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zoomed-out view of a spiral galaxy and an elliptical galaxy

Galaxies seem to have less matter than they should. Has the missing matter been found at last in the form of hot, sparse gas?

The Troubling Matter of Normal Matter

image of a spiral galaxy

The space surrounding a galaxy isn’t empty. Instead, it’s filled with tenuous circumgalactic gas invisible to optical telescopes. [NASA, ESA, CXC, SSC, and STScI; CC BY 4.0]

From tiny dwarf galaxies to enormous ellipticals, all galaxies have something in common: they all seem to have less baryonic matter — the stuff that makes up stars, gas, dust, and everything we can see and touch — than we expect. Some galaxies only have a few percent of the matter they should have.

Researchers suspect that the matter isn’t really “missing” but is instead “hidden,” present in a form that’s hard to observe. Recently, a major breakthrough happened when researchers zoomed out from the luminous, starry disks of galaxies to study the tenuous gas of the circumgalactic medium. There, they found immense reservoirs of cool (~104K) gas accounting for as much as 50% of the lost matter. In a recent article, researchers have turned up the heat on the search, seeking out a second, hotter (~106K) component of the circumgalactic medium.

cartoon of the observing setup

A diagram (not to scale!) of the observing setup. X-rays emitted by the quasar pass through the circumgalactic medium of a foreground galaxy. [AAS Nova/Kerry Hensley]

A Critical Alignment

A team led by Fabrizio Nicastro (Italian National Institute for Astrophysics) aimed to track down the remaining missing matter by observing the light from quasars as it passes through galaxies in the foreground. Quasars are the luminous centers of distant galaxies that are powered by supermassive black holes consuming matter. Superheated disks surrounding these active black holes emit enormous amounts of X-rays. As these X-rays travel from the quasar to our telescopes, they can be intercepted by galaxies along their path. If the sought-after hot gas is present in the surroundings of these galaxies, it’ll make itself known by absorbing some of the quasars’ X-ray light.

Nicastro and collaborators analyzed X-ray spectra of three quasars whose lines of sight pass through the circumgalactic medium of a foreground galaxy. None of the individual quasar spectra showed definitive signs of the elusive hot circumgalactic gas, but a signal emerged when the team combined observations of all three quasars. Using several different fitting methods and ways of combining the data, the team detected a signal at a significance ranging from 4.2 to 6.8 sigma.

Missing Mass Found

plot of an X-ray absorption line

Example of an X-ray absorption line identified after stacking the observations from all three quasars. [Adapted from Nicastro et al. 2023]

Based on how much the quasars’ X-ray light was absorbed by the gas in its path, the team estimated the mass of the hot gas surrounding the three galaxies in their sample — and it was a lot. Nicastro and collaborators estimated that 70% of the remaining missing matter can be found in the hot circumgalactic medium, and it might even contain the full amount that is missing.

The discovery of this supply of hot circumgalactic gas has significance beyond solving the missing matter problem: the newfound detection of hot gas can also tell us about how galaxies have exchanged gas with the circumgalactic medium throughout their lives.

Citation

“X-ray Detection of the Galaxy’s Missing Baryons in the Circumgalactic Medium of L* Galaxies,” Fabrizio Nicastro et al 2023 ApJL 955 L21. doi:10.3847/2041-8213/acec70

Hubble image of a star surrounded by a blue nebula

The universe is a surprisingly dusty place. New research takes a look at a rare system where massive stars make dust in their powerful winds.

Stardust Gets Its Start

Cosmic dust is typically born in the atmospheres of evolved stars, where it’s cool enough for carbon and silicon to condense into solid grains. A more dramatic example of dust creation can be seen in the binary system WR 137, where the clashing stellar winds of two massive stars create dust every time the stars approach each other.

Wolf–Rayet star

The dusty ejecta of Wolf–Rayet stars makes for fantastic images. These stars will eventually explode as supernovae. [NASA, ESA, CSA, STScI, Webb ERO Production Team]

Both of the stars in this binary system are extremely rare. One is a Wolf–Rayet star: a once-massive star that has lost its entire hydrogen envelope, leaving its scorching-hot core behind. Evolutionary modeling suggests that the Wolf–Rayet star in WR 137 once clocked in at 60 solar masses, but the combined forces of stellar evolution and mass loss have reduced the star to a mere 4.4 solar masses. The other star is a hot, massive O-type star that is rotating so fast that its atmosphere is being ejected into a disk around the star.

As these stars draw near each other every 13 years, the Wolf–Rayet star’s intense stellar winds (at 4.5 million miles per hour!) pummel the O star’s disk, creating a perfect environment for making dust.

infrared light curves of WR 137

Infrared light curves of WR 137 showing four brightness increases due to dust production. Click to enlarge. [Peatt et al. 2023]

When Stellar Winds Collide

Past observations show an increase in WR 137’s infrared brightness every 13 years. Because dust grains absorb light of many wavelengths and re-emit it in the infrared, this periodic increase in infrared light suggests that there is a periodic increase in dust formation as well. The next flurry of dust formation should happen in 2024, so a team led by Megan Peatt (Embry-Riddle Aeronautical University) seized the opportunity to observe the system at infrared wavelengths using the Stratospheric Observatory For Infrared Astronomy (SOFIA), which has now been decommissioned.

The observations, made in July 2021, February 2022, and May 2022, show a steady increase in infrared emission, marking the increase in dust production as the stars approach each other. In addition to the characteristic spectral lines from the Wolf–Rayet star’s powerful winds, the team also identified a weak emission line around 6.3–6.4 microns (1 micron = 10-6 meter). This feature grew stronger as the system brightened, suggesting that it’s linked to the formation of dust.

Signs of Dust Composition?

zoomed-in spectra showing the change in the 6.2 micron feature

Vertically offset spectra showing the slight migration of the 6.2-micron feature. Click to enlarge. [Peatt et al. 2023]

The precise location of this unidentified emission line appeared to shift to longer wavelengths over time. Peatt’s team suggested that it could reflect a change in the composition of the dust; 6.2-micron emission features could come from a class of molecules called polycyclic aromatic hydrocarbons, which contain rings of carbon atoms bonded to hydrogen atoms. A shift in the emission feature toward longer wavelengths could indicate a shift from hydrogen-rich molecules to hydrogen-poor ones.

This might happen because as the stars draw close, the carbon-rich, hydrogen-poor wind of the Wolf–Rayet star collides and mixes with the hydrogen-rich disk around the O star. As dust production begins, there are ample hydrogen atoms to be wrapped into dust molecules, but as it continues, the proportion of wind material to disk material rises, meaning fewer hydrogen atoms are available. The team emphasizes that this result is speculative, and we’ll need more observations of WR 137 to understand how it makes dust. Hopefully, we’ll learn more about this cosmic dust factory as it nears peak production in 2024!

Citation

“FORCASTing the Spectroscopic Dust Properties of the WC+O Binary WR 137 with SOFIA,” Megan J. Peatt et al 2023 ApJ 956 109. doi:10.3847/1538-4357/acf201

illustration of a quasar

Astronomers have long known that the universe has grown more metallic over time: in its younger, purer days, it was composed almost entirely of hydrogen and helium. Recently, however, researchers discovered a galaxy that was notably ahead of the trend and had already amassed a high metal content only a billion years after the Big Bang.

Building Starstuff

Nearly all of the atoms heavier than helium began their lives in a star, the forges of the cosmos responsible for crushing primordial materials into the rich array of elements we see today. These forges run day and night, constantly churning through the universe’s finite supply of hydrogen and helium. Consequently, the overall budget of hydrogen goes down over time, while the proportion of heavier elements (which astronomers call “metals,” regardless of their actual metallic properties) grows. When astronomers look back in time and observe the high-redshift universe, they expect to find mostly pure hydrogen and helium, unpolluted by the starstuff that makes up rocks and people and telescopes.

This prediction usually stands up to observations, and when looking at galaxies with redshifts beyond z = 4 (those born in the first roughly 1.5 billion years after the Big Bang), researchers most often observe clouds of gas with barely any metals. However, a collaboration led by Jianghao Huyan, University of South Carolina, recently discovered a surprising contradiction to this harmonious agreement: their observations of hazy galaxy at z = 4.7 revealed a metal fraction more than two orders of magnitude above the prediction for such a young source.

Mystery Metals

An 8-panel plot, each of which shows wavelength vs. flux for a different absorption line. The red model generally agrees quite well with the black data.

Zoomed-in regions of the measured spectrum centered on different metal absorption lines. The red curve represents the best-fitting model spectrum, while the black histogram is shows their data. [Huyan et al. 2023]

Huyan and colleagues made their discovery when observing a distant quasar named SDSS J002526.84-014532.5 that sits at a redshift of 5.07. Sitting between Earth and this luminous radiation source is a still distant, but slightly closer, galaxy at a redshift of 4.74. As the light from the quasar passed through the wispy gas of the intervening galaxy on the way to our telescopes, specific wavelengths were preferentially absorbed by the molecules and atoms it encountered along the way. By measuring the relative amount of this absorption across many wavelengths, the researchers could back out which elements had tried to block the light’s path, and how dense each species must have been within the gas.

They found that the galaxy possessed a substantial amount of carbon, oxygen, magnesium, and other heavy elements. In fact, just 1.2 billion years after the Big Bang, this galaxy already had a higher relative amount of carbon and oxygen than our own Sun that was born many billions of years later. This was a startling find: models of early galaxy formation expect a significantly smaller fraction of metals, even when accounting for the large uncertainties about the behavior of theorized-but-not-yet-seen first-generation stars.

A 2D plot showing a set of gently downward sloping curves, meant to indicate that metallicity should drop with larger redshifts. The point marking this galaxy lies near the upper right corner of the plot.

Previous measurements and predictions of galaxy metallicity as a function of redshift. The galaxy in question here is marked as the pink triangle that lies far above the model curve. Click to enlarge. [Huyan et al. 2023]

Like many of the most intriguing surprise discoveries, the authors currently have no explanation for what could lead to such a substantial metal enrichment. They concede that it’s possible this particular line of sight may have passed through an anomalously developed region of gas, and that on average, the galaxy as a whole may be as metal-poor as expected. Even in this scenario, however, they cannot explain how that small patch could have been processed to such an extent. Perhaps it is time to revisit the chemical evolution models of early galaxies; perhaps there is something special about this particular galaxy that remains to be uncovered. For now, though, astronomers have another mystery on their hands, and once again the universe has proved ready to challenge our attempts to explain it.

Citation

“Discovery of Super-enriched Gas ∼1 Gyr after the Big Bang,” Jianghao Huyan et al 2023 ApJL 954 L19. doi:10.3847/2041-8213/aceefe

JWST image of Pandora's Cluster

Dark matter, which makes up 85% of the matter in the universe, is thought to interact with normal matter only through gravity. If dark matter and normal matter could interact through collisions, what would that mean for our models of the universe?

maps of the cosmic microwave background anisotropy and local galaxy distribution

Measurements of S8 made from the fluctuations in the cosmic microwave background radiation (top) and the distribution of nearby galaxies (bottom) do not agree. [Top: ESA and the Planck Collaboration; Bottom: M. Blanton and the Sloan Digital Sky Survey]

Clumpiness in Conflict

Our best cosmological model, called lambda-CDM, describes a universe dominated by dark energy, dark matter, and normal matter, in that order. Lambda-CDM has weathered many challenges, but a couple of nagging disagreements have prompted some researchers to propose tweaks or outright rewrites of this leading theory of cosmology.

The first and most famous issue is the Hubble tension: the disagreement between different methods of measuring the expansion rate of the universe. The second issue, and the subject of today’s article, has to do with how clumpy or uniform matter is in our universe. The “clumpiness” parameter, called S8, measured in the nearby universe from observations of galaxy clusters is smaller than what we measure in the distant universe from the cosmic microwave background radiation — the oldest light in the universe, hearkening back to just 380,000 years after the Big Bang. In an attempt to relieve this tension, researchers have explored the effects of loosening one of our fundamental assumptions about dark matter.

Dark Matter Can Have a Little Interaction, as a Treat

In the lambda-CDM model, dark matter is only capable of interacting with normal matter through gravity. This means that dark matter can’t bump into normal matter; a dark-matter particle cruising through your body would have no effect at all. In a recent research article, a team led by Adam He (University of Southern California) suggested that tweaking this property of dark matter could solve our problems with the S8 tension.

plots of posterior distributions

Posterior distributions resulting from different data combinations under the interacting dark matter (IDM) model. [He et al. 2023]

He and collaborators explored the consequences of allowing a small fraction of dark-matter particles to interact with normal matter through collisions. These collisions would allow the two types of matter to exchange heat and momentum. The team used this dark-matter model to analyze observations of galaxy clusters from the Baryon Oscillation Spectroscopic Survey (BOSS) and found that allowing 5–15% of dark-matter particles to collide with normal matter reduced the S8 tension by 30%. Importantly, in the process of lessening the S8 tension, this change doesn’t worsen the Hubble tension — an issue that other models have struggled with.

A Preference for Interacting Dark Matter?

When the team looped in data from the Planck spacecraft, which measured the cosmic microwave background radiation, and the Dark Energy Survey, which has mapped the locations of hundreds of millions of galaxies, they found that the data actually slightly lean toward a scenario in which dark matter and normal matter lightly interact.

Commissioning images from the Euclid space telescope

Commissioning images from the Euclid space telescope. [ESA/Euclid/Euclid Consortium/NASA; CC BY-SA 3.0 IGO]

The results are not conclusive evidence that we need to revamp our understanding of dark matter. He’s team suggests that interacting dark matter lessens the S8 tension because it reduces the clumpiness of matter in the universe on small scales, and upcoming observations of small-scale structure may douse or stoke the interacting-dark-matter fire; the European Space Agency’s Euclid space telescope, which launched in July 2023, will develop a three-dimensional map of the universe, and Rubin Observatory will take a new census of dwarf galaxies, allowing us to map the structure of the universe on the scales required to study this issue further.

Citation

“S8 Tension in the Context of Dark Matter–Baryon Scattering,” Adam He et al 2023 ApJL 954 L8. doi:10.3847/2041-8213/acdb63

side-by-side images of the Sun, a galaxy containing a supernova, and a protoplanetary disk

To meet the challenges posed by our growing collection of data, researchers have devised increasingly sophisticated computing techniques. Today, we’re taking a look at three ways machine-learning techniques have been applied to astrophysical data.

Machine Learning in the Spotlight

Machine learning is a term that describes a collection of techniques in which computers explore data and develop their own algorithms. In astrophysics research, this often takes the form of training computers on a set of known inputs and outputs before introducing data from outside the training set and allowing the computer to derive outputs for those data. For example, researchers could train an algorithm using a set of stellar spectra coupled with known properties of those stars (e.g., spectral type, age, metallicity) and then use the resulting algorithm to classify other stars based on their spectra.

Machine learning and other artificial intelligence techniques are increasingly popular in many fields of science. Here we take a brief look at three recent research articles that describe how machine learning can help us model planet-forming disks, compare observations from different telescopes, and detect fleeting cosmic events.

A Rapid Disk Predictor

A team led by Shunyuan Mao (毛顺元) from the University of Victoria used an artificial neural network to model the interactions between planets and the disks of gas and dust they form in. Planet-forming disks show a wide variety of structures, such as rings and spiral arms, that appear to be linked to the presence, movement, and growth of young planets. By modeling these features, researchers can determine the properties of the planets embedded in protoplanetary disks, but the process can take hours of computing time. Luckily, machine learning appears to offer an easier, faster way to model these disks.

actual versus predicted surface density profile of a gap in a protoplanetary disk

One example of PPDONet’s performance, showing the actual (blue) and predicted (red) density profile of a gap in a disk. [Mao et al. 2023]

Mao’s team has introduced the Protoplanetary Disk Operator Network (PPDONet), which can predict the results of a disk and a planet interacting in less than one second — using a normal laptop. This enormous reduction in computing time is made possible by the team’s machine-learning methods, which recognize when modeling outcomes will be similar to previous runs, jumping ahead and eliminating the need to start every simulation from scratch and iterate through millions of timesteps. The team trained their model on fluid dynamics simulations of disks containing a single planet and found that the model accurately predicts the structure of the disks. The model is publicly available.

Matching Images Between Spacecraft

Researchers wanting to predict solar flares, coronal mass ejections, and other forms of solar activity often base their predictions on images of the Sun taken at extreme-ultraviolet wavelengths. Thanks to spacecraft like the Solar Dynamics Observatory (SDO) and the Solar and Heliospheric Observatory (SOHO), we have decades of solar images to work with, but the differences between telescopes can make it challenging to combine data from different sources into a single prediction — when different observations have different fields of view, spatial and temporal resolution, and noise levels, it’s hard to compare apples to apples.

Demonstration of the different fields of view and spatial resolution of SOHO (left) and SDO (right). [Chatterjee et al. 2023]

To make it possible to work with both SDO and SOHO data sets, Subhamoy Chatterjee (Southwest Research Institute) and collaborators trained a deep-learning model using data from the two spacecraft taken at the same time. The model translated the SOHO images to match the resolution and other characteristics of the SDO images. In an improvement over previous attempts to homogenize solar imaging data, Chatterjee’s team also used Bayesian statistical methods to estimate the uncertainty of the translated images — a critical piece of information for estimating the uncertainty of predictions based on those images.

A Faster Way to Track Down Transients

demonstration of the traditional transient search process

Example of the transient search process. The template image (left) is subtracted from the search image (center), resulting in a difference image (right) that clearly shows a transient source. Click to enlarge. [Adapted from Acero-Cuellar et al. 2023]

Every time we survey the night sky, we find fleeting flashes of light from exploding stars, cosmic collisions, and more. We can learn a lot from studying these events, known as transients, but the process of tracking them down can be time intensive and computationally expensive. A typical method for finding astronomical transients in survey data involves creating reference templates from multiple observations that are then altered to match the seeing conditions and observing setup of the comparison data. The scaled template is then subtracted from the new data, and the resulting image, called a difference image, is scoured for new sources. This method is effective but time consuming, and imaging artifacts, moving stars, and variable stars can all cause false positives. Tatiana Acero-Cuellar (University of Delaware and National University of Colombia) and collaborators suggest that machine learning can make this process faster and eliminate the need for human intervention.

Using data from the Dark Energy Survey, Acero-Cuellar’s team constructed two neural networks to test the possibility of eliminating the difference image altogether. Using one neural network that was trained to use all three images and one that was trained to use all but the difference image, the team found that eliminating the difference image does reduce the network’s ability to identify transients, but only slightly — the accuracy dropped from 96% to 91%. While these neural networks are time-consuming to train, especially when the difference image is not used, putting them into practice requires only a few seconds. This demonstrates the potential for neural networks to eliminate a time-consuming step while retaining a high level of accuracy, which could help us handle the enormous amount of data produced by current and upcoming surveys.

Citation

“PPDONet: Deep Operator Networks for Fast Prediction of Steady-state Solutions in Disk–Planet Systems,” Shunyuan Mao et al 2023 ApJL 950 L12. doi:10.3847/2041-8213/acd77f

“Homogenizing SOHO/EIT and SDO/AIA 171 Å Images: A Deep-learning Approach,” Subhamoy Chatterjee et al 2023 ApJS 268 33. doi:10.3847/1538-4365/ace9d7

“What’s the Difference? The Potential for Convolutional Neural Networks for Transient Detection without Template Subtraction,” Tatiana Acero-Cuellar et al 2023 AJ 166 115. doi:10.3847/1538-3881/ace9d8

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

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