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composite image of cygnus a

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

Title: Evidence for Strong Intracluster Magnetic Fields in the Early Universe
Authors: J. Xu and J. L. Han
First Author’s Institution: Chinese Academy of Sciences and University of Chinese Academy of Sciences
Status: Published in ApJ

Mysteries of the Magnetic Macrocosm

Magnetic fields are everywhere, from the vast, pristine emptiness of cosmic voids to the dense, galaxy-packed environments of massive clusters. Wherever we find plasma — the hot, ionized fluid making up 99.9% of the universe’s visible matter — we find magnetic fields shaping and stirring said plasma. Needless to say, magnetic fields have their fingers (or, rather, their field lines) in many pies. Yet, for a wide range of astrophysical situations, the question still remains: where did these fields come from?

Galaxy clusters — associations of hundreds to thousands of galaxies held together by the glue of gravity — are no exception to this magnetic mystery. Today’s article seeks to understand the origin of magnetic fields in the intracluster medium, the ultra-hot plasma permeating the space between cluster-bound galaxies. At surface level, knowledge of the magnetic fields in the intracluster medium is necessary for understanding the rich spectrum of radiation emitted by galaxy clusters (see, for instance, these three astrobites). On a grander scale, however, these clusters — as the largest gravitationally bound bodies in the universe — can also provide key insight into the history of our cosmos. By tracing the growth of intracluster fields back through cosmic time, we can probe how magnetic fields influenced the formation of structure in the infant universe and catch a glimpse of the earliest magnetic fields in existence. This is precisely what today’s authors set out to do.

Faraday Forecasts of Faraway Fields

So, how does one study magnetic fields that are millions to billions of light-years from Earth? Today’s authors leverage the power of Faraday rotation: when a polarized light wave passes through a magnetic field, the wave is rotated through an angle that depends on the strength of the field (as illustrated in this cartoon). Therefore, by observing the change in the polarization angle of incoming light and calculating the so-called rotation measure, one can deduce the strength of the magnetic field along the light’s path. This technique is invaluable in radio astronomy and has been used extensively to study the magnetic backdrop of our universe.

If we’re going to be using Faraday rotation to explore intracluster magnetic fields, all we need now is some radiating object to shine light through a galaxy cluster and into our telescopes. There’s one slight complication, though: the incoming light is sensitive to magnetic fields along its entire path of propagation — if we’re looking at light from a distant galaxy cluster, the wave will be rotated not only by the intracluster fields, but also by intergalactic fields between the cluster and the Milky Way and by galactic fields within the Milky Way. How, then, do we isolate the rotation due solely to intracluster fields?

image of radio galaxy hercules A

Figure 1: Composite photo of the radio galaxy Hercules A and its two prominent radio lobes from the Hubble Space Telescope and the Very Large Array. [NASA, ESA, S. Baum and C. O’Dea (RIT), R. Perley and W. Cotton (NRAO/AUI/NSF), and the Hubble Heritage Team (STScI/AURA)]

The authors ingeniously sidestep this issue by looking at close by pairs of light sources; by looking at the difference in Faraday rotation between two light sources embedded in the intracluster medium, we probe only the intracluster fields separating the two sources — the intergalactic and galactic contributions cancel out! Serendipitously, the universe has provided us with an abundance of double light sources in the form of radio galaxies, whose bright pairs of lobes naturally arise as material ejected from these galaxies interacts with the surrounding intracluster medium (see Figure 1). Figure 2 illustrates, schematically, the authors’ strategy to probe intracluster magnetic fields via the rotation measures of radio lobes.

cartoon of a person looking through the intergalactic medium toward a double-lobed radio source

Figure 2: Schematic diagram showing the observation of a radio galaxy embedded in the intracluster medium. Light emitted from the two radio lobes (labeled RM1 and RM2 to indicate their different rotation measures) passes through the magnetic fields of the intracluster medium, the intergalactic medium, and the Milky Way before reaching the observer (far left). [Xu & Han 2022]

Baffling B-fields from Bygone Bodies

Since the authors are interested in the evolution of intracluster fields across the lifetime of the universe, they comb through archived radio telescope data from both the NRAO VLA Sky Survey (NVSS) and from recent literature to obtain rotation measures for double-lobed radio galaxies across a wide range of redshifts (in this context, redshift just tells us how far into the past we’re looking). When compiling their data set of lobe pairs, the authors make careful cuts based on the distances between the lobes and the locations of the lobes relative to the Milky Way so as to minimize rotation measure contamination from intergalactic and galactic fields — when we take the difference between the rotation measures of a given pair of lobes, we want this difference to reflect only the contribution from intracluster fields.

plots of rotation measure difference as a function of redshift

Figure 3: Plots of the pairwise rotation measure (RM) differences (top two rows) and the statistical dispersion in these differences (bottom two rows, showing two different ways of quantifying the dispersion) vs. redshift for the radio lobe data set analyzed by the authors. Blue points represent pairs of lobes from the NVSS catalog, while red points represent pairs compiled from the literature. The right column shows a subset of the data with RM measurement uncertainties below a certain threshold. Click to enlarge. [Xu & Han 2022]

Ultimately, the authors select 387 pairs of lobes from NVSS and 197 pairs from the literature, with redshifts as high as 3 (meaning that the light we’re seeing from the farthest lobe is almost 11.5 billion years old). Plotting the pairwise rotation measure differences (and the statistical dispersion in these differences) yields Figure 3. To high confidence, the authors conclude that the rotation measure differences in higher-redshift clusters are statistically higher than those in lower-redshift clusters, thus implying that intracluster fields were stronger in the past.

The authors go a step further and use these rotation measures to estimate the typical intracluster field strength for clusters that existed more than seven billion years ago (roughly half the age of the universe) — but this only leads to more confusion: there was too little time between the beginning of the universe and the formation of these clusters for their strong magnetic fields to have grown via typical channels like dynamos. Thus, the authors conclude that strong magnetic fields must have existed in the early universe, prior to the formation of these clusters. While intracluster fields will provide useful constraints on the growth of magnetic fields in the early universe, the ultimate origin of these fields continues to elude us.

And thus, the universe’s grand magnetic mystery lives on.

Original astrobite edited by Catherine Manea.

About the author, Ryan Golant:

I am a second-year astronomy Ph.D. student at Columbia University. My current research involves the use of particle-in-cell simulations to study magnetic field growth in gamma-ray burst afterglows and closely related plasma systems. I completed my undergraduate at Princeton University, and I’m originally from Northern Virginia. Outside of astronomy, I enjoy learning about art history, playing violin and video games, and watching cat videos on the internet.

hubble image of spiral galaxy UGC 2885

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: Detection of a Superluminous Spiral Galaxy in the Heart of a Massive Galaxy Cluster
Authors: Ákos Bogdán et al.
First Author’s Institution: Center for Astrophysics | Harvard & Smithsonian
Status: Accepted to ApJ

Galaxy clusters contain hundreds of galaxies in a huge variety of shapes and sizes, ranging from irregular dwarf galaxies to giant ellipticals. The most luminous member of a cluster is known as the brightest cluster galaxy. Each brightest cluster galaxy is different, but there are some properties that they tend to have in common — most brightest cluster galaxies are found at the very centre of their host cluster and are large, elliptical galaxies, containing little gas and forming very few new stars.

The reason why most brightest cluster galaxies look so similar is well understood, as it is thought that these large galaxies form via a series of galaxy mergers. These are violent cosmic events that slowly increase the size of the galaxy, whilst also destroying any delicate disk or spiral arms that the galaxy may have (click here to see a simulation of two merging spiral galaxies). Additionally, mergers can lead to gas being expelled from a galaxy, resulting in the gas-poor, quenched brightest cluster galaxies that we see today.

However, today’s article presents an exciting twist to this story by presenting data from three galaxy clusters that do not appear to follow this trend, including one brightest cluster galaxy that doesn’t fit with our current theories at all.

Suspicious Spirals

The authors begin by introducing seven superluminous spiral galaxies, a recently discovered class of huge galaxies with spiral or lenticular shapes. The great size of these galaxies is what motivates the main question of today’s article: could these superluminous spiral galaxies actually be brightest cluster galaxies, despite not looking like them?

To answer this, we can look at the amount of X-ray radiation surrounding these galaxies. X-rays are emitted by the intracluster medium, a vast cloud of incredibly hot gas that fills a cluster, occupying the space between galaxies: if a cluster was a tasty chocolate chip muffin, the intracluster medium would be the cake, filled with chocolate chip galaxies. Using the X-ray telescope XMM-Newton, the authors found no X-ray emission surrounding two of their galaxies. However, as Figure 1 shows, the remaining five have large amounts of X-rays being produced nearby. This indicates the presence of the intracluster medium, meaning that these galaxies are nearby to a galaxy cluster.

x-ray observations of seven superluminous disk galaxies

Figure 1: X-ray observations of the region surrounding each of the seven superluminous disk galaxies. Regions of stronger X-ray emission are represented by lighter colour, and the centre of each X-ray region (i.e., the cluster centre) is marked by a green cross. The position of each superluminous disk galaxy is shown by the green circle. Note that the two galaxies in the bottom right (J11380 and J09354) have no associated clusters, and that the top-left galaxy (J16273) is located at the centre of its cluster. [Adapted from Bogdán et al. 2022]

It’s unusual to find spiral galaxies inside of clusters, but not unheard of. However, what makes this work so exciting is that in three of these clusters, there is not a single other galaxy that is brighter than the superluminous spiral — in other words, they are the brightest cluster galaxy. Finally, one of these galaxies (J16273 in Figure 1) is not only the brightest galaxy in the cluster, but is found directly in the cluster centre, in exactly the position that we would usually expect to find a brightest cluster galaxy!

Galaxy Mergers, but Not as You Know Them

The fact that J16273 is the brightest galaxy in a cluster and lives right in the cluster centre makes it look like a fairly typical brightest cluster galaxy. However, brightest cluster galaxies are elliptical because of the large numbers of galaxy mergers that they experience. How can we explain why this one is so different from all of those that we’ve seen before?

Surprisingly, one explanation is mergers themselves. The authors suggest that J16273 was previously a regular, elliptical brightest cluster galaxy that recently merged with a smaller gas-rich galaxy. Under the right conditions, this merger could spin up the elliptical galaxy, with the remnants of the gas-rich galaxy forming a brand new spinning disk.

In order to really understand these giant spiral galaxies, future work will need to look at many more than just seven of them. The authors acknowledge this and suggest that eROSITA, an ongoing X-ray survey of the sky, will be able to look at many more of these galaxies and determine whether they live in clusters, groups, or alone. eROSITA is due to release its first data at the end of 2022 and should help us to solve the mystery of how these huge spirals ended up in places we never expected to find them.

Original astrobite edited by Katy Proctor.

About the author, Roan Haggar:

I’m a PhD student at the University of Nottingham, working with hydrodynamical simulations of galaxy clusters to study the evolution of infalling galaxies. I also co-manage a portable planetarium that we take round to schools in the local area. My more terrestrial hobbies include rock climbing and going to music venues that I’ve not been to before.

composite X-ray and optical image of the galaxy Messier 51

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: COSMOS2020: Ubiquitous AGN Activity of Massive Quiescent Galaxies at 0 < z < 5 Revealed by X-ray and Radio Stacking
Authors: Kei Ito et al.
First Author’s Institution: The Graduate University for Advanced Studies and National Astronomical Observatory of Japan
Status: Accepted to ApJ

While most passive or “dead” galaxies we see today have had fairly passive lives, distant passive galaxies in the early universe may have had a more active path to passivity. Detailed studies of nearby quiescent galaxies have revealed they follow a simple evolutionary track: a burst of star formation early on in their life followed by a quiet existence with low rates of star formation. In contrast, recent discoveries have uncovered a new population of quiescent galaxies that get quenched faster and earlier on than should be possible if following this simple evolutionary track (for example, the distant quiescent galaxies covered in this astrobite and that astrobite). The existence of so many quiescent galaxies so early on in the universe is a problem for galaxy evolution models, and the intense starburst phase and rapid suppression of star formation has been difficult to reproduce with cosmological simulations.

A big unresolved question related to this problem is how the burst of star formation gets suddenly shut off or quenched in these galaxies. Are the streams of gas from the cosmic web that fuel star formation getting cut off? Or is the gas flowing in and being expelled by some feedback mechanism? One such possible feedback mechanism is triggered by the galaxy’s central supermassive black hole as it funnels in material and creates a disk of hot, luminous gas and dust around it, forming an active galactic nucleus (AGN). The AGN devours some of the gas and radiation, wind, and jets eject the rest.

In today’s article, the authors leverage the extensive multiwavelength COSMOS2020 catalog to explore the AGN activity in quiescent galaxies across cosmic time through two primary AGN signatures: X-ray and radio emissions. However, many of these galaxies and the possible AGN within them, especially those farthest away, are faint enough that they are not individually detected in X-ray and radio surveys. To both overcome this faintness and to focus on typical (rather than extremely bright) sources, the authors use a technique called stacking to characterize the average properties of a quiescent galaxy sample and a comparison star-forming galaxy sample. Beyond comparing the stacks of quiescent galaxies and star-forming galaxies, the authors create a grid of stacks spanning stellar mass (basically, how big the galaxy is) and redshift (how far away and therefore how early on in the universe the galaxy is) to investigate trends along these axes.

Galaxy Pancakes 

To better understand the stacking technique and the grid of stacks, imagine each galaxy is a pancake. Some pancakes are regular (quiescent) and the ones that have a little more going on are buttermilk (star-forming). Now let’s say all of the pancakes have berries in them, but eating a single pancake won’t get you a full serving of fruit. So, to portion out a daily fruit intake you make stacks of pancakes on each plate, separating out regular and buttermilk.

Besides the regular and buttermilk types of pancakes, let’s say they also come in different sizes, from silver dollar to the size of the plate — this represents the stellar mass axis. And of course, the pancakes weren’t made simultaneously: the stacks of pancakes made earlier are farther down the table from where you’re sitting, and the newer ones are right in front of you, similar to how more distant (i.e., higher redshift) galaxies represent conditions earlier in the universe than nearby galaxies.

To build their grid of galaxy pancake stacks, the authors used observations at wavelengths at which the galaxies were detected individually (optical and infrared) and redshifts from the COSMOS2020 catalog to decide which galaxies were star-forming versus quiescent as well as how massive each was. The authors then used observations at wavelengths at which the galaxies were not individually detected (X-ray and radio) to place stacked observations in a grid of stellar mass and redshift. The resulting sample is the largest, highest-redshift sample of typical quiescent galaxies created so far.

Taking an X-ray

plot of image stacks showing brighter and fainter detections in a grid of redshift, stellar mass, and quiescent versus star-forming galaxies

Figure 1: The grid of galaxy stacks showing the average X-ray detection for two different X-ray bands. The red images show the regular pancake quiescent galaxies and the blue images show the buttermilk pancake star-forming galaxies, with redshift (z) increasing from top to bottom and stellar mass increasing from left to right in each color bin. Click to enlarge. [Ito et al. 2022]

The first stacking analysis the authors conducted was with X-ray data, with some representative stacks shown in Figure 1.

Beyond identification of some general trends, physically interpreting these stacks requires understanding what is causing the X-ray emission. X-ray emission comes from two main sources in galaxies: X-ray binaries, which contain a dense stellar remnant energetically drawing material from a star in its orbit, and AGN. Returning to our analogy, the fruit content in the pancakes could come from whole berries scattered around the pancake (X-ray binaries) or from a berry jam filling in the center (AGN).

But if you only know the average amount of fruit in each pancake stack, how can you tell if it’s in the form of whole berries or a jam filling? Based on known relations between the star formation rate and stellar mass in a galaxy and the amount of X-ray binaries expected, the authors determined the relative contribution from X-ray binaries and AGN. With these models, they found that X-ray binaries could explain most of the X-ray emission for the star-forming galaxy stacks. On the other hand, for quiescent galaxies, the average X-ray emission in each stack was 5–50 times higher than expected from just X-ray binaries, implying that much of the X-ray emission came from AGN. Additionally, they found the biggest difference between the star-forming and quiescent samples in the highest redshift bin, providing hints that AGN may have a role in quenching star formation early in the universe.

Tuning In to the Radio

To further verify their findings, the authors then stacked data from the other major signature of AGN: radio emission. Similar to X-rays, radio emission comes from two main sources in galaxies: one related to ongoing star formation and one related to AGN. Taking an empirically known correlation between star formation rate and radio luminosity, the authors determined that the quiescent galaxy stacks have 3–10 times higher radio emission than expected from just star formation, while the star-forming galaxy stacks could be explained primarily by star formation. Consistent with the X-ray result, this suggests that faint AGN are ubiquitous in quiescent galaxies.

How to Quench a Pancake

How does this AGN feedback mechanism work to quench galaxies? In nearby galaxies, we know that quenching tends to occur with more active AGN. This is due to two processes: quasar-mode feedback and radio-mode feedback. In quasar-mode feedback, wind from a bright AGN expels gas from the galaxy and suppresses star formation. In radio-mode feedback, a typically fainter AGN heats the gas in and around the galaxy with radio jets, which prevents gas from cooling and forming stars. In this way, radio-mode feedback maintains quiescence rather than just reducing the possible star formation by tossing out fuel. The authors note their faint, typical sample is probably mostly undergoing radio-mode feedback, with some non-AGN environmental quenching coming into play at lower redshifts.

So what do these stacks tell us about galaxy evolution? The ubiquitous AGN signatures in both X-ray and radio give us an interesting clue about quenching: everyday quiescent galaxy pancakes are often filled with AGN berry jam, and feedback from faint AGN within them are likely the culprit for shutting off star-forming buttermilk berry galaxy pancakes so suddenly and early in the universe.

Original astrobite edited by Alice Curtin.

About the author, Olivia Cooper:

I’m a second-year grad student at UT Austin studying the obscured early universe, specifically the formation and evolution of dusty star-forming galaxies. In undergrad at Smith College, I studied astrophysics and climate change communication. Besides doing science with pretty pictures of distant galaxies, I also like driving to the middle of nowhere to take pretty pictures of our own galaxy!

photograph of a globular cluster

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: Detection of a 100,000 M black hole in M31’s Most Massive Globular Cluster: A Tidally Stripped Nucleus
Authors: Renuka Pechetti et al.
First Author’s Institution: Liverpool John Moores University, UK
Status: Published in ApJ

Intermediate-Mass Black Holes

Stellar-mass black holes — those with masses of tens of solar masses (M) — are thought to result from the collapse of massive stars. The formation of supermassive black holes — those with millions to billions of solar masses — is less clear. Given their large masses, there is not enough time for stellar-mass black holes to grow into the supermassive black holes that we know existed fairly early in the history of the universe. One possibility is that the “seeds” that grow into supermassive black holes lie somewhere in between 102 and 105 M, or what we’d call intermediate-mass black holes.

Despite their importance, intermediate-mass black holes remain elusive and their existence has not been quite confirmed. The best way to measure black hole masses is using the motions of stars around them, but this tactic may not work for intermediate-mass black holes since they have a smaller sphere of influence than supermassive black holes. Today’s article takes a look at a possible intermediate-mass black hole in a globular cluster in neighboring galaxy M31, also known as the Andromeda Galaxy.

Pinning Down the Mass

The globular cluster B023-G078 is the most massive cluster in Andromeda, and the velocity of stars within the cluster seems to indicate the presence of a massive central object. The authors of the article use images of the cluster from the Hubble Space Telescope and spectroscopic observations from Gemini to determine if this central mass could be an intermediate-mass black hole.

plot of root mean square velocity as a function of radius in arcseconds and parsecs

Figure 1: Root-mean-square velocity of stars in the cluster vs. radial distance to the center of the cluster. Points in red show the observations from the Gemini telescope. The black line shows the best-fit model for a massive black hole. The blue line shows the model assuming there is no black hole. [Adapted from Pechetti et al. 2022]

The authors use the Hubble images to come up with models for the mass of the black hole. They use a method called Jeans anisotropic modeling, which fits the Jeans equations to observations of a star cluster or galaxy. The high resolution of the Gemini data (and the proximity of the cluster) allows them to get information on the motion of individual stars within the cluster. Using integral field spectroscopy, the authors determine the root-mean-square velocity of stars at different distances from the center of the cluster, which depends on the central mass. The authors then compare their models to the observed velocities, shown in Figure 1.

The best-fit models give the central object a mass of 9 x 104 M, placing it firmly in intermediate-mass black hole territory!

It is possible that the central mass is actually several stellar-mass black holes rather than one intermediate-mass black hole. The main difference between the two possibilities would be that a collection of many black holes would look more extended than a single compact object. The authors investigate this possibility using their models, but any conclusions may require higher resolution observations.

However, there is something else that can give us a clue if this is indeed an intermediate-mass black hole: the origin of the globular cluster.

Remnants of a Small Galaxy? 

Because of the wide spread of metallicity measured for stars in the cluster, the authors consider the possibility that B023-G078 is the remnant of a small galaxy that underwent a merger with Andromeda, making it a stripped nuclear star cluster. The idea is that as small galaxies merge into larger galaxies (what is known as a minor merger), tidal forces pull apart parts of the galaxy, including the nuclear star cluster at the center that houses a massive black hole, leaving behind a globular cluster.

Given the mass of the cluster (~106 M), the authors estimate that the original galaxy had a mass of ~109 M. (For comparison, the mass of the Milky Way is ~1011 M.) Since the mass of a central black hole typically scales with the mass of the galaxy, this mass estimate means this nucleus is a good place to look for an intermediate-mass black hole.

The combination of the mass of the black hole from modeling and the evidence that this cluster is a stripped nuclear star cluster leads the authors of the article to favor the idea that there is indeed an intermediate-mass black hole in the cluster!

Original astrobite edited by Alex Pizzuto.

About the author, Gloria Fonseca Alvarez:

I’m a fifth-year graduate student at the University of Connecticut. My research focuses on the inner environments of supermassive black holes. I am currently working on measuring black hole properties from the spectral energy distributions of quasars in the Sloan Digital Sky Survey. As a Nicaraguan astronomer, I am also involved in efforts to increase the participation of Central American students in astronomy research.

side-by-side images of Venus's surface today and an imagining of what its surface might have looked like in the past

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: Was Venus Ever Habitable? Constraints from a Coupled Interior–Atmosphere–Redox Evolution Model
Authors: Joshua Krissansen-Totton, Jonathan J. Fortney, and Francis Nimmo
First Author’s Institution: University of California, Santa Cruz
Status: Published in PSJ

Where Oh Where Did the Water Go? (And Was It There To Begin With?)

Despite sometimes being called “Earth’s twin,” Venus isn’t very similar to Earth beyond its size and composition. With a thick toxic atmosphere filled with CO2 and a volcano-laden surface, it’s definitely more like Earth’s evil twin. Even spacecraft can only survive on its surface for a maximum of 2 hours before succumbing to the high pressure and temperature of a planet plagued by the runaway greenhouse effect.

But was Venus always such a hellish place? For a long time, we’ve theorized that Venus boasted an ocean of liquid water on its surface many billions of years ago, but being closer to the Sun, a runaway greenhouse effect took hold: as the Sun got brighter over time, more solar radiation hit the planet’s surface and led to an increase of surface water evaporation and more water vapor in Venus’s atmosphere. As the presence of water vapor heated the planet even more, intense radiation from the Sun split the water molecules, causing hydrogen to escape into space. This left room for carbon escaping from the planet’s surface to combine with some of the free oxygen left over and build up CO2 in the atmosphere, trapping even more heat and leading to a runaway greenhouse effect.

Different models of Venus’s climate evolution have led to conflicting stories about its past. Some models that incorporate the effects of clouds responding to the warming or cooling of the planet have found it possible for habitable conditions to have existed on the planet as late as 700 million years ago. Also, unlike the Moon or Earth, whose craters are weathered or otherwise degraded, most of Venus’s craters are in pristine condition and randomly distributed across its surface. From this, we infer that most of Venus’s geological history has been erased due to resurfacing events like volcanic outbursts and lava flows that happened very recently. This means that the surface we can see is very young (<1 billion years old), which makes it difficult to use observations of the surface to uncover information about Venus’s elusive history.

However, scientists have found evidence of felsic crust: igneous rocks on Venus’s surface that are relatively rich in feldspar and quartz, whose presence may indicate past surface water. That said, Venus’s atmosphere is almost completely devoid of molecular oxygen (O2). But if water vapor in the atmosphere was broken down by radiation and most hydrogen escaped into space, then this would mean there should be some leftover oxygen in the atmosphere. So what happened to all the oxygen?

Let’s Have PACMAN Eat All Our Doubts Away…

The authors of today’s article try to reconcile all the clues we have about Venus by using a coupled atmospheric–interior model called PACMAN (Planetary Atmosphere, Crust, and MANtle) to reproduce its climate conditions over time to see if the planet could have ever sustained liquid water on its surface. All this means is that they keep track of the conditions in both Venus’s atmosphere and its interior while accounting for any effect one system has on the other. People have used these kinds of models to study Venus before, but none of them have ever looked at the possibility of water on its surface.

The model is split into two phases (see Figure 1). Initially, Venus had a magma ocean on its surface created from impacts with other pieces of space rocks that were abundant during the planet’s formation. The magma ocean was a giant layer of molten, bubbly rock that you definitely wouldn’t want to dip your toes in. As this ocean cooled and released gases into the atmosphere, the temperature dropped to a point where this ocean “froze” and became a solid mantle, initiating phase two of the model.

schematic of the two phases that make up the authors' model

Figure 1: A simplified schematic of the PACMAN model the authors used. On the left is the magma-ocean phase that consists of (from innermost to outermost layer) the core, a solid mantle, magma ocean, and atmosphere. On the right is the solid mantle phase which occurs after the magma ocean solidifies, consisting of the core, solid mantle, and the atmosphere/hydrosphere. Different colored arrows show what components leave and enter each layer in the model. [Adapted from Krissansen-Totton et al. 2021]

The authors calculate quantities like the surface temperature, the amount of radiation emitted and absorbed by the planet, how much water vapor is in the atmosphere, and the amount of water on the surface during both phases. They also keep track of the abundance of various molecules containing carbon, hydrogen, and oxygen (carbon dioxide, water, O2, etc.) and calculate their flux between the atmosphere and the interior (i.e., how many of these molecules enter or exit over time). In addition, they also calculate the accumulation of 40Ar and 4He in the atmosphere, which tell us about the total magmatic activity and more recent magmatic activity, respectively. Together, these enable us to better determine whether a habitable or uninhabitable past is better at reproducing Venus’s current atmosphere.

There are lots of unknown parameters and initial conditions in the model such as CO2 pressure and planetary albedo (reflectiveness), so the authors run their model 10,000 thousand times to sample all 24 of these unknown parameters. Out of all of these runs, only 10% ended successfully in a state that mirrors Venus’s modern atmospheric and surface conditions and chemical abundances. What’s interesting about these successful models is that they suggest Venus’s current state is compatible with two different histories: some of the models tell us Venus was never habitable in its past, while others claim that Venus was transiently habitable, meaning it could have contained an ocean up to ~100 meters deep on its surface for anywhere between 0.04 and 3.5 billion years before succumbing to the runaway greenhouse effect. The latter scenario should have left salt or mineral deposits on the surface after all the water evaporated, leaving these materials potentially accessible to future remote sensing observations!

And the Winner Is…

So which model is correct? Unfortunately, there is no definitive answer since the authors found that both models are favored under different conditions. CO2 tends to make it difficult for hydrogen to escape if the water concentration is too low. Therefore, in the uninhabitable scenarios where no surface water is present, H2O in the atmosphere has a hard time escaping because CO2 continually dominates the atmosphere instead of being locked in the surface. That means that these scenarios can’t reproduce the modern water-less and oxygen-less Venus that we see today. But, if CO2 is allowed to radiatively cool the upper atmosphere, then water can condense on the surface and CO2 is removed from the atmosphere and stored away in the interior of the planet, giving Venus a chance to have a period of enhanced water loss that can then initiate the runaway greenhouse effect before the CO2 is outgassed back into the atmosphere.

On the other hand, most modern models assume that when the magma ocean phase ends, virtually all the carbon and water from the magma (so-called volatiles) live in the atmosphere. But it is possible that some of these volatiles are trapped in the resulting solid mantle instead. If this is allowed, then far fewer models allow for Venus to have been habitable. This is because it would take longer for water to then be released back into the atmosphere, making it hard to explain Venus’s current almost non-existent water abundance.

The bottom line here is that either of these two scenarios is possible and consistent with modern observations. Which scenario wins depends on our assumptions and model parameters. Though this might seem a bit anticlimactic, understanding and constraining Venus’s evolution is important for interpreting the atmospheres and histories of other exoplanets out there that might have gone through similar processes. JWST might be capable of constraining what the atmospheres of other so-called exo-Venuses are, like some of the TRAPPIST-1 system planets. Hopefully, our studies of both Venus and exo-Venuses can symbiotically help shine a light on planetary evolution!

Original astrobite edited by Ishan Mishra.

About the author, Katya Gozman:

Hi! I’m a second-year PhD student at the University of Michigan. I’m originally from the northwest suburbs of Chicago and did my undergrad at the University of Chicago. There, my research primarily focused on gravitational lensing and galaxies while also dabbling in machine learning and neural networks. Nowadays I’m working on galaxy mergers and stellar halos, currently studying the spiral galaxy M94. I love doing astronomy outreach and frequently volunteer with a STEAM education non-profit in Wisconsin called Geneva Lake Astrophysics and STEAM.

visualization of the milky way's magnetic field

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: Evolution of primordial magnetic fields during large-scale structure formation
Authors: Salome Mtchedlidze et al.
First Author’s Institution: Ilia State University, Georgia; University of Göttingen, Germany; and Abastumani Astrophysical Observatory, Georgia
Status: Accepted to ApJ

Magnetic fields — often denoted by a in physics shorthand — are ubiquitous throughout the universe, playing a part in the physics of planets, stars, galaxies, and beyond. But, where did these magnetic fields come from? Were they born in the Big Bang, or did they arise sometime later in cosmic history? The short answer: we don’t know! This question of cosmic magnetogenesis remains one of the most important unsolved problems in modern astronomy and is intimately connected to the underlying cosmology and fundamental physics of our universe.

The Birth of Magnetic Fields

Very broadly speaking, there are two competing avenues for cosmic magnetogenesis: the astrophysical scenario and the primordial scenario. In the astrophysical scenario, weak, small-scale magnetic fields are produced around local astronomical systems — like stars and galaxies — and are then amplified and spread across large scales; these initially tiny seed fields could be generated via naturally circulating electric currents (so-called dynamos), the turbulent flow of intergalactic or interstellar gas, or spontaneous processes in unstable plasmas. By contrast, magnetic fields in the primordial scenario are generated at the dawn of cosmic time — before stars, galaxies, or any structure in the universe came to be — and grow with the universe itself. Hypotheses of primordial magnetogenesis involve highly theoretical quantum phenomena, like the violation of fundamental symmetries of nature or the coupling and decoupling of the fundamental forces. Despite each of the primordial models taking place shortly after the Big Bang, the precise mechanism of field generation is hotly contested.

Nevertheless, the presence of magnetic fields in the early universe could have vitally important cosmological consequences. For one, these fields would tamper with the cosmic microwave background — the oldest, most distant light we can see — fundamentally affecting our inferences of the state of the infant universe. Primordial magnetic fields would also alter the thermal properties of the material between galaxies, thus shifting the time at which the universe transitioned from neutral to reionized. Recently, it’s even been suggested that early magnetic fields could explain the Hubble tension — the notorious mismatch between local and global measurements of the expansion rate of the universe — and, if these fields are sufficiently twisty (i.e., if the fields are helical), they could also explain why the universe contains so much more matter than antimatter. In other words, figuring out magnetogenesis could solve many of the universe’s biggest puzzles for the price of one!

Baby Photos of the Cosmic Magnetic Field

Evidently, primordial magnetic fields deserve some attention. As such, the authors of today’s article seek to understand how magnetic fields would evolve from the very early universe to the present day. In particular, the authors use computer simulations to trace how a primordial seed field would interact with the largest-scale structures in the universe — the components of the cosmic web, like massive galaxy clusters; long, thin filaments; and vast, empty cosmic voids — as they develop over cosmic time. By comparing current observations of large-scale magnetic fields to the patterns predicted by these simulations, we can rule out different models of primordial magnetogenesis.

The authors consider four different models for the primordial magnetic field:

  1. A completely uniform and homogeneous field that could be produced during the rapid inflation of the universe
  2. A scale-invariant field (a field possessing equal contributions from waves with small wavelengths and waves with large wavelengths) that could result from a different inflationary scenario
  3. A random, non-helical field that could originate from a phase transition in the early universe, when some fundamental force became independent from the rest
  4. A random, helical field that could also arise from a phase transition

These scenarios set the initial conditions of the authors’ simulations, and thus each model is expected to evolve in a different way.

maps of the temperature, mass density, and magnetic field strength at a redshift of z=0.02 for the four scenarios

Figure 1: Maps of the present-day cosmic web as predicted from simulations of primordial magnetic field evolution. From left to right: uniform magnetic field case, scale-invariant case, helical phase-transitional case, and non-helical phase-transitional case; from top to bottom: magnetic field, density, and temperature. Click to enlarge. [Mtchedlidze et al. 2022]

Magnetic Fields All Grown Up

Figure 1 shows the imprint of the simulated primordial magnetic fields on the present-day cosmic web with respect to field strength, density, and temperature. The authors find that the two inflationary magnetic field models develop stronger evolved fields than the two phase-transitional models, with the overall magnetization in galaxy clusters and in the bridges between clusters differing by orders of magnitude between the two field-generation scenarios. Additionally, the inflationary magnetic fields stretch to much larger scales than do the phase-transitional cases. While the helical phase-transitional fields evolve to higher strengths than the non-helical fields, the authors note that, at least according to their models, it should be difficult to distinguish between helical and non-helical fields observationally.

four-panel plot of simulated rotation measure

Figure 2: Predicted present-day rotation measure from simulations of primordial magnetic field evolution. From top to bottom: uniform magnetic field case, scale-invariant case, helical phase-transitional case, and non-helical phase-transitional case. The color bar is in units of radians per square meters. [Mtchedlidze et al. 2022]

The authors also produce simulated maps of the present-day rotation measure based on the evolved primordial fields (Figure 2). When a radio wave passes through a magnetic field on its way to an observer, its polarization is rotated by an amount proportional to the magnetic field’s strength; therefore, by measuring the degree to which an extragalactic radio wave’s polarization has been affected (quantified by the aptly named rotation measure) one can deduce the strength of astronomical magnetic fields. By comparing their rotation measure maps to recent observations, the authors find that the two inflationary magnetic field models, which produce larger magnetization levels in cosmic filaments, are favored over the phase-transitional models.

While their modeling of magnetic field evolution over cosmic time neglects some key physical processes, such as gas cooling, chemical evolution, and high-energy outflow from stars and black holes, the authors still decisively show that different models of primordial magnetogenesis leave unique imprints on the universe’s largest scales.

Since the Low-Frequency Array has already started taking rotation measure data of distant radio waves passing through cosmic filaments, it’s only a matter of time before we can start ruling out models of early magnetic field creation. Even better, when the Square Kilometre Array comes online in the next decade, it’ll collect exquisite rotation measure data from the entirety of the cosmic web. With the power of the Square Kilometre Array at our disposal, we’ll be solving the mysteries of magnetogenesis B-fore you know it!

Original astrobite edited by Zili Shen.

About the author, Ryan Golant:

I am a second-year astronomy Ph.D. student at Columbia University. My current research involves the use of particle-in-cell (PIC) simulations to study magnetic field growth in gamma-ray burst afterglows and closely related plasma systems. I completed my undergraduate at Princeton University, and I am originally from Northern Virginia. Outside of astronomy, I enjoy learning about art history, playing violin and video games, and watching cat videos on the internet.

picture of a globular cluster

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: NGC5846-UDG1: A galaxy formed mostly by star formation in massive, extremely dense clumps of gas
Authors: Shany Danieli et al.
First Author’s Institution: Princeton University
Status: Accepted to ApJL

Stars can find a home in many different places: some reside in galaxies, whereas others can reside in tightly bound star clusters, which themselves orbit galaxies. The stars that exist in the densest, oldest star clusters — globular clustersformed in extreme conditions at very early times in regions of space with extremely high gas pressures. Globular clusters are ancient relics encoding information on conditions for star formation in the early universe.

black-on-white image of UDG1

Figure 1: V-band image of UDG1 (diffuse grey points) and its globular cluster candidates (black points). [Adapted from Danieli et al. 2022]

The authors of today’s article use Hubble Space Telescope data to investigate the globular cluster population of the ultra-diffuse galaxy NGC5846-UDG1 (UDG1 for short; image of galaxy and its globular clusters shown in Figure 1). Ultra-diffuse galaxies are a type of low-surface-brightness dwarf galaxy that can be approximately the size of the Milky Way galaxy but up to a factor of 100 less luminous (see here and here for previous bites on ultra-diffuse galaxies). Exactly how these odd galaxies form is an open question in astronomy. So, what can UDG1’s globular cluster population tell us about galaxy formation?

A Galaxy Rich in Globular Clusters

A key result of today’s article is that UDG1 has a significantly higher number of globular clusters than would be expected for a galaxy of its mass, with the tally coming in at 54 (+/- 9)! In addition to the number of globular clusters, the authors also estimate the total light from the clusters relative to the total light in the galaxy itself. The authors find that the globular clusters currently emit 13% of the total light of the galaxy, meaning that 13% of this galaxy’s stars reside in globular clusters — this is the highest fraction of stars in globular clusters known for any galaxy to date, and it’s 100 times larger than the globular cluster fraction of the Milky Way!

While the current proportion of stars in globular clusters for UDG1 is 13%, this figure was likely much higher at earlier times due to the dynamical evolution of these clusters. Globular clusters are expected to interact with their environment and lose mass (or be completely destroyed) via tidal stripping over time. The stars that are lost from the clusters may end up contributing significantly to the stellar content of the galaxy itself (see this astrobite for more).

These mass-loss processes can lead to globular cluster systems losing up to 80–90% of their stellar mass content over their lifetime. The authors use an analytical model to estimate the original cluster fraction (before any mass loss processes occurred) from the current cluster fraction of 13%. They find that the original cluster fraction is likely to have been 65%, indicating that the majority of the stars in this galaxy at present day originally formed in bound clusters from extremely dense gas clumps.

These results are summarised in Figure 2, which displays the cumulative proportion of mass in globular clusters (currently observed clusters in black; the initial cluster model in purple (before mass loss effects); and Milky Way values in orange).

plot of galaxy mass in clusters versus stellar mass in clusters

Figure 2: The cumulative fraction of galaxy mass in globular clusters for NGC5846-UDG1 in comparison to the Milky Way. The black line shows the current observed values, the purple dotted line shows the modelled values expected at the time of formation, and the orange dashed line shows the current Milky Way values. Mcl refers to the total stellar mass of the globular clusters. [Adapted from Danieli et al. 2022]

Implications for Star Formation

The large difference between the Milky Way and UDG1 globular cluster fractions highlights the rare conditions under which UDG1 originally formed. Globular cluster formation (and subsequent destruction) is very likely to have been the dominant star formation mode for UDG1, with these stars originally forming from extremely dense, high-pressure gas clumps. This idea is supported by the fact that the spatial distribution and ages of the cluster stars versus the galaxy stars in UDG1 are very similar, indicating that the galaxy’s stars likely originated from disrupted globular clusters.

It is unlikely that UDG1 is the only galaxy of its kind. The authors note that there are hints that some ultra-diffuse galaxies in the Coma cluster have similarly high globular cluster fractions. The uncertainties on these measurements are high since the galaxies in the Coma cluster are located farther away, but it’s still a promising indicator that star formation through extremely dense clumps at early times may be a viable way to build a galaxy!

Original astrobite edited by Gloria Fonesca Alvarez.

About the author, Katy Proctor:

I am a first-year PhD student at the International Centre for Radio Astronomy Research at the University of Western Australia. My research is focused on using cosmological simulations to study the build up of stellar halos. Outside of research, I can usually be found climbing up walls or playing guitar.

hubble space telescope image of the core of Starburst Galaxy NGC 1569

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: Characterization of Two 2 mm detected Optically Obscured Dusty Star-forming Galaxies
Authors: Sinclaire Manning et al.
First Author’s Institution: The University of Texas at Austin and University of Massachusetts, Amherst
Status: Published in ApJ

Dusty star-forming galaxies could be important for learning what drives star formation, since they have been observed to have extremely high star-formation rates. These galaxies could also be predecessors to some quenched galaxies that have stopped forming stars at high redshift. However, due to observational challenges, it is relatively uncertain how common dusty star-forming galaxies are at high redshift.

Ultraviolet and optical light from young stars in dusty star-forming galaxies gets absorbed by large amounts of dust and re-emitted in the infrared. In some cases, there is so much dust that the optical emission from the galaxy is heavily obscured and all we can observe is the light re-emitted by the dust. When galaxies are undetectable (meaning they either can’t be observed or their presence can’t be confirmed) in both optical and near-infrared light, we call them OIR-dark galaxies.

At high redshift, dusty star-forming galaxies are observable at long wavelengths with telescopes like the Atacama Large Millimeter/submillimeter Array (ALMA). The authors of today’s article investigate two OIR-dark galaxies from the Mapping Obscuration to Reionization ALMA (MORA) survey and measure their redshifts and physical properties.

Dusty, Dark, and Distant

The two galaxies, MORA-5 and MORA-9, have been observed in multiple wavelengths with ALMA and other telescopes like the Hubble Space Telescope. What makes them interesting is that they are detected in ALMA observations but not in optical or near-infrared wavelengths, so they are considered OIR-dark. The availability of multi-wavelength observations allows the authors to study the galaxies’ spectral energy distributions (SEDs), which tell us how a galaxy’s energy is distributed over different wavelengths. The shape of a galaxy’s SED depends on properties such as the rate of star formation and gas mass, so fitting the SED can give us a clue about the physical environment of the galaxy.

Before they can determine the properties of these galaxies, the authors must obtain their redshifts. One way to measure redshift is using spectroscopy, since the wavelengths of galactic emission lines get shifted depending on their distance to us. Since spectroscopic data for these galaxies are not available, the authors use three different methods to estimate redshift from the SEDs. Figure 1 shows the SEDs of these two galaxies and the fitting results from these three methods.

Figure 1: Spectral energy distributions of MORA-5 (top) and MORA-9 (bottom). The blue line shows the fit to the OIR part of the SED from EAZY. The orange line shows the fit from MMpz to the far-infrared part of the SED. The black line shows the combination of these two models. The gray and purple lines show the MAGPHYS fitting of the entire range of the SED for 3 different redshifts. [Manning et al. 2022]

One method called MMpz fits the far-infrared part of the SED and is based on the relationship between the infrared luminosity and the wavelength at which the SED peaks. The two properties are anti-correlated, meaning that galaxies with higher infrared luminosity peak at shorter rest-frame wavelengths. This is helpful since knowing the wavelength at which the SED is expected to peak can help us figure out how much it has been redshifted. Another method uses the software EAZY to fit the (faint) OIR data. The third method, MAGPHYS, fits the entire SED (both OIR and far-infrared regions) to figure out the redshift. The best estimates from these methods place the galaxies at redshift z > 4 (within the first 1.5 billion years of the universe), potentially making them the highest redshift galaxies in the MORA survey.

Galaxy Properties

After estimating the redshifts, the authors estimate the physical properties of these galaxies. First, they once again use correlations between the infrared luminosity and other galaxy properties, but this time to estimate the star-formation rate. They estimate that in one year MORA-9 forms 200 solar masses of stars. MORA-5, on the other hand, forms roughly four times as many stars, with a star-formation rate of 830 solar masses per year! The authors also estimate the mass of the galaxy that comes from stars, gas, and dust.

Figure 2: Star-formation rate (in solar masses per year) vs. stellar mass (in solar masses) of dusty star-forming galaxies from different surveys. The blue and green diamonds represent the MORA galaxies discussed in this article. The gray shaded area represents the main sequence that galaxies follow for star formation at a redshift of z=3–5. [Manning et al. 2022]

Figure 2 shows how the star-formation rates for these two galaxies compare to other dusty star-forming galaxies, showing that the two galaxies fit in with two different populations. MORA-5 fits in with high-stellar-mass galaxies with high star-formation rates, while MORA-9 is a part of a more moderate population.

To figure out how common these galaxies are, the authors look at two other galaxies, MORA-3 and MORA-4, and estimate the volume density of OIR-dark galaxies to be 5 x 10-6 Mpc-3. The authors note that it’s unclear why some dusty star-forming galaxies are OIR-dark while others aren’t, and they suggest it could simply be dependent on their redshift. Regardless of what makes this class of galaxies OIR-dark, they provide a way to study the environments of distant star-forming galaxies that are otherwise invisible.

Original astrobite edited by Jamie Sullivan.

About the author, Gloria Fonseca Alvarez:

I’m a fifth-year graduate student at the University of Connecticut. My research focuses on the inner environments of supermassive black holes. I am currently working on measuring black hole properties from the spectral energy distributions of quasars in the Sloan Digital Sky Survey. As a Nicaraguan astronomer, I am also involved in efforts to increase the participation of Central American students in astronomy research.

photograph of CHIME

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 First CHIME/FRB Fast Radio Burst Catalog
Authors: The CHIME/FRB Collaboration
Corresponding Author’s Institution: Massachusetts Institute of Technology
Status: Published in ApJS

Fast radio bursts are just as their name suggests — short, millisecond-long bursts observed in radio wavelengths all over the galaxy. But, despite the first burst being found in 2007, astronomers still aren’t sure what type of sources produce them. We do know that there seem to be two types of bursts — those that repeat (repeaters) and those that occur only once (non-repeaters). One of the repeating sources has recently been associated with SGR 1935+2154, a galactic magnetar — a type of neutron star with a very powerful magnetic field. Another repeating source has been associated with the M81 galaxy.

In this article from the CHIME/FRB Collaboration, the authors present a new catalog of fast radio bursts (called “Catalog 1”), which more than doubles the number of known sources. Statistical studies of the entire population are thus much more viable, and as a result, astronomers can begin to address some of the open questions about these mysterious sources.

How Can We Tell Where Fast Radio Bursts Come From?

Although one repeater has been associated with a magnetar in the Milky Way, not all fast radio bursts come from objects within our galaxy. We can see the distribution of fast radio bursts (both repeaters and non-repeaters) in Figure 1.

plot of locations of fast radio burst sources

Figure 1: The distribution of fast radio bursts across the sky seen by the CHIME telescope. CHIME, located in Canada, observes the northern sky and a small band of the southern sky near the celestial equator (declination -11 to 90 degrees). There are 474 unique non-repeating sources (blue circles) and 18 known repeaters (red triangles). [CHIME/FRB Collaboration 2021]

examples of flux observed over time for two fast radio bursts

Figure 2: 2D histograms of the amount of radio waves (flux) received in different frequency bands by CHIME for two sample fast radio bursts, with name and dispersion measure in the top right of each panel. These “waterfall plots” show what the telescope observed for each source. The plot on the left has a more pronounced “swoop” pattern and a higher dispersion measure. Plots for the rest of the catalog are here. [Adapted from CHIME/FRB Collaboration 2021]

Fast radio bursts, especially the ones outside our galaxy, travel a long way to get to us on Earth. We can look at the dispersion measure to tell how far the burst has traveled. Space is not empty, and as the burst travels through the interstellar medium it gets scattered off electrons and other particles, which causes the “swoop”-like pattern seen in the data in Figure 2. The dispersion measure is the integrated number density of particles along the path of the waves and is a quantitative way to estimate the distance that the burst has traveled to get to the telescope.

Are Repeaters and Non-Repeaters From the Same Type of Source?

One of the major open questions about fast radio bursts is what type of source produces them. The differences between repeating and non-repeating sources leave open the possibility that the two types of bursts may originate from different sources. The nature of repeating sources rules out cataclysmic scenarios (situations where one or more objects collide or explode, such as a neutron star merger), but some of these might be possible for a non-repeating source.

The authors of the article start by looking at several characteristics (including distribution across the sky, dispersion measure, signal strength, flux, temporal width, and bandwidth) of both repeater and apparent non-repeater bursts to see if it is statistically plausible that they originate from the same distribution. If so, both repeaters and non-repeaters could come from the same underlying source population.

The authors find that repeaters and non-repeaters are distributed similarly across the sky and have similar dispersion measures. Repeaters and non-repeaters have similar flux and fluence distributions, meaning they give off a similar amount of radio waves over the course of the burst (fluence here refers to the integral of the flux over the duration of the burst).

histogram of the number of fast radio bursts as a function of duration

Figure 3: Distribution of (temporal) widths for the bursts. The distribution in blue shows the single non-repeater bursts, while the orange distribution chooses only the first observed burst for each repeater. These distributions do not look at all similar and showcase one of the differences seen between the types of fast radio burst sources. [Adapted from CHIME/FRB Collaboration 2021]

However, repeaters and non-repeaters also show some differences. Their temporal widths and bandwidths appear to differ, which is shown in Figure 3 (as also previously reported with lower statistics in two articles [1] [2]). The fact that the widths differ between the two types of bursts is interesting, because it seems that repeater bursts last longer than non-repeaters on average. The bandwidth (the spread of radio frequencies observed for the burst) is also different between the two types, which is an additional indicator that repeaters and non-repeaters may come from different types of sources.

So, What Is the Major Takeaway From This Article?

One of the most exciting aspects of this article is the increase in the number of sources and bursts now available for fast radio burst research. With higher statistics, studies of the entire population are more meaningful. Differences between repeaters and non-repeaters (with regards to their temporal width and bandwidth) leave open the possibility that these come from different populations. The authors estimate that 820 bursts occur over the full sky per day, which means that statistics will continue to increase over time, more sources may be found to repeat, and more pieces of the fast radio burst puzzle will continue to fall into place.

Original astrobite edited by Konstantin Gerbig and Ali Crisp.

About the author, Jessie Thwaites:

Jessie is a PhD student at the Wisconsin IceCube Particle Astrophysics Center at the University of Wisconsin-Madison. She studies possible astrophysical sources for high-energy neutrinos through multimessenger astrophysics. Outside of physics, she plays horn and enjoys spending time outdoors, especially skiing and biking.

photograph of an engineer standing in front of JWST mirror segments

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: Detecting Biosignatures in the Atmospheres of Gas Dwarf Planets with the James Webb Space Telescope
Authors: Caprice Phillips et al.
First Author’s Institution: The Ohio State University
Status: Published in ApJ

Across the World, Astronomers Are Excited…

…because the latest “Great Observatory” was successfully launched into space! The space telescope known as JWST has become infamous amongst space professionals for long and frequent delays in its deployment since its conception in 1996. Though its timeline and price tag are daunting, they are but sxmall consequences of the telescope’s exquisite engineering and mind-blowing potential. Most often cited as the instrument that will allow us to look deeper into the past than ever before, JWST might also be able to search for potential signatures of life on planets outside our solar system! The authors of today’s article explain how.

A Portrait of an (Exo)World and Its Atmosphere

On Earth, we are protected from the adverse effects of our solar system (like flying rocks and too much radiation) by our atmosphere. While our rocky home has just enough gravity to hold on to an atmosphere of mostly nitrogen (N), planets slightly larger than Earth but smaller than Neptune would likely live in a bubble of hydrogen (H). Known as gaseous dwarf planets, these potential homes are easier to identify and study thanks to their larger sizes. Because of their hydrogen-dominated atmospheres, chemical reactions with hydrogen that produce molecules like ammonia (NH3) are expected to be common on these planets.

However, ammonia doesn’t occur spontaneously; it requires either extremely high temperatures and pressures or the existence of a reaction catalyst, potentially one that arose to support life-sustaining chemical processes. Furthermore, ammonia is easy to destroy in volcanic environments, meaning that the presence of ammonia suggests the presence of a process that regularly replenishes it. Therefore, in these worlds ammonia is a possible biosignature: a clue that the planet in question might be hosting life!

Unfortunately, the presence of ammonia alone isn’t enough to detect life outside the solar system, but it would set the stage for follow-up observations and even more amazing science. The authors of today’s paper begin by considering seven close-by exoplanets of the right size and temperature range to have gaseous envelopes and be observable by JWST. In this article, we’ll focus on one of the seven: TOI-270 c, which the authors find to be best suited for future JWST observations.

How Would We Search for Ammonia?

One method for determining the contents of a planet’s atmosphere is transmission spectroscopy: measuring the spectrum of the planet’s host star as its light filters through the planet’s atmosphere on its way to the detector. Ideally, the measured spectrum will reveal the tell-tale dips and peaks of absorption and emission spectra that are associated with individual elements and molecules. An example of the signature of ammonia from planet TOI-270 c is given in Figure 1. Note that there are several ammonia features we could potentially look for (at 1.0–1.5, 2.0, 2.3, and 3.0, as well as 5.5–6.5 and 10.3–10.8 µm, which are not shown) and that the height of the peaks depends on how much ammonia there is in the atmosphere in parts per million (ppm). The more ammonia there is, the easier it will be to detect!

ammonia spectrum

Figure 1: The spectrum of ammonia in the atmosphere of TOI-270 c for different ammonia concentrations in parts per million (ppm). The height of the peak is given in terms of the ratio of the radius of the planet squared and the radius of the star squared (i.e., how much of the light is blocked). Note that there are at least six ammonia features to look for! [Phillips et al. 2021]

Another lever controlling the spectrum of TOI-270 c’s ammonia signature is the presence of a cloud deck in its atmosphere. Whether the planet is enveloped in clouds — and at what height they occur — will significantly change its ammonia signature, as shown in Figure 2. The authors show that the most identifiable signature will come from a cloudless atmosphere, or one with a cloud deck at high pressure (read: low to the ground). On the other hand, a cloud deck at 0.01 bar (really high up!) would flatten the signature almost completely, making it incredibly hard to detect ammonia.

spectra of ammonia

Figure 2: The spectrum of ammonia in the atmosphere of TOI-270 c as above, but now for different cloud decks. The most noticeable features emerge from an atmosphere with no clouds or with a cloud deck at 1.0 bar. Smaller features are noticeable in an atmosphere with a cloud deck at 0.1 bar, while a cloud deck at 0.01 bar flattens the spectrum to almost a straight line. [Phillips et al. 2021]

Could JWST See These Signals?

There are several instruments aboard JWST that could be used to hunt for ammonia-filled atmospheres; the authors find that Near-Infrared Spectrograph (NIRSpec) and the Near-Infrared Imager and Slitless Spectrograph (NIRISS) are particularly suitable. Together, these instruments cover the wavelength range of the ammonia features illustrated in Figures 1 and 2 above. The question is: are they sensitive enough to make the detection?

I won’t keep you in suspense: the authors’ results suggest that the answer is yes! The authors used two of their simulated spectra from above — assuming two different percentages of hydrogen in the atmosphere of TOI-270 c — and modeled what JWST observations of such an atmosphere might look like for 10 transits (see Figures 3 and 4). As might be expected, an atmosphere that is richer in hydrogen will produce a higher signal-to-noise ratio (S/N).

simulated spectrum of ammonia

Figure 3: Top: The spectrum of ammonia in the atmosphere of TOI-270 c for an atmosphere rich in hydrogen is shown in orange, while the different black points show simulated JWST observations with NIRISS and two NIRSpec filters (G235M and G395). The authors also show the wavelength range of each of the instruments below the spectrum. Bottom: The signal-to-noise ratio of the simulated spectrum and observations. [Phillips et al. 2021]

simulated spectrum of ammonia

Figure 4: The same figure as above, but this time with an atmosphere that is poorer in hydrogen. Note that this results in a lower signal-to-noise ratio than in Figure 3. [Phillips et al. 2021]

Conclusion

Under the right circumstances (hydrogen-rich atmosphere, lots of ammonia, no cloud deck), JWST is well-equipped to detect ammonia on gas dwarf planets — an exciting start on a long road towards determining whether we truly are alone in the universe!

Original astrobite edited by Lili Alderson.

About the author, Luna Zagorac:

I am a PhD candidate in the Physics Department at Yale University. My research focus is ultra-light (or fuzzy) dark matter in simulations and observations. I’m also a Franke Fellow in the Natural Sciences & Humanities at Yale working on a project on Egyptian archaeoastronomy, another passion of mine. When I’m not writing code or deciphering glyphs, I can usually be found reading, doodling, or drinking coffee.

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