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Editor’s Note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.

Title: Small and Close-In Planets are Uncommon Around A-Type Stars
Authors: Steven Giacalone and Courtney D. Dressing
Authors’ Institutions: California Institute of Technology and University of California, Berkeley; University of California, Berkeley
Status: Published in AJ

Some of the most interesting articles in exoplanet science concern the discovery of new exoplanets. After all, most of us get into this field with dreams of discovery. Today’s article is as fascinating as it is empty of planets: the authors searched roughly 20,000 stars and found a whopping zero planets. What’s the deal?

While the Kepler mission’s survey that stared at a single patch of sky for four straight years searching for exoplanet transits has dominated exoplanet statistics for more than a decade, Kepler focused primarily on host stars with spectral types F, G, K, and M (i.e., FGKM stars). These stars all have temperatures less than about 7000K, and while there is a lot of variation among these types of stars, they are considered relatively Sun-like (our Sun is a G-type star). Through observing these stars, we were able to build up robust statistics on the occurrence rates of different kinds of planets. Astrobites has covered occurrence rate studies in the past (see these bites on small planets around M-dwarf stars, occurrence of systems with similar architecture to our own, and big planets around small stars). Essentially, the occurrence rate measures how many planets of a specified type (like “super-Earths” or “hot Jupiters”) are likely to be found if you survey a certain number of stars with a certain spectral type.

However, the Kepler mission did not observe enough A-type stars to measure the occurrence rate of different kinds of planets around hosts of this stellar type. A-type stars are hotter than our Sun, with temperatures between about 7500K and 10000K. They are bigger in both mass and radius than our Sun, and they emit more of their light in the ultraviolet portion of the electromagnetic spectrum. Very little is known about planetary systems around A-type stars, in large part due to Kepler’s blind spot, but also because these stars make for very poor targets in radial-velocity surveys. A-type stars have far fewer spectral lines than Sun-like stars and they spin much faster than our Sun, both factors that decrease radial-velocity sensitivity greatly compared to the more Sun-like stars. Most of what is known of planetary systems around A-type stars comes from direct-imaging surveys, which are only sensitive to massive Jupiter-size planets at very large separations from their stars.

Then, when the Transiting Exoplanet Survey Satellite (TESS) came along with the plan to observe most of the night sky, an opportunity to search for planets around more A-type stars became available. That’s where today’s article comes in. The authors used the TESS dataset to search A-type stars for small planets (between 1 and 8 Earth radii) on short orbits (orbital period less than 10 days). To say that this search was an immense labor is almost an understatement. The authors wrote a custom software pipeline to search through the dataset, identify potential transits, and then apply a few rounds of vetting on each candidate.

The authors first had to identify all the A-type stars in the TESS catalog — about 20,000 stars. Of these, the pipeline identified 299 transit candidates. Looking more closely at these with a complementary dataset, the authors ruled out many as false positives (mostly obvious-by-eye eclipsing binaries), leaving only 88 candidates remaining. Next, they inspected the candidates for secondary eclipses, which would indicate the transit is not planetary but stellar; this effort ruled out another half, leaving only 44 candidates. Next they tested for background eclipsing binaries, which can mimic planet transits, cutting more candidates so that only 10 remained. Lastly, they performed statistical tests on the leftovers by analyzing the way nearby stars get brighter or dimmer during the time of the transits to see if anything is correlated. In the end, they found that not one candidate passed and therefore the pipeline found zero planets.

Despite finding no planets, this null result is still very important. In this study, the null result was used to place constraints on the occurrence rates of different kinds of planets that orbit A-type stars. In particular, the authors found that sub-Neptune-sized planets occur six times less frequently around A-type stars than they do around FGKM stars. The authors dive into this result and point to earlier studies that show a decrease in planet occurrence as the host stars get hotter. In fact, this result is well in line with these earlier studies, but now the trend is finally investigated for the even hotter A-type stars, as shown in Figure 1. This is a fascinating result: big hot stars seem to host fewer planets than smaller, cooler stars. Why?

plot of sub-Neptune occurrence rate as a function of host star temperature

Figure 1: The occurrence rate of sub-Neptune planets versus host-star temperature. Previous studies show that sub-Neptunes are common around smaller, cooler stars. As host-star temperature increases, the occurrence of sub-Neptunes goes down. This has been noted in earlier works, but this work extends the temperature regime greatly at the hot end and shows that sub-Neptunes are very rare around hotter A-type stars. [Adapted from Giacalone and Dressing 2025]

The authors note a few caveats to their result. These A-type stars are known to pulsate more often than cooler FGKM stars, and this pulsation is imprinted into the flux measurements of the star, which can make it more difficult to find planets. Accounting for these pulsations is another project in itself, but it could explain why the team didn’t find any planets. Furthermore, the authors acknowledge that A-type stars spin so fast that they actually flatten out at the poles and bulge out at the equator. This in turn makes the high latitudes of the star brighter and the equator dimmer, thus finding small planets that orbit at higher inclinations becomes even more difficult. This effect, called gravity darkening, is essentially not a problem for FGKM stars.

Next, the authors discuss what their result means for the big picture of our understanding of planet formation. First, they discuss if this could be part of an observing bias. There are two ways that A-type stars make finding transits more difficult. A-type stars emit lots of their light in the ultraviolet, which can strip the atmospheres of Neptune-like planets through a mechanism called photoevaporation. Perhaps all the gaseous planets that orbit A-type stars have been stripped of their atmospheres and all that is left behind are the relatively small rocky cores, which are very hard to detect via transits. Additionally, the existence of binary systems, to which many A-type stars belong, can make planets harder to detect. If what is thought to be a single star is in fact a binary, the second star’s extra light can make the transit depths even smaller and therefore even harder to detect.

Finally, the authors pose the question of whether A-type stars simply make fewer planets or are able to retain fewer planets. The disks of gas and dust that form around all stars when they are born, and from which planets are born, are dissipated by the strong A-type star’s stellar winds much faster than those of FGKM stars. Perhaps these disks don’t survive long enough to produce many planets. On the other hand, A-type stars are very massive and studies show that the large gas disks they produce when they are born should produce many planets and in particular many big planets. Perhaps these systems are born with many large planets that make the system gravitationally unstable and all the inner, small planets get flung out of the system.

In all, the authors provide a rigorous study of the occurrence (or lack thereof) of small planets in close orbits of A-type stars. This work sheds light on a severely understudied population and better rounds out our understanding of planet formation across stellar mass and temperature regimes. Sometimes finding nothing is just as meaningful!

Original astrobite edited by Maria Vincent.

About the author, Jack Lubin:

Jack received his PhD in astrophysics from UC Irvine and is now a postdoc at UCLA. His research focuses on exoplanet detection and characterization, primarily using the radial velocity method. He enjoys communicating science and encourages everyone to be an observer of the world around them.

close-up of the center of the active galaxy Centaurus A

Editor’s Note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.

Title: Extremely Dense Gas Around Little Red Dots and High-Redshift AGNs: A Non-Stellar Origin of the Balmer Break and Absorption Features
Authors: Kohei Inayoshi and Roberto Maiolino
Authors’ Institutions: Peking University; University of Cambridge and University College London
Status: Published in ApJL

Since its launch in late 2021, JWST has discovered all kinds of weird and wonderful objects. Its impressive sensitivity to infrared wavelengths has allowed astronomers to peer billions of years into the past and discover previously unseen populations of early galaxies. One distinct group of new galaxies was dubbed “little red dots,” or LRDs for short, and these galaxies were observed to be red and compact with distinctive “V”-shaped spectra. You can read more about LRDs here and here.

There’s been a lot of speculation about what kinds of galaxies LRDs might be. One of the most popular interpretations is that a little red dot is a galaxy hosting a supermassive black hole that’s being fed by a rapidly rotating disc of gas. This is known as an active galactic nucleus or AGN. One of the key signatures of an AGN is the presence of broad Balmer emission lines in the galaxy’s spectrum.

Electrons in an atom can only inhabit specific energy levels, and to jump down from one level to another, a photon with the exact same energy as the difference between the two levels must be emitted. Each element has unique energy levels, allowing astronomers to attribute different emission lines to specific transitions within specific elements. Wavelengths in the Balmer series are emitted when an electron in a hydrogen atom jumps from a higher energy level to the second energy level.

cartoon showing how Doppler broadening is produced

Figure 1: This diagram shows how a spinning disc creates Doppler broadening. The side that’s moving towards the viewer will emit blueshifted light and the side that’s moving away from the viewer will emit redshifted light. When you add up the slight shifts from each part of the disc, you end up with a broad emission line. [Nathalie Korhonen Cuestas]

But just having Balmer emission lines doesn’t tell us much — it just indicates that there’s hydrogen in the galaxy. Hardly surprising given that it’s the most common element in the universe! Normally, an emission line is narrow since light is being emitted at just one wavelength. However, if the gas is moving relative to the observer, then the Doppler effect kicks in, shifting light to different wavelengths. The breadth of the Balmer lines in LRD spectra can only be produced by a spinning disc of hydrogen (see Figure 1). From the observer’s point of view, one edge of the disc is moving towards you, and the other edge is moving away from you. As a result, light from the edge moving towards you will be blueshifted, and light from the edge moving away from you will be redshifted. Adding up the light from the entire disc results in a broad emission line, hence why a broad Balmer line is a hallmark of an AGN (although not all AGN are observed to have broad lines — you can learn more about these kinds of AGN here).

But there are other possible explanations for what LRDs might be. One explanation that’s garnered some attention is the possibility that LRDs are not AGN and are instead very dusty starburst galaxies. This explanation is supported by the presence of a Balmer break (sometimes also called a Balmer jump) in the spectra of some LRDs. A Balmer break refers to a significant dip in a spectrum for wavelengths shorter than the Balmer limit — or, the maximum wavelength of light that can ionise a hydrogen atom with an electron in the second energy level. Observing a Balmer break means that a significant fraction of hydrogen atoms with electrons in or above the second energy level have been ionised by high-energy photons. Typically, a Balmer break is associated with recent star formation, since you need lots of hot stars to be emitting photons beyond the Balmer limit.

There’s a problem with this explanation of LRDs. If LRDs are in fact dusty starbursts, then their spectra are consistent with stellar masses of tens of billions (or even up to hundreds of billions) of solar masses. In the local universe, these kinds of masses are pretty normal, but local galaxies have had 13.8 billion years to grow — an LRD at a redshift of z = 7 has not even had 1 billion years to grow. Our current understanding of the universe makes it seem pretty unlikely that this could happen.

Luckily, today’s authors are on the case and have shown how an AGN spectrum could have a Balmer break, allowing astronomers to assume a much lower stellar mass for LRDs. The authors suggest that if LRDs contain AGNs covered by a thick blanket of dense gas, then we could expect to see a Balmer break.

To test this idea, the authors use a photoionisation modelling code called Cloudy, which essentially calculates how many electrons should be in each energy level, given the temperature and density of the gas, as well as the light source illuminating the gas. The authors model the gas in the LRD and surrounding the AGN as a single slab of low-metallicity (10 times lower abundance of heavy elements than in the Sun) gas at a uniform temperature and density and use an AGN spectrum as the light source. They vary the density of the gas between 10 million and 100 billion atoms per cubic centimeter.

At low densities (see the magenta line in Figure 2), there’s no Balmer break because there just aren’t many electrons in the second energy level. As the density increases, collisions between particles become more common, and some electrons will excite to the second energy level due to these collisions. As a result, there are more electrons at the right energy level to absorb photons bluewards of the Balmer limit and photoionise. In Figure 2, you can see that the strength of the Balmer break increases as you go from 108 cm-3 to 1010 cm-3.

plots of spectra produced by different gas densities

Figure 2: This plot shows you the spectrum produced by slabs of different densities. You can see that the Balmer break (highlighted in yellow) becomes deeper at higher densities, although it becomes slightly shallower at the highest density. [Adapted from Inayoshi & Maiolino 2025]

At the highest density tested by the authors (1011 cm-3, the yellow line in Figure 2), the strength of the Balmer break actually decreases. This is because the equilibrium temperature associated with this density is slightly lower (7800K instead of 8000K), resulting in less frequent collisions and fewer electrons in the second energy level.

You can see from Figure 3 that the authors’ simulated spectra produced Balmer breaks that are just as strong as the Balmer breaks seen in LRDs. This means that the picture of LRDs as AGNs surrounded by very dense gas is consistent with observations! The authors also show that such dense gas can produce absorption features at the Balmer wavelengths and an oxygen emission line, which are also sometimes observed in LRD spectra.

Plot of Balmer break strengths as a function of density

Figure 3: Balmer break strengths as a function of density for a range of the authors’ simulated AGN spectra. The colorful horizontal lines show the Balmer break strengths actually observed in different LRDs. [Inayoshi & Maiolino 2025]

Further observations are needed in order to definitively say what LRDs are, and it’s possible that not all LRDs are the same kind of object. The results of today’s research article show that we don’t have to invoke large stellar populations in order to understand Balmer breaks in LRD spectra, but Balmer breaks are only seen in 10–20% of broad-line AGN observed by JWST, so astronomers will need to understand the different physical scenarios that produce the full range of LRD spectra.

Original astrobite edited by Storm Colloms.

About the author, Nathalie Korhonen Cuestas:

Nathalie Korhonen Cuestas is a second-year PhD student at Northwestern University, where her research focuses on the chemical evolution of galaxies.

spiral galaxy NGC 1672

Editor’s Note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.

Title: CEERS: Increasing Scatter Along the Star-Forming Main Sequence Indicates Early Galaxies Form in Bursts
Authors: Justin W. Cole et al.
First Author’s Institution: Texas A&M University
Status: Published in ApJ

In the local universe, galaxies tend to form stars at a slow and steady rate, but new observations suggest this is not the case in the early universe. JWST has allowed astronomers to peer further back into the universe’s history, and the galaxies that they’ve found seem to defy expectations. For example, astronomers keep observing more bright galaxies than initially expected. One explanation for the observations made by JWST is that galaxies in the early universe form their stars in bursts interspersed with periods of less intense star formation.

The average rate at which galaxies form stars varies over cosmic time: from the beginning of the universe, it rises for about 3.3 billion years, peaks at around a redshift of z = 2 (equivalent to about 10.4 billion years ago), and then begins to decline. While an individual galaxy might deviate from this general trend, it’s a good description of how an entire population of galaxies behaves over time. Importantly, this trend is slightly different depending on a galaxy’s mass. More massive galaxies tend to form more of their stars and reach their peak star formation rate earlier in the universe’s history.

The correlation between mass and star formation rate gives rise to a relationship known as the star-forming main sequence, or SFMS. The SFMS is an observed correlation between a galaxy’s stellar mass and its star formation rate — higher-mass galaxies generally form stars at a higher rate. Since the cosmic star formation rate changes with time, so does the SFMS. For example, the SFMS of galaxies close to the peak of cosmic star formation will be shifted towards higher star formation rates than the SFMS of local galaxies.

To make things more interesting, astronomers can use different features in a galaxy’s spectrum to estimate the star formation rate averaged over a different amount of time. For instance, you can use the luminosity of the Hα emission line to estimate the star formation rate averaged over the last 10 million years. This is because the Hα emission line is produced in ionised gas surrounding the most massive stars (O types), which only live for a couple million years. By contrast, the ultraviolet luminosity of a galaxy is more sensitive to the emission from lower-mass, longer-lived stars, giving astronomers a way to estimate the star formation rate over the past 100 million years.

The authors of today’s article use these two measures of star formation rate to investigate the star-forming histories of over 1,800 high-redshift galaxies. They find that the SFMS has a lot more scatter if you use a short-timescale, Hα-based star formation rate as opposed to a longer-timescale, ultraviolet-based star formation rate. This suggests that the star formation rate is more variable on short timescales and supports the idea of bursty star formation.

The galaxies used in the analysis were observed as part of the Cosmic Evolution Early Release Science (CEERS) survey using the Near Infrared Camera (NIRCam) on JWST. All of the galaxies are in a very well-studied region of the sky known as the Extended Groth Strip, so the authors were able to combine the JWST data with pre-existing Hubble data.

To determine how the SFMS evolves with time, the authors divided the sample into five different bins, each in a different redshift range. For each bin and each star formation rate indicator, they estimated the slope, normalisation (y-intercept), and scatter of the SFMS. Across all redshifts, the authors found that the shorter-timescale, Hα-based star formation rate generated an SFMS with more scatter (see Figure 1) and a lower normalisation than an SFMS based on a longer-timescale star formation rate. Larger scatter shows that the star formation rate varies more on short timescales than long timescales, reflecting a bursty star-formation history. The lower normalisation shows that the short-timescale star formation rate is, on average, lower than the long-timescale rate. So, the authors conclude that while star formation does happen in bursts, it’s more accurate to describe the star-formation history as interruptions to normal star formation (lulls or naps that last between 100 and 250 million years), as opposed to short periods of very intense star formation.

plots of star formation rate as a function of galaxy stellar mass

Figure 1: The left-hand panel shows a long-term star formation rate (averaged over 100 million years) and the right-hand panel shows a shorter-term star formation rate (averaged over 10 million years). You can see that the right-hand panel has a lot more scatter than the left. [Cole et al. 2025]

Today’s authors also found that the scatter of the shorter-timescale SFMS increased with redshift, suggesting that galaxies in the early universe had burstier star-formation histories. However, the normalization of the SFMS did not change significantly, suggesting that the intensity of the bursts was not significantly higher in the early universe.

plot of the ratio of short term star formation rate to long term star formation rate

Figure 2: Higher-mass (x-axis) galaxies have a lower short-term star formation rate as compared to their long-term star formation rate, suggesting that higher-mass galaxies have shorter periods of activity. Click to enlarge. [Adapted from Cole et al. 2025]

When the authors divided the sample into higher-mass and lower-mass galaxies, they found some interesting behaviour. On average, lower-mass galaxies had more similar long- and short-timescale star formation rates, while higher-mass galaxies showed a more marked difference between the two star formation rates, with the shorter-term star formation rate being lower than the longer-term rate. You can see this behaviour in Figure 2, which plots the ratio between the short- and long-term star formation rates on the y-axis and shows a negative correlation between the ratio and mass. This shows that lower-mass galaxies experience longer bursts of star formation than higher-mass galaxies. The authors estimate that star-forming bursts last around 60 million years in high-mass galaxies and about 110 million years in low-mass galaxies.

The results of today’s article give us an exciting insight into the early stages of galaxy evolution and will help us to model these newly observed baby galaxies. There are still lots of open questions regarding why star-formation histories seem to be so different in the early universe, so stay tuned as astronomers learn more about galaxy evolution!

Original astrobite edited by Will Golay.

About the author, Nathalie Korhonen Cuestas:

Nathalie Korhonen Cuestas is a second-year PhD student at Northwestern University, where her research focuses on the chemical evolution of galaxies.

galaxies known as little red dots

Editor’s Note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.

Title: Discovery of Local Analogs to JWST’s Little Red Dots
Authors: Ruqiu Lin et al.
First Author’s Institution: Shanghai Astronomical Observatory
Status: Published in ApJL

The universe is home to many fascinating objects! In today’s bite, let’s take a closer look at two of them:

Little red dots: These compact red galaxies are peppered throughout the early universe (as seen in JWST images), and evidence suggests that each little red dot hosts an active black hole (active galactic nucleus) at its center. This raises essential questions about how black holes got so huge in a very short time since the formation of the universe. You can read more about little red dots here and here.

Green pea galaxies: These are small and green (resembling peas!) and are found in the nearby universe (0.1 < z < 0.4). They appear green because a large fraction of light from these galaxies originates from bright, glowing gas clouds that emit light at specific wavelengths (such as emission from [O III], which falls in the part of the electromagnetic spectrum that corresponds to green color) rather than the broad spectrum of light and continuous colors emitted by stars in other galaxies. The presence of broad emission lines in the spectrum of green peas suggests that these galaxies could also host an active galactic nucleus. Interestingly, green peas were first discovered in 2007 by citizen scientists through the Galaxy Zoo project.

Wait, Are These the Same Thing?

The similarities between little red dots and green pea galaxies suggest that green peas could be early-universe little red dots that evolved with little change and survived to the present-day universe. The authors of today’s article set out to explore this possibility. A good approach is to list the characteristics of little red dots and determine whether green peas share those same traits. The authors start with a large sample of around 2,000 green pea galaxies and then systematically select the galaxies that have the closest resemblance to little red dots.

What Are the Defining Characteristics of Little Red Dots?

  1. They have broad H-alpha lines. One of the characteristic features of little red dots is the presence of broad hydrogen lines, which indicates that they host an active galactic nucleus. Thus, only the green peas with signatures of broad lines (full width at half-maximum of the emission line is greater than 1,000 km/s) were selected, narrowing the list down to 19 green peas with broad H-alpha lines.
  2. They are little! If the name wasn’t clear enough, little red dots have compact sizes. Fourteen out of the 19 broad-line green peas were compact, with the radius of the image being less than 2.5 arcseconds, which is similar to the size of little red dots.
  3. They have “V”-shaped spectral energy distributions. Perhaps the most defining feature of little red dots is that they have a spectral energy distribution that looks like a “V,” with a sharp rise in the ultraviolet part of the spectral energy distribution, which gradually slopes downwards and then rises steeply again in the infrared. While the increase in the infrared can be attributed to dust reddening, the ultraviolet spike is still poorly understood. Nevertheless, seeing that this is the defining characteristic of little red dots, the authors only select compact, broad-line green peas that have similar (based on the calculated slopes from little red dots) V-shaped spectral energy distributions, using ultraviolet data from the Galaxy Evolution Explorer survey and optical data from the Sloan Digital Sky Survey (Figure 1).
ultraviolet and optical spectra of a little red dot galaxy and a green pea galaxy

Figure 1: The ultraviolet–optical spectrum of a little red dot plotted in red with the spectrum of a green pea candidate overplotted in green to show the similarities between the two spectra. The broad H-alpha feature can be noticed in both spectra at around 6564 Å. The rising continuum in the ultraviolet for the little red dot spectrum is visible here, which is also seen in the green pea spectrum (indicated by the green dots). [Adapted from Lin et al. 2025]

After applying all these cuts, the authors narrowed down their sample to seven V-shaped, broad-line, compact green pea galaxies, which they believe are the best candidates for local analogs of the little red dots. They find similarities between their green pea and little red dot samples in terms of other properties, such as their ultraviolet magnitudes, and similar correlations between the slope and the black hole mass found for little red dots. They also find that the black holes in little red dots and green peas are likely more massive (Figure 2) than predicted for their small galaxy sizes when using established scaling relations.

plot of black hole mass versus stellar mass

Figure 2: A plot showing the stellar mass of the galaxy on the x-axis with the black hole mass on the y-axis. The various black lines are the expected masses from scaling relations. The little red dots (red diamonds, crosses, and triangles) and green peas (green stars, with the broad-line green peas with V-shaped spectral energy distributions highlighted with the blue box) have higher black hole masses than expected. [Lin et al. 2025]

Local analogs to high-redshift galaxies are very useful to study because they offer an unprecedented way to study the galaxies of the early universe. Being at low redshifts, we can study their spectra from ultraviolet to infrared at very high sensitivity, allowing us to fully understand how these galaxies form and evolve. Many of these green peas need to be studied in greater detail to know if they are indeed local analogs of little red dots. This will enable us to investigate the early growth and evolution of supermassive black holes in a local context.

Original astrobite edited by Erica Sawczynec.

About the author, Archana Aravindan:

I am a PhD candidate at the University of California, Riverside, where I study black hole activity in small galaxies. When I am not looking through some incredible telescopes, you can usually find me reading, thinking about policy, or learning a cool language!

Messier 82

Editor’s Note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.

Title: Quantifying Baryonic Feedback on Warm-Hot Circumgalactic Medium in CAMELS Simulations
Authors: Isabel Medlock et al.
First Author’s Institution: Yale University
Status: Published in ApJ

Simulating the Universe

Galaxies are complex ecosystems with a wide variety of physical processes impacting how they form and evolve. Studying these processes  is often quite difficult, in part because we can’t actually travel to galaxies and poke and prod at them to see what’s going on. So we have to turn to another handy tool in the astrophysicist’s toolkit: simulations. Cosmological hydrodynamic simulations can model how giant swaths of the universe have evolved since the universe’s early years, and take into account a wide range of processes ranging from dark matter structure formation to star formation to galaxy mergers. But because these simulations are so large and complex, there is a limit to how small of a scale can be resolved, and a lot of the activity falls into what we call “subgrid physics”: phenomena that occur on scales below a simulation’s resolution. Instead of modeling activity from foundational behavior, we have to approximate what impact the subgrid physics would have on the scales we can resolve. For example, instead of directly modeling how a molecular cloud collapses, we would say if a volume of gas reaches some critical density it automatically forms some amount of stars with some initial mass function. (If that sounds a bit hand-wavy, that’s because it is, but we gotta work with what we’ve got, and right now we don’t have the computing power to model all the scales of astrophysics at once.)

Some of the most important subgrid processes are stellar and black hole feedback. Stellar feedback refers to how star-related events like star formation and supernovae deposit energy into the interstellar medium, which can impact the galactic environment significantly, slowing down star formation and even ejecting material from the galaxy entirely. Black hole feedback (sometimes called active galactic nucleus or AGN feedback) comes from activity around the supermassive black holes found in the centers of practically every large galaxy. As material accretes around the black hole, the material can heat up or even be ejected at high velocities, causing large amounts of energy to be expelled into and around the host galaxy. Between stars and black holes, these feedback processes have a large impact, and without their inclusion in simulations the results look nothing like the universe as we observe it.

A Flock of Camels

There are many cosmological simulations widely used today, but comparing their differing feedback implementations can be difficult as other parameters (i.e., initial conditions, cosmological parameters, resolution and simulation volume) are varied. The simulations used in this work come from the Cosmology and Astrophysics with MachinE Learning Simulations (CAMELS) project, which provides a suite of model runs that change feedback parameters but keep everything else constant. CAMELS pulls feedback implementations from several simulations, but this work looks specifically at comparing the simulations SIMBA and IllustrisTNG. For each of these simulations, CAMELS identifies four parameters that drive feedback (two each for stellar and black hole processes) and provides a range of runs that span this parameter space. These parameters describe things like the mass loading and speed of winds, accretion rates, and energy and momentum flux.

SIMBA and IllustrisTNG are broadly similar in their feedback treatments, but they have a few key differences. To model stellar feedback, both codes heat and move around gas particles, but the means in which they calculate temperature gains and velocities are different. Both simulations have two modes of AGN feedback: one associated with low accretion rates around the black hole, and the other associated with high accretion rates. For IllustrisTNG, when accretion rates are high the feedback is all thermal, heating nearby gas particles. At lower accretion rates, the feedback is more kinetic, where particles are ejected with energies set by the CAMELS black hole feedback parameters. In SIMBA, both AGN modes have some form of kinetic feedback, with high accretion rates corresponding to lower velocities.

This work compares how the different feedback treatments in CAMELS-SIMBA and CAMELS-IllustrisTNG impact black hole growth, feedback energies, and gas distribution in and around galaxies. The authors do this by comparing two different values: fCGM and the closure radius. The quantity fCGM provides a ratio of the mass found in the circumgalactic medium (the region surrounding a galaxy) relative to the total mass found in the halo. The closure radius takes a different approach, by looking at the physical scale at which the ratio between the mass of baryons (gas, stars, and black holes) and all the mass (including dark matter) is the same as the ratio for the universe as a whole. The measurement provides a way of determining the scale at which matter is associated with a given galaxy.

Some Feedback on the Feedback

Relationship between the halo mass and closure radius

Figure 1: Relationship between the halo mass and closure radius in different simulations run with varying stellar feedback strengths. Here, we can see the ability of SIMBA (purple) to disperse gas to a larger radius than IllustrisTNG (green). [Adapted from Medlock et al. 2025]

Overall, this work found that the variations in feedback processes not only impacted the gas distribution and galaxy properties, but also that the links between stellar and AGN feedback are important and vary between different simulations. In general, the feedback implemented by the IllustrisTNG code had a higher energy than that from SIMBA. However, SIMBA had a greater impact on the baryon distribution, with larger closure radii than the IllustrisTNG runs, as shown in Figure 1. When examining links between stellar and black hole feedback, the authors found that improving the efficiencies of stellar feedback weakened AGN feedback in IllustrisTNG but slightly strengthened it in SIMBA. Figure 2 shows how varying one of the stellar feedback parameters changed the energy of an AGN feedback mode. Finally, the authors looked at changes over time; earlier in the universe, AGN feedback was rarer than stellar activity, and the complexities of their interplay did not become important until redshifts of z < 2.

Plot showing the relationship between the halo mass and energy

Figure 2: Relationship between the halo mass and energy coming from the thermal AGN mode in IllustrisTNG (left) and SIMBA (right). The different colors represent different strengths of stellar feedback, with purple being the lowest and orange the highest. Here we see how changing the stellar feedback parameters impacts the black hole feedback, with stronger stellar feedback corresponding to lower AGN energies in IllustrisTNG, but slightly higher energies in SIMBA. [Adapted from Medlock et al. 2025]

Overall, these results indicate that one cannot simply treat stellar and black hole feedback independently (insert Boromir meme here). These processes are related in complex ways that are currently not fully understood. The fact that different simulations have different interplay (sometimes with directly opposite results!) points to a need for further constraining of these subgrid models going forward.

Original astrobite edited by Lucie Rowland.

About the author, Skylar Grayson:

Skylar Grayson is an Astrophysics PhD Candidate and NSF Graduate Research Fellow at Arizona State University. Her primary research focuses on active galactic nucleus feedback processes in cosmological simulations. She also works in astronomy education research, studying online learners in both undergraduate and free-choice environments. In her free time, Skylar keeps herself busy doing science communication on social media, playing drums and guitar, and crocheting!

Illustration of the seven known planets in the TRAPPIST-1 system

Editor’s Note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.

Title: The Detectability of CH4/CO2/CO and N2O Biosignatures Through Reflection Spectroscopy of Terrestrial Exoplanets
Authors: Armen Tokadjian, Renyu Hu, and Mario Damiano
First Author’s Institution: NASA’s Jet Propulsion Laboratory
Status: Published in AJ

Are we alone in the universe? This is one of the longest-standing questions ever asked by humankind. Though extraterrestrial life is discussed as a philosophical thought exercise, the NASA Astrophysics flagship mission of the 2040s, the Habitable Worlds Observatory (HWO), will get us closer than ever to the answer. This mission aims to detect signs of life on exoplanets. When active, the HWO will observe the atmospheres of Earth-like worlds to see if there are any particular molecules that could reveal past or present life. On Earth today, oxygen gas (O2) is a large component of our atmosphere and is considered a biosignature, since it is primarily produced as a byproduct of photosynthesis by living organisms. However, Earth’s atmosphere looked very different billions of years ago, when life had just begun to form. The authors of today’s article put forth theoretical models of how habitable exoplanets might appear to HWO using the molecules present in the Earth’s atmosphere from earlier eons, and test if HWO will be able to detect these molecules in simulated observations.

It’s Not Just a “Phase,” Mom, It’s an Eon

We all look different from the pictures from our youth, and Earth is no exception. And since life has been flourishing on Earth for billions of years, we can expect that any potentially life-harboring exoplanet may look like Earth from a different stage of its life. The authors of today’s article take two snapshots of Earth from two different phases, or eons, of its youth — but instead of revealing awkward braces and self-cut bangs, they are looking for the different molecules present in its early atmosphere. The first signs of life — microbes such as bacteria and archaea — emerged about 4 billion years ago during the Archean Eon. At this time, Earth’s atmospheric makeup was mostly nitrogen (N), carbon dioxide (CO2), and methane (CH4). Of these, CH4 is particularly interesting to astrobiologists because the primary source of CH4 during the Archean Eon was methanogenesis, a process that produces methane as a byproduct during microbial respiration, making CH4 a strong biosignature. However, abiotic processes like volcanism can produce CH4 as well. Therefore, the co-presence of CO has been suggested to distinguish the production mechanism of CH4, as volcanic activity typically produces more CO than CH4. A detection of CH4 in an atmosphere with a high CO to CH4 ratio should be considered a false positive of the CH4 biosignature as it is likely to have been produced abiotically.

In the following eon, the Proterozoic (2.5 billion to 541 million years ago), the atmospheric oxygen level began to rise, and the first eukaryotic organisms evolved. Microbes using the nitrogen cycle became the predominant producers of dinitrogen oxide (N2O), a compelling biosignature. Even more enticing is that N2O has few abiotic sources, unlike CH4. The authors of today’s article produced atmospheric models based on the biosignatures present in both the Archean and Proterozoic eons, and from those models tried to infer the presence of CH4/CO2/CO and N2O, respectively.

Earth as a Model

For both cases, the authors generated spectra using radiative transfer models, inputting parameters such as the cloud coverage, surface albedo (i.e., the fraction of light that a surface reflects), and the amount of the molecules of interest (specifically the number of a specific gas molecule divided by total number of molecules in a given volume). To model the Archean Earth atmosphere, the authors fix the amount of CH4 to be the upper limit of CH4 that could have been present in the Archean atmosphere, and they consider three different amounts of CO, corresponding to CO/CH4 ratios of 1, 5, and 10, respectively.

To model the Proterozoic Earth atmosphere, the authors consider two examples with different amounts of N2O. The first model corresponds to the upper limit of N2O for a Proterozoic Earth-like planet around a G-type star, and the second model corresponds to the same kind of planet, but around a K-type star instead. The K-type host star allows for much higher amounts of N2O in the planet’s atmosphere. This is because G-type stars are hotter and therefore have a higher ultraviolet flux, which causes the N2O in the atmosphere to photodissociate. These parameters are used in their radiative transfer model, which will produce a synthetic reflected light spectrum — i.e., the spectrum of light that is reflected off a cool exoplanet, which resembles what the HWO aims to observe.

Show Me the Biosignatures

To see if the authors can detect these molecules from these synthetic spectra, they employ an atmospheric retrieval code. In short, the synthetic reflection spectra generated by their radiative transfer model is sampled many, many times to see which one best fits the parameters of interest (most importantly, the presence of potential biosignatures). The planet-to-star flux ratio as a function of wavelength for the forward model with CO/CH4 = 10 along with the retrieval result for the Archean Earth atmosphere is shown in Figure 1. The authors find that they are able to detect CH4 and CO2 in the atmosphere, but they fail to constrain the CO abundance for any of the three cases. This is illustrated in Figure 1, where the CO features are relatively weak or located within a stronger CO2 feature. Because of this, it will be challenging to know if a potential detection is a false positive for the CH4/CO2 biosignature pair with similar observations.

Plot of the Planet-to-star flux ratio versus wavelength for Archean Earth-like planets

Figure 1: Planet-to-star flux ratio versus wavelength for Archean Earth-like planets with CO/CH4 = 10. The model is shown by the red circles and the retrieval result is overlaid. Each molecule of interest is labeled in a separate color to show the individual molecular contributions. While CH4 and CO2 are well constrained by the retrieval, CO is not. [Tokadjian et al. 2024]

On the other hand, the authors report that they are able to robustly detect N2O for the Proterozoic Earth-like planet around a K-type star (i.e., the model with much more N2O present), though not for the G-type star. The planet-to-star flux ratio plot for the atmosphere of a Proterozoic Earth around a K-type star is shown in Figure 2. This result warrants optimism, as few abiotic sources can replicate that level of N2O production in a planet’s atmosphere, making it top a biosignature candidate for the HWO.

Plot of the Planet-to-star flux ratio versus wavelength for Proterozoic Earth-like planets

Figure 2: Same as Figure 1, but for the Proterozoic Earth-like planet around a K-type star. N2O is well constrained by the retrieval result. [Tokadjian et al. 2024]

This study is just one of many that astrobiologists will need by the time HWO launches in order to better understand its detection capabilities. It is important to look for Earth analogs at each stage of development, and not just analogs to the stage of Earth we all know and love now. Furthermore, as many abiotic processes can produce false positive biosignatures, we must be cautious in making any strong claims about potential detections of life. Nevertheless, this marks an exciting new era in astronomy, where we will be able to better understand the atmospheres of planets like our own and potentially see if life exists on other worlds.

Original astrobite edited by Kylee Carden.

About the author, Tori Bonidie:

I am a 4th-year PhD candidate studying exoplanets at the University of Pittsburgh. Prior to this, I earned my BA in astrophysics at Franklin and Marshall College, where I worked on pulsar detection as a member of NANOGrav. In my free time you can find me cooking, napping with my cat, or reading STEMinist romcoms!

illustration of a hot Jupiter exoplanet orbiting close to its host star

Editor’s Note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.

Title: The Roasting Marshmallows Program with IGRINS on Gemini South. II. WASP-121 b has Superstellar C/O and Refractory-to-Volatile Ratios
Authors: Peter C. B. Smith et al.
First Author’s Institution: Arizona State University
Status: Published in AJ

Our solar system has a very nice, ordered structure to it. Close to the Sun are the rocky, terrestrial planets, and then once you cross the asteroid belt you find the big, surface-less gas giants. But over the past 30 years, astronomers have learned that this is not a universal setup. A majority of the exoplanets we have discovered to date fall in the gas giant category, but not all of these gas giant planets lie at the outer edges of their systems. In fact, we’ve found so many gas giants that are close to their host star that they’re given their own name: hot Jupiters. These are large, gaseous planets that orbit closer to their star than Mercury does to the Sun, and how they came to be there is a bit of a mystery.

Divvying Up the Protoplanetary Disk

Planets form out of a swirling ring of gas and dust called a protoplanetary disk. But this disk isn’t the same at all radii. As you move away from the star and the temperature drops, certain compounds change from gas to solid phase, which can have a big impact on the types of planets that form there. One way of dividing up the disk is via “snow lines.” Interior to a snow line, volatile materials (like water) are in gas form, but once you cross this line it is cold enough for them to freeze, forming small solid particles that provide seeds for planet growth. To highlight the role this could play in the types of planets that form, it’s thought that the snow line for water for our solar system was around 3 au, which places it right in between Mars and Jupiter. With the presence of solid particles, it’s easier for planetary cores to grow quickly and thus reach a point where they can hold on to the large gas envelopes that make a planet a gas giant. Another line that’s grown in interest recently is the soot line, which similarly marks the transition into gas specifically for carbon molecules. The properties of planets, including their potential for habitability, can depend greatly on where they formed relative to these lines.

So this is great and all, but when we go and observe exoplanets, we aren’t able to see a nice highlight reel of their formation history. We don’t know where they formed, and we can’t tell how they might have migrated through the disk. But if we can measure their compositions, we can start to get some clues as to where they might have formed and how they came to be where they are.

WASP-121b: An “Ultra-Hot” Planet

That’s where today’s article comes in. This work looks at an exoplanet with the beautifully poetic name of WASP-121b, which is a bit bigger than Jupiter but orbits so close to its host star that it’s actually classified as an “ultra-hot Jupiter.” (Side note: the title of the article is “The Roasting Marshmallows Program” because the planets studied in the program are all hot and puffy.) Ultra-hot Jupiters are, as you may have guessed, hotter than hot Jupiters, and thus they have fewer clouds in their atmospheres. This allows for a better understanding of their composition as you don’t have to deal with pesky and still relatively poorly constrained cloud models.  Today’s work used observations from the Immersion Grating Infrared Spectrometer (IGRINS) instrument on the Gemini South telescope, observing the planet before and after its secondary eclipse — when the planet moves behind the host star. Figure 1 gives details about the system and shows the phases of the two observations.

schematic of the WASP-121 planetary system

Figure 1: Schematic of the system studied in this article. The blue and red highlighted regions show the phases of the planet’s orbit during the observations. Something fun about this diagram: it’s fully to scale, which really demonstrates just how close the planet is to its host star. [Adapted from Smith et al. 2024]

The observations of WASP-121b consisted of high-resolution spectra capturing the thermal emission of the planet. These data were then compared against models that included volatile (water, carbon monoxide, hydroxide) and refractory (Fe, Mg, Ca) species. Being able to constrain all these different compounds with the same observations is actually a pretty rare feat, as volatile and refractory species generally have to be studied in different portions of the electromagnetic spectrum. However, it’s possible to observe refractory species with high-resolution spectroscopy in the near-infrared, which is exactly what IGRINS provides. This is handy because it allows for direct comparisons without needing to worry about systematic effects from using different instruments.

Ratios to the Rescue

Posterior distributions of the C/O and R/V ratios

Figure 2: Posterior distributions (which can be understood as the probability for each value) of the C/O and R/V ratios in the atmosphere of WASP-121b. By comparing the values to the stellar abundance we can constrain where in the protoplanetary disk the ultra-hot Jupiter formed. [Adapted from Smith et al. 2024]

Thus, by comparing IGRINS observations against planetary atmosphere models, the researchers were able to place constraints on the presence of both volatile and refractory species in WASP-121b’s atmosphere. However, getting absolute abundance measurements was a trickier task given relative unknowns about other contributions to the continuum of the spectrum. So instead of trying to constrain absolute abundances, the research team calculated ratios. The two ratios that are important in this article are carbon to oxygen (C/O) and refractory to volatile (R/V). Not only are ratios better constrained than straight up abundances, they can also help place where in the protoplanetary disk WASP-121b formed. The C/O ratio will increase past the soot line as oxygen incorporates into silicates, and it will further increase past the snow line as oxygen freezes into water ice. The R/V ratio tracks the behavior of solid accretion, and if the R/V ratio is high, this suggests that the planet formed interior to the snow line where most of the accreted solids did not contain volatile material. Thus, a combination of the C/O and R/V ratios allows us to determine the location of planet formation relative to the water snow line and the soot line. Figure 2 shows the posterior distribution (which is a semi-complicated Bayesian statistical result that essentially just means probability) for the C/O and R/V ratios.

Overall, WASP-121b has high values for both the C/O and R/V ratios. This suggests that the planet was formed exterior to the soot line but interior to the water snow line, which is kind of an unexpected result. As discussed above, it’s thought that massive gas giants form exterior to the snow line, as they need the ices to build a big enough core. Though surprising, it’s not impossible for this ultra-hot Jupiter to have formed interior to the snow line. One interpretation is that the planet actually started as a super-Earth (exoplanet categories are so silly sounding — this means a planet between the sizes of Earth and Neptune) that could form interior to the snow line but still be big enough to sustain the runaway gas accretion that got it up to Jupiter size. And then over time, it would have migrated in towards the host star, heating up until it became the roasted marshmallow we know and love. While this is one explanation, there’s still a lot to understand about this formation pathway and more study is definitely needed. Overall, these results provide an interesting challenge to our understanding of planet formation. They also demonstrate the usefulness of near-infrared spectroscopy when it comes to constraining the atmospheres of ultra-hot Jupiters.

Original astrobite edited by Lucas Brown.

About the author, Skylar Grayson:

Skylar Grayson is an Astrophysics PhD Candidate and NSF Graduate Research Fellow at Arizona State University. Her primary research focuses on active galactic nucleus feedback processes in cosmological simulations. She also works in astronomy education research, studying online learners in both undergraduate and free-choice environments. In her free time, Skylar keeps herself busy doing science communication on social media, playing drums and guitar, and crocheting!

Illustration of the orbits of moons of Saturn

Editor’s Note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.

Title: Irregular Moons Possibly Injected from the Outer Solar System by a Stellar Flyby
Authors: Susanne Pfalzner, Amith Govind, and Frank W. Wagner
First Author’s Institution: Jülich Supercomputing Center; Max Planck Institute for Radio Astronomy
Status: Published in ApJL

Irregular Moons

Our solar system has many moons that are “regular.” They have circular, prograde orbits with low inclination, meaning they orbit their host planet in the same direction and plane as the planet orbits the Sun. These regular moons formed, along with their planets, from the protoplanetary disk. But there are some moons in the solar system with strange orbits, and we’re not sure yet why. These “irregular moons” orbit the outer, giant planets (Jupiter, Saturn, Uranus, and Neptune) on eccentric, inclined, and often retrograde (backwards) orbits.

Luckily, we have a clue. Irregular moons resemble a class of objects that have similarly strange orbits around the Sun out beyond Neptune, called trans-Neptunian objects (TNOs), so they may share a common origin. (When Pluto was demoted from planet status, it was reclassified as a dwarf planet, which is a type of TNO.) One origin theory is that TNOs and irregular moons are a result of the giant planet migration; as the giant planets drifted away from the Sun, they could have kicked TNOs into skewed orbits and captured some in their path as irregular moons. However, this theory cannot explain TNOs that are too distant to have been gravitationally influenced. Another theory is that a star passed through the outer solar system shortly after the solar system’s formation, gravitationally jumbling TNOs, some of which were captured by giant planets. This theory seemed too far-fetched, and it was mostly dismissed until recently, when the Atacama Large Millimeter/submillimeter Array showed that such stellar flybys are actually more common than we thought. Today’s article explores the stellar flyby theory using simulations to demonstrate that such a scenario could create the irregular moons we see today.

The Stellar Flyby Simulation

In a previous article, the authors ran a range of stellar flyby simulations to find the specific scenario that best reproduces the population of TNOs we find beyond Neptune. The best model was a parabolic flyby of a 0.8-solar-mass star at an inclination of 70° and a closest approach of 110 au from the Sun. In today’s article, the authors studied the effect this flyby has on the region near the giant planets, within Neptune’s orbit.

The simulation begins with a disk of undisturbed TNOs on circular orbits. For comparison, the authors modeled two disk sizes, 150 au and 300 au. As is commonly done in simulations for computational simplicity, the TNOs are represented by test particles. The flyby star is flung through the disk, and test particles are moved according to their gravitational interactions with the flyby star and the Sun. As the star passes through the disk, it pulls particles out of their orbits, creating the complex structure shown in Figure 1. The authors then let the simulation run for a billion years to see where the test particles ended up and how they interacted with the giant planets.

Snapshot of the simulation 200 years after the flyby star’s closest passage

Figure 1: Snapshot of the simulation 200 years after the flyby star’s closest passage, to the right of the Sun. The turquoise particles are those that will end up within Neptune’s orbit after the interaction has settled. [Pfalzner et al. 2024]

Comparing the Simulation to Observations

Distribution of injected TNO perihelions

Figure 2: Distribution of injected TNO perihelions for the 300 au disk (solid line) and the 150 au disk (dotted line). Click to enlarge. [Adapted from Pfalzner et al. 2024]

Immediately after the flyby, around 7.2% of the TNOs from the original disk end up inside Neptune’s orbit on highly eccentric orbits. Figure 2 shows the distribution of injected TNO perihelions, as well as the radii of the giant planets’ orbits. For both disk models, more TNOs end up near Saturn’s orbit than Jupiter’s, which matches observations — Saturn has 122 irregular moons while Jupiter has 87. It’s hard to say if this result makes sense for Uranus and Neptune, because it is difficult to detect moons at greater distances.

Most of the injected TNOs after the flyby had prograde orbits, and a significant fraction were at high inclinations. But interestingly, retrograde orbits dominated in the region inside 10 au. Around Jupiter’s orbit at 5.2 au, TNOs were 30% more likely to be retrograde than prograde, while around Saturn’s orbit at 9.5 au, it was 20% more likely.

Distribution of injected TNO perihelions for prograde and retrograde orbits

Figure 3: Distribution of injected TNO perihelions for prograde (blue) and retrograde (red) orbits, at 12,000 years (solid line) and a billion years (dashed line) after the stellar flyby. After a billion years, more TNOs with retrograde orbits remain. Click to enlarge. [Adapted from Pfalzner et al. 2024]

As the simulation progressed over the next billion years, 85% of the injected TNOs were eventually ejected. Figure 3 shows the distribution of prograde versus retrograde orbits as the simulation evolved. TNOs with retrograde orbits were more likely to avoid ejection than those with prograde orbits. The high fraction of resulting TNOs with retrograde orbits matches observations, as the giant planets have mostly retrograde irregular moons.

Finally, observed irregular moons are similar to observed TNOs in color, ranging from gray to red, except they lack very red objects. The original pre-flyby disk had a color gradient from red near the center to gray on the edges. Figure 4 shows the regions of the original disk that ended up near the giant planets, none of which cover the extremely red region.

The original TNO disk

Figure 4: The original TNO disk. The colored regions show the position of TNOs from the original disk that were injected inside Neptune’s orbit, none of which were extremely red. The colorbar represents the inclination of those resulting orbits. [Adapted from Pfalzner et al. 2024]

So far, the authors have shown that a stellar flyby could have perturbed TNOs to create the population of giant planet irregular moons. If a TNO orbiting the Sun on an eccentric, inclined, or retrograde orbit was gravitationally captured by a planet, its post-capture orbit would also be eccentric, inclined, or retrograde. The authors haven’t modeled the planets capturing the moons, but they reference another article that demonstrates that moon capture in such a scenario is likely. The plausibility of the stellar flyby and the similarities between TNOs and irregular moons demonstrate that this theory is promising and worth further investigation.

Original astrobite edited by Kylee Carden.

About the author, Annelia Anderson:

I’m an Astrophysics PhD candidate at the University of Alabama, using simulations to study the circumgalactic medium. Beyond research, I’m interested in historical astronomy, and hope to someday write astronomy children’s books. Beyond astronomy, I enjoy making music, cooking, and my cat.

Images of six "little red dot" galaxies from JWST

Editor’s Note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.

Title: The Small Sizes and High Implied Densities of “Little Red Dots” with Balmer Breaks Could Explain Their Broad Emission Lines Without an Active Galactic Nucleus
Authors: Josephine F. W. Baggen et al.
First Author’s Institution: Yale University
Status: Published in ApJL

Since its launch, JWST has been on a roll with observations that continue to shape our understanding of the universe. Recently, JWST saw a bunch of galaxies in the early universe that could each host a massive black hole. These galaxies are rightfully called “little red dots” as they are compact (i.e., have small radii) and appear red in the infrared as observed by JWST. However, we are not entirely sure about the identity of the little red dots. The spectra of these galaxies seem to contain features that suggest the presence of an accreting supermassive black hole or AGN. AGNs are known to produce X-ray emission, and we have not detected X-rays from the little red dots. So, what exactly are the little red dots? Do they have an AGN, or are they simply a collection of very massive and compact galaxies?

A recent finding showed that three little red dots have a relatively old and evolved stellar population. We know this because of the presence of a Balmer break — a jump in the galaxy spectrum at the Balmer line — which is predominantly seen in older stars. Does this mean that these galaxies are dominated by older, evolved stars, and does that rule out the presence of AGNs? The authors of today’s article argue that the three little red dots with a Balmer break have no AGNs and are just massive, compact galaxies. Have they truly unmasked the little red dots? Let’s find out.

These Are Some Extremely Dense Galaxies

The authors find that the galaxies are extremely compact, with small half-light radii around 100 parsecs (326 light-years), similar to ultra-compact dwarf galaxies in the local universe. While the galaxy spectra can give us an estimate of the stellar mass, the contribution from the AGN alone can be a significant fraction of a galaxy’s mass and must be accounted for. The authors fit three models to the galaxy spectra: 1) assuming most of the galaxy’s light comes from an old, evolved stellar population with no contribution from an AGN, 2) assuming maximum contribution from an AGN and minimal from the stars, and 3) a model that lies somewhere in between. The three models yield very different galaxy mass estimates, with the first model estimating 100 billion solar masses in stars, and the second model estimating just a billion solar masses.

The detection of the Balmer break points toward a source that has light mostly dominated by stars. Assuming the AGN contribution to the mass is minimal, the stellar mass density would be quite high. The authors show this in Figure 1, where the model with no AGN component is shown in purple and exhibits a very high stellar mass density near the galaxy’s center compared to other galaxies. This could be taken as evidence against the no-AGN model, but there is another observation we should consider.

plot of stellar mass density versus galactocentric radius

Figure 1: The stellar mass density profile — stellar mass density versus the distance from the center — for the three different models. The purple profile is for the no-AGN model. The black dotted line shows the profiles of galaxies from the local universe. The cyan and purple dotted lines show the profiles at higher redshifts. The no-AGN model’s stellar mass profile indicates a higher central density than other models. [Baggen et al. 2024]

What About the Broad Emission Line?

One of the clearest signatures of an AGN in the galaxy spectra comes from broad emission lines, which result from fast-moving gas near the supermassive black hole moving both toward and away from us. This causes a Doppler broadening of the emission lines. This associated velocity dispersion was measured in the three little red dot galaxies in today’s article.

The authors argue that the calculated velocity dispersion can be explained by the dynamics of gas and stars in the galaxy itself, without the need for an AGN. This is because of the galaxy’s extreme density, which can produce a spectral line width that is similar to that of an AGN’s broad emission line. In Figure 2, the authors show a plot of the observed velocity dispersion versus the predicted velocity dispersion, assuming it is the gas and stars present in the galaxy that are contributing to the Doppler broadening. The predicted and observed velocity dispersions seem to match very well for the model with no AGN contribution. This also provides a natural explanation for the lack of X-ray signatures in these galaxies, since we would expect to see X-ray emission if AGNs were present but no such emission from an ordinary stellar population.

plot of the observed vs predicted stellar velocity dispersion

Figure 2: Left: A plot of the observed versus predicted stellar velocity dispersion assuming the kinematics of the gas and stars in the dense galaxy contributing to the broad line emission. In the no-AGN model, the three little red dots (indicated by the purple square, rectangle, and pentagon) can explain the observed stellar velocity dispersion. Right: A schematic figure showing that the observed H-beta line widths can be accounted for by the velocity dispersion calculated from the kinematics. [Baggen et al. 2024]

Verdict: We Need More Evidence

However, further investigation is needed to determine if this scenario can explain every aspect of the little red dots. Additionally, the origins of such extremely dense galaxies are uncertain, and we need to understand how they evolve into the “normal” galaxies with lower central densities and larger sizes that we see today. The authors stress that the explanation of the little red dot spectra being dominated by AGNs rather than stars is still likely, and further observations are needed to uncover the truth.

Original astrobite edited by William Smith.

About the author, Pranav Satheesh:

I am a second-year graduate student in physics at the University of Florida. My research focuses on studying supermassive binary and triple black hole dynamics using cosmological simulations. In my free time, I love drawing, watching movies, cooking, and playing board games with my friends.

NGC 4151

Editor’s Note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.

Title: XRISM Spectroscopy of the Fe Kα Emission Line in the Seyfert Active Galactic Nucleus NGC 4151 Reveals the Disk, Broad-line Region, and Torus
Authors: XRISM Collaboration
Status: Published in ApJL

Today we’re going to be taking a high-resolution look at X-rays from close to a supermassive black hole! But before we get into the astrophysics of today’s article, we first need to discuss the instruments that were built to do this science. More than 50 years ago now, charge-coupled devices (CCDs) began revolutionizing astronomy, and they continue to be one of the most commonly used detectors on telescopes. CCDs rely on the photoelectric effect, through which an incoming photon can liberate electrons in some material (semiconductors in the case of CCDs). These electrons are trapped by strong potential wells and electric charge can be applied to move the charge along and read this signal (check out this Astrobite for more details). CCDs are particularly powerful in the X-ray band, where the number of electrons trapped in each pixel scales roughly with the photon energy. This means that you get energy information (i.e., a spectrum) for free with CCDs! However, CCDs have limited spectral resolution, meaning they can’t determine this energy very precisely and therefore cannot resolve and unlock the power of narrow emission and absorption lines.

X-Ray Microcalorimetry and 20/20 Vision

cartoon of a microcalorimeter

Figure 1: Schematic showing how a microcalorimeter works. An X-ray photon with energy E will produce a spike in the temperature of the absorber of E/C, where C is the heat capacity of the absorber. The thermometer is extremely sensitive to small changes in temperature, which means that we can get very accurate energies for each of the incoming X-ray photons. Therefore, a microcalorimeter can produce an X-ray spectrum with the best energy resolution of any current instrumentation. [NASA]

There are other ways to get better spectral resolution in the X-ray, including using gratings that will disperse your spectrum, as is commonly done with optical spectroscopy. However, even these techniques can’t reach the high spectral resolution needed; instead, new technology called a microcalorimeter has been engineered to solve this long-standing issue. As the name suggests, this instrument detects incoming photons by measuring tiny (micro) changes to the temperature (calorimetry) of the detector. Figure 1 shows the basic set-up of a microcalorimeter and how the energy of the photon is encoded in the strength of the resulting temperature fluctuation. In order to detect tiny changes to the temperature, microcalorimeters need to be extremely cold, 50 millikelvin to be precise! This is a huge engineering feat, but one that has recently been achieved by the X-ray Imaging and Spectroscopy Mission (XRISM)! XRISM is a JAXA/NASA collaborative mission, and it has two instruments on board: a CCD camera called Xtend and a microcalorimeter called Resolve. It was launched in September 2023, and its first science results are just starting to roll in!

Now, XRISM isn’t actually the first X-ray microcalorimeter to fly, but it’s the first to live through its commissioning phase! Although the X-ray microcalorimeter has been in the works since the 1990s, previous X-ray microcalorimeters have been cut from missions, lost to launch failures, and left unable to operate due to loss of coolant for the detector. In 2016, JAXA successfully launched and operated the first X-ray microcalorimeter on the Hitomi Satellite. However, unfortunately, shortly after taking a beautiful spectrum of the Perseus Cluster, one of the best-studied galaxy clusters in the local universe, communication was lost with the satellite and never recovered. XRISM’s Resolve instrument has been the most successful X-ray microcalorimeter so far, and it has allowed us to start looking at the universe with 20/20 X-ray vision!

Supermassive Science with XRISM

Today we’re going to put on our high-resolution X-ray spectroscopy glasses to look at one of the first XRISM targets: NGC 4151, one of the most well-known active galactic nuclei in the local universe. An active galactic nucleus consists of a supermassive black hole that is gobbling down gas from its surroundings through a process known as accretion. While we’ve known about active galactic nuclei for more than 50 years now, we still don’t really understand how they are fueled and what the structure is around them. XRISM can unlock this information indirectly by resolving some of the key X-ray emission and absorption lines. In particular, the most prominent emission line in the X-ray spectrum of an active galactic nucleus is a neutral iron Kα line at 6.4 kiloelectronvolts (keV), which arises from material around the supermassive black hole being illuminated by the light from the accretion process. This line holds the keys to probing the structure of the surrounding gas, as its dynamics can tell us about the structure of the accretion disk and trace gas in the torus that is thought to connect the local host galaxy to the accretion flow.

Figure 2 shows the XRISM Resolve spectrum of NGC 4151 from two separate observations. The spectrum shows a prominent 6.4 keV line that is resolved, meaning that the measured width of the line is greater than the instrument’s resolution limit. Additionally, the line cannot be fit with a single emission line and instead requires multiple lines, signaling multiple physical scales contributing to this emission line. The right panels of this figure highlight that there are three distinct components to this emission line with broad (magenta), intermediate (dark blue), and narrow (cyan) widths. Since gas that is closer to the black hole will be moving faster than more distant gas, the authors can use these line widths to estimate where this gas is located. They find that these three lines range from about 100 gravitational radii (about 100 times the size of the black hole) to about 10,000 gravitational radii. Determining the multi-scale nature of this line has been extraordinarily difficult to detect with other instruments due to their limited energy resolution!

XRISM resolve spectra of NGC 4151

Figure 2: XRISM Resolve spectra of NGC 4151. The left panels show the spectrum in the 5.8-7.2 keV range from two separate observations, with the data in black and the best fit total model in red. The right panels show a zoom in on the iron Kα 6.4 keV line with the three different components for the line also shown. The magenta model corresponds to the widest line, arising potentially from a warped disk, the dark blue model corresponds to the intermediate width line coming from the inner edge of the broad line region (BLR), and the cyan model corresponds to the most narrow line that arises from the inner edge of the dusty torus. [XRISM Collaboration et al. 2024]

Together these three components to the iron Kα line provide a compelling picture for the nuclear structure, which is shown in Figure 3. There are some additional pieces of evidence from the data that support this model as well. For example, the broadest line (magenta) shows variability on timescales of less than a day. This timescale corresponds roughly to the distance light could travel before reaching the magenta part of this figure, supporting the idea that there is a broad component associated with the disk. In addition to the location of the emitting gas, the dynamics and density can be constrained using the energy and shape of the line, respectively. In this source, the line is at the rest-frame energy and the shape is relatively symmetric, which together suggest that the emission comes from relatively optically thin gas that has not been accelerated to high velocities. Together, these diagnostics give one of the most in-depth pictures of supermassive black hole environments to date and will be crucial for testing our models of black hole feeding!

Schematic highlighting where each of the iron Kα emission lines arise from

Figure 3: Schematic highlighting where each of the iron Kα emission lines arise from. The magenta component corresponds to the broadest line, potentially from a warp in the disk. The dark blue component corresponds to the intermediate-width line and arises from the inner edge of the broad line region (BLR). The cyan component corresponds to the narrowest line and arises from the inner edge of the active galactic nucleus torus. [XRISM Collaboration et al. 2024]

What’s Next?

These XRISM observations are rich with information, and today’s article focused only on the 6.4 keV emission line. The authors are planning a series of further articles, including on the active galactic nucleus winds traced by the absorption lines (i.e., the major dips seen at ~6.7 and ~7 keV in the left panels of Figure 2), comparisons of the emission lines with optical emission lines, and looking for faint evidence of broader emission from even closer to the supermassive black hole. The next obvious steps are also to observe more active galactic nuclei to test whether this multi-zone emission is a common occurrence in active galactic nuclei. One thing’s for sure, this 20/20 vision is sure to reveal new secrets about the lives and environments of supermassive black holes!

Original astrobite edited by Roel Lefever.

About the author, Megan Masterson:

I’m a 4th-year PhD student at MIT studying transient accretion events around supermassive black holes, including tidal disruption events and changing-look active galactic nuclei. I primarily use multi-wavelength observations to study from the inner accretion flow to the obscuring material in these transients. In my free time, you’ll find me hiking, reading, and watching women’s soccer.

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