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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: The Discovery of Little Red Dots in the Local Universe: Signatures of Cool Gas Envelopes
Authors: Xiaojing Lin et al.
First Author’s Institution: Tsinghua University and University of Arizona
Status: Accepted to ApJ

Astronomers are using JWST to search the early universe for strange and captivating objects, and the results so far have exceeded all expectations. For every question we definitively answer, several deeper ones seem to emerge. The study of active galactic nuclei (AGNs), black holes that are actively consuming matter at the centers of galaxies across the universe, is no exception. Our pre-JWST understanding of AGNs has been completely shattered by the discovery of an astounding population of objects called little red dots (LRDs).

We know these objects host active black holes for several reasons. For one, they’re incredibly small despite being extremely energetic (that’s the “little” in their name). The smallest LRD we’ve seen appears to be less than 100 light-years across — for reference, the nearest star to the Sun is 4 light-years away — and yet is still as bright as a galaxy. The most plausible explanation for such brightness and compactness is a supermassive black hole that’s actively consuming matter. As further evidence for this hypothesis, we observe broad Balmer emission lines, spectral features that serve as a clear signature of AGN activity, in the vast majority of LRDs.

However, that’s where the similarities between LRDs and more typical AGNs seem to end. First off, astronomers have been unable to detect any significant X-ray emission from LRDs. This is shocking, since X-rays are a telltale signature of previously known AGNs. Another classic way to identify AGNs is by looking for changes in their brightness over time, since they can show drastic variability in their emission on the scale of years or even days. LRDs buck the trend by showing little to no fluctuation. Other classic AGN signatures that LRDs lack include radio emission and an abundance of hot dust. Explaining all of these unusual features has been incredibly challenging; nevertheless, coming up with a solution is crucial because there appear to be orders of magnitude more LRDs than any other kind of AGN in the early universe!

Wishing on a Star

One leading explanation for LRDs has recently gained significant traction. The hypothesis suggests that early black holes may have been cocooned in shrouds of extremely dense gas. This scenario helps explain the absence of many typical AGN signatures in LRDs. For example, this dense gas could absorb the X-rays generated by the central black hole. The gas-enshrouded model also explains a particular feature in the spectra of many LRDs known as a Balmer break. This feature is usually interpreted as coming from old stars with relatively cool atmospheres. The cocooned AGN hypothesis implies that Balmer breaks could actually be made by a black hole embedded in a cloud of gas, which somewhat resembles an enormous star!

This model helps explain the properties of some puzzling LRDs, including the most distant one we currently know of. However, their enormous distance makes them difficult to study in detail. It would be much easier to examine LRDs if they existed in the nearby universe, since they would appear much brighter and we could probe them on small spatial scales. Unfortunately, previous studies with JWST suggest that LRDs started becoming a lot less common around 12 billion years ago (corresponding to a redshift of about z = 4).

Luckily, less common doesn’t mean extinct! The authors of today’s article leverage data from the Sloan Digital Sky Survey (SDSS), which has taken millions of spectra of objects across the night sky over the past few decades. By mining this rich and diverse dataset, they identify a handful of LRDs in the nearby universe. Although they’re still far beyond our own Local Group, they’re practically in our backyard compared to the distant LRDs we’ve seen before. The authors followed up the new sources with some of the most powerful ground-based telescopes, enabling a detailed study of their properties.

A Chick That Forgot to Hatch

The first major result is that these local LRDs really do resemble many of the distant ones identified by JWST. They show a characteristic “V”-shaped spectrum in the ultraviolet and visible, a key feature in the definition of LRDs. They’re also quite compact. Using existing data from the Hubble Space Telescope, the authors measured the size of one source and found a tiny core embedded within a galaxy only a few thousand light-years across. For comparison, the Milky Way spans about 100,000 light-years.

Using spectroscopy from the Large Binocular Telescope, one of the Magellan Telescopes, and the MMT (formerly Multiple Mirror Telescope), the authors of this article find a rich variety of spectral features. This is where the proximity of these objects is extremely important. Taking spectra of the distant LRDs in this astounding level of detail would be at best prohibitively expensive (multiple full days of JWST observations) and at worst be completely impossible. In contrast, the nearby LRDs appear much brighter than their distant cousins. Their spectra are like intricate tapestries, with each thread revealing new details about their properties.

The authors find numerous broadened emission lines, signaling the presence of an active black hole. They also detect several intriguing absorption lines. These are most striking in one source, J1025+1402, nicknamed “The Egg.” In this object, there are clear absorption features from sodium, potassium, iron, and calcium. These elements appear in the observed states only under specific physical conditions. The authors interpret these findings as implying the presence of an envelope of cool gas surrounding the central black hole (see Figure 1). Such a scenario closely resembles the gas-cocooned AGN model proposed to explain distant LRDs!

schematic of a little red dot model

Figure 1: A schematic showing the authors’ interpretation of the nature of LRDs. This scenario is built up based on spectroscopic observations from several ground-based telescopes. Although the local sources differ slightly from the ones in the early universe, this model is strikingly similar to the ones invoked to explain some of the most extreme LRDs. The fact that such objects exist in the nearby universe is intriguing and raises further questions as to how black holes enter such a phase. [Lin et al., in press]

These observations provide direct evidence for a potentially new kind of AGN. These local objects do differ slightly from their more distant counterparts; for example, they show some emission from hot dust, which is absent in early LRDs. Still, this adds another piece to the puzzle of this cosmic mystery. The authors hope that future observations will further illuminate the inner workings of LRDs across cosmic time. For example, studies may reveal what objects “hatch” from a source like the Egg, offering new insight into the life cycles of these unusual AGNs.

Original astrobite edited by Niloofar Sharei.

About the author, Ansh Gupta:

I’m an astronomy graduate student at the University of Texas at Austin working with Steven Finkelstein. I use data from JWST to study the formation and growth of the first galaxies and black holes in the universe. In my spare time, I enjoy playing piano, reading, and making YouTube videos.

Fomalhaut debris disk

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: ALMA Reveals an Eccentricity Gradient in the Fomalhaut Debris Disk
Authors: Joshua B. Lovell et al.
First Author’s Institution: Center for Astrophysics | Harvard & Smithsonian
Status: Published in ApJ

Step 1: Understanding How to Carve Your Debris Disk

Let’s start with our solar system: the Kuiper belt, a large ring of icy asteroids, is believed to have been sculpted into its current shape by Neptune. Neptune may have previously scattered objects in the Kuiper Belt through gravitational interactions, but some of them (like Pluto) remain in an orbital resonance with Neptune. In the same way that Neptune shapes the Kuiper Belt, today’s authors believe a planet could be shaping an exo-Kuiper Belt around the star Fomalhaut.

What on Neptune is an orbital resonance, though? A planet orbiting a star has an orbital period, and the gravitational forces between nearby (astronomically speaking) objects can push these objects into a state where their orbital periods are multiples of each other. For example, Pluto and Neptune have a 2:3 orbital resonance, meaning Pluto completes two orbits for every three that Neptune does. The same can happen for the asteroids and planetesimals in the Kuiper Belt, so the same should happen in other star systems!

Step 2: Make Your Observations

If we understand how debris disks carved by exoplanets look — and we think we do — then we should be able to infer the existence of exoplanets! Today’s authors have used observations from the Atacama Large Millimeter/submillimeter Array (ALMA) of the debris disk around Fomalhaut and made some very clever calculations. We’ve known about this disk for a while, which is why today’s authors have studied it with a new analysis technique they developed.

Planeteismals — basically big rocks from a few to hundreds of kilometers across — that orbit in this disk do so with a certain eccentricity, and typically things in the same orbit would have the same eccentricity. But today’s authors were clever — they checked if there was an eccentricity gradient, meaning the planetesimals’ eccentricities depend on their semi-major axis (i.e., the mean orbit radius); we would typically not expect any eccentricity gradient for bodies orbiting a star unperturbed. The authors discovered that the gradient for planetesimals around Fomalhaut is negative, which implies the presence of a planet when you look at the maths behind gravitational interactions between planets and planetesimals.

A negative eccentricity gradient means the planetesimals gather up at the point on the orbit farthest from the star (the apocenter), and since there are more planetesimals in that region, they appear brighter in the ALMA data (see Fig. 1 left); the ring also appears slightly wider. If the eccentricity gradient were positive, the same thing would happen at the point on the orbit closest to the star (the pericenter). The authors term this phenomenon the “eccentric velocity divergence.”

observed and modeled brightness of the Fomalhaut debris disk

Figure 1: Left: The observed intensity of the Fomalhaut debris disk with ALMA. Middle: the authors’ model that fits the ALMA data the best. Right: The residual (data – model) between model and data. White means there is a close match to the data (which is better). [Lovell et al. 2025]

When the authors ran their eccentric velocity divergence calculations for the Fomalhaut disk model, they compared it to observations using a Markov Chain Monte Carlo algorithm. Figure 1 shows their best-fitting model, which fits remarkably well, based on the residual (i.e., the difference between model and data) you can see on the right — including the slightly wider ring at the apocenter!

The authors tested other scenarios with different gradients and allowed for the planetesimals to oscillate their eccentricity around their orbit, but they didn’t find a better-fitting scenario.

Step 3: Find a Carving Planet

Okay, so those were the details. The authors investigated a few scenarios to see what could be causing the observed debris disk and its negative eccentricity gradient, as well as an intermediate ring sitting between the main disk and the star that recently was seen with JWST. The authors tested two scenarios: one where a planet sits between the rings and evacuates the nearby region, and another where a planet is interior to the inner ring and clears the gap through orbital resonances (kind of like Neptune!). An illustration can be seen in Figure 2.

Illustration of possible planet-based scenarios that could create the observed debris disk around Fomalhaut

Figure 2: Illustration of possible planet-based scenarios that could create the observed debris disk around Fomalhaut. One features a planet between the observed debris disk rings, and another is where the planet is interior to both and carves the gap with orbital resonances. [J. Williams]

A planet was previously thought to exist around Fomalhaut, but it is now accepted there is not one we can currently observe. The authors point out that the possible planet sculpting this debris disk could be the same planet we thought existed previously, but at a lower mass (1–16 Earth masses; almost a Neptune mass). We can’t observe a planet with these parameters yet, but maybe with future observing facilities!

Finally, the authors stress, however, that it might not be a planet causing the observed structure — it could instead be the gravity of the planetesimals in the disk. Unfortunately, existing models are not equipped to explore this scenario, which is why the authors are planning to develop tools to investigate this next.

Original astrobite edited by Sandy Chiu.

About the author, Joe Williams:

I’m a third-year PhD student at the University of Exeter in the UK, and I study protoplanetary discs — mainly the tiny dust grains and their ices! In my spare time, I’m a climber, crocheter, and reader of sci-fi and fantasy books. My favourite sci-fi series is The Expanse!

Hubble image of a supernova in the galaxy NGC 2525

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: Supernova-Induced Binary-Interaction-Powered Supernovae: A Model for SN2022jli
Authors: Ryosuke Hirai (平井遼介) et al.
First Author’s Institution: RIKEN Pioneering Research Institute, Monash University, OzGrav: The ARC Centre of Excellence for Gravitational Wave Discovery
Status: Published in ApJ

There are thousands of dying stars that go bang in the night. These explosions are known as supernovae, and they are classified primarily on what spectral lines they show in the days and weeks following their initial explosion. Most broadly, they are separated into Type I (spectra that show little or no evidence of hydrogen) and Type II (spectra that show hydrogen). The physics leading to each type of supernova is different, and astronomers use all of the information at their disposal to learn about these cosmic fireworks.

One of the most useful tools for monitoring and learning about a supernovae is its light curve — the plot of the explosion’s brightness over time. This light curve usually appears as a steep incline up to a peak in brightness several days after the initial detonation, and then a shallow decline in brightness over the following weeks. There are, however, a few examples of supernovae that break this mould.

Today’s article seeks to understand the curious case of supernova SN2022jli. Unlike the typical gradual fade in brightness, the light curve of this supernova shows a periodic bump of brightness every 12.5 days (see Figure 1). The proposed mechanisms to explain this include the blown-apart supernova material interacting with concentric and regularly spaced shells of circumstellar matter, or a binary companion that interacts with the supernova remnant each orbital period. The authors of today’s article investigate the latter scenario by running 3D hydrodynamical simulations of an interacting, post-supernova binary to try to recover SN2022jli’s periodic light curve and constrain the orbital characteristics of the system.

light curve of SN2022jli

Figure 1: The light curve of SN2022jli shows periodic oscillations in the brightness, superimposed on top of the typical fading luminosity of a supernova. The left plot shows the light curve across various colours, and the associated bottom panel shows the oscillations with the typical fading supernova signal subtracted. The right plot shows the light curve of the authors’ proposed mechanism, where the different coloured lines are for different viewing angles on the system at the heart of the supernova. [Moore et al 2023 (left) and Hirai et al. 2025 (right)]

The authors only have the supernova’s light curve to go off of, so they explore a range of orbital scenarios to explain SN2022jli. They run a series of models evolving a neutron star — the degenerate core left over after a low-mass supernova — together with a main-sequence stellar companion, varying the orbital eccentricity and companion mass, while fixing the orbital period (at the observed 12.5-day oscillatory period) and neutron star mass to 1.4 solar masses — a typical mass for a newborn neutron star.

Why a neutron star? SN2022jli is a sub-class of Type I supernovae involving a “stripped” massive star which has lost its hydrogen — most likely through interactions with a close companion. These interactions typically circularise the pre-supernova orbit, and the subsequent explosion most commonly yields neutron stars. To explain the necessarily highly eccentric post-supernova orbit that may cause the periodic oscillation in SN2022jli, there was most likely a “natal kick” — a sudden and asymmetric burst during the supernova that rockets the neutron star in a random direction — which widened the orbit and de-circularised it, giving further evidence for a neutron star remnant.

Immediately following the supernova explosion, ejecta interacting with the main-sequence companion star would heat the star and cause it to swell up. This inflated companion then feeds matter to the orbiting dense neutron star, and the accretion is most intense when the stars are closest in their orbit (called the periastron). During accretion, however, mass is not perfectly absorbed onto the neutron star, and there is some feedback onto the surrounding gas due to the complex thermal and magnetic physics around the neutron star. In this study, the authors model cases where there is no feedback, feedback from thermal radiation due to accretion, or feedback from bipolar outflows via jets or disk winds. Snapshots from the simulation involving a 5-solar-mass companion on an orbit with eccentricity of 0.5 and bipolar feedback is shown in Figure 2.

simulations of a neutron star with an inflated main-sequence companion

Figure 2: Several density snapshots of the 3D hydrodynamic simulations show how the neutron star (blue point) in the binary accretes and sculpts matter from the inflated main-sequence companion star (bright diffuse blob in the centre of each panel). The xy panels are a top-down look onto the orbital plane, while the xz and yz panels show a cross section through the plane. [Hirai et al. 2025]

The authors evolve the simulation for several orbits and show that periodic bursts of accretion onto the neutron star are capable of producing the oscillatory behaviour seen in SN2022jli’s light curve. The scale of the brightness oscillations are best recreated if the plane of the binary’s orbit is aligned with our line of sight (shown with the +x red line on the right side of Figure 1). Still, the profile of the undulations is not exactly reproduced by the model, and the outcome is very sensitive to our viewing angle of the binary orbit.

SN2022jli is just one of an emerging class of oscillatory supernovae, with more and more being discovered with highly sensitive modern all-sky surveys. Today’s authors rigorously modelled one proposed scenario to explain this periodic oscillation in brightness and showed that it is viable. More modelling using this interacting-binary mechanism on other oscillating supernova light curves — those with longer periods and different supernova classifications — will show whether this explanation is ubiquitous, or if the population of dancing supernova light curves is due to some other process!

Original astrobite edited by Anavi Uppal.

About the author, Ryan White:

I am a masters student at Macquarie University in Australia, working mainly on binary/multiple systems with massive stars (Wolf–Rayets in particular!). Outside of study, I’m a novice film buff, baking sourdough all the time, probably drinking coffee, and trying to get more into reading and frisbee/squash. You can also find me procrastinating on Bluesky @astroryan.bsky.social.

field of stars containing RR Lyrae variables

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: Search for the Blazhko Effect in Field RR Lyrae Stars Using LINEAR and ZTF Light Curves
Authors: Ema Donev and Željko Ivezić
First Author’s Institution: XV. Gymnasium (MIOC)
Status: Published in AJ

RR Lyrae stars are a class of pulsating variable stars — similar to better-known Cepheid variables — that sit on the horizontal branch of the Hertzsprung–Russell diagram. Because of the regularity with which they pulsate, these stars are useful for a number of scientific applications, including standard-candle distancing (helping astronomers set the scale of distances in the universe) and as probes of very old star formation in nearby populations (because most RR Lyrae stars are at least 10 billion years old).

Today’s article studies the Blazhko effect in RR Lyrae stars. Simply put, the Blazhko effect is a long-term change of the duration (period) or strength (amplitude) of pulsation in some RR Lyrae variables. Fig. 1 shows an example from today’s article. While this effect was first observed as early as 1907, the physical mechanism for Blazhko modulation is still formally unknown, as is the percentage of RR Lyrae stars that exhibit it. Broadly speaking, there are three explanations for this effect: 1) nonlinear resonance between a star’s primary pulsation mode and some higher-level pulsation, 2) magnetic influence, or 3) cycles in the convection activity.

example of the Blazhko effect

Figure 1: An example of the Blazhko effect. Each panel shows data from ZTF for the same source for different seasons (at different times). The best-fit pulsation model for the total data set is shown in red. Over time, the actual pulsation of the source (black data) varies significantly from the average best fit due to Blazhko modulation. [Adapted from Donev and Ivezić 2025]

Today’s article searches for and identifies a population of Blazhko stars that may be used for future research into the Blazhko effect. Using data from the Lincoln Near-Earth Asteroid Research (LINEAR) asteroid survey and the Zwicky Transient Facility (ZTF) survey, the authors analyze around 2,857 RR Lyrae stars found in both data sets. The LINEAR survey was taken over a period of about 6 years, and the ZTF survey over about 5 years. On average, there is a 15-year difference between the LINEAR and ZTF observations. Using both, therefore, allows the authors to search for Blazhko modulation in each survey individually, as well as to compare between the two over the 15-year period. They additionally require a source to have at least 150 data points in both surveys to be considered.

From this initial set of RR Lyrae stars, today’s article identifies 531 potential Blazhko star candidates that are moved on to a visual inspection step. In order to identify the candidates for visual inspection, the authors establish two pre-selection methods based on the direct light curve and periodogram for each source:

  1. Light curve selection works by algorithmically assigning a score to each source, with higher scores indicating a greater expectation that the source is a Blazhko RR Lyrae star. The scores are associated with best-fit pulsation models. One way a given source could earn points was by having a very high reduced χ2 statistic in one or both data sets. Blazhko modulation changes the characteristics of the pulsations over time, meaning the best-fit model will be a poor fit to many of the pulsations within one or both data sets. Generally, poorer fits mean higher reduced χ2 values. In addition, candidates could earn points by having a moderately high reduced χ2 statistic in one or both data sets, as well as a significant change in pulsation characteristics of the best fit model from one data set to the next. Such a change between data sets is an indication of long-term Blazhko modulation. From this 479 of the 531 candidates are identified.
  2. Periodogram selection works by looking for interactions between the primary pulsation and Blazhko frequencies. First, the authors create a periodogram for the time-series data. In short, a periodogram plots a number of possible frequencies (or periods) of variability in the data versus the “power” associated with that frequency (or period), where higher power means the data vary more strongly at that frequency. When periodic data have only one associated frequency, the periodogram will show a single peak with high power. In the case where there are two effects of variation (in this case, the pulsation of the star and the Blazhko modulation) with disparate frequencies, a single, large peak will occur at the average frequency, with a smaller peak appearing to either side. Fig. 2 shows an example using simulated data. By identifying the location and strength of these side peaks, the authors are able to identify a handful of additional Blazhko sources (29), as well as estimate the frequencies of Blazhko modulation.
simulated Lomb–Scargle periodogram

Figure 2: A simulated Lomb–Scargle periodogram made using the sum of two sine functions with similar, but different, frequencies. Note the primary peak at the frequencies’ mean, and the smaller side peaks indicating the difference. [Adapted from Donev and Ivezić 2025]

From here, the authors visually inspect the 531 candidates and confirm that 228 of them exhibit convincing evidence of the Blazhko effect. They are able to place a lower limit on the percentage of RR Lyrae stars that are Blazhko sources at 11.4 ± 0.8%. In addition, they report that for a certain subclass of RR Lyrae stars, those that show the Blazhko effect have pulsation periods 5% shorter on average but no significant difference in amplitude. But a less common subclass of RR Lyrae stars shows no significant difference in period or amplitude when comparing Blazhko sources to the general population. Finally, the authors highlight that some sources show Blazhko modulation in one data set, but not the other, indicating that the modulation itself may change over time. Further research into this finding may help us better identify the most likely physical mechanism(s) for the Blazhko effect.

Original astrobite edited by Kylee Carden.

About the author, Catherine Slaughter:

Catherine is a PhD candidate in astrophysics at the University of Minnesota. Her research primarily deals with stellar population astrophysics in local dwarf galaxies, with particular focus on the intersection between computational and observational research methods. Prior to moving to Minnesota, she completed her BA in Physics and Astronomy, and MSc in Astronomy Research at Leiden University.

Illustration of a supermassive black hole enveloped in gas

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: Massive Black Hole Seed Formation in Strong X-Ray Environments at High Redshift
Authors: Kazutaka Kimura, Kohei Inayoshi, and Kazuyuki Omukai
First Author’s Institution: Tohoku University
Status: Published in ApJ

Environments of Massive Black Holes

Astronomers have found that nearly every observable galaxy hosts a central supermassive black hole (SMBH) — a black hole more than a million times more massive than the Sun. Using JWST, they have peered back into the early universe to observe some of the earliest galaxies, and these early galaxies also contain SMBHs. In fact, many of these galaxies seem to contain SMBHs that are more massive than we expected them to be this early in the universe.

We believe that most black holes form at the ends of the lives of massive stars, which collapse under their own gravity when they run out of fuel. We expect that this happened to the first stars too, forming “light” (as in “lightweight”) seeds of SMBHs with masses up to about 100 solar masses. Astronomers also think that the right conditions in the early universe could lead to the formation of a “heavy” seed with a mass on the order of 104 solar masses or more; this kind of black hole is called a direct-collapse black hole (DCBH).

In order to form a DCBH, a gas cloud needs to collapse without fragmenting to form star clusters and stars — if stars form, there won’t be enough mass funneled to the center of the cloud to form the DCBH. One important criterion for this to happen is the absence of coolants, like metals and molecular hydrogen (H2), which cause fragmentation. One way to eliminate H2 is with a particular type of ultraviolet light called Lyman-Werner radiation, which can destroy H2 molecules. However, recent results from the Hydrogen Epoch of Reionization Array (HERA) suggest that early galaxies emitted more X-rays than expected, which would increase the amount of H2 and make it less likely for DCBHs to occur. Today’s authors investigate the feasibility of forming DCBHs in these X-ray-bright environments.

Modeling Black Hole Formation

Today’s authors model the collapse of gas in dark-matter halos that have a nearby neighboring halo. This neighboring halo emits radiation like ultraviolet light and X-rays that impact the formation of molecules like H2, which affects the gas chemistry. This radiation is typically emitted by hot gas in star-forming regions. The authors also track quantities like the gas density and temperature as the gas collapses. Once the gas collapses enough, a star forms at the center and continues to accrete surrounding material. Once this star evolves and reaches the end of its life, it becomes a black hole.

The authors considered different intensities of X-ray radiation relative to a critical value of Lyman-Werner radiation believed to destroy enough H2 to form a DCBH, which is called J21. They explore values of the X-ray intensity Jx relative to J21, considering ratios of 0, 10-6, 10-5, and 10-4.

Another important consideration was the baryonic streaming velocity. This is the relative bulk velocity between baryonic (i.e., “normal”) matter and dark matter in the early universe. Higher streaming velocities can delay the collapse of gas in dark-matter halos, which affect gas properties like density, temperature, and chemistry. The streaming velocity depends on the redshift, z, which is given by σbsm = 30 km/s (1+z)/(1+z0), where z0 = 1,100. This “standard” relationship has been calibrated with measurements from observations. The authors consider two cases for the streaming velocity: vbsm = 0 and vbsm = 1σbsm, which are denoted in the figures below.

X-Rays and Seed Suppression

The first main results are shown in Figure 1. The authors find that a large number of intermediate-mass seeds form from H2 cooling, shown in blue, regardless of the X-ray intensity. A smaller number of higher-mass seeds form from H–H2 and H–H cooling, but these cooling processes become suppressed as the X-ray intensity increases. This is because the increased X-ray intensity converts more H to H2. Interestingly, accounting for the streaming velocity all but wipes out black hole formation via H2 cooling except at very high X-ray intensities.

histograms of seed black hole mass

Figure 1: Mass distributions of seed black holes. The top row uses a baryonic streaming velocity of 0 and the bottom row uses the standard streaming velocity. Columns represent increasing X-ray intensities (left to right). Colors represent the dominant cooling mechanisms: H2 (blue), atomic hydrogen H transitioning to H2 (orange), and solely H (green). [Kimura et al. 2025]

Another main set of results are the black hole mass functions (that is, the number density of black holes for a given black hole mass), shown in Figure 2. Note that this differs from Figure 1 because we are now accounting for the number density of black holes, not just the number, and we are no longer categorizing black holes based on their cooling mechanism. As before, we see that most massive black holes are eliminated with a high X-ray intensity and no streaming velocity, but a higher streaming velocity tends to produce more massive black holes regardless of the X-ray intensity. These black hole mass functions provide a useful comparison to observational results from telescopes like JWST.

histograms of seed black hole number density

Figure 2: Seed black hole mass functions for low and high X-ray intensities (left and right columns, respectively). Rows are the same as in Figure 1. [Kimura et al. 2025]

In summary, today’s authors find that a high X-ray intensity leads to less-massive black holes due to the increased amount of H2, but accounting for the baryonic streaming velocity can produce massive black holes even with a large amount of X-rays. The number densities of these black holes appear to agree with observations from JWST, meaning that this could be a promising formation mechanism for the observed SMBHs in the early universe.

Original astrobite edited by Sparrow Roch.

About the author, Brandon Pries:

I am a graduate student in physics at Georgia Institute of Technology (Georgia Tech). I do research in computational astrophysics with John Wise, using machine learning to study the formation and evolution of supermassive black holes in the early universe. I’ve also done extensive research with the IceCube Collaboration as an undergraduate at Michigan State University, studying applications of neural networks to event reconstructions and searching for signals of neutrinos from dark matter annihilation.

Andromeda Galaxy

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: Deep in the Fields of the Andromeda Halo: Discovery of the Pegasus VII Dwarf Galaxy in UNIONS
Authors: Simon E. T. Smith et al.
First Author’s Institution: University of Victoria
Status: Published in ApJ

Picture a galaxy. It’s big, right? Unfathomably big, huge even! But galaxies aren’t all extremely massive; they come in a variety of sizes and luminosities, and some galaxies are so small and so dim that astronomers may not even know they’re there, even when they’re basically in our backyard. The hunt for the smallest galaxies is an important one, as these systems give us extreme environments where we can test theories of dark matter and galaxy evolution. The authors of this article present a newly discovered ultra-faint dwarf galaxy dubbed Pegasus VII (Peg VII), found next to our nearest major galactic neighbour: Andromeda.

How Do You Find a Dwarf Galaxy?

Finding ultra-faint dwarf galaxies requires deep, long-exposure imaging across wide swaths of the night sky, something that is only possible through observational surveys. Some telescopes, like JWST, require astronomers to submit an application to point the telescope at a specific, known target and, if approved, will have some amount of “telescope time” to do their imaging. Others, like the Euclid space telescope, are part of large surveys in which an area of the sky is selected and systematically imaged over several years, regardless of if there are known objects there or not. Because of this, surveys are our best bet to discover faint systems like ultra-faint dwarf galaxies.

The survey these authors used is UNIONS (Ultraviolet Near Infrared Optical Northern Survey), which covers the same area of the sky as the Euclid space telescope but is done with three ground-based telescopes: the Canada-France-Hawaii Telescope, Pan-STARRS, and Subaru, all located on Maunakea in Hawaiʻi. The authors were originally focused on finding satellite galaxies in the outskirts of Andromeda’s gravitational influence and during their search were able to identify an overdensity of bright stars. They then applied for and were awarded telescope time with both the Canada-France-Hawaii Telescope and the Gemini Observatory to perform deeper follow-up imaging of the system to figure out what exactly it was.

Optical Illusion or Optical Observation?

With this deeper follow-up imaging they could confirm if these stars were really one coherent system or if they just appeared to be clumped together on the sky. When they plotted the stars on a colour–magnitude diagram (Figure 1) they were able to identify a main sequence, horizontal branch, and red giant branch, as would be expected if these stars all formed at roughly the same time and were allowed to evolve over billions of years. Congratulations, it’s a dwarf galaxy!

color–magnitude diagrams for stars identified as being within or outside of Peg VII

Figure 1: Left: The colour–magnitude diagram for all stars within 2 half-light radii of Peg VII. Those inside the orange dashed lines are identified members of Peg VII, and the red lines are the best-fit isochrones. The vertical red line is the main sequence, the curve at the top is the red giant branch, and the horizontal line is the horizontal branch. Below the black dashed lines are stars that are too dim to be reliably used in the analysis. Right: The colour–magnitude diagram for stars that appear to be around Peg VII that are not actually members. These stars don’t follow the main sequence at all. [Adapted from Smith et al. 2025]

All together, Peg VII only hosts around 82 stars and is physically 129 times smaller than Andromeda. This results in an absolute magnitude of −5.7 (in astronomy, the more negative the absolute magnitude, the greater the object’s intrinsic brightness). For context, Andromeda hosts more than 1 trillion stars resulting in an absolute magnitude of −21.5, making Peg VII 2 million times dimmer than its massive neighbour. This definitively makes Peg VII the dimmest known Andromeda satellite galaxy. See Figure 2 for more about Andromeda’s satellites.

locations of Andromeda Galaxy satellites and survey footprints

Figure 2: The spatial distribution of the Andromeda (M31) satellite galaxy system. Galaxies are coloured based on how bright they are (darker means dimmer). The dashed grey line denotes the area covered by PAndAS, a survey focused on Andromeda satellites that are more centrally concentrated. The dashed green line denotes the area covered by the UNIONS survey, with Peg VII on the right-hand side, more than 978,000 light-years from Andromeda. [Smith et al. 2025]

With their colour–magnitude diagram, the authors could also fit isochrone models to estimate other properties of the system. Essentially, they performed simulations of systems with a variety of ages and metallicities and determined what the colour–magnitude diagrams of these system would look like and compared them to Peg VII’s. They estimate Peg VII to be around 10 billion years old and very metal-poor, which is expected for a small galaxy like this.

How Are Baby Dwarf Galaxies Made?

The big question surrounding Peg VII is how exactly it got like this. Did it originally form with so few stars, or was it originally bigger and has since had stars stripped away due to tidal forces exerted on it by Andromeda? It’s difficult to answer this right now because the imaging the authors obtained doesn’t allow them to determine Peg VII’s orbital path and speed around Andromeda. But, they can look for indicators of tidal disruption, like spatial elongation of the stars’ distribution, or how “stretched out” the galaxy is.

They found that Peg VII is quite elliptical (oval-like) and its major axis is roughly pointed at Andromeda (Figure 3). This could indicate that Peg VII has interacted with Andromeda in the past, and this could have affected its mass and size. However, there are lots of other possibilities for Peg VII’s shape that wouldn’t involve Andromeda at all. Peg VII could have naturally formed like this in isolation and has only just recently been brought into Andromeda’s orbit. It’s also possible Peg VII is the byproduct of a merger between two even smaller dwarf galaxies, which has been shown to result in elliptical distributions.

spatial distribution of Peg VII member stars

Figure 3: The spatial distribution of Peg VII member stars (dark black dots). The dashed lines show where 1, 2, and 3 half-light radii are, showing how elliptical Peg VII is. The blue arrow shows the direction Andromeda is in, which roughly lines up with Peg VII’s major axis. [Adapted from Smith et al. 2025]

To answer all of these questions, more follow-up observations are needed. By combining different wavelengths of light, it will be possible to determine Peg VII’s velocities, its star-formation history, and its hydrogen gas content and distribution. Even without an answer about Peg VII’s evolution, its discovery alone is noteworthy for the Andromeda system. Our theories of galaxy formation predict that Andromeda should have way more small satellite galaxies like Peg VII that we haven’t observed yet, as many as 60! This study of Peg VII highlights the need for large, deep surveys to extend this search for satellites to further distances from Andromeda and perhaps down to dimmer magnitudes.

In the meantime, keep your eye on the sky for more news about Peg VII and its other ultra-faint dwarf galaxy buddies. As big surveys designed for finding faint systems like these start to ramp up, like the Euclid space telescope and the Vera C. Rubin Observatory, we may be hearing more big things about these little galaxies.

Original astrobite edited by Caroline Von Raesfeld.

About the author, Veronika Dornan:

Veronika is a postdoctoral research associate at the University of Edinburgh. Her research is in observations of globular star clusters and how they can be used to study the evolution of their host galaxies.

illustration of DART and LICIACube spacecraft

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: High-Speed Boulders and the Debris Field in DART Ejecta
Authors: Tony L. Farnham et al.
First Author’s Institution: University of Maryland
Status: Published in PSJ

The DART Mission: A First Step in Planetary Defense

In a groundbreaking experiment, NASA’s Double Asteroid Redirection Test (DART) became the first mission to intentionally crash a spacecraft into an asteroid to test whether such an impact could change the asteroid’s path. The target was Dimorphos, a small companion orbiting a larger asteroid called Didymos, located about 11 million kilometers from Earth.

Luckily, neither Dimorphos nor Didymos was ever on any sort of collision course with anything (and they’re still not). But by impacting an object orbiting another, the effect of the collision could more easily be measured and so Dimorphos was chosen as the target. You see, the goal wasn’t destruction, but deflection. The scientists wanted to see if a spacecraft could alter an asteroid’s orbit by striking it — and if so, by how much. When DART slammed into Dimorphos at over 22,500 kilometers per hour, it successfully shortened the asteroid’s roughly 12-hour orbit by about 33 minutes. This confirmed that hitting an asteroid with a spacecraft — in this case scientifically called a kinetic impactor — could be a viable method to redirect a potentially hazardous object in space.

But the change in momentum didn’t just come from the spacecraft itself. Much of the momentum transfer came from the plume of debris — called ejecta — that blasted out from the impact site. This debris, spreading out in multiple directions, added to the total push on the asteroid. Understanding how this ejecta moved exactly is key to calculating the full effect of the impact and was one of the primary goals for the DART mission.

With this exact purpose in mind, a small companion spacecraft called LICIACube (Light Italian Cubesat for Imaging of Asteroids), released by DART two weeks before the crash, flew past moments after the collision. It snapped several images of the debris cloud, as seen in Figure 1, as it raced past. It is with a reanalysis of these images that today’s authors have managed to track a number of boulders in the ejected material.

A GIF sequence of images that show the aftermath of the DART spacecraft impact on Dimorphos

Figure 1: Animated sequence of images taken by LICIACube as it flew past Didymos and Dimorphos a few minutes after impact by the DART spacecraft. The image is centered on Didymos with the smaller Dimorphos visible for most of the flyby. Large plumes of ejected material are visible, radiating out from the impact site where a very deep crater likely formed. Video provided by the author and subsequently converted to GIF and slowed for use in this bite. [Farnham et al. 2025]

Boulders, Boulders Everywhere!

You can say that DART really made an impact — pun intended. It kicked up a lot of material as it struck Dimorphos. In some parts the ejecta plumes are dense enough to block sunlight and even cast shadows on Dimorphos. In the images, like the one seen in Figure 2, the authors were able to make out more than 100 boulders, some as big as 3.6 meters in radius. Tracking the boulders allowed them to calculate velocities and their contribution to the momentum budget. Some of the boulders were ejected at speeds up to nearly 200 kilometers per hour and carried almost as much momentum as the DART craft itself.

Boulders detected in the ejecta from Dimorphos

Figure 2: The image shows part of the ejected material from the DART impact on Dimorphos at 2 minutes and 40 seconds after impact. A number of boulders are highlighted as they are tracked through the sequence of images from the LICIACube. The boulders are not uniformly distributed but mostly clustered in two distinct populations, with the South Cluster containing around 70% of the measured objects. [Farnham et al. 2025]

Using the assumption that the objects are somewhat spherical, the authors calculate that the total volume of all 104 boulders that they managed to identify and track amounts to 403 cubic meters of material, which is equivalent to a sphere 4.6 meters in radius. If you use the bulk density of Dimorphos, this suggests that the mass ejected in these objects is around 0.02% of Dimorphos’s total mass. And this is only for the boulders tracked — the study identified more boulder, but LICIACube was not able to track all of them and could only detect sizes down to 0.2 meter!

The authors also found that the boulders are clustered, suggesting that they were ejected in preferred directions. They conclude that this non-uniform distribution likely changed Dimorphos’s orbital plane. Additionally, the momentum imparted by boulder ejection also likely altered the rotational state of the asteroid, making it tumble around in its orbit.

So what happened? Although there are no constraints on their actual points of origin, the authors suggest that a likely scenario might be that the ejected rocks are the shattered pieces of two large boulders that were seen in some of the last images from the DART spacecraft itself. The two boulders on the surface — named Bodhran and Atabaque after their drum-like shapes — were likely struck by the solar panels as seen in Figure 3.

DART impact site and possible trajectories for the ejected boulders.

Figure 3: The right side of the figure shows the impact site for the DART spacecraft just before it hit the surface. The outline of the craft is superimposed on top with the main bus of the craft and with two extended solar panels. Also shown are the authors’ estimated paths for the 104 boulders tracked. They may be the remains of two surface rocks that were shattered upon impact. To the left is the radius distribution for the boulders that were tracked in the study. [Farnham et al. 2025]

The shallow angles and high ejection speeds are consistent with this scenario. Regardless of how they were ejected, though, it is clear that a significant amount of momentum can be found in the debris and that this process plays an important part in understanding how a kinetic impactor can deflect asteroids.

The European Space Agency’s Hera mission, now en route and arriving in 2026, aims to carry out an in-depth post-impact analysis. It could determine if Dimorphos is tumbling in its slightly modified orbit and help assess the momentum transfer from the impact debris. All this will contribute to a better understanding of just how much the impact changed the asteroid’s orbit and how much more momentum came from the ejected material than was contributed by the DART spacecraft to Dimorphos. These data will be key to refining asteroid deflection strategies in the case that we might need them in the future. After all, the dinosaurs didn’t have any, and look where it got them!

Original astrobite edited by Chloe Klare.

About the author, Kasper Zoellner:

I have a Master of Science in astronomy and I am currently working towards a PhD in physics and educational science. My greatest passion is the search for exoplanets and how stellar variability may influence the possibility of life. I am also interested in science outreach, education and discussing what sci-fi novel to read next!

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: Stars Born in the Wind: M82’s Outflow and Halo Star Formation
Authors: Vaishnav V. Rao et al.
First Author’s Institution: University of Michigan
Status: Published in ApJ

Outflows and Star Formation

Some galaxies, known as starburst galaxies, form stars at exceptionally high rates. High star formation rates mean, you guessed it, lots of new stars! The most massive stars live comparatively short lives and can die in a brilliant cosmic explosion known as a supernova. So, when you have a starburst galaxy, you get lots of young stars, a fraction of which produce supernovae. While these stellar explosions occur on relatively small scales, they can collectively drive galactic-scale expulsions of gas and dust known as outflows.

Outflows expel gas laden with metals out of the galaxy, playing a pivotal role in the evolution of galaxies. They can also drive additional star formation in the areas surrounding the galaxy, which is exactly what today’s authors are interested in. Messier 82 (M82), a quintessential local starburst galaxy, is the focus of today’s article (see Figure 1). M82’s proximity makes its spectacular outflows a prime testing ground for studying the impact of outflows on a galaxy and its surroundings.

starburst galaxy Messier 82

Figure 1: An image of the local starburst galaxy M82, taken by the Subaru Telescope. The Hubble Space Telescope field encompassing the Southern Arcs is highlighted as a green box. [Rao et al. 2025]

The Southern Arcs of M82

The main focus of today’s article are arc-like groups of stars called the Southern Arcs that are located near M82’s southern outflow. Using photometry from the Hubble Space Telescope, today’s authors derive star formation histories for the Southern Arcs, with the goal of understanding the impact that M82’s outflows have had on star formation in its halo. Figure 1 shows the Southern Arcs region of M82 highlighted in green alongside an image of M82.

Star Formation Histories

If you’ve ever taken an astronomy class, you’re probably familiar with the Hertzsprung–Russell (HR) diagram. HR diagrams are one of the most powerful tools available to astronomers, as they encode a ton of information regarding populations of stars and their formation. In practice, astronomers can construct HR diagrams using resolved stellar populations. That is, if you can resolve individual stars in a galaxy, you can construct a color–magnitude diagram, which is essentially the HR diagram you may be familiar with, but uses observed properties as a proxy for temperature (color) and luminosity (magnitude).

Today’s authors use the color–magnitude diagrams of the Southern Arc to derive star formation histories for the region. Star formation histories describe the star formation rate as a function of time, providing an insight into when and how stellar populations formed. To derive the Southern Arc star formation histories, the authors use the MATCH color–magnitude diagram fitting code, which determines the combination of stellar populations that reproduce the observed color–magnitude diagram, accounting for observational biases along the way. Figure 2 shows the star formation histories obtained using three different stellar evolution models. The authors find that about 85% of the stellar mass in the Southern Arc field formed sometime between 70 and 150 million years ago before star formation slowed down. About 30 million years ago, star formation picked up again, producing the rest of the stellar mass in the Southern Arc field.

star formation history and cumulative star formation history of the Southern Arc region

Figure 2: The star formation history (left) and cumulative star formation history (right) of the Southern Arc region. Different stellar evolutionary models are marked in different colors. The star formation history shows the star formation rate as a function of time, while the cumulative star formation history shows the buildup of the stellar mass as a fraction of the total mass observed now. [Rao et al. 2025]

So, What’s the Deal with the Southern Arcs?

The authors explore two mechanisms that could explain the star formation histories. In the first scenario, M82’s outflows trigger star formation when impacting the cooler circumgalactic gas. When the outflow shocks collide with the cooler gas, it causes it to collapse and form stars. In the second scenario, star formation is occurring within the outflows themselves. Figure 3 is a schematic of the two proposed mechanisms.

schematic outlining the two possible mechanisms for the formation of the Southern Arcs

Figure 3: A schematic outlining the two possible mechanisms for the formation of the Southern Arcs. The left panel shows the scenario in which shocks produced by the outflow collide with gas in the circumgalactic medium, triggering star formation. The right panel shows the scenario in which star formation is triggered within the outflow itself. [Rao et al. 2025]

The authors emphasize that distinguishing between the two scenarios requires further observations, specifically to determine metallicities of the stars in the Southern Arcs. If the two distinct stellar populations have different metallicities, it is more likely that the stars formed within the outflow in multiple gas clouds. If the populations have similar metallicities, it hints that the outflow shock triggered star formation, leading to similar stellar populations. So, as the age-old saying goes, “further data are needed” to better understand the origin of the Southern Arcs!

Original astrobite edited by Jessie Thwaites.

About the author, Drew Lapeer:

Drew is a first-year PhD student at the University of Massachusetts Amherst. They are broadly interested in the evolution of galaxies, with a focus on the impact of cosmic feedback on the galactic ecosystem. In their free time, they enjoy reading, rock climbing, hiking, and baking!

Illustration of stellar-mass black holes embedded within the accretion disk of a supermassive black hole

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: Tracing the Light: Identification for the Optical Counterpart Candidates of Binary Black Holes During O3
Authors: Lei He et al.
First Author’s Institution: University of Science and Technology of China
Status: Published in ApJ

Since the first detection of gravitational waves about a decade ago, gravitational wave science has been the gift that keeps on giving (for example, check out GW231123, announced earlier this year). It is also a field where more questions with unknown answers just keep coming. A key question at the forefront of gravitational wave science is about formation channels: how does the population of black holes seen through the LIGO–Virgo–KAGRA (LVK) detectors form? The two most popular of these formation channels are the “isolated binary” channel, wherein two stars of a binary stellar system each form black holes that then merge, and the “dynamical” formation channel, wherein two stars in a dense stellar cluster (usually a nuclear star cluster) become black holes and merge just through chance and the denseness of the environment they are in.

More recently, a third channel has been gaining popularity: the active galactic nucleus (AGN) disk channel. An AGN disk is a large disk of hot gas that surrounds a supermassive black hole in the center of a galaxy. The disk helps address two potential issues with stellar-mass binary black hole mergers that could pose a problem for the other two formation channels: delay time and mass. The delay time is the time between when the binary system forms and when the two black holes merge. By putting the system into the denser gaseous environment of an AGN disk, the orbits of the black holes decay faster, allowing the merger to happen on a quicker timescale. Regarding mass, the LVK detectors have detected mergers with masses above what would be expected from stellar-mass black holes alone, the so-called “lite” intermediate-mass black holes. One potential avenue through which these “lite” intermediate-mass black holes may form is in the gaseous disks of AGNs, where they can accrete material and gain mass before merging.

One potential astronomical benefit of a binary black hole merger happening inside the gaseous disk of an AGN is that the merger could disturb this disk and spark a flare of light. If we could spot such a flare and tie it to a gravitational wave event, we would then have both gravitational wave and electromagnetic observations of the same event. Prior research has suggested that some AGN flares detected by the Zwicky Transient Facility (ZTF) might be linked to binary black hole mergers observed during LIGO–Virgo’s third observing run. Today’s authors revisit this possibility by using additional years of data from ZTF combined with the public data through the third observing run.

The authors re-analyzed seven candidate flare–gravitational wave pairs originally flagged during the third observing run (see Figure 1). They examined each AGN’s long-term light curve from data from ZTF, looking for repeated flaring that might indicate a variable AGN rather than a one-time merger-driven flare. Using a Bayesian framework and an approach called Gaussian processes, they calculated the probability that each optical flare was physically connected to a specific gravitational wave event, considering both the spatial and temporal alignment of each of the two signals. Their framework allowed them to calculate a value they dub pflare, which is the probability that a given flare is indeed a genuine flare rather than a characteristic of the variability intrinsic to the AGN itself.

Sky localization of the seven AGN systems analyzed in this work

Figure 1: Sky localization of the seven AGN systems analyzed in this work. Red stars represent the locations of the AGNs with suspected flares, and colored lines represent the nine potential gravitational wave events that may be associated with each AGN flare. The white/black background represents the density of the distribution of AGNs in the Million Quasar Catalog. [He et al. 2025]

After analyzing the updated data, only two of the original seven flare candidates remained consistent with being merger-driven events, as the two associated AGNs showed no additional flares for three years after the initial brightening, suggesting that their activity was not just normal AGN variability.

The authors take this result one step further. One primary application of combining gravitational wave and electromagnetic signals is an independent measurement of the Hubble constant. The most straightforward way to measure the Hubble constant is to measure both the recessional velocity and the distance to a single source. Multi-messenger astronomy provides a straightforward approach to this, as the redshift can be determined from electromagnetic observations and the distance from gravitational wave observations. (GW170817 is a notable example of this.)

The authors show how this could be applied to their data using flares. They use the two most promising flare–gravitational wave associations to derive a new estimate of the Hubble constant. By combining the distance inferred from the gravitational wave signal with the redshift of the AGN host galaxy, they obtained a value of the Hubble constant of about 72 kilometers per second per megaparsec (roughly the peak of the green dotted curve on the bottom of Figure 2). This result is consistent with other measurements of the Hubble constant, which are the local distance-ladder measurements from supernovae and the cosmic microwave background estimates from Planck (the two measurements that are often cited as being in tension). The uncertainty from the author’s analysis is still quite large, but it provides a proof of concept. When they included the well-known GW170817 neutron star merger, they obtained a result that is slightly more informative and slightly greater than the result from GW170817 alone.

plot showing constraints on the Hubble constant from multiple methods

Figure 2: Constraints on the Hubble constant using multiple methods. For both figures, the x-axis represents the inferred value of the Hubble constant, and the y-axis represents the posterior probability of that value using hierarchical Bayesian analyses (for gravitational wave analyses). The vertical orange bands represent the measured value of the Hubble constant with five standard deviations of certainty using the “distance ladder” method as reported by the SH0ES collaboration. The vertical blue bands represent the measured value of the Hubble constant with five standard deviations of certainty by using measurements of the cosmic microwave background with the Planck spacecraft. The dotted black curve (both left and right) represents the posterior distribution of the Hubble constant as measured by the LVK collaboration from the first binary neutron star multi-messenger source, GW170817 (which peaks directly between the other two measurements). The purple and red curves (left) represent the posterior distributions of the two confident AGN/binary black hole pairs in this work, the green curve (right) represents the combined posterior distribution of the two confident AGN/binary black hole pairs in this work, and the solid purple curve (bottom) represents the combined posterior distribution of the two confident AGN/binary black hole pairs with GW170817. [He et al. 2025]

What makes this work so exciting is that it showcases a method for identifying black hole binaries in AGN disks, and once identified, turning the joint observation of the merger and AGN disk into a new astronomical and cosmological tool! If we can confidently identify a handful of these events, they can be used both to understand binary black hole formation channels and to independently measure cosmological distances, thereby expanding our understanding of the universe. With upcoming surveys like the Vera Rubin Observatory’s Legacy Survey of Space and Time, which will monitor vast numbers of AGNs with greater cadence and depth, the prospects for finding more such events are strong.

Original astrobite edited by Amaya Sinha.

About the author, William Smith:

Bill is a graduate student in the astrophysics program at Vanderbilt University. He studies gravitational wave populations with a focus on how these populations can help inform cosmology as part of the LIGO Scientific Collaboration. Outside of astrophysics, he also enjoys swimming semi-competitively, music and dancing, cooking, and making the academy a better place for people to live and work.

Illustration of a binary system containing two white dwarfs

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 Evolution of Hypervelocity Supernova Survivors and the Outcomes of Interacting Double White Dwarf Binaries
Author: Ken J. Shen
Author’s Institution: University of California, Berkeley
Status: Published in ApJ

Even stars get kicked around sometimes. When a white dwarf — a dense stellar corpse that’s run out of its nuclear fuel — gets pushed past its limit, the result is a spectacular thermonuclear explosion called a Type Ia supernova. Two stars are needed to trigger a Type Ia supernova explosion. In a binary system that gives rise to a Type Ia supernova, the more massive star, called the primary, steals mass from the less massive companion star. As the primary gains mass, it undergoes a runaway thermonuclear reaction and explodes. Despite the violent nature of the blast, it doesn’t always destroy everything in sight. While the primary star is obliterated, the companion star might survive the explosion by being launched, or “kicked,” into space at incredibly high speeds.

A few of these hyper-velocity survivors have been detected in surveys, particularly from the Gaia mission. Understanding how these stars survived a supernova provides important clues about how a supernova forms and explodes in the first place.

The D6 Model: (Double the Trouble) + (Double the Double the Trouble)

In today’s post, we explore the “dynamically driven double-degenerate double-detonation” model, fortunately shortened to the D6 model. (Try to say that ten times fast!) The D6 model describes a binary star system of two white dwarfs, each of which is generally composed of an even mix of carbon and oxygen. (Sometimes white dwarfs composed of helium or a mix of oxygen and neon are also possible.) As the white dwarfs spiral toward each other, the more massive primary star steals material, often helium, from the less massive companion star’s outer shell. If this mass transfer is violent enough, it can trigger a detonation within the helium shell of the primary star. This detonation then triggers an explosion deep within the core of the primary star, causing a supernova.

For a Type Ia supernova, it takes two stars to tango. What happens to the secondary (donor) star? One theory suggests that it can be blasted away from the scene of the crime at speeds ranging from approximately 1,000 to 3,000 km/s, becoming a so-called hyper-velocity star. Of these observed “survivors,” the hottest of the bunch have been theorized to be heated by the supernova explosion itself. But some of these survivors have been cooler (in temperature). If these companion stars were in the vicinity of the blast, how could they not be heated by the explosion? What can their temperature tell us about their origin?

Defining the Model

To explore the origins of these cool hyper-velocity stars, today’s author used the stellar evolution code MESA to model the evolution of different types of white dwarf companion stars after (presumably) surviving a Type Ia supernova. A key focus was on the Kelvin–Helmholtz mechanism, which is the process by which a star cools and therefore contracts over long periods as it radiates away its internal heat.

Because white dwarfs come in many flavors, they explored a range of possible elemental compositions for the companion white dwarf: helium-rich (He-rich), carbon/oxygen-rich (C/O-rich), and oxygen/neon-rich (O/Ne-rich).

One important detail of these models is that the stars were assumed to be fully convective, which is a common property of low-mass stars with masses less than around 0.4 M☉. (You can think of a fully convective star almost like a massive lava lamp where the heat source is at the center of the sphere.) Extensive mass loss can cause a more massive non-convective star to become less massive and convective, which in turn makes it susceptible to cooling quickly and avoiding a fiery death as a supernova. (Hotter gas rises to the surface, where the particles lose kinetic energy by doing work, gradually slowing down and cooling off.) This is key if we want to produce cool, fast-moving stars.

Detonate First, Simulate Later

For helium-rich survivors, the simulations suggest that if the companion star loses enough mass either before or during the explosion, the star’s natural Kelvin–Helmholtz evolution can potentially explain why we observe some cool hyper-velocity survivors. In the case of a star called D6-2, the simulations reproduced its low temperature and luminosity, assuming that it began its life as a helium white dwarf that was shredded but not entirely destroyed. This produces a tiny, convective, hyper-velocity supernova survivor.

D6-2 is an interesting object, however, because it has a fairly low velocity for a potential hyper-velocity survivor. Its velocity is estimated to be about 1,050 km/s, which simulations suggest should be higher based on its estimated mass. It’s likely that either the simulations and models need refining or D6-2 had an altogether different origin.

What About the Others?

So far, we’ve mainly talked about He-rich stars, but what about those C/O-rich ones? These stars would likely appear cooler and redder since they evolve at a nearly constant temperature before moving onto the standard white-dwarf cooling track. (This is called the Hayashi track.)

Red objects are harder for telescopes to detect and often get mistaken for other stars or objects, partially due to a pesky phenomenon called dust extinction, or interstellar reddening. To expand the search for these hyper-velocity candidates, surveys like Gaia might benefit by expanding their search limits to include redder stars, although this opens up the possibility of increasing the number of false positives.

A different class of fast-moving, faint stars — like LP 40-365 — could be related to this population. They are theorized to be remnants of white dwarfs around 1.4 M☉ that only partially exploded. Low-mass, O/Ne-rich survivors might match the observed temperatures and luminosities of LP 40-365, but the ages don’t line up as well as one might hope. (More work will be needed to figure out this particular puzzle.)

The Odds of Stellar Survival

Based on these simulations, it’s estimated that about 2% of Type Ia supernovae might leave behind a He-rich hypervelocity star like D6-2 (see Fig. 1). If we consider C/O-rich survivors (like D6-2’s cousins D6-1 and D6-3), the rate drops to about 0.2%, which is intriguingly close to the estimated rate of SN 2003fg-like events: unusually bright, slowly evolving Type Ia supernovae that often display tell-tale signs of unburned carbon and oxygen in their spectra.

A histogram of detectable hypervelocity survivors from the He-rich track

Figure 1: A histogram of detectable hyper-velocity survivors from the He-rich track assuming every Type Ia supernova produces a companion with a mass of 0.02 solar mass. These survivors are assumed to be ejected from their Type Ia supernovae at a velocity of 1,050 km/s, matching D6-2. The x-axis, tmidplane, describes the apparent travel time from the midplane of the simulation’s galaxy. A negative value means the survivor was observed before it passed through the plane. A total count of 43 hyper-velocity survivors from every Type Ia supernova yields an estimated survival rate of about 2%. [Shen 2025]

The presence of these hyper-velocity survivors might be linked to rare types of supernovae, which would provide an interesting way to look into the origins of these oddball events.

Choose Your Own (Stellar) Adventure

This article suggests different evolutionary paths for how binary white dwarfs might evolve. Depending on the properties of the merger, the system might do any of the following:

  1. Trigger nuclear reactions within the outer shell of the primary without detonating the primary’s core
  2. Become a normal Type Ia supernova
  3. Leave behind a hyper-velocity remnant
  4. Become a SN 2002es-like supernova, which is a fainter type of Type Ia supernova

There are lots of potential outcomes for these types of binaries, and hyper-velocity survivors indicate just a tiny fraction of a particular configuration of binary white dwarfs (see Fig. 2).

diagram of the speculative outcomes for a white dwarf–white dwarf binary

Figure 2: A diagram of the speculative outcomes for a white dwarf–white dwarf binary. The y-axis shows the secondary’s initial mass, whereas the x-axis shows the primary’s initial mass. The dashed lines indicate where the combined white dwarf mass is 1.4 M☉. The plot illustrates the wide variety of outcomes of white dwarf–white dwarf binaries, of which hyper-velocity survivors (HVS) are rare. sdB/sdO = subdwarf B/O stars; R CrB = R Coronae Borealis, variable star; Fe CC = iron core-collapse supernovae; HVS = hyper-velocity survivor; NS = neutron star. [Shen 2025]

These stellar survivors are rare and fast. Cooler hyper-velocity survivors, in particular, are fascinating because they may originate from white-dwarf binaries with unusual properties. Studying these fast-moving stars helps piece together our understanding of Type Ia supernovae as a whole. With improved surveys and simulations, we may discover more about these elusive escapees.

Original astrobite edited by Chloe Klare.

About the author, Mckenzie Ferrari:

I’m currently a PhD grad student in the Geophysical Sciences program at the University of Chicago. While I now study the atmosphere and oceans of Earth, most of my previous undergrad and grad research focused on simulations of Type Ia supernovae and galaxy formation and evolution. In my free time, I foster cats for a local organization, enjoy cooking, and can often be found running along Lake Michigan.

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