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JWST Field - 20,000+ galaxies

A new study explores the conditions in an extremely distant galaxy to understand how early galaxies may evolve to form the objects we see now in the local universe.

Early Universe Galaxies

Over the past few years since JWST went online, astronomers have discovered some of the most distant galaxies in the universe. To be visible from over 13 billion light-years away, these galaxies must be extremely bright and thus powered by an intense energy source — typically either active massive central black holes known as active galactic nuclei (AGN) or very strong star formation that causes a burst in stellar light.

The universe’s earliest galaxies are astronomical fossils, providing clues to how nearby galaxies and objects have evolved to their current state. These high-redshift galaxies may be progenitors of globular clusters or beginnings of growing galaxies — objects whose origins are not well understood. Further characterizing the newly discovered galaxies may reveal how objects in the local universe came to be.

ALMA detection of 88 micron line

ALMA detection of the 88-micron emission line from doubly ionized oxygen. The left-hand panel plots the intensity map of GHZ2 as seen on the sky, where the yellow in the center corresponds to the galaxy. The right-hand panel shows the ALMA spectrum with the 88-micron emission line highlighted in yellow. Click to enlarge. [Zavala et al. 2024]

Star Formation or AGN?

One such high-redshift galaxy is GHZ2, which was recently discovered with JWST and lies at a redshift corresponding to ~400 million years after the Big Bang. To complement the JWST spectroscopic observations, Jorge A. Zavala (National Astronomical Observatory of Japan) and collaborators observed the galaxy using the Atacama Large Millimeter/submillimeter Array (ALMA). These observations aimed to measure far-infrared emission lines that are critical in determining if a galaxy is powered by star formation or an AGN. The authors targeted and successfully recovered the 88-micron (1 micron = 10-6 meter) emission line from doubly ionized oxygen — an emission line known to correlate with a galaxy’s star formation activity. From the detection, the authors derived a redshift that agrees with the redshift determined from JWST.

With the ALMA observations and prior measurements from JWST, the authors compare GHZ2 to other galaxies. They find that GHZ2 exhibits characteristics similar to other galaxies that are dominated by star formation. Additionally, this very high-redshift galaxy appears to share characteristics with giant star-forming regions, which further suggests that GHZ2 is likely driven by extreme star formation. The authors find that, though some AGN contribution fraction cannot be definitively ruled out, GHZ2 is most likely dominated by metal-poor and young stellar populations.

How Will GHZ2 Evolve?

stellar mass-velocity dispersion relationship

The stellar mass–velocity dispersion relationship with data points plotted for different object types including globular clusters (light-purple circles), ultra-compact dwarf galaxies (solid purple circles), dwarf ellipticals (down-pointing triangles), compact ellipticals (blue solid circles), and elliptical and S0 galaxies (green squares). The target galaxy GHZ2 is shown with a yellow star, which falls in closest agreement with compact ellipticals and ultra-compact dwarf galaxies in this relationship. Click to enlarge.
[Zavala et al. 2024]

What does the dominant star formation in GHZ2 imply about its evolution? With powerful star formation, GHZ2 could be a proto-globular cluster, though the overall mass of the galaxy is very high compared to typical globular clusters seen in the local universe. Along with its high mass, GHZ2’s size is also a bit larger than reported for other proto-globular clusters. Given the galaxy’s high mass and extended radius, the authors suggest that GHZ2 could be composed of multiple massive star clusters that could evolve into multiple globular clusters, or it could be the compact core of a growing, more massive galaxy.

Though the exact evolution of GHZ2 is unclear, this study underscores the significance of ALMA and JWST as a powerful pair of instruments capable of characterizing the most distant galaxies in the universe. Future observations with both instruments will continue to clarify what the early universe looked like and how it has evolved over time. 

Citation

“ALMA Detection of [O iii] 88 μm at z = 12.33: Exploring the Nature and Evolution of GHZ2 as a Massive Compact Stellar System,” Jorge A. Zavala et al 2024 ApJL 977 L9. doi:10.3847/2041-8213/ad8f38

globular cluster Terzan 5

The star SOS1 is not like its neighbors. Using chemical and dynamical data, stellar sleuths have tracked this star from its current home in the Milky Way’s central bar back to its likely origin in one of the most massive globular clusters in our galaxy.

A Star in a Bar

barred spiral galaxy NGC 1300

The Milky Way, like the galaxy NGC 1300 shown here in an image from the Hubble Space Telescope, has a central bar of stars. [NASA, ESA, and The Hubble Heritage Team (STScI/AURA); Acknowledgment: P. Knezek (WIYN)]

Today, the Milky Way has an intricate and interlocking structure: thin and thick disks of stars surrounded by an extended halo, with a bulge of old stars at the center. A bar of stars cuts across the center of our galaxy, and globular clusters — ancient collections of thousands to millions of stars — dot the galactic bulge and halo. These structures didn’t always exist, and a major goal for galactic research is understanding when and how the many components of our galaxy were assembled.

One piece of the puzzle might be provided by the star 2M17454705-2639109, also known as SOS1. This star is located in the busy galactic downtown of the Milky Way’s center, orbiting within the central bar of stars. Data from the Apache Point Observatory Galactic Evolution Experiment (APOGEE) show that SOS1 has a curious chemical composition that sets it apart from its neighbors. Now, researchers have shown that these chemical differences may be evidence that SOS1 originated far from its current location — and that many other stars in the galactic bar might have completed similar journeys.

Globular cluster Liller 1

The reddish stars of the globular cluster Liller 1 glow behind bright blue stars in the foreground. [ESA/Hubble & NASA, F. Ferraro; CC BY 4.0]

SOS1: Far from Home?

A team led by Stefano Souza (Leibniz Institute for Astrophysics Potsdam; University of São Paolo; Max Planck Institute for Astronomy) investigated SOS1’s origins by first comparing its chemical abundance pattern to those of different populations of stars in the Milky Way. The observed pattern of low carbon, high nitrogen, and high aluminum matches expectations for second-generation stars in globular clusters: densely packed, roughly spherical collections of thousands to millions of stars.

But how would a star born in a globular cluster end up in the Milky Way’s central bar? Souza’s team highlighted two possible scenarios: SOS1 might have been ejected from its home cluster by a gravitational interaction with a binary star system, or — deemed more likely — it could have been stolen from its home cluster by the tidal forces of the Milky Way.

Candidate Clusters

Souza’s team used N-body simulations to determine if SOS1 once called one of the existing globular clusters home. (The team notes that it’s possible that SOS1’s parent star cluster no longer exists, having been pulled apart by the Milky Way’s powerful tidal forces.) The likeliest candidate is Terzan 5, which is among the most massive and most ancient globular clusters in the Milky Way. The simulations suggest that SOS1 might have been bound to this cluster 353 million years ago.

chemical and age comparison between SOS1 and Terzan 5

Chemical (left) and age (right) comparison between SOS1 and stars in the globular cluster Terzan 5. Click to enlarge. [Souza et al. 2024]

The chemical abundances of SOS1 support this hypothesis, since SOS1’s curious chemical makeup is consistent with that of the oldest and most metal-poor stars in the cluster. The final clue would be a comparison of the ages of Terzan 5 and SOS1. Though the data did not allow for a precise determination of the star’s age, the preliminary analysis suggests that it is of a similar age to the cluster.

The chemical similarities and dynamical properties make it likely that SOS1 once resided in a globular cluster, possibly Terzan 5. Its current residence in the Milky Way’s central bar supports the idea that ancient globular clusters contributed stars to the bar through tidal stripping.

Citation

“Tracing Back a Second-Generation Star Stripped from Terzan 5 by the Galactic Bar,” Stefano O. Souza et al 2024 ApJL 977 L33. doi:10.3847/2041-8213/ad91af

36 images of supernova host galaxies

Another year is drawing to a close, and we’re looking back on all the discoveries that we’ve covered on AAS Nova this year. The top stories offer an astronomical smorgasbord and an in-depth look at some of the most recognizable objects in our galaxy. Without further ado, here are the top 10 most-read posts of 2024:

the Milky Way’s central supermassive black hole in polarized light

The first image of the Milky Way’s central supermassive black hole in polarized light. [EHT Collaboration; CC BY 4.0]

10. A New Way of Looking at the Milky Way’s Supermassive Black Hole

In 2022, the Event Horizon Telescope collaboration released the first image of the Milky Way’s supermassive black hole, Sagittarius A*. But the collaboration’s work didn’t stop there, and two years later they released an image of our hometown black hole in an entirely new light: polarized light, to be exact. The polarization of light — the orientation of the light waves as they travel through space — informs researchers as to the magnetic field conditions close to the black hole as well as in between the black hole and Earth. These results revealed a high degree of linear polarization as well as a lesser amount of circular polarization, suggesting a strong, orderly magnetic field.

An illustration of an exoplanet being engulfed by its home star, as 8 UMi b somehow has not been

An illustration of an exoplanet being engulfed by its home star. The planet 8 Ursae Minoris b has somehow escaped this fate. [International Gemini Observatory/NOIRLab/NSF/AURA/M. Garlick/M. Zamani; CC BY 4.0]

9. Astronomers Reopen the Mystery of a Planet That Shouldn’t Exist

Researchers thought they had solved the mystery of the exoplanet 8 Ursae Minoris b: unsure how the planet had survived its host star ballooning into a red giant, they proposed that the star had swallowed its one-time stellar companion, changing its evolution in a way that saved its planet from certain doom. But new data analyzed by Huiling Chen and collaborators upended this tale of a planet saved by a stellar merger: Ursae Minoris b is simply too young to have merged with a companion star. Luckily, the team’s research brought to light another possible resolution to the mystery.

supernova SN 1994D in the galaxy NGC 4526

The supernova SN 1994D illuminates the outskirts of its host galaxy, NGC 4526. [NASA, ESA, The Hubble Key Project Team, and The High-Z Supernova Search Team]

8. The Quest to Watch a Supernova in Real Time

The sooner researchers spot a supernova explosion, the more they can learn from it — but is there a way to know when a supernova is coming? The ejecta from a supernova explosion is so dense that light from the explosion is delayed on its way to our telescopes, but nearly massless particles called neutrinos can escape the blast and, in theory, announce the explosion before the light reaches us. Yuri Kashiwagi and collaborators examined how the upgraded Super-Kamiokande detector can be used to alert astronomers of an impending supernova, enhancing our ability to learn from these explosions.

7. How Common Are Solar Systems Like Our Own?

illustration of the planets in our solar system and their orbits

Illustration (orbits not to scale) of the planets in our solar system. [NASA/JPL]

When researchers began to discover exoplanet systems, it immediately became clear that not all systems are arranged like our own. What remains unclear is how many exoplanet systems are similar to the solar system. One important feature of our solar system is the presence of small planets — like Earth — orbiting interior to large planets, like Jupiter. Astrobites’s Jack Lubin reports on work by Marta Bryan and Eve Lee that searches for this configuration in distant planetary systems, helping to understand how common solar system–like arrangements are in the galaxy.

An icy surface overlaid by a liquid water ocean. Beams of light are streaming through the water from the ocean's top towards the ice below.

An artist’s impression of light shining through the ocean of a “water world,” possibly like LHS 1140b. [NASA]

6. What Kind of World is LHS 1140b?

LHS 1140b is a bright, nearby star that is known to host two planets. The nature of the inner of the two planets, LHS 1140b, is unclear, despite the bevvy of telescopes that have observed this world. As Charles Cadieux and collaborators have shown, LHS 1140b might be the smallest known mini-Neptune exoplanet, or it might be a water world with a surface of ice and oceans.

illustration of Betelgeuse

Artist’s impression of the red supergiant star Betelgeuse. Though depicted solo, new research suggests that Betelgeuse might have a tiny companion. [Adapted from ESO/L. Calçada; CC BY 4.0]

5. Hiding in Plain Sight: Betelgeuse’s Binary Buddy

Betelgeuse is a highly recognizable red supergiant in the constellation Orion. Astrobites’s Alexandra Masegian reports on two AAS journal articles that independently arrived at the same conclusion: that Betelgeuse is not a single star, but rather a member of a binary system. While the two articles find slightly different masses and orbital separations for the proposed companion star, they both point to Betelgeuse’s long secondary pulsation period as evidence of the companion.

cartoon showing different types of exoplanets

Cartoon showing a variety of exoplanet types. Figuring out whether a planet is rocky or gaseous can be a challenge, as is the case for K2-18b. [NASA/JPL-Caltech/Lizbeth B. De La Torre]

4. K2-18b May Not Be Habitable After All

The 8.6-Earth-mass exoplanet K2-18b made headlines when researchers reported that the planet might be a rocky world covered in oceans. Even more eyebrow raising was the potential detection of a faint signal from dimethyl sulfide, a compound that on Earth is only associated with the presence of life. Nicholas Wogan and coauthors used models to interpret JWST data of K2-18b, finding that the planet is instead most likely an uninhabitable gas-rich world — though they didn’t entirely rule out the inhabited ocean world scenario.

The Cassiopeia A supernova remnant as seen by JWST

The Cassiopeia A supernova remnant as seen by JWST. [Rho et al. 2024]

3. Featured Image: A New Portrait of Cassiopeia A

The Cassiopeia A supernova remnant has sat for countless astronomical portraits, each of which reveals new details about this remnant of an exploded star. This portrait from JWST’s Mid-Infrared Instrument and Near-Infrared Camera highlighted electrons spiraling around magnetic field lines as well as light from argon and carbon monoxide, allowing a team led by Jeonghee Rho to study the connections between the formation of molecules like carbon monoxide and the creation of cosmic dust.

infrared image of the supergiant star Betelgeuse and its surroundings

Red supergiant star Betelgeuse, pictured here in an infrared image from the Herschel Space Observatory, has ejected a considerable amount of material. [ESA/Herschel/PACS/L. Decin et al.]

2. Monthly Roundup: Betelgeuse, Betelgeuse, Betelgeuse

Everyone’s favorite red supergiant makes two appearances in this list! This article summoned three perspectives on Betelgeuse, which has made frequent news appearances over the past few years because of its pronounced, prolonged dimming episode in 2019–2021. Though the star has returned to its normal brightness, questions linger about the star’s future behavior and its uncertain past — including whether Betelgeuse is the product of a stellar merger.

1. The Odds of the Unthinkable

Radar images of the near-Earth object Apophis

Radar images of the near-Earth object Apophis. [NASA/JPL-Caltech and NSF/AUI/GBO]

By far the most widely read article on AAS Nova in 2024 concerned an asteroid with an unsettling name: Apophis, named for the Egyptian deity that embodies disorder, destruction, and darkness. On 13 April 2029 — for the superstitious among you, the 13th happens to be a Friday — Apophis will zoom between Earth and the Moon. Apophis’s passage bears no threat to Earth, but an article by Paul Wiegert explored the possibility that gravitational nudges from other asteroids could change all that, sending Apophis careening catastrophically toward our planet in the future.

Thank you for joining us for another year of astronomy news — we hope to see you in 2025 for more discoveries. Happy New Year!

photograph of clouds exhibiting the kelvin-helmholtz instability

Editor’s Note: For the remainder of 2024, we’ll be looking at a few selections that we haven’t yet discussed on AAS Nova from among the most-downloaded articles published in AAS journals this year. The usual posting schedule will resume January 3rd.

First Direct Imaging of a Kelvin–Helmholtz Instability by PSP/WISPR

Published March 2024

Main takeaway:

solar Kelvin-Helmholtz instability

One of several frames in which the turbulent eddies are clearly visible in images from the Wide-field Imager for Parker Solar Probe (WISPR). Each vortex-like feature is labeled. Click to enlarge. [Adapted from Paouris et al. 2024]

Evangelos Paouris (George Mason University; The Johns Hopkins University Applied Physics Laboratory) and collaborators discovered evidence of the Kelvin–Helmholtz instability in the outer solar corona. This is likely the first time this phenomenon has been directly imaged so far out in the Sun’s atmosphere, and this groundbreaking observation was made possible by the unique vantage point and sensitive instruments of the Parker Solar Probe.

Why it’s interesting:

The Kelvin–Helmholtz instability arises at the interface between two fluids, or between fluid regions that are moving at different velocities. While that might sound obscure, Kelvin–Helmholtz instabilities are thought to be common throughout planetary and stellar atmospheres, and you may have seen images of the wave-like clouds (pictured above) that form through the instability. Although researchers knew that the Kelvin–Helmholtz instability happens in the Sun’s atmosphere, they didn’t expect to see evidence of it in the Sun’s middle and outer corona — the hot and tenuous upper atmosphere. They believed that the wave-like pattern of the instability would simply be too small to pick out, even from a nearby spacecraft like the Parker Solar Probe.

More about the discovery and what it might enable:

Paouris and collaborators spotted the instability at the boundary between the solar corona and a coronal mass ejection: a massive outburst of solar plasma and magnetic fields. The team applied multiple tests to the data to confirm that the observed pattern of eddies was consistent with expectations for the Kelvin–Helmholtz instability. This discovery opens up the possibility of using these observations to study the rarefied plasma of the solar corona, as well as the movement of coronal mass ejections through the corona and the outflowing solar wind. Understanding the motions of coronal mass ejections is important for forecasting whether these destructive events might strike Earth, which has practical value: the energetic particles of a coronal mass ejection threaten spacecraft electronics, power grids, and astronauts floating beyond the protective shield of Earth’s magnetic field.

Citation

Evangelos Paouris et al 2024 ApJ 964 139. doi:10.3847/1538-4357/ad2208

illustration of a tidal disruption event

Editor’s Note: For the remainder of 2024, we’ll be looking at a few selections that we haven’t yet discussed on AAS Nova from among the most-downloaded articles published in AAS journals this year. The usual posting schedule will resume January 3rd.

A New Population of Mid-Infrared-Selected Tidal Disruption Events: Implications for Tidal Disruption Event Rates and Host Galaxy Properties

Published January 2024

Main takeaway:

A team led by Megan Masterson (Massachusetts Institute of Technology) has scoured data from the Near-Earth Object Wide-Field Infrared Survey Explorer (NEOWISE) to find tidal disruption events (TDEs): stars that are torn apart by the tidal forces of a supermassive black hole. The search turned up a dozen TDEs that represent a new population of mid-infrared-detected TDEs. This new population helps to relieve some of the tension between the expected and observed numbers of TDEs.

Why it’s interesting:

The existence of TDEs was first theorized in the 1970s, and the first event in this class was identified in the 1990s. Since then, the number of known TDEs has skyrocketed thanks to sensitive, high-cadence surveys. Most TDEs are identified by their rapid brightening at optical or X-ray wavelengths. Based on existing detections, TDEs appear to be more common in galaxies that fall in between star forming and quiescent, or in galaxies that were highly star forming in the past but are now quiescent. It’s not yet known if TDEs are truly less common in star-forming galaxies, or if dust in the centers of these galaxies simply hides TDEs from view.

How the team collected their sample:

host galaxies of TDE canddiates

Host galaxies for each of the selected TDE candidates. The 12 galaxies in the clean “gold” sample are marked with stars; the remaining six TDE candidates may instead be weak active galactic nuclei. Click to enlarge. [Masterson et al. 2024]

To find potential dust-obscured TDEs missed by other surveys, Masterson’s team used observations from NEOWISE, which scans the sky at infrared wavelengths to find potentially hazardous near-Earth asteroids. The team searched for transient signals from the centers of galaxies, selecting signals that met certain criteria, such as having the short, sudden increase in brightness and longer, steadier decrease in brightness that is characteristic of TDEs. After removing contamination from known supernovae and active galactic nuclei, the team settled on a sample of 12 events, including the closest known TDE to date. Most of the host galaxies of this TDE sample appear to be actively forming stars, suggesting that dust was indeed to blame for the under-representation of star-forming galaxies among TDE hosts. The newfound sample also brings the observed rate of TDEs into closer alignment with theoretical estimates, and future searches at infrared wavelengths may reduce the discrepancy further.

Citation

Megan Masterson et al 2024 ApJ 961 211. doi:10.3847/1538-4357/ad18bb

JWST image of the galaxy cluster SMACS

Editor’s Note: For the remainder of 2024, we’ll be looking at a few selections that we haven’t yet discussed on AAS Nova from among the most-downloaded articles published in AAS journals this year. The usual posting schedule will resume January 3rd.

Which Came First: Supermassive Black Holes or Galaxies? Insights from JWST

Published January 2024

Main takeaway:

Joseph Silk (Sorbonne University; The Johns Hopkins University; University of Oxford) and collaborators have theorized that supermassive black holes are responsible for the high rate of star formation seen in many young galaxies. The proposed framework flips the conventional narrative of black hole and galaxy formation, suggesting that supermassive black hole growth spurred the formation of new stars rather than lagging behind the formation of stars.

Why it’s interesting:

How and when supermassive black holes formed in the early universe is one of the key unanswered questions in astronomy. The conventional theory of supermassive black hole formation suggests that galaxies formed first: gas clouds collapsed to form the first stars, which left behind stellar-mass black holes when the stars expired. A series of collisions between these stellar-mass black holes slowly built the first supermassive black holes while star formation continued busily in the background. The team’s new theory suggests that black holes and galaxies grew in tandem instead, with black hole growth playing an important role in the formation of new stars.

Solving a cosmic chicken-and-egg problem:

diagram showing the redshifts at which supermassive black holes spur and quench star formation

Diagram showing the redshifts at which supermassive black holes spur and quench star formation in their host galaxies, in the proposed framework. Click to enlarge. [Silk et al. 2024]

Observations from JWST have revealed the presence of extremely bright galaxies in the early universe, leading astronomers to wonder how these galaxies became so brilliant so quickly. Within the framework proposed by Silk’s team, the extraordinary brightness of these young galaxies is a natural consequence of the supermassive black holes at their centers; as the growing supermassive black holes accreted gas from their surroundings, they shot out powerful outflows that slammed into the surrounding gas, compressing it and triggering an explosive burst of star formation. This theorized powerful burst of star formation doesn’t last forever, though; about 1 billion years into the universe’s history, a shift in the outflowing winds of the supermassive black holes cast out the gas that fueled star formation, bringing it to a halt. Testing the predictions of this theory is likely to be difficult, though sensitive observations and intricate simulations may provide a path forward.

Citation

Joseph Silk et al 2024 ApJL 961 L39. doi:10.3847/2041-8213/ad1bf0

locations of stars in two newly discovered Milky Way substructures

Editor’s Note: For the remainder of 2024, we’ll be looking at a few selections that we haven’t yet discussed on AAS Nova from among the most-downloaded articles published in AAS journals this year. The usual posting schedule will resume January 3rd.

Shiva and Shakti: Presumed Proto-Galactic Fragments in the Inner Milky Way

Published March 2024

Main takeaway:

Illustration of the structure of the Milky Way

Illustration of the structure of the Milky Way. Click to enlarge. [Left: NASA/JPL-Caltech; right: ESA; layout: ESA/ATG medialab]

Using data from the Gaia spacecraft, Khyati Malhan and Hans-Walter Rix (Max Planck Institute for Astronomy) discovered two stellar structures within the Milky Way. The two structures, named Shiva and Shakti, each contain more than 10 billion solar masses’ worth of stars and orbit in the inner Milky Way. Though the origins of the two structures are not yet known, current data suggest that they formed more than 12 billion years ago — before the Milky Way’s spiral arms and stellar disk came to be.

Why it’s interesting:

The Gaia spacecraft makes precise measurements of the positions, distances, velocities, and chemical compositions of stars in our galaxy. Using these data, researchers have discovered many new structures and stellar populations in and around our galaxy, such as stellar streams and satellite galaxies. These structures hold important clues to the formation and evolution of our galaxy, revealing a complex history of accretion, assimilation, and migration of gas and stars.

Potential origins of the newfound structures:

Stellar structures within our galaxy can develop in one of two ways: in situ, meaning forming out of gas and stars already present in the Milky Way, or through accretion from outside the galaxy. The orbits and chemical abundances of the stars in Shiva and Shakti tell a conflicting story about the origins of these two structures. The orbits point to an accretion origin, while the chemical abundances suggest in situ formation; this combination of features has never been seen before. Malhan and Rix suggest that these opposing conclusions can be reconciled in one of two ways: 1) the stars of the Shiva and Shakti structures belonged to the Milky Way’s diffuse, extended halo and became trapped in a resonance with the Milky Way’s central bar of stars before migrating to their current positions, or 2) the Shiva and Shakti structures are truly ancient, representing fragments of proto-galaxies that formed before the Milky Way was constructed. While each of these hypotheses has its strengths and neither is fully compatible with the data, Malhan and Rix favor the proto-galactic fragment idea. Luckily, upcoming surveys and instruments will give an even deeper look into the structure of the Milky Way, helping to clarify how our galaxy was assembled.

Citation

Khyati Malhan and Hans-Walter Rix 2024 ApJ 964 104. doi:10.3847/1538-4357/ad1885

asteroid 243 Ida

Editor’s Note: For the remainder of 2024, we’ll be looking at a few selections that we haven’t yet discussed on AAS Nova from among the most-downloaded articles published in AAS journals this year. The usual posting schedule will resume January 3rd.

Detection of Molecular H2O on Nominally Anhydrous Asteroids

Published February 2024

Main takeaway:

Using the Stratospheric Observatory for Infrared Astronomy (SOFIA) — a now-decommissioned Boeing 747 that toted an infrared telescope to 40,000 feet for more than a decade — Anicia Arredondo (Southwest Research Institute) and collaborators investigated the surface compositions of four asteroids. The spectra of two asteroids in the sample contained a feature unambiguously attributed to water, marking the first time water has been detected on the surface of an asteroid.

Why it’s interesting:

Asteroids represent material left over from the formation of the solar system. By studying asteroids, researchers hope to learn about the solar nebula from which the planets coalesced. Of particular interest is the distribution of water in the early solar system, which can tell us about how Earth got its water, as well as how planets in other star systems might develop. The four asteroids examined in this study are S-type asteroids, which are thought to form in the inner solar system, where volatile materials like water are scarce. The discovery of water molecules on these supposedly dry asteroids provides an important constraint on theories of solar system and planet formation.

spectra of four asteroids

Spectra of the four asteroids examined in this work. Two of the asteroids, (7) Iris and (20) Massalia, clearly show the 6-micron water feature, while the other two asteroids do not. Click to enlarge. [Arredondo et al. 2024]

On the definitive spectral feature:

Detecting a particular molecule in space can be difficult because the spectral features from different molecules often overlap. Many asteroids exhibit a spectral feature at 3 microns (1 micron = 10-6 meter) that arises from stretching of the chemical bond between an oxygen atom and a hydrogen atom. This bond is present in a water molecule — meaning that this feature could be due to water — but it’s also present in many other molecules. To make their definitive detection of water, Arredondo’s team searched instead for a spectral feature at 6 microns, which is solely due to water and has been detected previously on the Moon. The water molecules discovered in this study might be trapped in glass beads on the asteroids’ surfaces, adsorbed onto silicon, or bound up in minerals. The team plans to use JWST to continue their search for water on asteroid surfaces.

Citation

Anicia Arredondo et al 2024 Planet. Sci. J. 5 37. doi:10.3847/PSJ/ad18b8

faintest known Milky Way satellite Ursa Major III/UNIONS 1

Editor’s Note: For the remainder of 2024, we’ll be looking at a few selections that we haven’t yet discussed on AAS Nova from among the most-downloaded articles published in AAS journals this year. The usual posting schedule will resume January 3rd.

The Discovery of the Faintest Known Milky Way Satellite Using UNIONS

Published January 2024

Main takeaway:

A team led by Simon Smith (University of Victoria) discovered a collection of stars orbiting the Milky Way in data from the Ultraviolet Near Infrared Optical Northern Survey (UNIONS), which is carried out by three telescopes in Hawaiʻi. The newly discovered Milky Way satellite, named Ursa Major III/UNIONS 1, contains about 57 stars and has a total mass of 16 solar masses. Ursa Major III/UNIONS 1 is the faintest known Milky Way satellite.

Why it’s interesting:

The advent of sensitive surveys has revealed much about the Milky Way’s neighborhood, including a steadily growing population of satellites. These surveys are discovering fainter and fainter satellites that blur the line between the largest star clusters and the smallest dwarf galaxies. Dwarf galaxies and star clusters have significant overlap in their masses and are distinguished by the presence or absence of dark matter: dwarf galaxies are thought to form in individual dark matter halos, while star clusters are not.

properties of Ursa Major III/UNIONS 1 compared to different types of Milky Way satellites

The brightness and size of Ursa Major III/UNIONS 1 (orange square) relative to dwarf galaxies (blue circles), globular clusters (red circles), and ambiguous satellites (black diamonds). Click to enlarge. [Smith et al. 2024]

On the nature of this faint satellite:

It’s not yet clear whether Ursa Major III/UNIONS 1 is a dwarf galaxy or a star cluster. This fact is reflected in the system’s name: newfound dwarf galaxies are named for the constellation in which they appear, while star clusters are named for the survey in which they were discovered. This satellite currently bears both kinds of names. Smith’s team found evidence that the velocities of stars in Ursa Major III/UNIONS 1 are spread fairly widely around the mean velocity of the system, suggesting that the system sits in its own dark matter halo and thus could be an astoundingly small dwarf galaxy. However, this finding is highly sensitive to the number of stars included in the analysis, highlighting the need for careful follow-up observations of this faint Milky Way satellite — and many others.

Citation

Simon E. T. Smith et al 2024 ApJ 961 92. doi:10.3847/1538-4357/ad0d9f

artist's impression of a quasar

Editor’s Note: For the remainder of 2024, we’ll be looking at a few selections that we haven’t yet discussed on AAS Nova from among the most-downloaded articles published in AAS journals this year. The usual posting schedule will resume January 3rd.

Quaia, the Gaia-unWISE Quasar Catalog: An All-Sky Spectroscopic Quasar Sample

Published March 2024

Main takeaway:

map of quasars in the Quaia catalog

The on-sky distribution of quasars in the Quaia catalog. The dearth of objects across the center of the image is due to the Milky Way’s disk. The team also developed a more refined sample of 755,850 quasars with especially reliable distance estimates. Click to enlarge. [Adapted from Storey-Fisher et al. 2024]

Beginning with 6,649,162 quasar candidates identified by the Gaia mission, Kate Storey-Fisher (New York University) and collaborators have constructed a catalog of 1,295,502 quasars spread across the sky. The catalog, named Quaia, includes redshifts for each quasar, enabling detailed studies of the large-scale structure of our universe over the course of cosmic history.

Why it’s interesting:

Quasars — short for “quasi-stellar radio sources” — are extremely luminous galactic centers powered by accreting supermassive black holes. Quasars are thought to reside in regions of dense dark matter, making them important probes of the unseen dark-matter structures that suffuse the universe. In addition to their cosmological importance, studying quasars can also clue us in to the physics of accretion, the growth of supermassive black holes, and the evolution of massive galaxies.

More details about the data:

The Gaia satellite, which began its mission in 2013, was designed to obtain precise positions, distances, and velocities for stars in the Milky Way. Fortuitously, Gaia also studiously documented its observations of objects that are far brighter and more distant than stars in our galaxy, like quasars. The initial 6.6-million-object sample of quasar candidates, however, was riddled with objects mistaken for quasars, and many of the distances to the objects — critical for cosmological studies — were wildly inaccurate. Storey-Fisher’s team incorporated data from other sources, like the Wide-field Infrared Survey Explorer and the Sloan Digital Sky Survey, to boost the purity of the sample and provide better distance estimates. You can explore the final sample for yourself at this link.

Citation

Kate Storey-Fisher et al 2024 ApJ 964 69. doi:10.3847/1538-4357/ad1328

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