focused on the Laser Interferometer Space Antenna (LISA) — a proposed space-based gravitational wave observatory that would consist of three spacecraft trailing Earth in its orbit — which should detect individual extreme-mass-ratio inspirals as well as a background signal composed of thousands of events too faint to be detected individually. During the proposed 4-year LISA mission, the observatory could detect hundreds of individual sources.

Observing gravitational waves from a stellar-mass black hole as it spirals toward a supermassive black hole can help us understand many aspects of how supermassive black holes grow and merge. In particular, these observations may help us put a number on how many companions a supermassive black hole is likely to have; do these behemoths mostly fly solo, or are pairs, triples, or quartets more likely? Hopefully, it’s just a matter of time before LISA is in place in its berth in space — the planned launch date is 2037 — and ready to open a new window onto gravitational waves.

Citation

“The Enhanced Population of Extreme Mass-Ratio Inspirals in the LISA Band from Supermassive Black Hole Binaries,” Smadar Naoz and Zoltán Haiman 2023 ApJL 955 L27. doi:10.3847/2041-8213/acf8c9

Kuiper Belt object Arrokoth

Large mounds abound on the surface of the Kuiper Belt object Arrokoth. Using images from the New Horizons flyby, researchers have pieced together a story of how these features came to be.

A Close Look at a Distant Object

New Horizons image of Arrokoth

New Horizons image of Arrokoth, with its two distinct lobes and several other prominent features labeled. [Stern et al. 2023]

After New Horizons made its historic flyby of Pluto in 2015, the spacecraft set its sights on another first: a close flyby of an object in the Kuiper Belt — the ring of icy objects orbiting beyond Neptune. On New Year’s Day in 2019, New Horizons flew within 3,500 kilometers (about the distance between Washington, DC, and Los Angeles) of an object named Arrokoth, giving us our first close look at a Kuiper Belt object.

Arrokoth consists of two separate bodies, or lobes, that fused together at some point in the past. Though the two lobes, named Wenu and Weeyo, appear spherical from the flyby images, observations taken from farther away suggest that they’re actually rather flat, more like walnuts or pancakes. (You can explore Arrokoth’s shape using an interactive three-dimensional model here.) In addition to being curiously flattened, the larger lobe is covered with a series of interlocking mounds, raising even more questions about how this oddly shaped object was assembled in the cold, dark outskirts of our solar system.

Mapping Mounds

Alan Stern (Southwest Research Institute) and collaborators analyzed two New Horizons images of Arrokoth to assess the origins of the mounds. In total, the team identified 12 mounds on the larger lobe, Wenu. The mounds are roughly the same size and color and have similar ratios of length to width, suggesting that they share a common origin. Using computer simulations, Stern’s team explored two scenarios that could account for Wenu’s lumpy appearance: 1) multiple objects about 3 kilometers wide colliding with a larger object, and 2) a rotating cloud of many 5-kilometer-wide objects gently collapsing to form a single object.

Comparison of model output and Wenu's structure

Comparison of the final model output for the second scenario (left) and the structure of Wenu (right). Click to enlarge. [Stern et al. 2023]

The first scenario generated an object that is too uniform, the mounds having been splattered and flattened in the collision. The second scenario, though, resulted in a distinctly Wenu-like shape; because the objects came together gently, the mounds remained raised rather than flattened. This scenario also predicts other characteristics of the Wenu lobe, such as mounds of similar area that are arranged in an orderly way. How exactly a gravitationally bound, rotating group of 5-kilometer-wide objects might arise in the first place remains unknown, but future high-resolution simulations should provide clues as to whether it’s plausible.

As Wenu, So Weeyo?

New Horizons images of Arrokoth and maps made in this study

New Horizons images of Arrokoth (left column) and resultant maps created in this study (right). Mound regions have labels beginning with “t”. Click to enlarge. [Stern et al. 2023]

Wenu appears to have formed from multiple smaller objects coming together — could Weeyo be made the same way? At first glance, the geology of the two lobes is very different, possibly because of Weeyo’s single large impact crater, the creation of which blanketed the nearby surface with ejected material. Stern’s team picked out three possible mounds along the visible edge of the lobe, farthest from the influence of the crater, but this designation is only tentative.

With no missions to the Kuiper Belt currently planned, our best hope of learning more about Arrokoth is by studying similar objects targeted in upcoming missions: the trojan asteroids in Jupiter’s orbit, which will be visited by NASA’s Lucy mission, and a comet approaching Earth’s orbit, visited by the European Space Agency’s Comet Interceptor.

Citation

“The Properties and Origin of Kuiper Belt Object Arrokoth’s Large Mounds,” S. A. Stern et al 2023 Planet. Sci. J. 4 176. doi:10.3847/PSJ/acf317

Messier 101 aka the Pinwheel Galaxy, site of SN 2023ixf

It’s not every day that a supernova happens in our backyard! Earlier this year, astronomers discovered a supernova in the galaxy Messier 101, which is relatively close at just 21 million light-years away. The explosion, dubbed SN 2023ixf, is the nearest known supernova in recent years.

Discovery images and location of the newfound supernova in its home galaxy

Discovery image (bottom left) and last image in which SN 2023ixf is not visible (top left), as seen from Koichi Itagaki’s observatory. The right-hand image shows SN 2023ixf’s location within Messier 101. [Hiramatsu et al. 2023]

A Supernova Next Door

Using a 0.35-meter telescope, amateur astronomer and prolific supernova sleuth Koichi Itagaki spotted a rapidly brightening newcomer on the outskirts of a spiral galaxy on 19 May 2023. After announcing the discovery of supernova SN 2023ixf on the Transient Name Server, the race was on. Telescopes across Earth and in space pointed toward Messier 101, also known as the Pinwheel Galaxy, to monitor the rise and fall of the newfound supernova. The behavior of the supernova’s light curve in the early days after discovery gives us critical information about the exploding star and its surroundings. What did we learn about SN 2023ixf in these early days?

light curves and color evolution of SN 2023ixf

Light curves for SN 2023ixf showing the evolution of the supernova’s brightness at wavelengths between infrared and ultraviolet in the first month after detection. The bottom panel shows how the color changed over time; the supernova became bluer as it brightened and redder as it faded. Click to enlarge. [Adapted from Hiramatsu et al. 2023]

In the Days After Discovery

In a recent publication, a team led by Daichi Hiramatsu (Center for Astrophysics ∣ Harvard & Smithsonian; NSF AI Institute for Artificial Intelligence and Fundamental Interactions) outlined their observations made in the month after the supernova’s discovery. Analyzing light curves and spectra from multiple telescopes, Hiramatsu’s team found that the supernova rose from obscurity to its eye-catching peak brightness in just five days before declining more gently, fading by 0.03 magnitude each day. Its spectra showed numerous bright emission lines that mark the interaction of the expanding supernova shock with gas surrounding the star.

These light curves and spectra paint a picture of a massive star collapsing as its core nuclear fusion dried up, the star’s outer layers rebounding off its condensed core in an explosion that outshone its home galaxy — a core-collapse supernova. The data also hint at something unusual: there was so much gas packed into a dense shell around the star that it delayed the escape of the shock wave that emerged from the center of the star.

The team used models to investigate the origin of this dense circumstellar material, exploring scenarios in which 1) a strong, steady stellar wind carried material away from the star before the explosion or 2) random outbursts or eruptions preceded the eventual supernova. The team found that the observations were compatible with either scenario, and in both cases, the star likely lost up to a solar mass of material in its last 1–2 years — showing that the final years of the star’s life were anything but calm.

Moving Forward by Looking Back

Astronomers will likely study SN 2023ixf for years to come. Of particular importance will be identifying and characterizing the progenitor star; ideally, we’d be able to monitor supernova progenitor stars in the years or decades before they explode to link their properties before the explosion to what happens in the aftermath. Usually, our investigations of supernova progenitor stars go in the opposite direction: we detect a supernova and then go looking through our increasingly large treasure trove of data to find the star it came from.

Already, several searches for SN 2023ixf’s progenitor star have been published. Multiple teams, using different data sets and analysis methods, have independently identified the same red supergiant as the most likely progenitor for the explosion. To clinch the candidate as the source of the supernova, we’ll have to wait to see if the fading glare of the explosion reveals that the star has disappeared. To learn more about SN 2023ixf, its possible progenitor star, and ongoing investigations of this rare nearby supernova, check out the complete list of AAS journals articles regarding SN 2023ixf here.

Citation

“From Discovery to the First Month of the Type II Supernova 2023ixf: High and Variable Mass Loss in the Final Year before Explosion,” Daichi Hiramatsu et al 2023 ApJL 955 L8. doi:10.3847/2041-8213/acf299

A computer rendering of a brown planet, covered with patches and smears of bright read lava, suspended against a black background.

Imagine standing on a small rock, surrounded on all sides by a sea of boiling lava. Above you, an enormous star looms across most of the sky. Take a deep breath in: what do you smell, what’s in the air? While no humans have yet been in this situation, astronomers are making progress towards answering this question using instruments back here on Earth.

Worlds Beyond Imaginings

Ours is an era of wonder, one in which we are just beginning to discover planets beyond our own solar system and just beginning to realize how strange these worlds can be. Exoplanet astronomers have confirmed that over the years, science fiction writers largely underestimated the universe: included in the menagerie of the first discovered 5,000 planets are worlds with not one, or two, but three stars; planets that are fluffier than cotton candy; and planets where the clouds are made of rock. Each of these worlds offers the chance to contextualize our own home and to study how our galaxy creates and maintains planets.

One extreme and somewhat famous exoplanet discovered back in 2004 is named 55 Cancri e. In some ways, this world might remind us of our own. It’s a little bigger than Earth (it has a radius about twice as large), and it’s also likely rocky, with a bulk density a little higher than the terrestrial value. The comparisons to our home world abruptly end, however, when we consider where the planet is: it nearly skims the surface of its host star, whipping around it on an orbit that takes only about 17 hours. It is so close that were you to stand on the surface of the planet, the star would dominate your view of the sky. Also, you’d likely be standing in lava, since the ambient temperature is high enough to melt the upper crust.

Astronomers have been understandably fascinated with this molten world, and over the past 20 years numerous groups have labored to characterize it. Unfortunately, it is difficult to measure something as small as a planet from 41 light-years away — current estimates of the surface pressure range from a near-Earth-like 1.4 bar all the way to the pressures felt more than a mile undersea. Even so, progress has been made, and the latest step came recently from the first analysis of 55 Cancri e’s emission spectrum using a high-resolution instrument.

New Observations

A wavelength vs electron counts plot. The data appear as several dozen side-by-side Gaussian looking bumps, each of which is outlined with a black line.

One of the high-res spectra collected by MAROON-X of 55 Cancri. The black lines mark the borders between different echelle orders. [Rasmussen et al. 2023]

A team led by Kaitlin Rasmussen and Miles Currie, both University of Washington, collected observations using an exquisitely sensitive spectrograph named MAROON-X with the goal of measuring which elements were present in 55 Cancri e’s atmosphere. Before jumping straight to data analysis, however, they first determined which elements they could detect, if any were truly there. This was a valuable check: even though theories suggest that some combination of Mg, SiO, Na, K, H2O, and CO2 should be present, the researchers found that their data were insufficiently sensitive to detect these species. Happily, however, their data would be sensitive to iron: if any was present floating above the magma oceans, MAROON-X would be able to sniff it out.

A 3-panel plot of heatmaps showing radial velocity on the X axis and Kp on the Y axis, both in km/s. The bright central feature fades as you move from left to right.

The actual data (right), an injection of what an iron signature was expected to look like (middle), and an injection of an iron signal 10x stronger than expected (left). Only a very weak signal was recovered in the real data, which implies that there is very little iron present. [Rasmussen et al. 2023]

Confident they could detect iron should it be there, the team then turned to their real data and found very little sign of it. This left them confident that 55 Cancri e probably does not have a thick iron atmosphere, and gives the broader astronomy community the first constraint from high-resolution emission spectroscopy on whatever is going on in the air above this fiery world. It will likely not be the last, since 55 Cancri e is an upcoming JWST target. That brings us back to a common refrain these days in exoplanet science: to better understand this strange new world, we’ll have to wait for JWST to take a look.

Citation

“A Nondetection of Iron in the First High-resolution Emission Study of the Lava Planet 55 Cnc e,” Kaitlin C. Rasmussen et al. 2023 AJ 166 155. doi:10.3847/1538-3881/acf28e

Artist’s impression of a fast radio burst traveling through space and reaching Earth

The repeating fast radio burst FRB 20190520B traveled through an unusually large amount of matter on its journey to Earth. Could unidentified galaxy clusters in the billions of light-years that separate us from the burst’s source explain why?

An Astrophysical Mystery

Fast radio bursts are among the most mysterious events in the universe. Most of these powerful, milliseconds-long radio blips occur just once, each burst an astronomical flash in the pan that leaves researchers puzzling over its origin. In rare cases, fast radio bursts repeat, giving us a clue that at least some sources of these mysterious bursts survive the event.

signal from the first fast radio burst ever detected

The signal from the first fast radio burst ever detected. The highest frequencies arrive first, and the lower frequencies follow. [Wikipedia user Psr1909; CC BY-SA 4.0]

Fast radio bursts illuminate gas and dust as they travel across millions to billions of light-years, providing a way to study matter along their paths. This is reflected in what researchers call the dispersion measure, which is related to the amount of matter the burst travels through from its origin to an observer on Earth. Researchers determine the dispersion measure of a fast radio burst by measuring the delay between when its highest and lowest frequencies arrive.

The matter that delays the arrival of the lowest-frequency radio waves in a burst — free-roaming electrons are the best at holding up low-frequency waves — can be located anywhere along the burst’s path: in the immediate vicinity of the source, in the source’s host galaxy, in intergalactic space, or in our own galaxy. To disentangle the contributions from these different regions, researchers must take a wide view of the situation.

Surveying a Superlative Burst

The dispersion measure of the repeating fast radio burst FRB 20190520B is more than twice as large as expected given its distance. This unusually high value caught the attention of a team led by Khee-Gan Lee (Kavli Institute for the Physics and Mathematics of the Universe), which is carrying out the Fast Radio Burst (FRB) Line-of-sight Ionization Measurement From Lightcone AAOmega Mapping survey, or FLIMFLAM. This survey aims to map the distribution of luminous matter in the universe by searching for galaxy groups that are revealed by fast radio bursts.

Plot of newly identified galaxy clusters and other galaxies in FRB 20190520B's field

Snapshot of an interactive figure showing the locations of the newly identified galaxy clusters relative to FRB 20190520B’s location. Click to enlarge. You can interact with this figure here. [Lee et al. 2023]

The team spectroscopically determined the distances to galaxies in the field of view surrounding FRB 20190520B’s location and used a group-finding algorithm to identify galaxy groups and clusters. They found multiple galaxy groups in the field of view, including two galaxy clusters that lie directly between us and FRB 20190520B. By using models to estimate the masses of these galaxies and their halos, Lee’s team determined how much these intervening galaxy clusters contributed to the burst’s dispersion measure.

A Revised Estimate

Based on FRB 20190520B’s extremely high dispersion measure, previous research estimated its host galaxy’s dispersion to be the highest of any known fast radio burst, a fact that has been difficult to reconcile with other observations of the galaxy. Now, with the new estimate of the foreground galaxies’ contribution, FRB 20190520B’s host galaxy has been assigned a more moderate value that aligns with its observational properties. This study demonstrates that even when focusing closely on a single fast radio burst, it’s still important to zoom out and consider the big picture!

Citation

“The FRB 20190520B Sight Line Intersects Foreground Galaxy Clusters,” Khee-Gan Lee et al 2023 ApJL 954 L7. doi:10.3847/2041-8213/acefb5

Hubble image of a star-forming region in the Small Magellanic Cloud

Earlier this year, researchers using JWST discovered a galaxy that stopped forming stars just 700 million years after the Big Bang. Cosmological simulations provide a way to study sudden star-formation shutdowns in early galaxies like this one.

Hubble and JWST image of side-by-side elliptical and spiral galaxies

This image of the VV 191 galaxy pair combines data from the Hubble Space Telescope and JWST. Elliptical galaxies, like the left-hand galaxy in this image, usually have little or no star formation, while spiral galaxies, like the one on the right, are alight with new stars. [NASA, ESA, CSA, Rogier Windhorst (ASU), William Keel (University of Alabama), Stuart Wyithe (University of Melbourne), JWST PEARLS Team, Alyssa Pagan (STScI); CC BY 4.0]

Young Galaxies in the Early Universe

In the first few million years after the Big Bang, stars came to light and galaxies assembled, steadily brightening the infant universe. But star formation cannot continue forever, as evidenced by the many galaxies in the universe today whose star formation has drawn to a close.

For galaxies to exhaust their star-forming gas after 13 billion years may not be surprising, but JWST has revealed a galaxy that stopped its star formation far sooner, less than one billion years after the Big Bang. The galaxy, JADES-GS-z7-01-QU, provides an opportunity to study processes that halt star formation — either temporarily or permanently — in the early universe.

Stop-and-Go Star Formation

To understand why a galaxy might stop forming stars so early in the universe’s history, a team led by Viola Gelli (University of Florence and Italian National Institute for Astrophysics/Arcetri Observatory) turned to cosmological simulations. Using the Sᴇʀʀᴀ simulations, Gelli and collaborators gathered a sample of 130 galaxies with redshifts and masses similar to JADES-GS-z7-01-QU. The team found that about 30% of these galaxies had no star formation, and less massive galaxies in the sample were less likely to be star forming than more massive galaxies.

simulation snapshot showing active and quiescent galaxies

Snapshots of the Sᴇʀʀᴀ simulations at a redshift of 6 showing the stars (left) and gas (right) of typical active and quiescent galaxies. Click to enlarge. [Gelli et al. 2023]

A quick peek through the galaxies’ star-formation histories showed that all galaxies in that era with masses less than one billion solar masses (about a thousand times less massive than the Milky Way) had experienced periods when star formation flourished and periods when it stalled. These galaxies undergo bursts of star formation that gradually diminish, consistent with incremental heating of the interstellar gas by supernova explosions, which prevents new stars from forming. Once the gas has cooled, star formation begins again.

Abrupt Transition Needed

Plot showing the spectral energy distributions of simulated galaxies compared to a galaxy observed by JWST

Spectral energy distributions of simulated galaxies compared to JADES-GS-z7-01-QU. The simulated galaxy singled out for further study is named Lilium. Click to enlarge. [Adapted from Gelli et al. 2023]

But when Gelli’s team focused on a single simulated galaxy with almost exactly the same mass and redshift as JADES-GS-z7-01-QU, it became clear that this picture of stop-and-go star formation doesn’t fully explain JADES-GS-z7-01-QU’s behavior. Despite the similarities between the two galaxies, their spectral energy distributions — a measure of how energy output is distributed across the electromagnetic spectrum — didn’t agree. The team brought them into alignment by adding a sharp cutoff in the simulated galaxy’s star formation just 5 million years after it started.

What could be the cause of such a sharp shutoff in star formation? Although heating of star-forming gas by supernovae appears to be ubiquitous in low-mass galaxies in the early universe, the lag between when massive stars form and when they explode as supernovae means a shutdown due to supernovae can’t happen less than 30 million years after star formation starts. In the case of JADES-GS-z7-01-QU, something else must be at play — perhaps the powerful winds of young stars or an accreting supermassive black hole at the galaxy’s center could be responsible for rapidly shutting off star formation in JADES-GS-z7-01-QU. As JWST observes more quiescent galaxies in the early universe, we’ll be able to investigate the causes of stalled star formation further.

Citation

“Quiescent Low-Mass Galaxies Observed by JWST in the Epoch of Reionization,” Viola Gelli et al 2023 ApJL 954 L11. doi:10.3847/2041-8213/acee80

elliptical galaxy NGC 4150

Discovery of a bright, rapidly evolving source led researchers to propose a new class of transients called luminous fast coolers. The physical origin of these rare events, which arise in quiescent elliptical galaxies rather than spiral galaxies like many transients, is still unknown.

Plot of flux versus rest days from explosion showing the large, sudden increase in flux that marks the discovery of a new transient

ATLAS data of AT 2022aedm. [Adapted from Nicholl et al. 2023]

Something New in the Universe

In 2022, the Asteroid Terrestrial impact Last Alert Survey (ATLAS) detected a rapidly brightening source in a quiet elliptical galaxy. The event, labeled AT 2022aedm, was tentatively tagged as a supernova at first, but its fast evolution soon revealed it to be something other than an ordinary exploding star. In a recent research article, Matt Nicholl (Queens University Belfast) and collaborators described the object’s dynamic light curve, investigated its spectrum, and pondered its origins.

Spectra of AT 2022aedm and its host galaxy. The spectra are largely featureless and the shift of the peak wavelength to longer wavelengths shows the rapid cooling of the source.

Spectra of AT 2022aedm at multiple points in time after discovery, plus a spectrum of its host galaxy (gray). Click to enlarge. [Adapted from Nicholl et al. 2023]

Assembling the Puzzle Pieces

In the weeks after AT 2022aedm’s discovery, Nicholl’s team collected data to characterize the event’s swiftly changing light curve. Optical data show that it reached its peak brightness just nine days after its onset, rocketing up to a spectacular peak magnitude of −21.5, before fading nearly as quickly. To put that into context, this means that this single event was about 2.5 times brighter than our entire galaxy! This incredible brightness placed AT 2022aedm in the realm of superluminous supernovae, which make up just 0.1% of supernovae. But other pieces of the puzzle didn’t fit this explanation; AT 2022aedm’s brightness increased and decreased too quickly and its spectrum never gained the tell-tale broad spectral lines of a supernova.

Even stranger than AT 2022aedm‘s light curve and spectrum is where it was found: in a massive elliptical galaxy with very little active star formation. Most core-collapse supernovae happen in lower-mass spiral galaxies, where ongoing star formation is constantly creating new massive stars to fuel these explosions, and superluminous supernovae especially seem to be confined to star-forming spirals.

A Class of Three

Location of AT 2022aedm within its host galaxy.

The location of AT 2022aedm within its host galaxy is marked with magenta crosshairs. [Adapted from Nicholl et al. 2023]

Based on the key properties of the event — extremely high peak brightness and rapid fading — the team identified two more likely members of the same class among previous observations and coined the term “luminous fast coolers” to describe them. The other two events, named Dougie and AT 2020bot, also occurred in the outer parts of elliptical galaxies, further hinting at a common origin.

Despite the similarities between the light curves and spectra of the three events, Nicholl and collaborators were unable to find a physical explanation that fit all three events. Among the options considered were tidal disruption events (when a star is shredded by a massive black hole), exploding white dwarfs, supernovae giving birth to extremely dense, highly magnetized stellar remnants called magnetars, and colliding neutron stars. Each of these scenarios encountered serious roadblocks during the team’s analysis, but the team found promising signs that with further modeling, either a star interacting with a stellar-mass black hole or a shock colliding with a gas cloud could someday explain the origins of these newly described events.

Citation

“AT 2022aedm and a New Class of Luminous, Fast-cooling Transients in Elliptical Galaxies,” M. Nicholl et al 2023 ApJL 954 L28. doi:10.3847/2041-8213/acf0ba

JWST image of the Chamaeleon I molecular cloud

Astronomers have made the first detection of interstellar glycolamide, a molecule closely related to the simplest of the amino acids necessary for life on Earth. Glycolamide is the latest interstellar molecule detected in the G+0.693–0.027 molecular cloud near the center of the Milky Way.

An Elusive Acid

model of the structure of a glycine atom

A model of a glycine molecule. Carbon atoms are shown as dark gray, hydrogen atoms are light gray, oxygen atoms are red, and nitrogen is blue. [Ben Mills; Public Domain]

Amino acids are among the most important molecules for life as we know it, providing the means for all life on Earth to build proteins. Beyond Earth, researchers have discovered amino acids in meteorites, comets, and the hot gas surrounding protostars. If amino acids can form in the sparse gas of the interstellar medium, as observations suggest, these critical molecules can be inherited by protoplanetary disks and, eventually, planets themselves.

The simplest of the amino acids necessary for life on Earth is glycine, which contains just 10 atoms. Glycine has been detected in various places in our solar system, but we’ve yet to detect it definitively in the interstellar medium. In a recent article, a research team led by Víctor Rivilla (National Institute of Aerospace Technology–Spanish National Research Council) took the search for interstellar glycine in a new direction by widening the investigation to include its chemical cousins.

Isolating an Isomer

A glycine molecule contains two carbon atoms, two oxygen atoms, five hydrogen atoms, and one nitrogen atom. But there’s more than one way to arrange these atoms into a molecule, and glycine has many isomers: molecules with the same atoms but arranged in a different way. To search for glycine and its isomers, Rivilla and collaborators turned radio telescopes toward G+0.693–0.027, an interstellar molecular cloud near the center of the Milky Way that is already known to host a number of complex organic molecules.

Structure of a glycolamide molecule and one of the emission lines detected

Left: Example of an emission line from glycolamide detected in this work. Right: The structure of the glycolamide molecule. The molecule is similar to glycine except the carbon atom that is doubly bonded to an oxygen atom is adjacent to the nitrogen atom. [Adapted from Rivilla et al. 2023]

Even in this molecule-rich cloud, glycine remained elusive. However, Rivilla’s team was able to detect one of its isomers for the first time, a molecule called glycolamide, and place upper limits on the abundance of glycine and several of its other isomers.

Tracing Chemical Pathways

Rivilla and collaborators used the measured abundances to explore the likeliest ways glycolamide is created in the interstellar medium. These molecules likely form on the surface of dust grains, where atoms and molecules can gather and link up in the sparse environment of a molecular cloud. For the case of G+0.693–0.027 specifically, ultraviolet photons might create an abundance of highly reactive molecules called radicals, which could interact in this environment to form glycolamide.

graphic showing the proposed way to create glycolamide in the interstellar medium

Pathway proposed in this work to form glycolamide (NH2C(O)CH2OH) in the interstellar medium. Click to enlarge. [Rivilla et al. 2023]

And as for glycine? Comparing the abundances of similar molecules suggests that only small amounts of this critical amino acid exist in G+0.693–0.027 — maybe 3–8 times less than the upper limit measured in this study. Given the low abundance, detecting glycine in the interstellar medium will remain difficult with our current instruments.

Citation

“First Glycine Isomer Detected in the Interstellar Medium: Glycolamide (NH2C(O)CH2OH),” Víctor M. Rivilla et al 2023 ApJL 953 L20. doi:10.3847/2041-8213/ace977

An image of the milky way oriented horizontally, with overlaid blue blobs along the plane corresponding to areas of large neutrino emission.

We live in a universe teeming with neutrinos, tiny particles that zip around near the speed of light and pass right through most solid objects. Where are they all coming from, and what could be responsible for so many bizarre little bits of matter? Astronomers still aren’t sure, but researchers at the IceCube Neutrino Observatory are beginning to sketch out an answer.

Lawless Speeders

Since 2013, astronomers have known that Earth, its telescopes, and all of its inhabitants are constantly weathering a continuous but nearly undetectable barrage of speeding neutrinos. What they are still less sure about, however, are where all of these ultralight and intrusive particles are coming from. Five years ago they discovered and interrogated a possible suspect, a faraway active galactic nucleus with the catchy name TXS 0506+056; however, although they found that this temperamental blazar was indeed responsible for at least some of the high-energy neutrinos zipping through our solar system, it was a small-time player that couldn’t account for the vast majority caught darting through.

An equatorial projection of the sky. Each of the detections is shown as a set of nested contours, each of which denotes a level of confidence.

The on-sky location and uncertainty of each of the detected high-energy neutrino events. [R. Abbasi et al. 2023]

To catch the real culprits, in principle astronomers try to follow their literal tracks. When a high-energy neutrino careens through the earth, sometimes it bumps into the nucleus of an intervening atom. Charged particles created in the aftermath of these rare collisions create a streaking flashes of light pointing along the direction of the incoming neutrino. If scientists can measure the orientation of this streak before it fades, they can trace it backwards and draw a line into the sky pointing towards its origin. This is one of the many goals of the IceCube Neutrino Observatory, a kilometer-sized instrument at the South Pole that looks for tell-tale neutrino flashes through the Antarctic ice.

In reality, measuring these tracks is much easier said than done. While IceCube has detected a few dozen bright streaks since construction finished in 2010, it hasn’t been able to measure any of their orientations accurately enough to guide other telescopes confidently to their sources. Luckily, however, IceCube detects more than just the highest-energy neutrinos, so astronomers have other evidence to consider. It also records millions of smaller, tamer events; with some careful filtering to ignore the neutrinos that are created within the atmosphere by local processes, astronomers can extract a much larger catalog of astrophysical neutrinos, each of which is tagged with a rough estimate of its source location.

Coincidence or Coincident?

The IceCube Collaboration, a vast network of hundreds of scientists, recently dove into these lower-energy events to answer an interesting question: did any of these neutrinos come from the same direction as their brighter counterparts? If they did, that might suggest that whatever is creating all of these racing particles does so at a steady rate, even if it only sends out the heavy-hitters on rarer occasions. Alternatively, if every neutrino came from its own random direction, that would imply that most are created during short outbursts from otherwise quiet sources.

A photograph of a building sitting on a large ice sheet in front of a pink sky.

The IceCube Neutrino Observatory, located at the geographic South Pole [Sven Lidstrom, IceCube/NSF]

After an intensive statistical analysis, the Collaboration found no significant correlation in the direction of neutrino tracks across a wide range of timescales. Even within minutes of a high-energy event, most of the detected low-energy tracks came from random directions, implying that whatever causes a large burst likely doesn’t produce a shower of lower-energy particles at the same time. In other words, neutrinos that arrived at the same time did so by accident: they probably traveled alone from different sources.

Though astronomers still haven’t pinned down the object or process responsible for the background flux of neutrinos, they are making progress building the profile of what they must be like. As studies like this accumulate and the picture improves, eventually the community will settle on an explanation. For, now though, the neutrinos will keep coming, and IceCube will keep watching for flashes.

Citation

“Constraints on Populations of Neutrino Sources from Searches in the Directions of IceCube Neutrino Alerts,” R. Abbasi et al 2023 ApJ 951 45. doi:10.3847/1538-4357/acd2ca

image of the "dragon scale" texture on the surface of Mars

New laboratory experiments suggest that salty water mixed with Martian surface material can remain a liquid under colder and drier conditions than water alone. This means that liquid water might be found over a larger area of Mars’s surface than previously thought, as well as throughout more of the Martian year, with important implications for habitability and exploration.

The Search for Water on Mars

dark streaks on sandy slopes on Mars

Warm temperatures on Mars are associated with the appearance of dark streaks on sloping terrain. On Earth, these streaks are caused by water, but on Mars they may be caused by shifting sand grains instead. [NASA/JPL-Caltech/UA/USGS]

Mars’s sinuous riverbeds and dry lake basins tell a tale of a planet once awash with water, but what about today? Proving the presence of liquid water on Mars’s surface has been tricky, and claims of evidence for modern-day liquid water often find themselves rebutted; for example, the dark streaks thought to indicate subsurface water seeping through the sand were reinterpreted as sand sliding down steep slopes (say that five times fast!).

But the search continues, with evidence mounting that liquid water might exist in the form of brine: a concentrated mixture of water and salt. Martian brine can form in several ways including by water vapor collecting on the surface of salt crystals. In the lab, researchers have tested the conditions under which brine remains a liquid, rather than freezing or evaporating in Mars’s cold, dry climate. But brine on Mars doesn’t exist in isolation. Instead, it’s muddled together with regolith: the loose mixture of rocks, sand, and dust that coats the planet’s surface. Could the mashup of these two materials help water remain a liquid on Mars’s surface?

photograph of Martian soil

An image of Martian soil scooped up by the Phoenix Mars Lander. For this study, the team used simulated Martian soil made from volcanic rocks in the Mojave Desert. [NASA/JPL-Caltech/University of Arizona/Max Planck Institute]

Throwing Regolith into the Mix

To explore this question, Andrew Shumway (University of Washington) and collaborators measured the properties of regolith–brine mixtures in a lab. Since we don’t yet have actual Martian regolith to experiment on, Shumway’s team used a simulated regolith that was originally developed to help NASA scientists test the navigation and sample-collecting skills of the Mars rovers. For their Martian brine, the team swirled together water and a salt called magnesium perchlorate (magnesium and perchlorate are common components of Mars’s surface material).

The team measured two key factors for each of their regolith–brine samples: 1) the freezing point, which partly determines where on the planet’s surface the mixture can remain a liquid, and 2) the amount of water that’s available to participate in chemical reactions and other processes important for life.

Briny Findings

Plot showing the melting points of samples with various concentrations

Melting temperature of frozen regolith–brine samples. Samples with a lower melting temperature also freeze at lower temperatures, making them remain liquid under colder conditions. Click to enlarge. [Shumway et al. 2023]

Shumway’s team found that mixtures of brine and regolith have more water available and freeze at a lower temperature than brine alone, and water can persist when the ambient air is drier, as well. This means that liquid water might be found across more of the Martian surface and during more of the Martian year than previously thought. While this is exciting news for the prospect of finding life on Mars, it also means that we’ll need to be even more careful not to spread earthly microbes to the Martian surface, as water helps to support Earth life as well!

Citation

“Regolith Inhibits Salt and Ice Crystallization in Mg(ClO4)2 Brine, Implying More Persistent and Potentially Habitable Brines on Mars,” Andrew O. Shumway et al 2023 Planet. Sci. J. 4 143. doi:10.3847/PSJ/ace891

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