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8 September 2025

A Dust Curve That Breaks the Mold

Title: The AURORA Survey: The Nebular Attenuation Curve of a Galaxy at z = 4.41 from Ultraviolet to Near-Infrared Wavelengths
Authors: Ryan L. Sanders et al.
First Author’s Institution: University of Kentucky
Status: Published in ApJ

Imagine a galaxy from 13 billion years ago, long before the Milky Way took its current form. Now picture trying to uncover the secrets of that ancient galaxy. How quickly did it create stars and evolve? Here’s the tricky part: its light has journeyed across an unimaginable distance to get here. On its journey, cosmic dust — tiny grains of carbon and silicates — has scattered and dimmed the light, making the task harder.

Dust doesn’t just block light — it changes it. Ultraviolet (UV) light gets scattered much more than light at longer wavelengths, which is why galaxies seem redder and dimmer than they truly are. This dual nature can be seen in the “Pillars of Creation,” in Figure 1. A new near-infrared image from NASA’s JWST, shown in the right panel below, helps us peer through more of the dust in this star-forming region. The thick, dusty brown pillars are no longer as opaque, and many more red stars that are still forming come into view.

Pillars of creation as seen by Hubble and JWST

Figure 1: On the left, NASA’s Hubble Space Telescope captures the iconic Pillars of Creation in stunning visible light (2014). On the right, JWST unveils a breathtaking infrared view, revealing even more details hidden within the cosmic dust. [NASA, ESA, CSA, STScI; Joseph DePasquale (STScI), Anton M. Koekemoer (STScI), Alyssa Pagan (STScI)]

To quantify how dust affects light at different wavelengths, astronomers use dust curves — tools that reveal how much light is blocked or scattered, helping them correct for dust-induced distortions. For instance, the Milky Way’s curve stands out with a distinct “bump” around 2175 Å, which comes from tiny carbon-based particles in the dust. But the Small Magellanic Cloud (SMC) tells a different story. Its curve is steeper in the UV and doesn’t have that bump at all. These differences aren’t just random — they give us clues about the size, makeup, and distribution of dust grains in each galaxy. (See Figure 3 below for a visualization of these dust curves.)

But here’s the catch: most of the dust curves we have were designed for galaxies much closer to us, like the Milky Way or the SMC, so they may not apply in the exact same way to distant galaxies. Whether those same curves work for the much younger, chaotic galaxies of the early universe is an open question. The dust in nearby galaxies is well studied and relatively predictable, but does dust behave the same way in galaxies billions of years in the past?

This question drove Ryan Sanders and collaborators to study GOODSN-17940, a starburst galaxy located at a redshift of z = 4.41, when the universe was just 1.36 billion years old. Observed as part of the Assembly of Ultradeep Rest-optical Observations Revealing Astrophysics (AURORA) survey using JWST, this galaxy provided an unprecedented opportunity to investigate how dust behaved in the distant past. Using JWST’s NIRSpec instrument, the team constructed a unique dust curve by studying 11 hydrogen emission lines from Hα to H12 (the twelfth line in the rest-frame optical Balmer series), shown in Figure 2. These lines act like lighthouses, their brightness giving away how much light is being absorbed by dust at each wavelength. By comparing the observed brightnesses of these lines, the researchers created a detailed dust curve tailored to GOODSN-17940.

Figure 2: Spectrum of GOODSN-17940 from the AURORA survey, showing hydrogen Balmer lines. These features help estimate the galaxy’s dust and ionization conditions. The top panel presents the 2D spectrum, showing spatially resolved emission that traces the galaxy’s structure, while the bottom panel shows the corresponding 1D spectrum. [Adapted from Sanders et al. 2025]

GOODSN-17940’s dust curve is shown in Figure 3. The y-axis represents the factor by which light is blocked/scattered by dust compared to this amount at 9550 Å. For example, a value of 2 means that light at that wavelength is attenuated two times more than light at 9550 Å.

dust attenuation curve of a redshift 4.41 galaxy compared to nearby galaxies

Figure 3: The dust curve of GOODSN-17940 (solid red line) at z = 4.41, derived in this work, compared to established curves for the Milky Way (green dashed), Small Magellanic Cloud (SMC, dotted cyan), Calzetti et al. (blue dot-dashed), and Reddy et al. z ~ 2 galaxies (purple solid). This curve is steeper in the near-infrared and flatter in the UV, highlighting unique dust properties in this galaxy. [Sanders et al. 2025]

This dust curve is surprising. In the near-infrared, it is much steeper than the curves of the Milky Way or SMC. This suggests the galaxy’s dust grains might be smaller or distributed differently. But in the UV, the curve flattens out, showing less absorption than classical models predict. And unlike the Milky Way’s dust, there’s no 2175 Å bump at all. These findings hint at a galaxy with truly unique dust properties, shaped by its extreme youth and chaotic star-forming environment. But here’s the twist — GOODSN-17940 isn’t just any galaxy. Its star formation rate is 40 times higher than typical galaxies at its redshift. While rare at redshifts of z ∼ 2–4, galaxies like this are more common in the epoch of reionization, suggesting that what we’ve learned from GOODSN-17940 could apply to reionization-era galaxies — the ones that played a key role in shaping the early universe.

So, why does this even matter? If you used the Milky Way’s dust curve to adjust for the dust in a galaxy like GOODSN-17940, you’d be way off. In fact, you’d underestimate its star formation rate by up to 50%! This isn’t just a minor detail — it could explain why star formation rates derived from Hα don’t always match those derived from UV luminosity. Such details can completely change how we interpret a galaxy. It’s the difference between a galaxy steadily forming stars and one caught in a dramatic starburst phase. That’s why astronomers focus on creating dust curves tailored to individual galaxies. This study isn’t just about one galaxy — it’s about rethinking how we interpret dust, star formation, and galaxy evolution in the early universe. Before JWST, isolating individual emission lines from distant galaxies was challenging, but it has become much more efficient now. As more galaxies like GOODSN-17940 are studied, we might find that many early galaxies had dust curves breaking the mold, forcing us to refine how we measure star formation and understand cosmic history.

Original astrobite edited by Hilary Diane Andales and Delaney Dunne.

3 September 2025

Weather Forecast for the Habitable Worlds Observatory: Cloudy with a Chance of Biosignatures

Title: Clouds Can Enhance Direct-Imaging Detection of O₂ and O₃ on Terrestrial Exoplanets
Authors: Huanzhou Yang, Michelle Hu, and Dorian S. Abbot
First Author’s Institution: University of Chicago
Status: Published in ApJ

In the hunt for life beyond Earth, studying exoplanet atmospheres is one of our most powerful tools. The atmosphere of an exoplanet acts as a window into its environment and allows astronomers to take a guess at what processes are shaping the planet’s climate, chemistry, and thus, its potential for habitability. The gases present in the planet’s atmosphere impart a unique spectral fingerprint on the light that passes through the atmosphere and allows us to identify what gases might be present and look for possible signs of life.

JWST is already hard at work looking at distant worlds and gathering spectroscopic data for us to analyse. It has already observed several exoplanets and recorded the light that passed through their atmospheres. This has not only sparked a debate over what could be interpreted as the first possible signs of biological activity, but has also given us a glimpse of worlds that are remarkably different from our own.

However, JWST is mostly limited to observing the atmospheres of transiting exoplanets. That means planets that orbit close to their host star and are more or less directly edge-on, as viewed from us, so that they transit across the face of the star. The only planets that the telescope is able to observe directly are big, young, and hot planets farther from their host star, at distances more resembling those of the outer planets in our solar system, like Saturn or Uranus. If only we had a telescope powerful enough to image a potentially habitable planet directly and analyse the light reflected from it.

The Habitable Worlds Observatory

This is where the Habitable Worlds Observatory (HWO) will come in. Now, I know what you are thinking. Why must astronomers always look towards ~~greener pastures~~ bigger telescopes when we already have perfectly functional space telescopes in orbit or practically on the launch pad? Well, it is true that the next flagship space telescope from NASA, the Nancy Grace Roman Space Telescope, is designed to directly image exoplanets and analyse their atmospheres. But space telescopes can take decades to plan and are often built on lessons learned from previous missions. Roman, with its 2.4-meter primary mirror and coronagraph, will be able to observe planets similar in size and orbital distance to Jupiter and will get us some of the way, for sure, but not all the way. At three times the mirror size, HWO, NASA’s proposed successor to Roman that is planned for the 2040s, will carry the torch even further and be able to directly image Earth-like planets in the habitable zone, where liquid water can exist on the planet’s surface.

But It Might Be Cloudy Tomorrow

This brings us to today’s article. If the HWO is going to potentially observe Earth analogues, we might expect it to run into the same issue that plagues everyone living in the UK for most of the year — clouds. Clouds in the atmosphere of an exoplanet can sometimes cause trouble when trying to analyse the light. Cloud particles scatter different wavelengths of light differently, and cloud variability creates dynamic changes in the observed brightness that can make it hard to know what exactly you are looking at. Clouds can also mean varying levels of opacity to look through depending on cloud composition, and they can hide underlying atmospheric properties such as water content beneath a thick cloud layer. For transmission spectroscopy, even a thin layer of clouds can disrupt the measurements as the light has to travel a long distance through the atmosphere. However, for direct imaging, the effect of clouds is perhaps more nuanced. While a thick cloud deck will block the signal from whatever gas lies beneath it, it will also increase the overall albedo of the planet and make it easier to detect the gas above. This means that cloud height and type are both critical variables in understanding atmospheric properties of exoplanets, which is what the authors of today’s article set out to investigate.

To understand how clouds affect the ability to detect signs of life on distant Earth-like planets, the authors used computer simulations. They focused on two gases, molecular oxygen (O₂) and ozone (O₃). Both gases are often regarded as biosignatures, although their astrobiological significance has been debated. The authors first used a tool that simulates how a telescope like the HWO might observe an Earth-like planet around a star 15 parsecs away. This tool predicts how light bounces off a planet’s surface and atmosphere, and whether the two gases would show up clearly in the reflected light. Next, they created realistic cloud conditions using a cloud microphysics model. This model calculates how clouds might form and behave depending on factors like surface pressure or how much sunlight the planet gets. With this they were able to change things like cloud height and particle size. They then combined the two tools by adding the simulated clouds into the observations model and building a mathematical model to help explain what they saw in the simulations. The sketch for their model can be seen in Figure 1.

diagram showing how clouds affect the amount of reflected light from an exoplanet

Figure 1: Clouds can affect the amount of reflected light (albedo) from the imaged planet. Depending on the cloud height and particle size, this can have a big impact on the observability of different species of gas in the atmosphere. Here the blue arrows indicate the amount of light entering the atmosphere and being reflected. The arrows get thinner from tail to head, representing absorption in the atmosphere. Depending on the presence of clouds, a varying amount is reflected back. The red colour indicates the assumed distribution of ozone, O₃, which is concentrated at a high altitude. [Yang et al. 2025]

A Cloudy Sky Can Actually Be a Good Thing

The researchers discovered that low clouds can actually make gases like oxygen and ozone easier to detect, because these clouds reflect more light without blocking the gases above them. On the other hand, high clouds can make it harder since they can hide the gases from view. They tested different cloud types using real data from Earth as seen in Figure 2. They looked at five common cloud types, like stratus (flat and low), cirrus (high and wispy), and deep convective clouds (tall, thunderstorm-like), and compared them to their model clouds (labelled as CARMA in the figure).

plot demonstrating how different types of clouds affect our ability to detect oxygen and ozone on an Earth-like exoplanet

Figure 2: The figure shows how different types of clouds affect our ability to detect oxygen (blue) and ozone (red) on an Earth-like planet. The dashed lines show the results without clouds, while the bars show what happens when clouds are present. Solid bars represent larger cloud droplets, and striped bars represent smaller ones, which reflect more light. The figure shows that most cloud types — especially low clouds like stratus and thick storm clouds — actually make it easier to detect these gases, particularly when droplets are small. Only high, thin clouds (cirrus) slightly reduce oxygen detection, and even then, the effect is small. This is the article’s main finding: while clouds can block light in some observation methods, for direct imaging of reflected light, they often help us see signs of life better. The cloud types are based on real Earth data while the authors’ simulated cloud model is labelled as CARMA. [Yang et al. 2025]

They found that most clouds improved detection, especially if the cloud droplets were small (as they reflect more light). Stratus clouds and deep convective clouds gave the strongest boost to detection, because they have a lot of water and are either low (stratus) or extend through many layers (deep convective). Ozone saw the biggest bump in detectability as it is concentrated higher in the atmosphere, which means that it is in the region above most clouds where the increased reflection from the cloud layer below makes it easier to spot. This means that unlike the case for transit spectroscopy where clouds interfere with the detection of gases, for direct observations of the reflected light, clouds may actually be a good thing. This insight into the role of clouds in exoplanet atmospheres not only refines our current understanding but also helps guide the design of future telescopes, like the HWO, and our search for life on these remote worlds.

Original astrobite edited by Maria Vincent.

25 August 2025

From Globs to Gravitational Waves: A Simulated Cosmic Choreography

Title: Formation and Evolution of Compact Binaries Containing Intermediate-Mass Black Holes in Dense Star Clusters
Authors: Seungjae Lee et al.
First Author’s Institution: Seoul National University
Status: Published in ApJ

A delicate dance goes on in astronomy between theory and observation. Most astronomy research, by and large, fits into one of two categories. In one category, you start with a theory, model, or simulation, and attempt to figure out what observations you might expect given those initial assumptions. The other approach begins with raw data and attempts to determine what kinds of theories, models, and fundamental astrophysical assumptions might give rise to it. Accepted knowledge usually happens when these two approaches agree and reinforce each other, but a lot of the actual science occurs when one side dances ahead of the other — either we have theories and models that make predictions that cannot yet be observationally tested, or we have data that defy attempts at fitting underlying models.

Another dynamical dance goes on in globular clusters between binary black holes. Today’s article investigates intermediate-mass black holes, or IMBHs, a pristine example of an astronomical topic in which theory has danced ahead of observations. IMBHs fill the gap between stellar mass and supermassive black holes. We can observe black holes in multiple ways, usually from the electromagnetic radiation of the gas disks surrounding some black holes (either stellar or supermassive), or the gravitational waves of two orbiting or merging black holes. Unfortunately, each method requires some extra component — either gas to accrete or a companion to orbit — and IMBHs are far more challenging to observe through these methods. Hence, we have a lot of ideas about how they might form, and scarce observational evidence for their existence.

That hasn’t stopped many researchers from trying to make better models and predictions. One of the most commonly proposed mechanisms for forming IMBHs is in dense globular or nuclear star clusters (up to a million times denser than our region of the Milky Way). In these dense environments, stars and the black holes they produce might have a chance to find and run into each other often enough to reach the masses of the IMBH regime through mergers. Today’s authors use a series of N-body simulations to study these dense stellar clusters, into which they embed IMBHs. Their study investigates the detectability of gravitational waves from intermediate-mass-ratio inspirals (IMRIs). Because so many mergers occur in dense clusters, it allows for more mergers between objects with greater mass differences. The mass ratio between the two objects (usually denoted as ‘q’) is one of the most important parameters for characterizing a merger, and a high q value creates a gravitational wave signature that is different from a low q, even if the total mass of the system is the same.

To investigate the formation and detectability of IMRIs, the authors conducted a suite of direct N-body simulations of star clusters with total masses between 5×10⁴ and 10⁵ solar masses, containing ~10⁵ particles. They include initial conditions for how densely packed the particles should be, given how far they are from the cluster’s center (called a Plummer profile). They also include a rule for how the initial masses of the stars are distributed (called a Kroupa initial mass function). Then, they use a code called Stellar EVolution for N-body (SEVN) to follow the evolution of the stars in the system for 100 million years.

In that time, massive stars in the simulation can evolve into neutron stars and stellar-mass black holes. In addition to simulating the clusters’ stars, the authors embed an IMBH of 300–5,000 solar masses into the clusters. The key to this article is that some of the stellar-mass binary black holes will interact with the IMBH, creating an IMRI, which gives off gravitational waves. This article’s primary goal is to characterize these IMRIs and to test how well current and future gravitational wave detectors will measure them.

In the simulation, when a stellar-mass binary black hole and an IMBH come close, the authors must create a “merger criterion” — essentially, how it is decided whether the two objects merge, and what the subsequent “gravitational wave” would look like. Once they model the wave itself, they turn to five current or future detectors to see how well they will measure those waves if they were at particular astronomical distances from Earth.

One of the primary relationships the authors look at in the cluster is the half-mass radius velocity dispersion of the cluster and the number of IMRI encounters between the IMBH and stellar-mass objects. One can think of the velocity dispersion as a more complicated “average velocity” of the stars in the cluster (a high velocity dispersion means, on average, the stars are moving faster, the half-mass radius is the distance from center that separates the inner half of the stars and outer half of the stars, and we use it because it is more representative of stellar motion in the overall cluster than the packed center or the mellow edge). Figure 1 shows the relationship between the number of IMRI events (y-axis) vs. the velocity dispersion (x-axis) of the cluster where the IMBH sits. Note that the y-axis is logarithmic, so a small increase in velocity dispersion actually results in a large increase in the number of IMRIs. This tells us that the number of events is much more dependent on the properties of the larger cluster than the IMBH itself.

plot of IMRI event rate as a function of cluster half-mass velocity dispersio

Figure 1: IMRI event rate as a function of cluster half-mass velocity dispersion (σ_hm) for 1,000-solar-mass IMBHs in the simulations. The IMRI merger rate is highly sensitive to the velocity dispersion of the cluster as a whole, making it a key observable of how efficiently IMRIs can form and merge in a given cluster. [Lee et al. 2025]

In addition to analyzing the dynamics of the clusters and the mergers within them, the authors go further and attempt to characterize what specific detectors will see. They chose five detectors. The first is advanced LIGO (aLIGO), the current incarnation of the LIGO detector network. Next is the Einstein Telescope (ET), a future proposed European ground-based detector. Following that is the Laser Interferometer Space Antenna (LISA), a space-based detector set to launch in the 2030s that is designed to observe supermassive black hole mergers. Next are two detectors that are less established, the advanced Superconducting Omni-directional Gravitational Radiation Observatory (aSOGRO) and the Deci-hertz Interferometer Gravitational Wave Observatory (DECIGO). aSOGRO is a proposed Earth-based detector that could theoretically measure gravitational waves at lower frequencies than the current LIGO-Virgo-KAGRA (LVK) detectors, and DECIGO is a proposed space-based detector designed to observe gravitational waves in the frequency range between the LVK network and LISA.

How do the authors translate the gravitational wave signal from the IMRI into what might be observed on Earth? First, they must pick a signal-to-noise ratio threshold, which is the ratio of the gravitational wave signal to the background noise in the detector. In this case, they chose a signal-to-noise ratio of 8, a commonly chosen threshold in the gravitational wave literature. From the signal-to-noise ratio, the authors can calculate a horizon distance. The horizon distance is the distance that a detector can measure a specific gravitational wave event with a given set of properties at or above the threshold chosen. Figure 2 shows the horizon distances (y-axis) for each detector for sets of gravitational wave sources with different properties vs. the mass of the IMBH generating the gravitational wave (x-axis). The solid and dotted lines represent two chosen masses of the IMBH’s stellar-mass companion. The figure shows that aLIGO (blue circles) can detect only low-mass IMBHs (≲ 300 solar masses) at a redshift of z ≲ 0.05. ET (red triangles) is sensitive to lower-mass IMBHs, and LISA (green diamonds) excels at detecting higher-mass IMBHs. aSORGO (purple triangles) covers a broad mass range but at lower redshifts. DECIGO (yellow Xs) offers the broadest mass range at the highest redshift.

plot of horizon redshift versus intermediate-mass black hole mass

Figure 2: Horizon redshift vs. IMBH mass for several gravitational wave observatories (aLIGo represented by blue circles, ET by red triangles, LISA by green diamonds, aSOGRO by purple triangles, and DECIGO by yellow x‘s). Solid vs. dotted lines represent the mass of the stellar-mass black hole interacting with the IMBH, with the dashed line representing a 10-solar-mass companion and the solid lines representing a 60-solar-mass companion. [Lee et al. 2025]

Today’s authors offer a glimpse into globular clusters as dynamic environments where we might see IMRIs. They also predict how those signals might be received by our current, future, and proposed gravitational wave detectors. As next-generation observatories prepare to listen for these cosmic mergers, this work helps chart a path for where and how we might first detect the elusive IMBH.

Original astrobite edited by Kasper Zoellner.

13 August 2025

Ultra-High-Energy Neutrino Emission on the Extragalactic Express: A Mystery

Title: ASKAP and VLASS Search for a Radio-Continuum Counterpart of Ultra-High-Energy Neutrino Event KM3–230213A
Authors: M. D. Filipović et al.
First Author’s Institution: Western Sydney University
Status: Published in ApJL

The Scene of the Crime

On Galentine’s Day this year, an ultra-high-energy neutrino attempted to sneak through the Mediterranean Sea, likely expecting she wouldn’t be caught. The odds were in her favor; neutrinos, ghostly particles with no electric charge and infinitesimal mass, only very rarely interact with matter. However, what she failed to account for was the awaiting undersea neutrino detector, KM3NeT, and the clever lepton within who would finally notice her. She slammed into the lepton, spewing charged particles everywhere at speeds greater than the speed of light in the water. While no particle can outrun a photon in a vacuum, water slows light down, giving us the familiar effect of refraction; similar to supersonic jets creating a boom when they break the sound barrier, these charged particles produced a distinctive blue light, known as Cherenkov light, exposing the neutrino’s position to astronomers and physicists everywhere. Busted.

The Investigation Begins

However, the neutrino was only the messenger; of even more interest is the astrophysical object that produced her. It’s not easy to generate such a high-energy particle, and no one can create a neutrino from thermal emission alone, indicating that wherever she originated, something extreme was going on. To date, only three astrophysical sources have been caught emitting neutrinos at all, and none of them are extragalactic: the Sun, although this is old news (in the 1960s, detections of solar neutrinos showed definitively that the Sun is powered by nuclear fusion, resolving the issue of how the Sun has burned long enough for life to evolve on Earth); the nearest core-collapse supernova to our galaxy in modern times, SN 1987A; and the galactic plane.

Theoretical models predict a much wider variety of objects, including extragalactic sources, to produce neutrinos, usually via cosmic-ray production: supernova remnants, star-forming galaxies, gamma-ray bursts, supermassive black holes (which are found at the centers of most galaxies), active galactic nuclei (a particularly fussy subset of supermassive black holes that are eating their host galaxies), and blazars (an extreme subset of active galactic nuclei that emit jets of radio light directly at Earth). The reason we have not detected their predicted neutrino emission is that neutrino astronomy is a new field, extragalactic sources are super far away, and neutrinos are both difficult to detect and difficult to trace back to their origin.

Rounding Up Suspects

With this in mind, today’s authors embark on a quest to catch the culprit, starting in the radio band. Radio emission, like neutrino emission, is usually an indicator of non-thermal radiative processes, and one such process, synchrotron radiation (emitted by relativistic electrons getting spun around in powerful magnetic fields), can be distinguished from other types of radiation based on its radio characteristics. Conveniently, the region our neutrino hails from is spanned by multiple radio surveys conducted with the Very Large Array (VLA) and the Australian Sub-Kilometer Compact Array Pathfinder (ASKAP), and so our authors use these surveys to round up all the radio riffraff. Unfortunately, the long wavelengths of radio photons and the scarcity of neutrinos result in reduced resolution for both compared to traditional optical telescopes, and our authors find over a thousand radio emitters in the region. Of course, no one can question that many sources, so our authors limit their investigation to objects with at least two radio brightness measurements, which can be used to calculate the brightness as a function of radio wavelength (the spectral energy distribution, which tells us about what type of radiation we see) and/or as a function of time (a light curve, which tells us if our source is variable). Our authors settle on a lineup of 10 likely blazars, any of whom could have emitted our ultra-high-energy neutrino, as well as a shortlist of prime suspects warranting further investigation: Phaedra, a spiral galaxy; Hebe, a radio galaxy; and Narcissus, an unusual compact radio emitter (see Figure 1).

radio emission from the region of sky from which the neutrino originated

Figure 1: Radio emission detected by ASKAP in the region of the sky in which the neutrino originated. Every yellow dot should be considered suspect, but the three colored squares identify the primary guilty parties: Phaedra (in blue), Hebe (in yellow), and Narcissus (in pink). [Filipović et al. 2025]

Phaedra: A Spiral Galaxy with a Secret?

Phaedra (Figure 2), the most radio-luminous in the area, exhibits plenty of behavior typical of a galaxy guilty of neutrino emission. For starters, she has two regions of highly concentrated radio emission, and these regions are offset from her center, making them look suspiciously like active galactic nucleus jets, which are excellent particle accelerators. Furthermore, infrared observations suggest she is a starburst galaxy, churning out stars faster than a bestselling author with a team of ghostwriters churns out books. This intense star formation could have easily been triggered by jet activity. Even more suspiciously, she is closely associated with an X-ray binary, and where there are high-energy photons, there are likely to be other high-energy particles like neutrinos and cosmic rays. Phaedra’s prospects of beating the neutrino emission allegations are not looking good; these high-energy phenomena produce buckets of high-energy particles, and even if they produce only cosmic rays, the cosmic rays are bound to crash into the surrounding dense gas and photons, creating neutrinos anyway.

Figure 2: Radio image of Phaedra, one of our suspects. The east and west components are the likely radio jets, and the third bright blob is the radio counterpart to the X-ray binary, SXPS J062657.7-082939. [Adapted from Filipović et al. 2025]

Hebe: A Simple Radio Galaxy, or Something More?

Hebe (Figure 3), the nearest extended radio source, isn’t exactly innocent-looking either. She is one of a triplet of galaxies sharing a common envelope, like peas in an extragalactic pod. Galaxies, unlike peas, however, are so massive that they can’t help but interact dynamically in such close quarters, causing a commotion that could totally produce ultra-high-energy neutrinos. She likely also has an active galactic nucleus jet, giving her the same neutrino-wielding powers as Phaedra.

Figure 3: An infrared image of Hebe that clearly shows the common envelope surrounding the triplets. The white contour lines denote levels of polarized intensity, which indicate the presence of a magnetic field. [Adapted from Filipović et al. 2025]

Narcissus: Double Active Galactic Nucleus?

Our final suspect, Narcissus (Figure 4), consists of not one, but two active galactic nuclei. One appears to exhibit the classic synchrotron spectral energy distribution, and the other is likely a blazar, based on his notable radio variability and infrared observations.

Figure 4: Infrared image of Narcissus, with the purple contours outlining the two radio sources that are likely active galactic nuclei. [Adapted from Filipović et al. 2025]

Solving the Mystery

So, who really emitted the ultra-high-energy neutrino? For now, our authors can’t jump to any firm conclusions — they’d never risk condemning an innocent galaxy — but they will continue to closely monitor the suspects and gather more evidence. In the meantime, Phaedra, Hebe, and Narcissus should find themselves a good defense attorney experienced in neutrino emission cases.

Original astrobite edited by Sandy Chiu.

5 August 2025

What’s Going On Inside GJ 486b?

Title: Unveiling the Interior Structure and Thermal Evolution of Super-Earth GJ 486b
Authors: Chandan K. Sahu et al.
First Author’s Institution: National Institute of Science Education and Research, India
Status: Published in ApJ

Planets between the sizes of Neptune and Earth, known as super-Earths, are the most frequently found planets in the galaxy. With no analog in our solar system, these worlds can offer a unique look at planet formation and evolution, and potentially life in the universe. To truly understand a planet’s environment and assess its potential for habitability, we must look at every part of it. This includes not just the atmosphere, but also the interior. Rocky planets, such as the inner solar system planets, typically don’t have enough gravitational pull to retain their primary atmospheres. Thus, only their secondary atmospheres remain. These secondary atmospheres are directly linked to the interior of the planet and are usually formed by volcanic outgassing, material from impacts on the planet’s surface, and other interior processes. By observing a rocky planet’s atmosphere and determining its composition, we can learn something about the interior of the planet as well.

The subject of today’s article, GJ 486b, is a warm super-Earth orbiting an M-dwarf star every 1.5 days. The transmission spectrum of the planet was recently observed by JWST, and when the data were analyzed, a potential water-rich atmosphere was discovered! Today’s authors take these recent discoveries of GJ 486b’s atmosphere and use them to model the interior of the planet.

One Model to Rule Them All

The interiors of rocky planets are not very well known. This is because the best way to understand what’s inside a rocky planet is through seismology. In order to do this, one must put a seismograph on a planet’s surface and measure how seismic waves move through the material inside the planet. Only three bodies have had seismographs landed on them: Earth, the Moon, and Mars. Rocky exoplanet interiors are even harder to understand when the only properties we know are the planet’s mass, radius, and certain aspects of its atmosphere. To understand exoplanet interiors, we have to use modeling to infer properties of the inside of the planet.

The authors of today’s article use the 1-D modeling package SERPINT (“structure and evolution model for rocky planet interiors”) to model GJ 486b’s interior based upon JWST observations of its atmosphere and assumptions made about rocky planets. Their model is broken down into four different parts, as illustrated in Figure 1.

Figure 1: Flowchart of SERPINT and the four different modules used in the overall model of GJ 486b’s interior. Arrows between each property indicate that they’re dependent on one another. [Sahu et al. 2025]

In the structure module, the authors model the layers in the planet. They assume an Earth-like interior structure with an iron core, an inner mantle, and an outer mantle. The crust is not modeled as it’s considered a very complex structure and outside the scope of the article. The thermal module takes into account how the host star’s light changes over time and the radioactive decay of elements in the planet’s interior. The volatile module models the water and oxygen in the planet and how they are exchanged between the interior, surface, and atmosphere. Finally, the escape module looks at how the atmospheric escape of elements such as hydrogen and oxygen affect the atmosphere (and therefore the interior) over time.

Taking all of these factors into account is important when creating a comprehensive picture of a planet. Each module models different processes that influence the planet’s interior, surface, and atmosphere, making sure that we’re taking all of these things into account when trying to understand the planet. It truly is one model to rule them all.

Outgassing and Photolysis and Escape, Oh My!

The authors start the model for the planet and star together and allow them both to evolve for 10 billion years. The planet starts with a mantle temperature of 5000K, which ensures the surface is molten, creating a magma ocean. This is typically how we think all rocky planets, including Earth, started their formation. Figure 2 illustrates how the mantle solidifies over time, influencing the overall structure of GJ 486b. Both layers of the mantle start to cool after the start of the simulation, and the lower mantle cools and solidifies first, followed by the upper mantle. This is due to the different composition in each layer and thus, a different temperature at which they will solidify. Cooling is delayed for a while due to internal heating from radioactive decay of the elements. This can be seen in Figure 2b with the shallow slope of the orange line. The orange line represents the mantle’s equilibrium temperature, and the yellow shaded region is the time in which the mantle has not yet solidified. As the mantle cools, it turns into a thick mush and then eventually into solid rock (as shown in Figure 2d). This indicates the mantle has solidified, and we move from the yellow shaded region on the graphs into the orange shaded region.

evolution of different parameters associated with the modeling the mantle

Figure 2: Evolution of different parameters associated with the modeling of the mantle (outer layer) of GJ 846b. Plot (a) shows how the host star changes its total luminosity (blue) and X-ray and ultraviolet luminosity (red) over time. Plot (b) shows how the mantle equilibrium temperature (orange) evolves towards the planet’s equilibrium temperature (blue). The red dashed line indicates the temperature at which the mantle completely solidifies. Plot (c) shows how the boundary between the mantle and other layers shifts over time as the mantle solidifies. This is shown in green. The magenta line in this plot shows the evolution of the radius at which the mantle is solidified. Plot (d) shows how the mantle viscosity changes over time as the mantle solidifies. The yellow shaded region indicates a time in which the mantle is not solid while the orange shaded region indicates a time after the mantle has solidified. [Sahu et al. 2025]

The authors start the simulation with 10 Earth oceans of water on the planet. This water is initially inside the magma ocean, but as the mantle cools and solidifies, some of the water becomes solid while the rest of it is transferred to the atmosphere in the form of water vapor. Water vapor outgassing from the interior of the planet stops when the mantle becomes solid. At this same time, the luminosity of the host star shrinks (as shown in Figure 2a), causing the atmospheric escape of hydrogen and oxygen to stop and for atmospheric water vapor to stop being broken down by radiation from the host star (known as photolysis). This results in a buildup of oxygen and water over time. This final atmospheric composition matches the transmission spectroscopy data from JWST. This exchange between outgassing, the breaking down of atmospheric water vapor and hydrogen/oxygen escape impacts the planet’s sources of water (shown in Figure 3) and influences the atmosphere we see on GJ 486b today.

Figure 3: Evolution of the sources of water on GJ 486b. The blue line shows the mass of water in the solid part of the mantle. The orange line shows the mass of water in the melting part of the mantle. The green line shows the mass of atmospheric water, and the red line shows the escaping atmospheric water. Each of these sources of water is crucial to understanding the life cycle of water on this planet. [Adapted from Sahu et al. 2025]

Today’s article summarizes the possible interior structure of the super-Earth GJ 486b. While the modeling is able to reproduce the atmospheric data obtained with JWST, there are many limitations to the model. First, it assumes a simplified version of Earth’s interior structure, when in reality, Earth’s structure has a much more complex composition and these minerals are distributed non-uniformly across Earth’s layers. This can result in different temperatures and pressures for each layer, something that was not modeled in this article. It’s challenging to determine the composition of rocky exoplanets, and because super-Earths are much larger than Earth, the pressures inside of the planet will be more extreme, leading to unknown effects on the materials inside. Furthermore, GJ 486b is expected to be tidally locked, presenting a whole new set of assumptions and challenges to modeling its interior. And lastly, even though there is evidence to suggest that GJ 486b could have a water-rich atmosphere, the JWST transmission spectrum could also be attributed to starspots on the host star, making the assumption of a water-rich atmosphere wrong. While this is a good first step, more data and new ways to understand the interiors of exoplanets will be needed to truly understand what is going on inside GJ 486b.

Original astrobite edited by Tori Bonidie.

Artist's impression of a group of black holes in a globular cluster

29 July 2025

The Black Hole Tango: Kicks and Spins in Hierarchical Mergers

Title: Gravitational-Wave Kicks Impact Spins of Black Holes from Hierarchical Mergers
Authors: Angela Borchers, Claire S. Ye, and Maya Fishbach
First Author’s Institution: Max Planck Institute for Gravitational Physics
Status: Published in ApJ

The room is large and densely packed. As you twirl across the floor, someone extends their hand, inviting you to dance. The two of you spin around each other, the tension building with every step. As you draw closer and closer, you realize this isn’t a ballroom, and you’re not people. This is outer space, and you and your partner are black holes, spiraling toward one another in one of the most energetic events in the universe: a black hole merger.

In this cosmic dance, black holes in a binary system orbit each other while also spinning around their own axes. Their spins are inherited from the angular momentum of the massive stars that collapsed to form them. After the two black holes merge, the remnant black hole also spins. The final spin depends on the spins of the two merging black holes and the ratio of the mass of the larger black hole to that of the smaller one. The higher the mass ratio, the more the larger black hole dominates the final spin. To quantify how fast black holes spin, physicists use a dimensionless spin parameter that ranges from 0 (not spinning at all) to 1, the maximum allowed by general relativity.

Observing the distribution of black hole spins helps us understand how binary systems form. For example, binaries formed through isolated channels, where the black holes are born and evolve together, are likely to have similar spin magnitudes, and the directions of their spins tend to align with their orbital motion. In contrast, black holes that form separately and later become a pair through dynamic encounters in dense environments typically have randomly oriented spins (see Figure 1). To determine which formation channels are more common, we need accurate predictions of the expected spin distributions for each scenario to compare them directly to merger observations from detectors like LIGO/Virgo or future gravitational-wave observatories.

infographic showing formation channels for black hole binaries

Figure 1: Graphic showing two main formation channels of black hole binaries. [Reproduced with permission from Shanika Galaudage/Space Australia]

In a dense environment, such as a globular cluster, black holes can undergo multiple consecutive mergers, known as hierarchical mergers. With this in mind, we assign “generations” to black holes: first-generation (1G) black holes have never merged, second-generation (2G) black holes are the product of one previous merger, and so on. After several generations, the spin distribution of black holes has been shown to peak at approximately 0.7, independent of the mass ratios and spins of the first-generation binaries. However, studies have often overlooked an important factor that may impact this distribution: recoil kicks.

After a merger, the newly formed black hole receives a “kick” velocity — a shove that sends it moving through space. This happens because gravitational waves carry away linear momentum asymmetrically during the merger, and to conserve momentum, the remnant black hole recoils in the opposite direction. If its kick exceeds the escape velocity of its cluster, the remnant is ejected, making it unlikely to merge again. However, remnants that receive smaller kicks are retained and can go on to merge again, contributing to the spin distribution over multiple generations. The authors of this article study the final spin distribution of black holes that remain within a cluster after forming from hierarchical mergers.

Simulating Binaries in a Globular Cluster

The authors simulate a first-generation population of one million black holes that are assumed to all have the same initial spin, labeled a_i. The authors use data from globular cluster simulations to determine the properties of binaries found in dense environments. The final spin, a_f, and kick velocity of the black holes are calculated using a high-accuracy model of black hole merger remnants. Only black holes with kick velocities less than the escape velocity of typical globular clusters are kept in the population for future mergers. For their analysis across multiple merger generations, the authors test escape velocities of 50, 100, and 200 km/s.

Some initial binary configurations are more likely to produce high kick velocities that could eject the remnant from the cluster. Figure 2 shows that binaries with isotropic (randomly oriented) spins tend to produce larger kicks than those with spins aligned with the orbital motion.

plot of kick velocity as a function of mass ratio

Figure 2: Kick velocity as a function of mass ratio for three types of black hole binaries: ones with random spin directions (blue), ones with spins aligned with the orbital motion (pink), and ones with zero spin (yellow). Each point shows the average from 10,000 simulated binaries, with spin magnitudes up to 0.8. Solid lines show the average kick velocity; shaded regions show the spread. [Borchers et al. 2025]

Spin Distribution of 1G + 1G Mergers

Figure 3 shows the final spin distribution from mergers between two first-generation black holes for different initial spin values. It compares all merger remnants (global population) to those that stay in the cluster (retained population). As expected, the distribution peaks around 0.7 for the global population. However, the peak shifts to higher spin values when only retained remnants are considered.

plot of spin distribution of black hole merger remnants

Figure 3: Spin distribution of 1G+1G merger remnants with different initial spin magnitudes and randomly oriented spins. The dashed lines show the distribution for all remnants, while the solid lines show the distribution for those that remain in the cluster. For the a_i = 0.0 case, the vertical line is centered at a_f = 0.69. [Borchers et al. 2025]

This happens because, as Figure 2 shows, binaries with aligned spins tend to produce lower kick velocities, making them more likely to stay in the cluster. At the same time, aligned spins lead to remnants with higher final spins, so the retained population is biased toward higher spins.

The authors also examine the effect of escape velocity on the spin distribution. Higher escape velocities make the distribution more similar to the global population, while lower escape velocities lead to broader distributions with more support at higher and lower spins.

Spin Distributions of Higher-Generation Mergers

Figure 4 shows how the spin distribution changes over several generations of black hole mergers. As more generations pass, the spin distribution of the retained black holes starts to look quite different from the global one. In particular, it spreads out and shows a broader range of spin values.

plot of probability distribution of final black hole spins

Figure 4: Final spin distribution for different merger generations. The dashed lines show the distribution for all remnants, while the solid lines show the distribution for those that remain in the cluster. [Adapted from Borchers et al. 2025]

The authors also test different starting spin values and escape velocities. While the global spin distribution tends to settle into a consistent shape after a few generations, the authors find that the retained spin distribution does not. Instead, the spin distribution of black holes that stay in the cluster depends on several factors: the binaries’ initial spins and mass ratios, what generation they belong to, and the escape velocity of the cluster. So, there isn’t a single “universal” spin distribution for retained black holes — it changes depending on the environment and merger history.

Hierarchical mergers are considered a pathway to forming intermediate-mass black holes, potentially explaining events like GW190521. Current gravitational-wave detectors are observing more and more black hole mergers, and future detectors will measure their spins more precisely. Accounting for the spin distributions of black holes that remain in clusters rather than the universal distribution will help us better understand and measure how hierarchical mergers contribute to the gravitational-wave population.

Original astrobite edited by Ryan White.

Authored by Viviana Cáceres.

22 July 2025

A GLIMPSE of the First Galaxies?

Title: A Glimpse of the New Redshift Frontier Through Abell S1063
Authors: Vasily Kokorev et al.
First Author’s Institution: The University of Texas at Austin
Status: Published in ApJL

JWST has opened up a new era of early universe astronomy. Of particular interest is the hunt for the first galaxies, which are thought to have formed just a couple hundred million years after the Big Bang. But finding these early galaxies has proved to be somewhat of a challenge. While there have been many bright galaxies discovered by JWST at a redshift (z) greater than 10, no galaxies at z > 15 have been confirmed to date. Today’s article goes hunting for these very high-redshift galaxies in the GLIMPSE field, which is home to the strong lensing cluster Abell S1063, shown in Figure 1.

Figure 1: JWST image of the field used for this study. [Kokorev et al. 2025]

The process of identifying galaxies at high redshifts is not exactly a simple one. In this article, all the sources in the field are slowly winnowed down through a series of steps with the ultimate goal of finding extremely high-redshift galaxies.

Step 1: Cut Galaxies by Color

Cutting galaxies by color involves finding the Lyman break. High-energy light is absorbed by neutral hydrogen in the galaxy and in the space between the galaxy and us. Thus, when you look at a galaxy’s spectral energy distribution, there is a drop-off at short wavelengths called a “break.” For distant galaxies, the location of this drop-off gets redshifted, allowing for an estimation of a galaxy’s distance. By comparing how bright sources are between two filters that are adjacent in wavelength, we can look for steep drop-offs that are caused by the Lyman break. Figure 2 shows a color–color plot (showing the difference in brightness between different filters), where the dashed line shows the region galaxies must be in to be considered Lyman-break galaxies at these high redshifts. In the whole field, the authors detect 38 objects in this region.

Figure 2: Color–color plot showing the selection criteria for high-redshift galaxies due to Lyman break. The upper-left region enclosed by the black dashed line represents the region that satisfies the high-redshift criteria. The various lines show properties of different populations. Solid are potential tracks for starburst galaxies at z = 16, dashed are lower-redshift quiescent galaxies, and dotted are cool stars and brown dwarfs, which can look like high-redshift galaxies in some situations. The two red diamonds show the final two high-redshift candidates in this work. [Kokorev et al. 2025]

Step 2: Calculate Redshift

However, this isn’t the end of the story. Color isn’t the only way to estimate distances to galaxies, as we can also use photometric fitting codes. These codes compare observations to templates of what we would expect galaxies to look like at different redshifts and see which templates fit best. This allows for an estimation of the redshift of each source identified in the field. All sources that the photometric fitting codes thought were at z > 16 were cross-referenced against the 38 color-identified objects, which narrowed the list to eight high-redshift candidates.

Step 3: Hunt for Imposters

But wait, there’s more! Both of these methods (looking at colors and calculating redshift photometrically) are susceptible to false positives. Lower-redshift galaxies, particularly star-forming dusty galaxies, can look just like high-redshift sources. This is apparent in Figure 2, where z ~ 4.5–5.5 dusty galaxies can also be found in the color-selected Lyman-break region. Thus, we need to explore the possibility that the sources might not be at high redshift at all. In this article, the authors do this by fitting their eight sources with a lower-redshift dusty starburst template, finding only five galaxies that match the high-redshift template better than the starburst template.

Step 4: Keep Significant Detections

The authors make one final cut to the last five sources, only keeping those that aren’t at the edges of the observation and thus have strong signals, where they can be confident we’re actually detecting something and it’s not just a hot pixel or noise. So, after all of that, they are left with only two z > 16 candidates in this field.

Figure 3 shows the images and spectral energy distributions of each of these candidates, given the wonderfully poetic names of 70467 and 72839. While there is still a lot to learn about these candidates, with the JWST data presented in this article we can uncover some things about the galaxy properties. Both sources appear to be compact, but still resolved, with effective radii of roughly 650 light-years. These observations can also tell us about the ultraviolet brightness of the two candidates, finding that they are most likely dominated by extended stellar emission, as opposed to active galactic nucleus activity. In general, these galaxy candidates are not very ultraviolet-bright, which is somewhat surprising, as JWST has found many bright galaxies at z ~ 12–14. However, it is not out of the question that galaxies at this slightly earlier epoch are just fainter than we expected, and then formed stars and increased in brightness relatively quickly.

Cutouts and spectral energy distributions for two high-redshift galaxy candidates

Figure 3: Cutouts and spectral energy distributions for the two high-redshift candidates in this work. Squares show images in nine JWST filters, as well as an image of several filters combined, with a cutout width of 1.5″. Spectral energy distribution plots show different template galaxies for high-redshift (orange and red) and low-redshift dusty star-forming galaxy interlopers (blue and green). You can see how these sources are only strongly detected in a few filters, with many detections being just upper limits. [Kokorev et al. 2025]

Overall, these results mark the finding of potentially some of the first galaxies in the universe. But for now, they remain candidates. The photometric data have served their purpose, allowing us to find these candidates and get some information about their properties, but now we need targeted spectroscopic observations to get a more accurate determination of their redshift.

Original astrobite edited by Bill Smith.

18 July 2025

Written in the Gas: Jet Feedback in a Voorwerp Galaxy

Title: Jet-Mode Feedback in NGC 5972: Insights from Resolved MUSE, GMRT, and VLA Observations
Authors: Arshi Ali et al.
First Author’s Institution: Savitribai Phule Pune University
Status: Published in ApJ

Voorwerp Galaxies as Laboratories for Active Galactic Nuclei

Supermassive black holes are believed to reside at the centers of nearly all massive galaxies. As supermassive black holes accrete matter, they can release enormous amounts of energy in the form of radiation, winds, and relativistic jets — powering active galactic nuclei and influencing their host galaxies. This process, known as active galactic nucleus feedback, can regulate star formation and the properties of the gas in the interstellar medium, making it a key mechanism for shaping how galaxies form and evolve over cosmic time.

Of the many diverse manifestations of active galactic nuclei, radio “loud” galaxies stand out as powerful laboratories for studying feedback in action. Not only do these galaxies have characteristic [O III] emission lines, but they also host intense radio-emitting lobes of gas that extend well beyond the structure of the galaxy, indicative of jets. (Today’s article refers to these lobes as the extended emission-line region or EELR.)

Originally discovered by a schoolteacher, Voorwerp galaxies are a special category of radio galaxies that are candidate sites of active galactic nucleus activity in terms of their emission-line signatures. Moreover, they exhibit intriguing clouds of ionized gas that may have originated from jets. Figure 1 shows some examples of Voorwerp galaxies. Exploring these galaxies can shed light on how jets interact with the interstellar medium, a major player in active galactic nucleus feedback.

Figure 1: Voorwerp galaxies are compelling laboratories for exploring the interaction between jets and the interstellar medium. Examples are Hanny’s Voorwerp (left), NGC 5972 (middle), and the Teacup Quasar (right). [NASA, ESA, W. Keel (University of Alabama), and the Galaxy Zoo Team; NASA, ESA, and W. Keel (University of Alabama, Tuscaloosa); NASA, ESA, and W. Keel (University of Alabama, Tuscaloosa)]

Today’s article focuses on the physical and kinematic properties of one particular Voorwerp galaxy, NGC 5972. This galaxy is interesting for its bizarre, helical structure of ionized gas, as well as hints of a rich history of active galactic nucleus activity. Could jet feedback have played a significant role in the evolution of this galaxy?

Evidence for Relativistic Jets in NGC 5972

To study the interstellar medium of the Voorwerp galaxy, the authors combined observations from multiple instruments that probe different phases of the galaxy’s gas. Specifically, they used data from the Very Large Telescope (VLT), the Giant Metrewave Radio Telescope (GMRT), and the Very Large Array (VLA) to create maps of the ionized gas emission, allowing them to trace the structures and movement of the gas and jet material. Together, these datasets offer a comprehensive, multi-wavelength view of how the radio jet shaped the surrounding interstellar medium. The maps of NGC 5972, shown in Figure 2, display an impressive spiraling structure that could be the result of prolonged active galactic nucleus activity.

Figure 2: The radio emission of NGC 5972 reveals the extended helical, spiral structure of the ionized gas, where the inner jets are connected to the outer lobes (top: VLA, bottom: GMRT). This suggests that the intriguing shape of NGC 5972 could have originated from active galactic nucleus jets. [Ali et al. 2025]

When the authors examined the extended emission-line region, they found that the velocity profiles (shown in Figure 3) are closely aligned with the direction of where we would expect to find a radio jet. The enhanced velocities around the jet region likely represent gas that is outflowing due to feedback. The expected amount of energy from a radio jet would have been capable of driving outflows at these velocities.

velocity profiles of the OIII emission lines

Figure 3: The velocity profiles of NGC 5972’s [O III] emission lines, a characteristic signature of the active galactic nucleus’s extended emission-line region. The velocities are aligned and enhanced along the jet axis, indicating that active galactic nucleus feedback influenced the gas by powering jets and outflows. [Adapted from Ali et al. 2025]

While the structure and movement of the ionized gas give promising hints of jet feedback, astronomers use a tool called Baldwin–Phillips–Terlevich (BPT) analysis to figure out what exactly is ionizing the gas. By comparing the ratio of different emission lines, we can categorize whether the galaxy’s gas was primarily ionized by radiation from an active galactic nucleus, stars, or other processes. The results of the BPT analysis are shown in the left panel of Figure 4.

Figure 4: The left panel shows the ionizing sources of the gas in NGC 5972. While the gas along the jet axis is ionized by radiation from the active galactic nucleus (orange in the diagram), the regions transverse to the jet are ionized by shock waves (blue). Moreover, the gas in the jet regions is more turbulent, as shown in the right panel. [Adapted from Ali et al. 2025]

In the case of NGC 5972, the radio jet is mostly ionized by radiation from the active galactic nucleus! However, the gas perpendicular to the jet shows a different signature — one that matches shock waves rather than strong radiation. In addition, the gas in the shock region is more turbulent (right panel of Figure 4). This suggests that the radio jet is not just a byproduct of the active galactic nucleus — it actively disturbs and heats the surrounding gas by way of shocks. This dual process of jet-induced shocks creates a richer picture of how supermassive black holes interact with the interstellar medium.

Putting the Story of NGC 5972 Together

These findings suggest that jet-driven feedback plays a crucial role in shaping the extended emission-line region of NGC 5972 (see Figure 5 for a summary). The interaction between the radio jet and the surrounding gas not only sustains ionization but also influences the gas structure. To further unravel the jet’s impact, future high-resolution radio observations will be essential, providing deeper insights into how active galactic nucleus–driven jets function.

Figure 5: Cartoon schematic diagram showing various mechanisms at play in NGC 5972, including the active galactic nucleus jets, shocks, and outflows. [Ali et al. 2025]

NGC 5972 is more than just a bizarre-looking galaxy — it’s a time capsule, preserving the memory of a powerful supermassive black hole and revealing how its energy continues to shape the galaxy.

Original astrobite edited by Alexandra Masegian.

8 July 2025

You Shall Not Pass! Active Galactic Nucleus Jets Through the Interstellar Medium

Title: You Shall Not Pass! The Propagation of Low-/Moderate-Powered Jets Through a Turbulent Interstellar Medium
Authors: Olga Borodina et al.
First Author’s Institution: Center for Astrophysics | Harvard & Smithsonian
Status: Published in ApJ

The central black holes of some galaxies are surrounded by accretion disks of hot infalling gas. In cases when the accretion disk is massive and highly energetic, this central system forms an active galactic nucleus that outshines the rest of the galaxy. Active galactic nucleus feedback is the process of energetic interactions between the central supermassive black hole and its surroundings. Active galactic nuclei are essential for galaxy quenching, and correctly modeling their contribution is important for accurate cosmological simulations.

Active galactic nuclei release the energy from their accretion via winds, which are widespread outflows, and jets. Jets are collimated, fast outflows of baryonic matter that can spread out from hundreds to tens of thousands of parsecs from their origin. There’s a big range in power for jets, ranging from a low end of around 10³⁸ erg/s to a high end of 10⁴⁶ erg/s. For context, our Sun emits at about 10³³ erg/s.

As jets pass through their host galaxy, they interact with the gas and dust of the interstellar medium. There isn’t a lot of observational evidence for what goes on in that interaction, as the scale of resolution needed is so small — on the order of parsecs within the galaxy. Cosmological simulations don’t model scales small enough to accurately simulate these interactions, and they instead rely on feedback models that implement the macroscale effects. This means we could be missing part of the picture. Today’s article considers galaxy-scale numeric models for jets passing through a turbulent interstellar medium to study how the jet is affected by the gas and dust.

To do this, the authors used the Arepo code, which has an adaptive mesh setup that improves the resolution in areas that need it. They set up a 2-kiloparsec-cubed box full of gas with properties based on real galaxies’ interstellar media. This study is unique compared to other works because the authors use a filamentary structure for the interstellar medium, while previous studies used a clumpy structure. This filamentary structure reflects our actual Milky Way’s interstellar medium and creates more low-density cavities for the jet to interact with. The authors studied three jet powers on the low to intermediate end, of 10³⁸, 10⁴⁰, and 10⁴³ erg/s. This is also unique to the study, as many works look at higher-powered jets, which produce large-scale radio galaxies, despite lower powers being more common. The jets were launched along the x-axis and had a tracer set up to track their positions through time.

propagation of jets into the interstellar medium

Figure 1: From top to bottom, the high to low power jets, which are traced in color against the background interstellar medium density in grayscale. The lower power jets are stalled by the interstellar medium and do not make it as far out in the simulation space. [Adapted from Borodina et al. 2025]

This study found three different outcomes for the three jet powers (see Figure 1), and these general conclusions held up even when the authors changed the initial randomized structure of the interstellar medium.

The highest-power jets, 10⁴³ erg/s, were easily able to pass through the turbulent environment and reach the outer boundary of the simulation. They mostly traveled along the axis that the jet was launched along, with some lateral expansion.

The intermediate-power jets, 10⁴⁰ erg/s, were highly disrupted and redirected from the axis they were launched along. The jets bent, filled the low-density cavities between the filaments, and produced bubble-like shapes. These jets did make it all the way to the simulation boundary, but at later times and not along the axis they were initialized on. These intermediate-power jets don’t have enough ram pressure — the pressure against the gas due to the jet’s motion — to push through the turbulent structure of the interstellar medium, and they have to fill pre-existing cavities rather than plowing their own. The authors developed an analytic model for the minimum energy needed for jets to be able to plow through the interstellar medium, at approximately 10⁴¹ erg/s.

The jets in the lowest-power case, 10³⁸ erg/s, were stalled out by the interstellar medium within the first kiloparsec of travel. They couldn’t penetrate the higher-density regions of the interstellar medium the way the 10⁴⁰ erg/s case could, and they couldn’t make it to lower-density cavities to expand into.

All three of these cases were slower and shorter than the same power of jets run in a non-turbulent simulation, with the most significant disruption in the lowest-powered jets. This work demonstrates that a turbulent interstellar medium–jet interaction can be the sole cause of observed asymmetric and bent active galactic nucleus jets. By improving our understanding of this small-scale interaction, the authors are contributing to making better active galactic nucleus feedback models in cosmological simulations.

Original astrobite edited by Ansh Gupta.

Artist's depiction of two black holes nearing a merger.

1 July 2025

Giving Justice to Intermediate-Mass Black Hole Mergers

Title: Intermediate Mass Ratio Inspirals in Milky Way Galaxies
Authors: Jillian Bellovary et al.
First Author’s Institution: Queensborough Community College
Status: Published in ApJ

Galaxies like our Milky Way are not just serene places hosting stars, gas, and dark matter. They also undergo a host of violent activities. Evidence strongly suggests that our Milky Way interacts with nearby dwarf galaxies, pulling them in entirely or tearing them apart through tidal forces. Dwarf galaxies, though small, often host black holes with masses between 10³ and 10⁵ solar masses (intermediate-mass black holes). When these dwarfs fall into the Milky Way’s gravitational pull, tidal forces strip them of stars and gas, leaving their black holes to roam the galaxy halo. Some of these “wandering” black holes spiral toward the central supermassive black hole. This leads to an event called inspiral, where black holes slowly spiral together before merging.

Mergers between an intermediate-mass black hole and a supermassive black hole are called intermediate mass ratio inspirals or IMRIs. They represent an intermediate case between major mergers involving equal-mass black holes and extreme mass ratio inspirals (EMRIs), where a stellar-mass black hole merges with a supermassive black hole. IMRIs are expected to generate gravitational waves that are detectable by the next-generation space-based gravitational wave detector, the Laser Interferometer Space Antenna (LISA). However, their waveforms remain challenging to model as the origins and development of these IMRIs are not fully understood.

Tracking IMRIs with Simulations

In today’s article, the authors use high-resolution simulations to study how IMRIs form and evolve, offering critical insights for the upcoming LISA mission. They used the DC Justice League simulation suite (the individual simulations are all named after women who have served on the US Supreme Court: Sandra, Ruth, Sonia, and Elena!) to model four Milky Way–like galaxies at high resolution to trace the origins of IMRIs. These simulations track the formation and evolution of massive black holes in dwarf galaxies that eventually merge into larger galactic systems.

What Did the Simulations Reveal?

The study uncovered key features of IMRIs in Milky Way–like galaxies:

- Prevalence: About half of all massive black hole mergers were IMRIs, emphasizing their importance in galaxy evolution.
- Timing of mergers: Most IMRI events occur early in the universe, approximately 3 billion years after the Big Bang, when galaxy mergers and interactions were more frequent (Figure 1).
- Inspiral timescales: The duration of the inspiral process depends heavily on the compactness of the dwarf galaxy. Dense, compact dwarfs have faster inspiral times, whereas diffuse dwarfs slow down the process. A more compact galaxy can plunge deeper into the galaxy before being disrupted, resulting in a massive black hole that is closer to the center and will inspiral more quickly (Figure 2).
- Orbital evolution: While some IMRIs become more circular over time, others maintain eccentric orbits until the merger.

Figure 1: The yellow stars represent the merger times for IMRIs, while the purple circles show other types of black hole mergers, such as EMRIs and major mergers. Most IMRIs occurred in the early universe when black holes were smaller and mergers between galaxies were more frequent. [Bellovary et al. 2025]

dwarf galaxy compactness versus inspiral time

Figure 2: The correlation between the compactness of the dwarf galaxy (y-axis) and the inspiral time (x-axis). More compact dwarf galaxies (indicated by larger values on the y-axis) have shorter inspiral times. Here, the IMRIs are indicated by the yellow stars. [Bellovary et al. 2025]

Why Does This Matter?

IMRIs offer a unique opportunity to study black hole demographics and galaxy assembly. The mass ratios and orbital eccentricities of these events are sensitive to the early conditions of black hole formation and the dynamics of galaxy mergers. However, IMRIs present challenges for gravitational wave detection. Unlike major mergers or EMRIs, their waveforms cannot be easily modeled using existing methods. The authors stress the need for a hybrid approach that combines post-Newtonian (commonly used for major mergers) and perturbative techniques (widely used for EMRIs) to simulate IMRI signals effectively.

The study also highlights the importance of preparing for LISA, slated to launch in the 2030s. As a space-based gravitational wave observatory, LISA will be sensitive to low-frequency waves produced by IMRIs at unprecedented distances. Accurate waveform models are essential for detecting these signals and extracting their astrophysical information. LISA’s detections of IMRIs could constrain the masses of black holes in dwarf galaxies, shed light on supermassive black hole seed formation mechanisms, and enhance our understanding of galaxy evolution across cosmic time.

Looking Ahead

While the authors of today’s article provide a detailed glimpse into the dynamics of IMRIs, they also highlight limitations. The small sample size and simplified black hole merger models call for broader studies using more comprehensive simulations. Future work must refine the physics of inspirals and develop robust waveform libraries to maximize LISA’s scientific return.

IMRIs are not just a niche class of black hole mergers; they are a treasure trove of information about the cosmic past. With LISA on the horizon, our ability to unlock these secrets is closer than ever!

Original astrobite edited by Tori Bonidie.

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