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X-rays, dark matter and galaxies in cluster Abell 2744

6 February 2024

Sloshing in the Universe’s Biggest Bathtub

Title: Deep Chandra Observations of Abell 2495: A Possible Sloshing-Regulated Feedback Cycle in a Triple-Offset Galaxy Cluster
Authors: Luca Rosignoli et al.
First Author’s Institution: University of Bologna and National Institute for Astrophysics, Italy
Status: Published in ApJ

Galaxy clusters are the biggest gravitationally bound objects in the universe. They have three main components: galaxies, their surrounding dark matter halos, and hot gas between the galaxies known as the intracluster medium. It’s the interaction between these components that makes galaxy clusters so interesting, and also (we think) significantly affects galaxy formation and evolution as a whole. As an example of this interaction, many galaxy clusters have a supermassive black hole inside their central galaxy (known as the brightest cluster galaxy) that’s acting as an active galactic nucleus — it’s being fed so much material by the galaxy and the cluster that it can’t contain it all, and it’s spitting material and energy back out into the cluster.

Thanks to gravity, each of these three components in a galaxy cluster should be centered on the same point in space — the dark matter forms a kind of gravitational “pit” that both the intracluster medium and the galaxies themselves can’t help but be pulled into. Other forms of energy (like heat keeping the intracluster medium puffed out, or leftover kinetic energy keeping the galaxies orbiting) prevent the intracluster medium and galaxies from collapsing entirely into the center of the pit, but they’re still definitely centered on where the pit is deepest. It’s very interesting, therefore, when the mass centers in galaxy clusters aren’t fully lined up. In today’s article, the authors explore one of these cases.

A Misaligned Galaxy Cluster

The cluster explored in this study, called Abell 2495, has been known about for a long time — it was discovered in 1998 through an X-ray search for clusters using the ROSAT satellite. Since then, a wide variety of data have been collected about this cluster at many different wavelengths that trace different components of the cluster. The data that are most relevant here are radio data from the Very Large Array at a frequency of 5 gigahertz (tracing activity from the active galactic nucleus inside the central brightest cluster galaxy) and optical images (showing the positions of the galaxies in the cluster) from the Hubble Space Telescope.

In addition to the ROSAT observations used to discover the galaxy cluster, there were also Chandra X-ray Observatory data of Abell 2495. However, the observations from both telescopes had very low sensitivity, making it difficult to distinguish any features in the X-ray-emitting gas. The authors present six new deep Chandra observations in this article, vastly improving the sensitivity of the X-ray observations (which trace the hot gas of the intracluster medium). Figure 1 combines several of these different types of observations, plotting them together as contours. X’s mark the various centers of the galaxy cluster components.

Figure 1: Multiwavelength observations of the galaxy cluster Abell 2495. The greyscale (and black contours) shows the X-ray emission from the hot gas in the galaxy cluster, the red contours show hydrogen gas (Hα), and the green contours show radio emission coming from the supermassive black hole inside the cluster’s central galaxy. The X’s show the center of the cluster determined in different ways: the black X is the center of the X-ray emission, the red X is the center of the Hα emission, the yellow X is the center of mass of the cluster (determined using the positions of the individual cluster galaxies), and the green X is the center of the brightest cluster galaxy. [Rosignoli et al. 2024]

A Gassy History

In order to understand why the galaxy cluster isn’t centered properly, we need to look back at the history of its movements. Luckily, there’s an easy way to do that — the X-ray light, which traces the intracluster medium, contains signatures of any strange happenings in the cluster’s recent history. This is because the intracluster medium is normally smooth and symmetric, but it takes a while to settle after disturbances. The authors’ new Chandra observations are thus perfect for finding out what’s going on in Abell 2495.

Some of the more interesting features the authors found in the X-ray observations are shown in Figure 2. The four cavities (the dark regions in Figure 2a) are indicators that the active galactic nucleus inside Abell 2495’s brightest cluster galaxy had periods in the past where it was extra active, emitting enough energy to blast holes in the intracluster medium. What’s particularly interesting is how some of these cavities line up with the current position of the emission from that active galactic nucleus (shown with the blue contours). The authors also found a significant density jump on one side of the intracluster medium (the light region in Figure 2b), suggesting that some force has piled up the gas on one side of the cluster in a dense cold front.

depictions of the cavities and cold front seen by the authors in this work

Figure 2: Interesting features in the X-ray observations of Abell 2495. In the left panel, the four cavities discovered by the authors are circled in green. These are areas of the otherwise smooth intracluster medium where very little material is present. In the right panel, a fitted profile of the intracluster medium has been removed, showing a large asymmetry with a significant jump — a “cold front.” [Adapted from Rosignoli et al. 2024]

temperature map of the intracluster medium of Abell 2495

Figure 3: A binned temperature map of the intracluster medium inside Abell 2495, measured using the X-ray emission. The regions outlined in black are cold enough to condense significantly in a reasonable timescale. [Adapted from Rosignoli et al. 2024]

Finally, the authors bin up their X-ray image in 2D space so they can use the spectra of the X-rays to calculate the temperature of the intracluster medium gas in each bin. Figure 3 shows the resulting temperature map. The regions of the map outlined in black have gas temperatures below a key threshold that means it can likely condense enough to fuel the active galactic nucleus in the center of the cluster. This also means that the cluster is cool-core (see an astrobite about this here). Interestingly, there’s also an extended tail of cold intracluster medium spiraling away from the center of the cluster, in a position that almost lines up with the cold front discussed above.

The Cosmic Bathtub

The authors of this article believe that all of this evidence points to one very interesting phenomenon: the galaxy cluster is sloshing. This is a phenomenon that’s appeared widely in simulations and has also been observed several times. It happens when some gravitational disturbance, typically a small “sub-cluster” of galaxies, passes by the cluster. This disturbance pulls both the dark matter and the intracluster medium in its direction. The dark matter can pass right through anything in its way, but the intracluster medium of the sub-cluster collides with the other intracluster medium of the other cluster, creating cold fronts, and so the two components get separated. This results in offsets between the centers, just like the ones observed in Abell 2495! As the disturbance passes, the dark matter and intracluster medium fall back towards the center of the cluster, setting up an oscillation just like water sloshing in a bathtub.

The Scientific Relevance of Baths

The reason why sloshing is so fascinating is because it could be regulating active galactic nucleus feedback. Normally, galaxy clusters act out the feedback cycle described in the introduction — the intracluster medium cools and collapses onto the active galactic nucleus, which in turn gets extra active and heats the intracluster medium back up again. In this case, however, the authors believe that the sloshing in this galaxy cluster is driving the whole feedback cycle. There is some quantitative evidence for this — the time the cluster takes to slosh is fairly consistent with the time between the formation of the different cavities in the intracluster medium, and the size of the X-ray cavities is consistent with the amount of energy that would be involved. If this is the case, sloshing could drive the whole life cycle of the cluster.

Sloshing Towards the Future

The authors have done a lot of analysis to come up with these findings, but there are some limitations in the data that mean significant uncertainties remain. Better X-ray data could uncover more details about the X-ray cavities, helping to narrow down when they were formed and how much energy was required to create them. A clearer picture of the cold front would allow the intracluster medium in the cluster center to be examined in more detail. It would also be helpful to get a larger sample of clusters with active galactic nucleus feedback potentially regulated by sloshing to understand if this is a common process, or if Abell 2495 is unique. Either way, this is a fascinating addition to our picture of one of the most extreme places in the universe — the center of a galaxy cluster.

Original astrobite edited by Lina Kimmig.

photograph of the Small Magellanic Cloud

30 January 2024

Two Small Magellanic Clouds Are Better than One

Title: A Galactic Eclipse: The Small Magellanic Cloud is Forming Stars in Two, Superimposed Systems
Authors: Claire E. Murray et al.
First Author’s Institution: Space Telescope Science Institute
Status: Published in ApJ

The Small Magellanic Cloud is one of the most-studied galaxies in our universe due to its proximity (just 200,000 light-years away), and because its structure and elements are so distinctly different from our own Milky Way galaxy. For example, it has a lower metallicity and is therefore an excellent laboratory for understanding the physics of the interstellar medium at lower-metallicity conditions. The interstellar medium is the gas and dust between stars within a galaxy (as opposed to the circumgalactic medium, which is the gas and dust between galaxies).

Despite its potential utility as a nearby parallel probe of galaxy evolution, the Small Magellanic Cloud’s structure is still relatively unknown. For example, stars with different ages appear to be distributed through the galaxy differently. The oldest stellar populations appear to be distributed spherically without rotating, whereas younger stars are rotating. Furthermore, the structure of the Small Magellanic Cloud’s interstellar medium indicates that it has potentially been severely disrupted by recent interactions with the Large Magellanic Cloud. Modeling the evolution and interaction history of the Small Magellanic Cloud from its conflicting observational constraints will help inform our understanding of its future.

The authors combined observations of neutral hydrogen (HI) gas emission and radial velocity (velocities along our line of sight) measurements to model the Small Magellanic Cloud’s evolutionary history. Because the interstellar medium of the galaxy is dominated HI emission (hydrogen gas that has not been ionized by any astrophysical source like a star), observing this atomic gas allows us to trace the bulk properties of the interstellar medium. As shown in Figure 1, the radial velocities derived from the emission of the HI gas show two distinct structures moving away from us — one at a higher speed and one at a lower speed.

radial velocity map of the Small Magellanic Cloud

Figure 1: The radial-velocity map derived from HI emission of the Small Magellanic Cloud shows two distinct structures moving away from us, one at high radial velocity (~170 km/s) in red on the left, and one at low velocity in blue on the right (~130 km/s). The magenta cross represents the center of the Small Magellanic Cloud. [Adapted from Murray et al. 2024]

Although the HI emission can provide a window into the velocity structure of the Small Magellanic Cloud, it cannot probe the relative distances of sources. The authors instead use a map of extinction from dust to look at the relative spatial order of stars along the line of sight (i.e., in “front” or “behind” the dust). A star at a location with high extinction is behind more dust than a star at a location with low extinction. If you also assume the dust and stars are located in similar locations, then you can trace the actual locations of the stars. For example, a star behind a lot of dust is probably farther away from us (“behind”) than a star in front of the dust (“front”). To estimate the extinction towards each source, the authors use the Rayleigh Jeans color excess method, which assumes the amount of dust based on an observed color. A star with a redder color is assumed to suffer from higher extinction.

maps showing the locations of the stars in the front and behind structures

Figure 2: The stars in the “front” structure and stars in the “behind” structure, derived from the extinction and radial velocities of these stars. [Murray et al. 2024]

Figure 2 shows the final inferred maps of the “front” and “behind” structures of the Small Magellanic Cloud. Using metallicities derived from the APOGEE survey, the authors also determined that generally the stars in the front component are higher metallicity than the stars in the behind component.

In conclusion, these results indicate the Small Magellanic Cloud is clearly composed of two distinct star-forming systems. One possible explanation for these results is that these two systems are actually remnants of different galaxies, potentially indicated by the fact that they have different metallicities. Alternatively, the authors propose that the “behind” structure is actually tidal debris from an interaction with the Large Magellanic Cloud. Ultimately, further observational constraints, such as direct distance measurements to the interstellar medium components and simulations of the Small Magellanic Cloud’s evolution, will help paint a more detailed picture of its history and future.

Original astrobite edited by Jessie Thwaites.

Illustration of a rogue planet floating through space

23 January 2024

Tilted Tatooines Can Serve as a Source of Strange Solo Planets

Title: Tilted Circumbinary Planetary Systems as Efficient Progenitors of Free-Floating Planets
Authors: Cheng Chen et al.
First Author’s Institution: University of Leeds
Status: Published in ApJL

All planets orbit stars.

Okay, that was a lie. But a hundred years ago, it seemed super true — we only knew of the planets around our own Sun.

Then, later, it still seemed decently true. Astronomers started finding planets around other stars in the 1990s (which the opening statement supported!). At this point, though, the field had at least begun to consider that there could be a population of “free-floating planets”: planet-sized bodies wandering through space, unaffiliated with any particular star.

Because our methods of finding planets, at that point, relied on the dimming or shifting of a host star’s light, these rogues seemed unknowable (if they existed at all). So perhaps the statement could be modified to read “all planets that we can detect orbit stars,” which would still maybe hold a little truth.

As our telescopes (and our techniques) have improved, though, it has become very clear that the statement is… irredeemably untrue. Since the turn of the century, largely through a more direct observational technique called microlensing, we’ve gradually uncovered a rich population of free-floating planets dancing in the vast cosmic dark. (Just in the past few months, JWST revolutionized yet another sub-field with its discovery of free-floating pairs of planets, but that’s a topic for another ’bite.)

Young, Wild, and Free: The Instability of Newborn Planetary Systems

But where do these unbound beauts come from? There are a few proposed mechanisms, but one of the most likely methods involves planets forming around stars (like normal!) but subsequently being violently ejected from their bound orbits.

The period immediately following the formation of the planets is often suspected to be pretty messy. In the solar system, we think there was a major instability right after planets formed, in which our eight major planets interacted strongly with each other before settling into roughly the orbits they have today.

In fact, simulations of such an instability suggest that there could have originally been a fifth giant planet in the outer solar system, formed alongside Jupiter, Saturn, Uranus, and Neptune. During the period of unrest (often called a “late instability” due to its occurrence after planet formation was complete), this planet would have been “kicked” off of its original orbit — and ejected from the solar system altogether — through interactions with the other giants. Any such planet would, at this point, be a free-floater.

Today’s article talks about these sorts of “planet–planet scattering” ejections, but it doesn’t talk about the solar system (nor any particular observed planetary system). Instead, in a common theorist tactic, the authors consider a whole class of planetary systems: those existing around two stars rather than one.

Two Stars Are Better than One (for Planetary Ejections)

Just as planets can orbit stars, stars can orbit each other. And if their orbit is small enough, protoplanetary disks — and, thereafter, planets — can form around the pair of them. One of the most famous (fictional) examples of such a system is Star Wars’ Tatooine, which enjoys an iconic double sunset courtesy of the binary stars around which it orbits. A diagram of planets orbiting a pair of stars is shown in Figure 1.

diagram depicting the setup described in this work

Figure 1: In the setup described in this work, two planets (green) orbit around a binary pair of stars (yellow). The planets’ orbits are separated by a dimensionless distance parameter Δ — the distance between the orbits in terms of their mutual sphere of influence. The orbits start off aligned with each other (though this can change over the course of a simulation!), but they aren’t necessarily aligned with the orbit of the stellar binary. The authors test the stability of systems spanning a range of different inclinations (i_p) and separations (Δ). [Mark Dodici]

Ejections in planetary systems around single stars, though possible, probably don’t happen often enough to explain how common free-floating planets are. In dynamics, things can get a lot more complicated when you add just one extra body (see, e.g., the n-body problem!), so the authors of today’s article wonder if adding another star will provide enough complication to eject more planets.

(This isn’t an unreasonable addition; binary pairs — and even binaries close enough to allow for planet formation — make up a significant fraction of all stars, so they’re definitely worth modeling.)

Specifically, the authors simulate planetary systems where two planets, separated by some distance, trace out orbits that are inclined relative to the orbit of the central binary stars. (See Figure 2 for examples of different orbital inclinations.) They test systems with a range of initial inclinations, for a range of planet separations, for a select few choices of unequal planet masses. Through these simulations, they cover a broader range of parameter space — i.e., the ranges of values that a real-life system could have for its properties — than previous work.

Figure 2: Examples of planetary systems with a few different inclinations, from prograde to retrograde. From this work, the orbits that most commonly lead to ejections are neither prograde, nor polar, nor retrograde — that is, systems with i_p ≈ 45° see ejections most often. [Mark Dodici]

For all planet masses, the systems most likely to yield ejections involve close-together planets with orbits that are neither well-aligned (“prograde” or “retrograde”) nor perpendicular to the binary (“polar”). (Results are presented in Figure 3.) Systems are much more likely to yield ejections when they have at least one planet more massive than Neptune, though the less-massive planet, if there is one, is always the one ejected. Eccentric binary stars — those orbiting each other on oblong, elliptical paths rather than perfect circles — also boost the ejection rate of the planets around them.

Plots showing which parameters resulted in an ejection and which parameters resulted in a stable system

Figure 3: For a given system, the authors reported whether each set of parameters was stable (blue) or resulted in an ejection (light red). On the left, we see a range of systems with different inclinations and planet separations around circular binaries. On the right, we see the same range of inclinations and separations around eccentric binaries, with e = 0.8. All systems in these plots have a Jupiter-mass outer planet with an Earth-mass planet orbiting interior to it. Dashed, colorful lines on these plots point out a few mean motion resonances (see text), which are known to be unstable in tilted planetary systems. In the article, the authors show similar plots for different combinations of planet masses; these generally show similar trends in ejection vs. stability for various inclinations and separations. [Adapted from Chen et al. 2024]

There are some fun planetary dynamics underlying these results! From the basic law of gravity, we know that more-massive, closer planets have more of an impact on others in their own system; it makes sense, then, that more-massive, closer planets cause more ejections. Oddly inclined orbits (i.e., not prograde, polar, or retrograde) often lead to long-term oscillations in inclination; if the two planets take on different inclinations over the course of their lifetime, von Zeipel-Kozai-Lidov cycles can sometimes increase their eccentricity until one of them becomes unbound. And although ejections were most common among close-together planet pairs, they aren’t impossible with a bigger gap; instability can occur near mean motion resonances, where the periods of the two planets are integer multiples of each other (e.g., at the 2:1 resonance, the inner planet orbits twice as quickly as the outer).

A Few More Free-Floaters

That all said, this isn’t an article about planetary dynamics — it’s about free-floating planets. These results show that a decent variety of planets can be ejected from planetary systems around binary stars, yielding a decent total occurrence rate for free-floaters from this mechanism. This is a decent win for planet–planet scattering as a potential source of unbound planets!

As in any theory article, there’s work to be done to constrain exactly how often we expect these Tatooine-like systems to eject planets, as well as the range of masses we would expect these ejected planets to have. And in reality, the total population of free-floating planets almost certainly comes from some combination of several effects, each contributing their own subpopulations. But with recent and upcoming observations from Subaru Hyper Suprime-Cam, TESS, Roman, and JWST, there will certainly be no lack of data to which models like these can be compared.

Original astrobite edited by Sahil Hedge.

Artist's impression of a protoplanetary disk

16 January 2024

What Drives Chemistry in a Protoplanetary Disk?

Title: Disentangling CO Chemistry in a Protoplanetary Disk Using Explanatory Machine Learning Techniques
Authors: Amina Diop et al.
First Author’s Institution: University of Virginia
Status: Published in ApJ

As the birthplaces of planets, protoplanetary disks hold many useful clues for how planets form and evolve. One of the pieces of information we can gather from disks is composition — to date, more than 35 different molecules have been detected in disks (see this bite for a summary of how these molecules are detected). Ideally, we want to be able to connect the abundances of different molecules to disk properties, such as total mass. This is more easily said than done, however, as a lot of different factors can alter the observed abundances of different molecular species. For example, one of the molecules we arguably have the best chemical understanding of is CO (carbon monoxide), which has been studied in a number of protoplanetary disks. Nonetheless, measured CO abundances tend to be lower than expected, indicating that effects such as local variations in temperature and dust distributions throughout the disk may have a significant impact on the observed abundances.

In order to try to understand the connection between local disk environments and molecular abundances, studies typically simulate a disk many times over, running through a huge range of different disk properties. As you might imagine, this approach is slow and takes a lot of computational resources. Today’s authors are trying a new method, using machine learning to more quickly find connections between disk properties and abundances. The goal is to use machine learning to search large parameter spaces and understand which physical disk properties are most important to creating and destroying CO in disks.

Figure 1: The initial CO abundances relative to hydrogen throughout the disk. You can think of r and z as x and y coordinates in the disk, where the star would be at (0,0). [Diop et al. 2024]

Making a Disk Model

The authors start with a disk around a T Tauri star (a type of young variable star that could be a typical host to disks). They assign a corresponding temperature and X-ray luminosity to the star, and they assume the disk is a mix of gas and dust, with both large and small dust grains. After assuming some initial compositions for the disk, they then apply a complex chemical code, including several thousand (!) different reactions to the modeled disk to calculate the CO abundances over 3 million years. Figure 1 shows the initial distribution of CO in one of these models.

Bring on the Machine Learning!

The authors consider each CO abundance calculated in the disk as a separate sample (so each point in Figure 1 is a sample) and check how each point’s CO value compares to local disk properties at that point, such as temperature, ultraviolet flux, and gas density. Figure 2 shows one example, with CO abundances plotted for specific temperatures and gas densities throughout the disk.

CO to H abundance ratio as a function of gas density and temperature

Figure 2: The disk from Figure 1, a million years later. The points now show the CO abundance ratio for the corresponding temperature and gas density at each point in the disk. [Adapted from Diop et al. 2024]

To test the relationship between CO and the selected disk parameters, the authors assume their data follow a polynomial regression, where the CO abundance is a function of the different disk parameters, each multiplied by some coefficient. They then use a machine-learning algorithm to solve for the coefficients that best recreate their modeled disk. The resulting coefficients indicate which parameters are most important to determining the resulting CO abundances.

The Top Contributors

After running their model and machine-learning algorithms, the authors found that the log of gas density is the most important factor in determining CO abundance. They note that this makes sense because the abundance of CO is typically related to the abundance of hydrogen (H₂) gas in a disk. Interestingly, they also find that the log of gas density squared decreases with increasing CO. This is because at higher densities there is a competing effect of depleting CO as it freezes into ice in dense, cold regions of the disk.

Figure 3: The coefficients indicating positive or negative correlation of the parameters listed on the left with CO abundance. T_gas and n_gas correspond to the gas temperature and density, while F_UV is the ultraviolet flux from the star, ζ_cr describes the cosmic ray rate, and ζ_xr is the X-ray rate. [Diop et al. 2024]

All together, 10 factors are considered as disk parameters, and Figure 3 shows the different coefficients for each, where positive coefficients indicate positive correlations with CO abundance and negative coefficients indicate negative correlations. The authors find that the negative correlations (aside from the strong effects of gas density) are stronger than the positive ones, indicating that disks may tend to destroy CO overall. They also find that other aspects such as the initial carbon-to-oxygen ratios and rates of cosmic rays can also influence the relative strengths of the different factors.

Know Thy Disk

This study provides a great proof of concept for the ways in which machine learning can be used to identify complex relationships in datasets — a particularly useful tool for studying the large number of entwined reactions involved in chemistry. It also brings up the important issue that molecular abundances can be influenced by many different factors, so we should consider disks as holistically as possible and always think about how multiple parameters may interact for a particular disk!

Original astrobite edited by Lucas Brown.

poster advertising the Rainbow Village at AAS 243 and the logos of the four supporting organizations

3 January 2024

The Rainbow Village at AAS: A Gathering Place for People of Color in Astronomy

Written and edited by Rainbow Village partners Arianna Long, Nicole Cabrera Salazar, Ashley Walker and Junellie Gonzalez Quiles.

Are you a person of color or an ally to people of color in astronomy and related fields? Have you wanted to connect with other people of color at the American Astronomical Society (AAS) meetings but found it difficult to do so? Have you wanted a space throughout the entire meeting to connect with other folks and receive support and resources for your career? If so, you are not alone!

This January, we are launching The Rainbow Village at #AAS243, an initiative born out of the need to provide a permanent space throughout the AAS meeting where people of color can gather, support each other, and obtain access to organizations that are directly serving people of color in astronomy.

The Rainbow Village in Context

Fred Hampton speaking at a rally in Chicago in 1969. [Wikipedia]

After recognizing the overlap in class struggles among different racial groups, Fred Hampton, the leader of the Chicago Black Panther Party, began to form alliances with organizations like the Latinx group, the Young Lords Organization, and working class white people of the Young Patriots. This alliance was named the “Rainbow Coalition” and it targeted the structural inequalities that Chicagoans faced in the system through community building and collective action.

Following this historical precedent, Dra. Nicole Cabrera Salazar, an astronomer and STEM equity advocate at Movement Consulting, came up with the new initiative of the Rainbow Village at AAS. The Rainbow Village was created with the similar intention of bringing people of color from different demographics to gather, meet, and support each other. It will be a place to form meaningful connections and obtain direct access to resources about how to navigate the field of astronomy as a person of color.

The Rainbow Village is a collaboration of four organizations who have partnered to provide this space at AAS:

AAS Committee on the Status of Minorities in Astronomy (CSMA): A committee of the AAS dedicated to enhancing the participation of underrepresented minorities in astronomy at all levels of experience.
#BlackinAstro: A community and grassroots organization formed to celebrate and amplify Black scientists and engineers within the space community.
VanguardSTEM: An online platform and empowered community devoted to connecting and uplifting emerging and established women of color, girls of color, and non-binary people of color in STEM.
League of Underrepresented Minoritized Astronomers (LUMA): A peer mentoring community for Black, Indigenous, and Latinx women of color in their graduate degrees or beyond in their careers in astronomy, physics, and related fields.

Logos of the organizations that have partnered to create the Rainbow Village

Logos of all four organizations that have partnered to create the Rainbow Village at AAS.

Each organization will provide resources for people of color to succeed in their careers while highlighting their meaningful contributions to creating a safe and inclusive place in the field of astronomy.

How can I participate in the Rainbow Village at AAS?

The Rainbow Village is a furnished booth and will be located next to the AAS Pavilion at the New Orleans Convention Center Exhibit Hall. The space is designed for folks to relax between sessions, connect with each other, share and celebrate AAS presentations, and grow together through exploratory salon-style discussions. We will have a variety of scheduled and ongoing programming, including:

A Community Calendar: Giving a talk? Presenting your first poster? We will have a digital and physical calendar of events that showcase our community members. Tell us about yours so we can cheer you on as your astro siblings, cousins, and fam!
Community Discussions: Join us for daily salon-style discussions on topics that are important to our community (e.g., radical mentorship, activism, climate justice). These will be group discussions led by our own community members. Stay tuned to learn more about the topic schedule!
Walls of Support: How are you navigating self-care while conferencing? What words of kindness would you give to yourself the first time you attended an AAS meeting? You can share these and other tips/affirmations in written form on the Rainbow Village walls.

I am not a person of color, but I would love to get involved. How can I help?

We will be advertising AAS meeting talks, posters, and events led by people of color at the Rainbow Village and on social media with the hashtags #AAS243 and #RainbowVillage. We would love for folks to signal boost and make sure to attend those events. If you will be present in person at the January 2024 meeting and would like to volunteer, please fill out this form.

Please also stop by our booth in the Exhibit Hall to listen to our community discussions and offer resources for people of color. We would love to have you!

Starting in December, Astrobites has released a series of articles highlighting the representatives of the Rainbow Village and their respective organizations. Please stay tuned to learn more about these organizations and get updates on the programming available for all throughout the AAS meeting in New Orleans this January 2024!

poster advertising the Rainbow Village at AAS 243

Artwork by Arianna Long.

Original Astrobite edited by Sahil Hedge.

illustration of the structure of the Milky Way

2 January 2024

Dusting for Spectral Fingerprints to Determine the Origins of Stars in the Milky Way Halo

Title: SPLUS J142445.34-254247.1: An r-process-enhanced, Actinide-Boost, Extremely Metal-Poor Star Observed with GHOST
Authors: Vinicius M. Placco et al.
First Author’s Institution: NSF’s National Optical-Infrared Astronomy Research Laboratory (NOIRLab)
Status: Published in ApJ

Have you ever looked at the periodic table and wondered where all the elements come from? Nearly all of the hydrogen, helium, and some of the lithium present in the universe formed in the first three minutes after the Big Bang. Beyond that, most of the elements (those that are heavier than helium are called metals by astronomers) were created in nuclear reactions that take place in the heart of stars. Up through iron, the formation processes happened systematically, where heavier elements were formed from the fusion of lighter elements. Beyond iron, about half of the elements are created by a slow and gradual process of capturing neutrons (called the s process, with s for slow).

Elements beyond iron are created in extreme events, such as stellar explosions (see Figure 1 for an overview). Such cataclysmic events cause neutrons to rapidly bombard atomic nuclei, forming heavier elements. This rapid capture of neutrons is called the r process (with r for rapid). There is a constant exchange of elements between the stars and the gas surrounding them. This eventually leads to the formation of more stars that are enriched in metals, setting up different generations of stars.

periodic table of the elements indicating the origins of each element

Figure 1: The astronomer’s periodic table indicates the elements’ astrophysical origin. [NASA’s Goddard Space Flight Center]

Studying the s and r processes can give us valuable insight into the nature and conditions prevalent in the universe when stars form. The s-process elements are believed to originate from low-mass stars as they evolve throughout their lifetime. The r-process elements come from dramatic events, such as supernovae or neutron star mergers. The mechanism through which a star’s r-process elements were created is determined mainly by modeling, as direct observational evidence of such events is rare.

If we obtain high-resolution spectroscopy of a star and map the abundances of all its elements, it would paint a picture of the events that led to its formation. In today’s article, the authors try to understand how stars were formed in the halo of the Milky Way by recreating the formation scenario based on the fingerprints in the high-resolution spectrum obtained from a Milky Way halo star called SPLUS J142445.34–254247.1, or SPLUS J1424−2542 for short.

Recreating the Formation Scenario

The star’s chemical abundances are determined from absorption features in its spectrum. If a particular element is present, it will absorb the starlight passing through the cold gas at a characteristic wavelength, and the extent of absorption can be used to determine how much of the element is present in the star. Looking at the elemental abundances of SPLUS J1424−2542, the star showed signs of being poor in iron (atomic number Z = 26) but being enhanced in elements with atomic numbers 26 < Z < 38 compared to the standard values measured from the Sun. This indicates the star formed from a gas cloud polluted by two distinct populations of stars in a multi-enriched process.

The abundances of the heavier elements indicate that the primary process involved in the formation is the r process. This is not uncommon for old stars in the galactic halo. The s-process elements are formed from the death of low-mass stars, which would not have occurred when such old stars were formed in the halo. Most elements in halo stars are created through the r process, even those typically formed by the s process. However, the authors found that certain elements, such as strontium, do not agree with the predicted values from either the r process or s process (Figure 2). Other elements, such as barium, are overproduced, indicating contributions from both s and r processes. The team also found an overabundance of thorium, which could indicate a possible contribution from a separate r-process event. Thus, this star is metal poor with enhanced heavy elements produced by r-process events.

observed elemental abundances compared to expected values from the r process and the s process

Figure 2: Observed abundances (red circles) with the expected values from the r process (blue line) and s process (yellow line). Most points lie on the blue line, indicating a more significant contribution from the r process. [Placco et al. 2023]

Modeling the formation scenarios predicts that the lighter elements (Z ≤ 30) were likely produced by a metal-free star with a mass in the 11.3–13.4-solar-mass range that exploded with low energies, a characteristic property of older Population III stars. The heavier elements (Z ≥ 38) were likely formed from the merger of two neutron stars with masses of 1.66 and 1.27 solar masses, indicating that at least two progenitor populations enriched the star.

Mysterious Circumstances Prevail

The authors derived the star’s kinematic properties (such as orbital dynamics, velocities, etc.), which are displayed in Figure 3. They found that the proposed formation scenario and the derived kinematics do not connect the star with any known structure in the Milky Way. This highlights that a distinct star formation mechanism may occur in the galactic halo. We must continue studying similar stars with high-resolution spectroscopy to help us understand the formation of old stars in the Milky Way halo.

comparison of the properties of the star studied in this article to those of stars in known Milky Way substructures

Figure 3: The yellow star indicates the star studied in this article. The upper panel shows it does not fall into the expected Milky Way streams. The bottom panel shows that it has distinct properties from other stars in a similar position on the upper panel. [Placco et al. 2023]

Original astrobite edited by Roel Lefever.

illustration of planets around an M-dwarf star

12 December 2023

Siblings or Only Children: M-dwarf Planets

Title: A Comparison of the Composition of Planets in Single-planet and Multiplanet Systems Orbiting M dwarfs
Authors: Romy Rodríguez Martínez et al.
First Author’s Institution: The Ohio State University
Status: Published in AJ

The most common type of star in the universe is the M dwarf, making up ~70% of all stars. Because M dwarfs are so common, and because we know exoplanets are common, it is only natural that we have found many exoplanets orbiting M-dwarf stars. In fact, Astrobites covered seminal articles on the occurrence rate of planets around M dwarfs and their compositions; read them here and here. But with all these planets, we can take our knowledge one step further and begin to ask deeper questions. For example, in M-dwarf planetary systems, how do single-planet systems (only child) and multi-planet systems (siblings) compare?

A new study seeks to answer this question, focusing on three key parameters: planet bulk density, planet core mass fraction, and host-star metallicity. In particular, the authors wish to investigate whether siblings and only-child systems are two outcomes of the same formation process, or if they truly form differently. In other words, are they from the same population, or are they two distinct populations of planets?

Bulk Density

First, bulk density, or the average density of the planet as a whole. We know that Earth is made up of many different materials (rocks, water, gases, etc.) and each one of these has its own density. But for exoplanets, we cannot explore the details of different materials and so instead we measure bulk density by simply taking the total mass of the planet and dividing by the total volume (assuming the planet is a sphere). The authors compute bulk density for a sample of planets around M dwarfs, computing this quantity for both the single-planet systems and all the planets in multi-planet systems.

Next they apply a statistical test (the Kolmogorov–Smirnov test) to determine if the two sets of planets are truly distinct populations or are consistent with one population (see Figure 1 top panel). The result overwhelmingly shows that these are two different populations. However, the authors caution that this result may be biased. Many of the single-planet systems in the sample are giant planets, which are naturally lower density than smaller planets are because they have higher gas fractions. Removing the gas giants from the sample and re-running the test, the authors find that actually the siblings and only-child planets are consistent with coming from the same population (see Figure 1 bottom panel).

plots of cumulative distribution function vs planet density

Figure 1: Top: The results of the statistical test when including all planets to determine if the two populations are distinct. The gap between the single and multis suggests they are indeed two populations. Bottom: The same as the top panel but for the sample that excludes giant planets. Here the finding of two populations is less statistically significant. [Adapted from Martínez et al. 2023]

Core Mass

Next, planet core mass. The mass of a planet is generally meant to include everything that makes up the planet. However, the core mass is just that, the mass of the core of the planet alone. Core masses are valuable pieces of information because it is thought that the size of the core, which is the first to form, can determine how big the planet eventually grows to be. Bigger cores are better at gravitationally attracting material, including gas, to grow the planet. While we cannot directly measure the core mass of a planet, we can use models that are tuned to Earth’s parameters to estimate planet core mass based on a few things we can measure, like mass and radius. Now taking only the planets that are likely to be rocky and again splitting by single versus multi-planet systems, the authors find that planets in single-planet systems have, on average, larger core masses than those in multi-planet systems. They further test if core mass correlates with orbital period but find no correlation.

Metallicity

Lastly, the authors explore the host star, particularly its metallicity, or the percentage of the star’s composition that is made up of “metals” (astronomers define “metal” as anything heavier than helium!). Host star metallicity is thought to correlate with the kinds of planets and number of planets in the system, the thinking being that since planets form out of the same disk of material as the host star, if there are more heavy materials in that disk (which would appear as higher metallicity in the host star) then there is more opportunity to make more and bigger planets. The authors here find that host stars of single planets are more metal rich than those hosts of multi-planet systems. This is counterintuitive, but the authors hypothesize this could be because more metal-rich stars might produce more and bigger planets, which may gravitationally interact in the early days of the system and fling out all but one planet. On the other hand, metal-poor stars cannot build big planets and instead build small planets that are dynamically “quiet.”

In all, the authors find that single- and multi-planet M-dwarf systems are likely two distinct populations. This could have large implications for how we understand the formation and evolution of planetary systems.

Original astrobite edited by Mark Popinchalk.

a photograph of the Sun and an illustration of a pulsar

5 December 2023

Forget X-ray Vision: Using Gravitational Waves to Peer Inside the Sun

Title: Probing the Solar Interior with Lensed Gravitational Waves from Known Pulsars
Authors: Ryuichi Takahashi
First Author’s Institution: Hirosaki University
Status: Published in ApJ

Forget X-ray vision — how about gravitational wave vision? In today’s article, a team of researchers examine the possibility of using gravitational waves from distant pulsars to learn about our own Sun. Much like the terrestrial seismologists and geologists who peer into Earth by listening carefully to the ways waves are distorted as they travel through its many layers, these imagined future gravitational heliophysicists would use the subtle distortions of gravitational waves that have passed through the Sun to learn about its inner contents.

Multi-messenger Seismology?

The idea of using the interaction between waves and normal matter to peer under the surface of otherwise opaque objects has a longstanding history in physics. Think for example of the idea of seismic tomography, one of the main tools used by seismologists and geologists to understand the makeup of Earth’s interior. As powerful shock waves move through Earth’s interior after seismic events like earthquakes, they come into contact with regions of material that may differ in their composition, density, temperature, and so on. Depending on the wave frequencies and the properties of the materials with which the waves come into contact, these waves will reflect, refract, diffract, or be otherwise altered from their initial waveform. Measuring the properties of these waves when they make contact with the surface again at different points around the world can thus tell us a great deal about what they may have encountered on their journey through the depths.

Similarly, when electromagnetic waves (light) encounter solid objects, they can undergo a number of interactions: certain wavelengths will be scattered or absorbed, and others may pass directly through an object. This is, roughly speaking, how X-ray imaging works: certain high-frequency electromagnetic rays can easily pass directly through your soft skin, but they will be reflected upon encountering denser material like bones, allowing us to reconstruct images of the interior of our own bodies without the need for invasive surgeries (thanks, science!).

Today’s article examines the feasibility of doing similar reconstructions but with waves of a different kind: gravitational waves. Gravitational waves, which have been discussed at length in previous Astrobites, are a hot topic right now within the astrophysics community given that the technology to directly detect their subtle presence has only come to maturity within the past decade or so. This is because gravitational waves, which are produced by the asymmetric motion of massive objects like black holes in binary orbits, produce extremely subtle effects here on Earth due in part to the weakness of the gravitational force and in part to the large distances the waves have typically traversed to reach us. Now that we are measuring these faint signals with regularity in ground-based gravitational wave detectors like LIGO, VIRGO, and KAGRA, interest has been growing in finding more and more exotic ways to use this brand new window into the universe to uncover its many secrets.

To understand the particularly out-there idea behind today’s article, we need to introduce at least one further concept: gravitational lensing. Gravitational lensing occurs when particles (or waves) travel close to a massive source and as a result have their trajectories altered. When a massive source (like a galaxy cluster) sits more or less directly between Earth and some distant object (like an individual galaxy), this deflection can act like a lens, focusing the light from the background galaxy towards Earth to make the galaxy appear bigger, brighter, or even more emoji-like. Additionally, as waves of any kind travel past such a lens, they will appear to take longer to reach the other side than they would have if there were no massive source. This effect is known as gravitational time delay, and it can sometimes manifest in ways not too dissimilar from the way in which light appears to “slow down” when passing through dense mediums like water. Gravitational lensing can cause all sorts of waves including gravitational waves to undergo many distorting effects similar to light traveling through media of varying densities or seismic waves traveling through Earth’s interior.

Taking all of these effects into account, we can begin to see why the idea of using gravitational waves to probe the interior of the Sun isn’t so far-fetched. While electromagnetic waves cannot typically pass into and out of the Sun due to their interactions with dense solar material, gravitational waves from a source behind the Sun would pass through easily, only experiencing distortion due to the aforementioned lensing effects caused by the varying density of the Sun along the line of sight between detectors on Earth and the source of the gravitational waves. While this idea has been explored before, today’s authors attempted a comprehensive analysis of what these distortions might look like for a set of real sources and examined how feasible it would actually be to detect them in present or future gravitational wave observatories. So can it be done? As it turns out, with some upgraded detectors, a dash of new millisecond (very fast-spinning) pulsar discoveries, and a bit of luck — it can!

Looking for Magic Millimeter Mountains

diagram showing the alignment of Earth, the Sun, and a distance source of gravitational waves that passes directly behind the Sun

Figure 1: Depiction of the arrangement of an Earth-based gravitational wave detector and an object that emits continuous gravitational waves that is of interest to the authors of today’s article. When gravitational waves from the source have to pass directly through the Sun to reach Earth, they can be deflected from their original path and distorted in ways similar to how light is distorted when passing through a traditional lens. [Takahashi et al. 2023]

Unlike with light waves or even some seismic waves, humans do not have the capacity to generate gravitational waves of sufficient power to be measured and manipulated for the purposes of doing experiments. Instead, if we want to use these new waves to our benefit, we have to get clever with what nature has provided for us. Firstly, what we will need is a source of continuous gravitational waves. Unlike most of what LIGO sees right now, which are the signature “chirps” of compact objects undergoing their last seconds of merging, the continuous waves (meaning gravitational wave signals that are continuously emitted from a source and detectable for some appreciable amount of time) that are expected to be seen by ground-based detectors are most likely to come from tiny (sub-millimeter scale) deformations (sometimes called “mountains”) on the surface of rapidly rotating pulsars. Once gravitational wave detectors increase in sensitivity enough to finally detect these waves, researchers will want to find sources that occasionally pass behind the Sun from our perspective here on Earth (Figure 1). Over the course of several hours as Earth moves along its orbit, detectors on Earth may be able to observe how this continuous signal changes as it appears to pass behind different parts of the Sun, like watching a straw appear to bend when lowered into a glass of water.

The authors of today’s article employ mathematical formulas (that are not too dissimilar from what one would see in an introductory optics course!) to calculate the amount of deflection, convergence, and time delay experienced by gravitational waves passing behind the face of the Sun at various angles relative to its center. Further wave-optics calculations are employed to find the corresponding amplification factors and phase offsets that waves of different frequencies would experience at these various angles. Their results are clearly shown in Figure 2: as each candidate pulsar (labeled by the different colored lines) appears to pass behind the face of the Sun, its corresponding gravitational wave signal will be amplified and offset in phase in complicated ways determined in part by the frequency of each gravitational wave and how closely the signal gets to passing directly behind the center of the Sun. By understanding how strong these effects are for continuous waves with different frequencies and amplitudes, the authors can begin to assess what it will take to detect them.

modeled amplification and phase offset of a gravitational wave signal passing behind the Sun from our perspective

Figure 2: Models for the amplification (left) and phase offset (right) of a potential continuous wave signal as their line of sight appears to pass behind the Sun from four pulsars that are known to be eclipsed in this way once each year. [Takahashi et al. 2023]

As it turns out, the ideal continuous-wave-emitting pulsars are those with high rotational frequencies (>10 Hz) that pass as close behind the Sun’s center as possible. When applying this cutoff to catalogs of known pulsars, only four currently fit the bill. While this doesn’t sound ideal, the authors go on to acknowledge that there are expected to be thousands more fast-spinning millisecond pulsars within our own galaxy that we have yet to discover, many of which could also turn out to pass behind the Sun on occasion.

diagram of the locations of annuli used to estimate the solar density

Figure 3: Estimates of the solar density profile will have to be made at different chosen slices called “annuli” (depicted here as the numbered concentric circles). Choices for how to pick these annuli affect how accurately their densities can be recovered for a given continuous wave pass. [Takahashi et al. 2023]

With all this background knowledge in hand, the primary question left to tackle is this: can these slight deformations in continuous waves actually be detected with enough confidence to infer the density of different layers of the Sun? As to whether the lensing signal could be detectable at all, the authors of today’s article find that such a detection could be made with a high degree of confidence using known pulsars with about one year’s worth of observation time given a signal-to-noise ratio of around 100 or greater. The signal-to-noise ratio can depend on many factors including the loudness of a given source, the sensitivity of the detector, and the timespan of data collection, but to put this number in perspective, current signal-to-noise ratio upper limits for continuous wave detections from LIGO searches are estimated to be around 10.

Unfortunately, accurately measuring the solar density at several different solar depths is even trickier, as the accuracy of each measurement depends on a variety of factors including how many total layers of the Sun one attempts to measure and how one sets the distance between each layer (Figure 3). For one year of continuous wave observation of the three best pulsar candidates, the signal-to-noise ratio needed to accurately measure the solar density at two different layers is found to be ~10⁴, which is an order of magnitude higher than is even expected from the next-generation gravitational wave detectors Cosmic Explorer and the Einstein Telescope. To measure densities across 6 or 10 different layers of the Sun’s interior, the signal-to-noise ratio requirements grow to ~10⁶ and ~10⁷, respectively (Figure 4), well beyond the capabilities of planned detectors unless more rapidly spinning pulsars passing behind the Sun can be found and loudly heard in these future detectors.

uncertainty in solar density measurements at various solar depths using the expected lensing signatures of three known pulsars as they pass behind the Sun

Figure 4: This plot depicts the uncertainty in solar density measurements at various solar depths using the expected lensing signatures of three known pulsars as they pass behind the Sun. The solid black line depicts the expected solar density as a function of solar radius (measured here as the angular position on the face of the Sun relative to its center). In the case where one attempts to measure six distinct densities with a very high signal-to-noise ratio of 10⁶ over one year of observation (left plot), uncertainties can become fairly low close to or far away from the Sun’s center. To achieve similar uncertainties across 10 annuli, a comparative signal-to-noise ratio of 10⁷ is necessary. [Takahashi et al. 2023]

Prospects for Gravitational Wave Vision

In recent years, combining gravitational waves with gravitational lensing has been proposed as a way to learn all sorts of new things about our universe, from constraints on the Hubble constant to independent measurements of the masses of distant stars. While it may take many more years for this sort of analysis to mature to a point where it can give us useful information about the Sun’s density profile, the fact that it may be possible at all is remarkable. For many decades the detection of gravitational waves was thought to be impossible. Now, not only are we detecting them with regularity, but we are finding all sorts of new ways to learn about our universe with each passing day — and that’s something to be excited about, even if you won’t be seeing gravitational wave vision goggles in stores anytime soon!

Original astrobite edited by Jessie Thwaites.

28 November 2023

Let’s Be Real: A Call for Physically Realistic Assumptions

Title: Spatially-Resolved Temperature and Density Structures of Nearby HII Regions
Authors: Yifei Jin et al.
First Author’s Institution: Australian National University
Status: Published in ApJ

Despite playing a fundamental role in shaping the emission-line spectra of HII regions, the assumptions about electron temperature and density that are used in most photoionization codes are oversimplified and quite far from reality. In today’s article, the authors analyzed the detailed electron temperature and density structures of four HII regions to show — spoiler alert! — that the prevailing assumptions are not physically realistic.

For Real

Photoionization, a key process in astrophysics, is the physical mechanism that converts electrically neutral atoms (or molecules) into ions through interactions with energetic photons. This phenomenon plays a crucial role in determining the characteristics of HII regions, where intense ultraviolet photons from massive stars lead to the ionization of the surrounding hydrogen gas.

Much of our knowledge about objects in the universe comes from spectroscopy (here is Astrobites’s Guide to Spectroscopy and Spectral Lines). The spectra of HII regions are dominated by emission lines that trace the characteristics of the emitting gas. Among the physical quantities influencing the emission-line spectra of HII regions are the electron temperature and density. These parameters serve as crucial diagnostics for determining the metallicities of the interstellar medium.

Reality Check

Figure 1: Diagram illustrating an idealized or simplified spherical HII region. [Kerry Hensley/AAS Nova]

To interpret the spectra of HII regions and understand the complexities of such ionized environments, astronomers rely on photoionization codes. These codes model the interactions between the ionizing radiation emitted by the ionizing source(s) and the surrounding gas, and they provide insight into three key factors:

the nature of the central star
the physical conditions of the ionized nebula
the chemical abundances that characterize the nebula

Photoionization codes tend to underestimate the complexity of the internal nebular structures of HII regions by assuming an isothermal condition (constant temperature) or a uniform electron density distribution across the extent of these regions. Additionally, in most models, the nebular geometry is assumed to be spherically symmetric, a simplification that often does not align with the real objects — as is evident if you compare the HII-region diagram in Figure 1 with the images from observations of the four HII regions studied in the article in Figure 2!

What’s Real vs. The Models

The authors of today’s article utilized integral field unit data from the Wide-Field Spectrograph (WiFeS) on the Australian National University 2.3-meter telescope to analyze the detailed temperature and density structures of four HII regions in the Magellanic Clouds (Figure 2). Integral field unit instruments enable simultaneous imaging and spectroscopy observations, generating a spatially resolved data product called a data cube that contains spectral information at each spatial pixel (spaxel). This article represents the first investigation into the nebular internal structures with a sample of extragalactic HII regions using integral field unit data with a high spatial resolution.

Figure 2: Integrated-flux (in erg s^-1 cm^-2) images of the four HII regions with relative positions (in arcseconds), showcasing the different nebular morphologies. [Adapted from Jin et al. 2023]

The authors used the temperature-sensitive [O III] line ratio to derive electron temperatures from the data and the density-sensitive [O II] and [S II] line ratios for the electron densities. The MAPPINGS V photoionization code was employed to create models, incorporating stellar atmosphere libraries as input for the ionizing sources’ spectra and using spherical nebular geometry assumptions for the shapes of the HII regions. MAPPINGS V generated a set of HII-region models with line fluxes for all the emission lines within the set threshold (specified in the source file).

Figure 3 compares the radial profiles of the temperature- and density-sensitive line ratios between the observed and modeled values derived from the best-fit models (those with minimal χ² values) for all four HII regions. The models predict that the density-sensitive [O II] and [S II] line ratios increase (top row) and remain flat (bottom row), respectively, with the distance from the center of the nebula. On the other hand, the observational data show a decreasing trend for [O II] line ratios, while [S II] ratios increase. Meanwhile, the observed radial profiles of the temperature-sensitive [O III] line ratios are flat (middle row) for all four regions, but the models predict decreasing profiles. These discrepancies suggest that none of the simple HII-region models is able to reproduce the observed electron temperature and density structures.

plots of radial distribution of line ratios in H II regions

Figure 3: The [O II], [O III], and [S II] line ratios as functions of normalized radius of the nebula with 0.0 being the center of the nebula. The yellow points represent the values from the observational data and the red dashed lines show the modeled radial distribution. [Adapted from Jin et al. 2023]

Let’s Be Real

Real HII regions exhibit notable radial variations in electron temperatures and densities, challenging the isothermal and/or constant density assumptions in photoionization codes. Codes like MAPPINGS V require assumptions about the shape of HII regions as input. Today’s authors have demonstrated in their article the pressing need to model realistic HII regions for accurate nebula modeling, emphasizing the importance of considering the arbitrary geometries inherent in these regions. As our understanding advances, the demand for a more realistic modeling approach grows. Ongoing developments in three-dimensional photoionization codes with Monte Carlo radiative transfer techniques (for more reading about the basic principle of this technique, this page is a good start) hold the promise of a new era in astrophysical simulations, providing a more physically realistic portrayal of these fascinating objects!

Original astrobite edited by Roel Lefever.

14 November 2023

A Cosmic Game of LEGO

Title: Revisiting Galaxy Evolution in Morphology in the COSMOS field (COSMOS-ReGEM): I. Merging Galaxies
Authors: Jian Ren et al.
First Author’s Institution: Chinese Academy of Sciences
Status: Published in ApJ

Galaxies are a bit like LEGO. At least, if you simplify down one of the fundamental theories of Lambda-CDM cosmology, our current best model of the universe, what you could say is that all structures in the universe today formed from the merging of smaller structures, much like building up a LEGO masterpiece. So, to build a massive galaxy like our own Milky Way, you need to merge lots of smaller galaxies together first (e.g., see this bite). One day, our galaxy will collide with our nearest neighbour, the Andromeda Galaxy, to form an even more massive galaxy. If you wind back the clock of our universe, what this simple, fundamental idea predicts is that galaxy collisions were more frequent back in the early universe. However, this has been a difficult prediction to prove as it requires observations much deeper into the universe.

Whilst it might seem easy to identify these colossal collisions with nearby galaxies, this gets very difficult for more distant galaxies, i.e., galaxies in the early universe (since distant = higher redshift = longer lookback time). At these distances, we need better instruments and bigger telescopes to resolve enough detail and collect enough light to really identify these more distant and therefore fainter mergers. The authors of today’s article have used observations of the Cosmic Evolution Survey (COSMOS) field — an area of the sky that has been extensively observed by multiple cutting-edge telescopes — to identify merging galaxies and investigate their properties.

How to Spot a Merger

There are a variety of different methods to identify merging galaxies. If you have sufficient spatial resolution, meaning you can see the extent of a galaxy rather than just a point source, you often see the evidence just by eye. In the featured image above, you can see a beautiful example of a local merger. These are called the Mice Galaxies, or NGC 4676. This merger is close enough for us to see it in action — the two galactic nuclei are clearly distinguishable, and we can even see a bridge connecting the two, as well as bright tidal tails of stars and gas streaming away from the collision. The authors of today’s article identified similar features in the COSMOS field. Some examples of these features are shown in Figure 1. In total, they identified 3,594 galaxies out of 33,605 galaxies as mergers by visually inspecting the images and searching for these clues.

Figure 1: Examples of some merging galaxies visually identified in this article. Each square is a different pair of merging galaxies. Each pair is at a different redshift, z, and stellar mass. [Ren et al. 2023]

Alternatively, you can also search for galaxies that are close together, since the expectation is that gravity will already be pulling these galaxies toward an imminent collision. This is useful if you can’t resolve much detail about the galaxies and can only see them as point sources. However, it is not enough to just find two points of light that look close together on an image; if you look up at a patch of the night sky, you might see two galaxies that appear close together, but in reality, one galaxy could be a million times farther away from us than the other. To check for this pesky problem, you need to know the redshift of the galaxy (again, remember that redshift is often used as a measure of distance from us, the observers). By checking the distances between sources in both the two-dimensional plane of the sky and according to their redshift, the authors identified 1,737 galaxy pairs.

Sometimes, however, even this method is too difficult. Redshifts can be hard to obtain (e.g., see this bite) and sometimes distances are just too great to even resolve galaxy pairs. For this reason, today’s authors used their impressively huge sample of mergers, pairs, and non-interacting galaxies to investigate some alternative methods that could be useful for a higher-redshift sample. They find that two parameters in particular, M20 and A0, are especially good at identifying mergers. M20 is a measure of how the brightness is spread across a galaxy, and A0 measures the asymmetry of the outskirts of a galaxy. The images of merging galaxies typically have much higher M20 and A0 values than the images of non-merging galaxies. This makes some sense — merging galaxies should show much less structure and symmetry since these collisions are violent and chaotic.

Mergers Across Cosmic Time

$plot of merger fraction over cosmic time$

Figure 2: How the merger fraction, which is calculated according to the visually identified mergers (blue points) and the number of pairs (black points), varies as a function of redshift, z. Estimates from other works are also shown. Click to enlarge. [Adapted from Ren et al. 2023]

The merging galaxies identified in this work span a redshift range of z = 0.2–1 — in terms of time, that’s from 2 billion to 8 billion years ago. Counting up all of the galaxies detected in the article’s observations, and labelling them as merging or not merging via the above methods, we can see in Figure 2 that the merger fraction (number of mergers out of the total galaxy sample) increases as a function of redshift. As predicted, there are indeed more mergers earlier in the universe. From this evolution, it is estimated that a massive galaxy in the redshift z < 1 universe will experience, on average, one major merger every 10 billion years.

Future efforts by the next generation of cutting-edge telescopes, as already begun by JWST, will help to extend this investigation to higher and higher redshifts. Seeing back earlier in time will paint a picture of galaxy mergers at the beginning of the universe, where the cosmic game of LEGO was likely even more intense and chaotic.

Original astrobite edited by Storm Colloms.

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