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illustration of a protoplanetary disk

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

Title: Formation of Dust Clumps with Sub-Jupiter Mass and Cold Shadowed Region in Gravitationally Unstable Disk around Class 0/I Protostar in L1527 IRS
Authors: Satoshi Ohashi et al.
First Author’s Institution: RIKEN Cluster for Pioneering Research, Japan
Status: Published in ApJ

When a cloud of gas in space has enough mass, the gravitational forces from all the gas overwhelm the gas pressure keeping the cloud puffed up, and the cloud collapses under its own gravity to form a star. If the cloud is initially rotating, the contraction of the gas will magnify that rotation due to the conservation of angular momentum — imagine spinning on a desk chair and pulling your legs in towards your body. The rotation also drives material towards the equatorial plane, ultimately resulting in a so-called protoplanetary disk — a flattened disk of leftover gas and dust orbiting the newly formed star.

The protoplanetary disk that birthed the planets in our solar system is long gone, so we need to look to stars much younger than our Sun to study these planetary nurseries. Today’s authors present a detailed analysis of a particular protoplanetary disk — one that is gravitationally unstable.

Remember the gravitational instability that formed the star from a cloud? Well, the disk can be unstable to its own gravity, too, when the pressure and rotational forces are too small to prevent collapse. This can occur if the disk is very massive and also very cool. Gravitational instability in disks is one possible way of manufacturing giant planets. It causes the disk to fragment into many small blobs of gas, which then collapse into planets. Thus, understanding how gravitational instability begins is an important piece of the puzzle in understanding the formation of the diverse range of planetary systems discovered over the last 20 years.

Observing an Edge-On Protoplanetary Disk

Two excellent tools for observing protoplanetary disks are the Atacama Large Millimeter/submillimeter Array (ALMA) and the Jansky Very Large Array (JVLA). Both use an array of radio dishes that look at the target in unison, acting as one massive telescope. Both can observe at different wavelengths, called bands, which can be combined to produce a more complete picture of the disk.

The disk observed by today’s authors is around the very young (less than 100,000 years) star Lynds 1527 (L1527) IRS in the Taurus molecular cloud at a distance of 447 light-years. The disk is viewed nearly edge on, its host star is still accreting, and the disk has not yet fully formed. Each band penetrates the disk to a different depth, so the observation will look very different depending on the filter. Figure 1 shows three ALMA images (bands 3, 4, and 7) and one JVLA image (Q band).

Four observations of the protoplanetary disk in different bands.

Figure 1: ALMA images (Band 7, 4, and 3) and JVLA image (Q band) of the L1527 protoplanetary disk viewed edge on. The clumps detected in the Q band are marked with black crosses in the other panels. The white ellipse in the bottom left of each panel indicates the image resolution. [Ohashi et al. 2022]

cartoon sketch of the L1527 disk

Figure 2: Sketch of L1527’s disk as viewed from Earth. The hot regions (red) on the near side are obscured by the flared outer disk (blue), so the near side appears slightly hotter in temperature maps. [Ohashi et al. 2022]

Viewed head on, the center of the disk is expected to be symmetrical in temperature, so any temperature asymmetry in this region can be used to measure the disk inclination. The regions closest to the host star receive the most radiation, so they are the hottest. However, since the disk is flared (i.e., it becomes thicker with increasing distance from the star), the inner regions on the near side should be mostly obscured, whereas the inner regions on the far side are more visible. This is sketched in the schematic in Figure 2. Because of this asymmetry, the near side appears hotter in Figure 1. The authors used this asymmetry to estimate the disk inclination at around 5°, with an additional warping potentially being present.

Assessing Gravitational Instability

A gravitationally unstable disk is characterized by a distinctive spiral structure. The problem is we can only view this disk edge on, so we can’t see the spiral structure — much like how the Milky Way’s spiral structure isn’t visible from Earth.

The authors resolved this issue by assessing the stability of the L1527 disk using Toomre’s stability analysis with measured values for temperature and surface density. They find that the disk is expected to be gravitationally unstable. The left panel of Figure 3 shows the spiral structure typical of a gravitationally unstable disk in the face-on view. If we were to look at this model disk from an edge-on, 90°-rotated view, we’d see two high-density regions flanking the center of the disk (right panel of Figure 3). This almost reproduces the shape of the Q-band observation (rightmost panel, Figure 1), so the authors conclude that L1527’s disk is indeed likely to be gravitationally unstable.

A model of a gravitationally unstable protoplanetary disk.

Figure 3: Model of a gravitationally unstable disk. If a massive disk cools enough such that its gas pressure cannot withstand the gas’s self-gravity, it starts to fragment and form a spiral structure (left panel, face-on view). The spiral structure projects two clumps on the edge-on view (right panel). [Ohashi et al. 2022]

One caveat of this assessment is that the surface density — which is a crucial quantity in Toomre’s stability analysis — has to be inferred indirectly by combining dust temperature measurements and opacity models that have some uncertainty attached to them (opacity is the ability of material to block photons). However, if truly unstable, the L1527 disk would be one of the youngest systems to be subject to gravitational instability, suggesting that this young star could have giant planets forming around it much sooner than expected.

Original astrobite edited by Sasha Warren.

About the author, Konstantin Gerbig:

I’m a PhD student in Astronomy at Yale University. I’m interested in the theory of (exo)planets and protoplanetary disks and do hydro simulations thereof. I also like music, as well as dancing salsa and tango.

collage of images of the Sun throughout the course of the solar cycle

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: HD 166620: Portrait of a Star Entering a Grand Magnetic Minimum
Authors: Jacob K. Luhn et al.
First Author’s Institution: University of California, Irvine
Status: Published in ApJL

Have you ever gazed at the Sun using a pinhole viewer? With only tape, a piece of foil, and a thumbtack, you can turn an old shoe box into a simple telescope that creates a projection of our parent star that’s safe to view. At first, the image it creates might look just like a smooth disk, but if you look closer, you can see smudges moving across it — sunspots! These dark, transient features are cooler parts of the Sun’s surface that correspond to areas with strong magnetic activity. If you had a really, really good pinhole viewer and a few decades of spare time, you might notice that the average number of sunspots slowly changes.

Usually these changes repeat every 11 or so years in what we call the solar cycle, and the changes in the number of sunspots are accompanied by variations in flares and other coronal activity. Once in a while, though, something weird happens. Records show that between 1645 and 1715, the Sun went through a long period with very few sunspots, an event that astronomers call the Maunder Minimum. It’s really, really weird, and so you might wonder: do other stars do the same thing? Today’s article presents new data on the Sun-like star HD 166620, for which the answer seems to be yes!

Like the Sun, HD 166620 exhibits long, slow cycles of activity, and so to show that it’s entering its own Maunder Minimum, the team needed decades of data. Fortunately, there are troves of observations by the Mount Wilson Observatory (Figure 1) from 1966 to 1995 on HD 166620 and a slew of other stars, as well as newer observations by the Keck-HIRES spectrograph from 2004 to 2020. These data track a particular type of emission from calcium atoms in the star’s outer layers — a good proxy for stellar activity, including sunspot numbers. Stronger emission correlates with more stellar activity.

black and white photograph of a telescope

Figure 1: The 100-inch Hooker telescope is best known for its use by Edwin Hubble in the 1920s to show that the universe is expanding, but it also provided data for the first decade of the Mount Wilson calcium survey, used by the authors of today’s article to provide evidence of HD 166620’s previous cycles. [Image courtesy of the Observatories of the Carnegie Institution for Science Collection at the Huntington Library, San Marino, California.]

Put together, the older Mount Wilson observations show HD 166620 slowly traveling through its decades-long cycle, while the newer Keck-HIRES data shows it at a constant, reduced level of activity for roughly the past decade and a half. However, there was an almost decade-long gap between the two datasets, a key period during which the star should have transitioned to this period of low stellar activity.

That’s where today’s article comes in. The team found more Mount Wilson data on calcium emission, this time covering the range from 1995 to 2002. What’s more — tada! — these observations show a smooth, satisfying transition from the star’s cycle to this new low-activity state. In addition, the team obtained photometry from the T4 Automated Photometric Telescope. These measurements of the star’s brightness, taken from 1993 to 2005 and 2015 to 2020, complement the measurements of calcium emission and show the exact same trend. It is, as the authors put it, “unambiguous” evidence that HD 166620 is entering a Maunder Minimum–like period (see Figure 2).

plots of the changes in the activity of HD 166620

Figure 2: Four plots illustrating the datasets the team used. The top includes only the previously published spectroscopic calcium measurements, while the second adds in the newly found observations. The third shows only the photometric data, while the fourth shows everything combined — a clear demonstration that HD 166620 has entered a longer duration minimum. [Luhn et al. 2022]

There are some other takeaways. The first is that the levels of emission/photometry during this new minimum aren’t significantly lower than the levels during the previous minima of the regular cycles. This indicates that there haven’t been any major changes in the structure of the magnetic field — nothing cataclysmic has happened. It looks a bit like one of the star’s typical minima, just stretched out significantly. The second is that other observations have painted a picture of HD 166620 as an older, less active cousin of the Sun, possibly with what the authors describe as a “faltering” dynamo.

Long-term surveys can turn up some interesting trends — and the decades of Mount Wilson data on HD 166620 have uncovered a new first. Several other stars have been proposed as candidates for entering or exiting these grand minima; hopefully, with the knowledge gained from the study of HD 166620’s calcium emission and photometry, we’ll soon have a clearer understanding of how these processes work and how common these phenomena are.

Original astrobite edited by Konstantin Gerbig.

About the author, Graham Doskoch:

I’m a graduate student at West Virginia University, pursuing a PhD in radio astronomy. My research focuses on pulsars and efforts to use them to detect gravitational waves as part of pulsar timing arrays like NANOGrav and the IPTA. I love running, hiking, reading, and just enjoying nature.

Mariner 10 image of Venus

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 Outer Edge of the Venus Zone Around Main-sequence Stars
Authors: Monica R. Vidaurri et al.
First Author’s Institution: Howard University, NASA’s Goddard Space Flight Center, and Stanford University
Status: Published in PSJ

Planetary Habitability: Water, Water, Everywhere?

Classically, the benchmark for planetary habitability is liquid water: if a planet’s surface temperature is above freezing and below boiling, then it lies within its star’s habitable zone. Earth, obviously, is our best example of an inhabited, habitable planet in the habitable zone. Mars, now too cold for liquid water, is thought to have historically been much warmer, habitable, and possibly even inhabited! The evidence for this is strong enough that searching for past Martian life is a main science goal of the Perseverance rover. However, there’s another planet in our solar system on the other end of the habitability spectrum: Venus.

Venus

In April, as part of Earth Week ⨉ Astrobites, we heard from David Grinspoon about comparative planetology, which is a way of looking at the differences between the environments of Earth, Mars, and Venus. If Mars is the end state of a planet unable to sustain a greenhouse effect (not just a result of carbon emissions, and necessary to maintain liquid water), then Venus is the end state of a planet with a runaway greenhouse effect. Venus absorbed more radiation from the Sun than the planet could emit, which heated the planet to the point where all liquid water (if it had any) evaporated, and the water was then lost to space through photodissociation. Historically, the orbital radius at which a planet would be subject to this effect from its host star has defined the inner edge of the habitable zone. Any closer, and the planet would lose its water. However, this isn’t a hard limit, and the actual orbital radii at which Venus analogs can be found vary significantly with both stellar and planetary parameters. In today’s article, the authors run a grid of simulations to determine the exact location of the “Venus zone,” which overlaps with the classical habitable zone, for stars of varying masses and temperatures.

How to Build an Uninhabitable Planet

In order for a planet to undergo the runaway greenhouse effect, there must be sufficient greenhouse gases in its atmosphere. The specific amount is different for every planet, but for any particular planet, the amount primarily depends on how much radiation the planet receives from its host star. As the planet is irradiated by the star, different atmospheric species absorb and emit different amounts of starlight at different pressures/altitudes, heating and cooling the atmosphere. By increasing the amount of carbon dioxide relative to nitrogen and water (starting with present-day Earth values for CO2 and N2 and maintaining a certain humidity level), the authors can eventually induce a runaway greenhouse on the planet, producing a “Venus analog” planet. The authors then decrease the stellar flux, effectively moving the planet further away from its host star, until the runaway greenhouse can no longer be sustained. This yields the outer edge of the “Venus zone,” past which no increase in the amount of carbon dioxide will cause a runaway greenhouse effect.

This process is repeated for each stellar type in the grid: an F star (7200K), a Sun-like G star (5780K), a K star (4400K), and two M stars (3400K and 3000K). Habitable-zone planets around M stars are particularly tempting targets for further study as they have significant observational benefits, such as short orbital periods due to their close-in habitable zones and deep transits compared to planets around larger Sun-like stars.

Including a variety of stellar types in the model is important, not only because of their different brightnesses, but also because of their different spectra. As seen in Figure 1, larger and hotter stars are less efficient heaters of planetary atmospheres than smaller, cooler stars. An F star emits much more of its light as shorter-wavelength radiation compared to an M star, which emits much of its light in the infrared. The higher infrared flux from an M star is better absorbed by greenhouse gases in planetary atmospheres, which means that an M star will heat planets more than an F star.

plot of atmospheric temperature and pressure for planets orbiting stars of different temperatures

Figure 1: Temperatures of planets receiving 1 Seff, the amount of radiation that Earth receives from the Sun. The planetary temperatures are inversely related to the temperatures of the host stars, since the planets’ atmospheres mostly absorb infrared radiation, while the spectra of hotter stars peak at shorter wavelengths. [Vidaurri et al. 2022]

Is Every Habitable Planet a Potential Venus?

Luckily, the authors found that while the Venus zone has significant overlap with the classical habitable zone, there are still places where habitable planets can exist with zero risk of runaway (see Figure 2). However, since there is so much overlap between the habitable zone and the Venus zone, future investigators must be careful to take the possibility of runaway greenhouses into consideration when studying potentially habitable planets. In addition, there is significant uncertainty in Venus’s atmospheric and geological history, though new in-situ measurements by the upcoming fleet of Venus missions (NASA’s DAVINCI+ and VERITAS, as well as the European Space Agency’s EnVision) will be able to uncover much of this unclear history. Just like the atmospheres of these simulated planets, the search for habitable planets is heating up. Work like this will hopefully ensure that the future Earth 2.0 isn’t actually a Venus 2.0!

plot showing the location of the Venus zone and the habitable zone as a function of stellar temperature and amount of stellar radiation received by the planet

Figure 2: The boundaries of the Venus zone (VZ) compared to the conservative habitable zone (HZ). The y-axis shows stellar temperature, which relates to how efficiently the stars can heat the planets’ atmospheres. The x-axis shows effective stellar flux, where 1 Seff is the amount of radiation Earth receives from the Sun. Between the green and blue dashed lines is the conservative habitable zone, while the gray area is the overlap region between the Venus zone and the habitable zone. [Vidaurri et al. 2022]

Original astrobite edited by Catherine Clark.

About the author, Yoni Brande:

I’m a third-year PhD candidate at the University of Kansas, working on exoplanet discovery and characterization. I primarily work with TESS transit data and Hubble Space Telescope exoplanet transmission spectroscopy data, and I’m also interested in enabling more collaborative science with open source astronomical software tools. When I’m not doing research or writing astrobites, I can be found in a sci-fi streaming binge, running, lifting, cooking, or on Twitter @YoniAstro.

artist's impression of galaxies during the epoch of reionization

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: CLEAR: Boosted Lyα Transmission of the Intergalactic Medium in UV-bright Galaxies
Authors: Intae Jung et al.
First Author’s Institution: The Catholic University of America and NASA’s Goddard Space Flight Center
Status: Published in ApJ

Sometimes the things we don’t see can still give us insight. This strategy of getting clues from both detections and non-detections is common in astronomy, and the non-detections studied in today’s article are used to understand the process of reionization. Sometime during the first billion or so years of the universe, a period of transition called the epoch of reionization took place. During this epoch, the first stars and galaxies formed and began emitting high-energy light that ionized the then mostly neutral hydrogen gas filling the universe. Ionizing radiation can kick off electrons from neutral hydrogen atoms, and in the epoch of reionization this occurred enough to ionize the universe’s gas nearly completely.

A Whodunit Mystery

Early galaxies are a major source of ionizing photons and perhaps the main drivers of this ionization process, so the properties of early galaxies and how they evolved over the first billion years of the universe have great implications for processes within the epoch of reionization. However, understanding how many photons were produced during this epoch and whether they escaped their galaxies and ionized the neutral gas around them is highly dependent on the physical conditions of each galaxy, and it’s therefore challenging to constrain and predict these factors. These challenges lead to more challenges in determining precisely when and where reionization occurred, as well as what kinds of galaxies were primarily responsible.

Tracing the strength of emission from the Lyman-α (n=2 to n=1) transition of hydrogen from early galaxies can give us a sense of the who and where: what sorts of galaxies produced more of the ionizing photons, and were they clustered together or spread out? This line of questioning corresponds to the spatial evolution of reionization. By tracking the fraction of gas that was ionized over time, we can also constrain the temporal evolution of reionization.

Today’s article seeks to get at the whodunit of reionization, focusing on galaxies in the epoch of reionization. More specifically, the authors of today’s article aim to distinguish between brighter and fainter galaxies, particularly within the ultraviolet (UV) range where photons have enough energy to ionize hydrogen. By determining trends between a galaxy’s capacity to emit ionizing photons and the reionization near that galaxy, the authors can test the idea that UV-bright galaxies sit within highly ionized bubbles of gas, and that reionization is accelerated in bubbles containing large numbers of galaxies (illustrated in Figure 1).

Cartoon depiction of inhomogeneous processes in reionization

Figure 1: Representation of the varied processes during reionization, with the more UV-bright galaxies (larger symbols) sitting in larger ionized bubbles (black) within the neutral gas (white). The ionized bubbles create an environment for the Lyman-α photons to escape and ionize the surrounding gas more easily. More UV-faint galaxies likely exist in the galaxy overdensities within the bubbles but are too faint to be detected with the current dataset. [Jung et al. 2022]

Inequivalent Equivalent Widths

The article seeks to answer one main question: is there any evolution of Lyman-α emission in epoch of reionization galaxies with respect to the UV brightness of those galaxies? To help answer this, the authors measure the strength of Lyman-α emission with a quantity called equivalent width as function of both redshift and the intrinsic UV brightness of the galaxy. Their sample contained a few hundred galaxies with detailed spectroscopic observations as well as new data from the Hubble Space Telescope. With these data, the team searched for any signal (continuum) or Lyman-α emission lines, but they found no convincing Lyman-α emission or continuum-detected galaxies.

Still, these non-detections can help constrain the strength of Lyman-α emission coming from the galaxies. Given the sensitivity of the observations, the authors could (or even should) have detected galaxies if there is no redshift evolution of equivalent width before and after the end of the epoch of reionization (redshift z ~ 6). This basically rules out the existence of strong Lyman-α emission (in other words, high equivalent widths) in this sample, which included more UV-faint galaxies than the sample detected in previous work.

By comparing the detected and non-detected sources and running simulations of mock observations, the authors find some evidence that the Lyman-α emission line strength evolves differently for bright and faint galaxies through the epoch of reionzation. Their analysis is consistent with a picture where reionization is spatially inhomogeneous with large ionized bubbles made by bright galaxies with boosted Lyman-α transmission (Figure 1). The authors note that reionization is probably fairly complicated, with large spatial and temporal variations. Nonetheless, while we can learn something from what we don’t see, the now-operating JWST and other next-generation telescopes will be sensitive to fainter and more distant galaxies, allowing us to get a clearer picture of the epoch of reionization.

Original astrobite edited by Evan Lewis.

About the author, Olivia Cooper:

I’m a third-year grad student at UT Austin studying the evolution of massive galaxies in the first two billion years. 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!

composite radio and X-ray image of the galaxy cluster Abell 194

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: Intracluster Magnetic Filaments and an Encounter with a Radio Jet
Authors: L. Rudnick et al.
First Author’s Institution: University of Minnesota
Status: Published in ApJ

The intracluster medium is wracked with the battle scars left behind over the history of a galaxy cluster: cavities, shocks, sound waves, and radio lobes. The blast zones of relativistic jets and the fronts left behind by the imperial ambitions of galaxies are etched onto the intergalactic landscape. But there is an order to this chaos. Out of the fog, long and slender networks of filaments coalesce, lit up by flows of charged particles like neurons of a vast cosmic brain.

radio image of galaxy cluster Abell 194

Figure 1: Filtered radio image of the galaxy cluster Abell 194, showcasing the many magnetic filaments embedded in its plasma. [Rudnick et al. 2022]

These filaments are thought to be a natural consequence of turbulence in plasma with a high ratio of gas pressure to magnetic pressure. A weak magnetic field can be strengthened several orders of magnitude by turbulent motion on large scales, such as the sideswipes and collisions of galaxies. Sheets of electric current are torn apart by the churning of the plasma sea, forming bundles of fibers. The fibers’ fields strengthen as they are stretched by the shearing motions until the entire galaxy cluster is draped in magnetic noodles.

One such galaxy cluster, the cluster Abell 194, is shown in Figure 1. Today’s article uses radio and X-ray observations to map out a particular subset of the cluster’s filaments and their interactions with a supermassive black hole’s fury.

A Cluster Wracked by Extremes

Abell 194 is a galaxy cluster containing a pair of interacting galaxies, 3C40A (aka NGC 541) and 3C40B (aka NGC 547), shown in Figure 2. As the two galaxies draw closer together under their mutual gravity, gas is dislodged from its usual orbits, spiraling into the waiting maws of their supermassive black holes. The resulting jets billow out across millions of light-years, radio waves whispering of powers terrible and gross. The two galaxies’ violent ejections have a huge impact on their surroundings, filling the cluster with cosmic rays and triggering a starburst where 3C404A’s jet collides with another galaxy. The jets’ impact is also felt in the intricate web of magnetic filaments threading through the galaxy cluster. Two 650,000-light-year-long strands in the east, nicknamed the E filaments (see Figure 2), are warped by the jet from NGC 3C40B. The authors decide these filaments deserve further investigation.

radio map showing the locations of several galaxies and filamentary structures

Figure 2: Radio-emission map showing the locations of 3C40A/NGC541, 3C40B/NGC 547, their twisted jets, and the eastern filaments (E filaments) trailing away from 3C40B’s jet. The region where the E filaments intersect 3C40B’s jet is marked with a green circle. The larger region of interaction considered in Figure 3 is marked with a red ellipse. Note how the E filaments and the jet bend where they encounter each other. 100 kiloparsecs (kpc) is about 320,000 light-years. [Adapted from Rudnick et al. 2022]

Following the Field Lines

Radio waves are just a long kind of light waves, so they have electric fields that oscillate. The direction in which the electric fields point while they oscillate is called the polarization of the wave. As the wave travels through a magnetized plasma, its electric field (and hence polarization) rotates. This is known as Faraday rotation. The amount of rotation depends on the magnetic field strength along the line of sight, so the polarization of radio waves can be used to map out the structure of magnetic fields. The authors take great advantage of this effect. They find that the E filaments have a field strength of around 1.4 x 10-10 Tesla (for comparison, Earth’s magnetic field is 3.05 x 10-5 Tesla). The field is oriented vertically at the intersection between the E filaments and 3C40B’s jet, but the field rotates to nearly horizontal farther along the filaments’ length. While bright in radio, the filaments are faint in X-rays, suggesting the fields are strong enough to block the flow of the hotter (X-ray emitting) plasma in the cluster.

But are the fields strong enough to impede 3C40B’s jet? The jet bends where it intersects the E filaments, the frothing rage of a supermassive black hole seemingly blunted by flimsy filaments. However, the authors note the jet is supersonic and, in this case, should be able to pass through the filaments without being deflected. Some other reason for the jet’s bending must be found. Another scenario (shown as the middle row in Figure 3) is that the magnetic filaments are embedded in a dense cloud. The jet pushes the cloud aside as it strikes, and the cloud drags the filaments along with it.

However, this cloud would not be stationary in real life. It would be orbiting the center of mass of the galaxy cluster at a speed of around 200 km/s, fast enough to cross the width of 3C40B’s jet in only 7 million years. The authors calculate that, in contrast, it would take 30 million years for the jet to push its way through the cloud. This means the motion of the cloud will be important on the timescales of its interaction with the jet (bottom row in Figure 3) and the jet will bend as a result of the cloud’s motion. The moving cloud scenario also handily explains why the jet bends just before it encounters the E filaments as well as at the exact point of intersection (look closely at Figure 2) — the cloud is moving eastward and shoving the jet in that direction.

cartoons illustrating the three proposed interaction scenarios

Figure 3: Cartoon of the region highlighted by the red ellipse in Figure 2, showing the three scenarios that could explain why the jet and E filaments bend when intersecting each other. The first panel depicts the jets being deflected by the magnetic filaments themselves. The second panel depicts the jets being bent by a stationary cloud that is then moved by the jet to bend the E filaments. The third panel depicts the jets and E filaments both being bent by a moving cloud. The authors favor the moving cloud scenario. [Rudnick et al. 2022]

In the end, if the authors’ argument is correct, then the title of this article is somewhat of a lie. The filaments and jets do not clash straight against each other; rather, both are impacted by the interloping of a passing cloud. In the great Gigayears-long struggle between galaxies to determine which shall become the dominant galaxy of its cluster, this little mystery may not mean much. But the drama lasts long enough that we have a chance to observe it, pause, and wonder about the clashes of forces far grander than even our own Milky Way in the night sky.

Original astrobite edited by Ben Cassese.

About the author, Lynnie Saade:

I’m a PhD student at UCLA who works on X-ray observations of supermassive black holes and their active galactic nuclei. I have an unusual hobby of drawing comics and writing stories about personified natural phenomena. I really want to see a story with a black hole being used as an actual character, just as how they (almost) are characters with great impact on their galaxies in real life.

Artist's impression of a hot Jupiter exoplanet near its host star

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

Title: Constraining the Planet Occurrence Rate around Halo Stars of Potentially Extragalactic Origin
Authors: Stephanie Yoshida, Samuel Grunblatt, and Adrian Price-Whelan
First Author’s Institution: Harvard University
Status: Published in ApJ

More than 5,000 planets have been discovered orbiting stars in the Milky Way, but astronomers have not yet confirmed detections of any planets in other galaxies. Most known exoplanets reside within a few thousand light-years of the solar system (less than the distance from us to the center of the Milky Way), so how can we find exoplanets at extragalactic distances? Today’s article describes a search for planets that formed in a dwarf galaxy that has since merged with the Milky Way.

(Not) Finding Extroplanets

A few extragalactic exoplanet, or “extroplanet,” candidates have been identified in the past, though none have been confirmed yet due to the observational difficulties of following up detections that far away. For example, in 2010, researchers announced the discovery of a planet orbiting HIP 134044, a star left over from a small galaxy that the Milky Way absorbed. Further study has since refuted this claim, noting errors in the analysis, and there is no longer evidence that such a planet exists. Today’s article continues the streak of not discovering extroplanets, but the authors invoke a statistical analysis to calculate how common planets may be around halo stars of extragalactic origin.

In the Milky Way’s outer halo, there is a unique population of stars with motions and elemental abundances that are different from stars that formed in the Milky Way. These stars are believed to have formed in a dwarf galaxy called Gaia–Enceladus that merged with the Milky Way 8–11 billion years ago. The authors of today’s article used measurements of stellar motions from the Gaia satellite’s second data release to identify stars moving in ways inconsistent with stars that formed in the Milky Way. These Gaia–Enceladus stars tend to have low or negative rotational velocity in the frame of the Milky Way, unlike typical Milky Way stars that rotate in the disk. The authors also set limits on stellar magnitude, color, and radius, combining Gaia and Transiting Exoplanet Survey Satellite (TESS) data to select low-luminosity red giant branch stars for this study. This allowed them to make direct comparisons to a previous study of similar stars with Kepler data.

TESS is searching for exoplanets that pass in front of their stars, periodically causing the stars to appear dimmer. The authors produced light curves for their sample of 1,080 stars from TESS images and searched them visually for any of these transit dips. No planet candidates were identified, so today’s article focused on using this non-detection to put a limit on how common planets could be around the stars in their sample.

Upper Limits

While a non-detection may sound disappointing at first, it can still teach us something. By calculating a study’s “completeness,” or what fraction of such objects it could detect, researchers can use a non-detection to place a constraint, or upper limit, on how common the objects may be.

The authors used an injection-recovery method to calculate the completeness of their search. They inserted simulated transit signals into their light curves (example in Figure 1), then used a Box Least Squares search to try to identify the signals within some precision in orbital period and transit depth. They found that roughly 30% of their injected signals were recovered, that the recovery rate was highest for planets with short periods and large radii, and that planets smaller than half the size of Jupiter were essentially undetectable.

comparison of a real light curve with no transit signal with a light curve with a synthetic transit signal inserted

Figure 1: Left: TIC455692967’s observed light curve. Right: TIC455692967’s light curve with a simulated transit signal (marked with the red arrow). [Yoshida et al. 2022]

Takeaways

The final upper-limit calculation found that less than 0.52% of low-luminosity red giant halo stars should host hot Jupiters (planets similar in size to Jupiter with orbital periods of 10 days or less). This finding agrees with a previous estimate that put the occurrence rate at roughly 0.5%. The occurrence rate of hot Jupiters correlates with stellar metallicity, generally measured as the ratio between iron and hydrogen abundances in the star. Halo stars typically have very low metallicities, suggesting that hot Jupiters should be about ten times rarer around halo stars than around other stars in the galaxy.

These upper limits are only the maximum possible occurrence rate, so these planets may be even less common than the percentages given in Figure 2. How will we actually find these rare planets of extragalactic origin? By studying more stars! The recent third data release from Gaia and ongoing TESS observations are moving astronomers in the right direction.

a table listing upper limits on planet occurence rates for low-luminosity red giant stars in the Milky Way halo

Figure 2: Upper limits on planet occurrence for low-luminosity halo red giants for ranges of planet radius and orbital period. Undefined limits are the result of zero injected signals in the given radius and period range being retrieved. [Yoshida et al. 2022]

Original astrobite edited by Sahil Hedge.

About the author, Macy Huston:

I am a fourth-year graduate student at Penn State University studying Astronomy & Astrophysics. My current work focuses on technosignatures, also referred to as the Search for Extraterrestrial Intelligence (SETI). I am generally interested in exoplanet and exoplanet-adjacent research. In the past, I have performed research on planetary microlensing and low-mass star and brown dwarf formation.

hubble image of SN1987a

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: 3D Hydrodynamics of Pre-supernova Outbursts in Convective Red Supergiant Envelopes
Authors: Benny T.-H. Tsang, Daniel Kasen, and Lars Bildsten
First Author’s Institution: University of California, Berkeley
Status: Published in ApJ

All right, I know what you’re thinking: “What do my digestive problems (for the hopefully very few of you) have to do with a star’s last, quite spectacular, goodbye?” Unlike most humans, stars don’t possess working intestines, and their outbursts — supernovae — are far more impressive than anything we can manage. There are many different kinds of supernovae, but they are broadly split into two categories: Type I and Type II. We typically see no emission lines of hydrogen in the spectra of Type I supernovae (here’s an example of why), while the spectra of Type II supernovae do contain hydrogen lines. We will talk about this latter type in today’s bite.

Red Supergiant Burps and Interacting Supernovae

Before red supergiants go supernova, they are prone to breathtaking belches called pre-supernova outbursts. These outbursts push huge amounts of gas from the star out into the so-called circumstellar medium — the material in the star’s direct neighborhood. If this neighborhood is filled with enough gas when a star dies, the expanding supernova will push against and interact with this material. This interplay between the material around the star and the supernova can actually be observed from Earth, giving rise to what is known as a Type IIn or interacting supernova.

How visible this interaction is depends mostly on how much material is in the stellar neighborhood. This depends again on how much gas the star decides to throw out, and also on how these pre-supernova outbursts (or burps) are actually formed. The authors of today’s article show that this has a lot to do with convection in the red supergiant.

Red Supergiant Boiling Pot

Simulating these red supergiant outbursts shortly before they go supernova is not new, so we already know the causes of these pre-supernova outbursts:

  • Increasingly unstable nuclear fusion in the core of the star causes powerful gravity waves (not to be confused with gravitational waves)
  • Large-scale convection in the red supergiant carries around material in the star, which can destabilize the nuclear fusion in the core, giving a very variable energy output
  • Pair instability can cause the core’s energy output to go through cycles of drops and spikes
  • A binary companion star can disturb the red supergiant enough to cause the star to temporarily become unstable

The bottom line is that some process releases a large amount of extra energy inside the star, which, depending on how the star reacts to this energy release, can lead to different outbursts of gas. Until now, the simulations of these outbursts have usually been spherically symmetric, meaning that the simulation of the outburst looks exactly the same from any direction. You can also see this as a simulation along a single line of sight from the outside of the star inwards (i.e., one-dimensional).

The problem with this approach is that you cannot simulate convection this way. To deal with convection, the authors of today’s article took the brute-force approach and did a fully 3D simulation. They simulated the region of the star outside the nuclear core (called the envelope) and started with a large energy release at the innermost part of their simulation. The authors considered different styles of energy release in the envelope. These included:

  • A large, sudden energy release, comparable to the energy needed to keep the star together by gravity. This can cause a mass ejection, quite like the Sun but on much larger scales.
  • A slow release of energy, which causes a much steadier stream of mass flowing away from the star instead of an explosive loss of mass.
  • Varying direction of energy release, which influences how (and where to) the pre-supernova outburst will occur.

A snapshot of the authors’ simulation is shown in Figure 1. Here, we see both the envelope density on the left and the velocity of the envelope gas in the radial direction on the right. In the velocity graph, we can see zones both moving away from the star and falling back towards the core. These are the same as convection cells we can find in daily life — like in a pot of boiling water.

two plots of model ouput, showing the density and radial velocity

Figure 1: Left: Density slice of the star’s outer layers, with radius (R) vs. the distance from the core to the pole (z). Right: Velocity in the radial direction (away from the core) slice with the same axes as on the left. [Tsang et al. 2022]

The convection cells leave “holes” or channels of lower density in the envelope from the outside to inner parts of the star. Through these channels, much more gas can escape than would be possible without convection.

We can also see this in Figure 2: the simulation in the left panel, which included the convection, resulted in much more mass loss than the simulation in the right panel, which did not. These channels of low density appear where most of the mass escapes in the convection simulation.

projected images of the simulated star's surface under different conditions

Figure 2: Two images of the star’s surface in Mollweide projection, showing how much mass has escaped. On the left is a model with convection, where the colors indicate the amount of mass lost per direction (or, specifically, solid angle). On the right is a simulation without convection. [Tsang et al. 2022]

This article shows the necessity of taking convection in 3D into account, where the loss of mass from the pre-supernova outbursts has mostly been underestimated. This increases the amount of gas in the neighborhood of the red supergiant, ultimately affecting how the interacting supernova will look to us on Earth.

Original astrobite edited by Sasha Warren.

About the author, Roel Lefever:

Roel is a first-year PhD student at Heidelberg University, studying astrophysics. He works on massive stars and simulates their atmospheres/outflows. In his spare time, he likes to hike/bike in nature, play (a whole lot of) video games, play/listen to music (movie soundtracks!), and to read (currently The Wheel of Time, but any fantasy really).

an image of the Sun's disk with a large group of sunspots

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: Modeling Stellar Surface Features on a Subgiant Star with an M-dwarf Companion
Authors: Maria C. Schutte et al.
First Author’s Institution: University of Oklahoma
Status: Published in AJ

The authors of today’s article search for polka dots of concentrated magnetic flux on the surface of a subgiant star.

A Menagerie of Astrophysical Phenomena

The surface of a star hosts a menagerie of astrophysical phenomena: flares that dramatically increase the brightness of the star, coronal mass ejections that spew plasma into space, and cold, dark regions that grow and decay called starspots (check out this astrobite to learn more about stellar activity). Understanding this stellar activity is not only imperative for understanding stellar evolution, but also for studying the planets that orbit these stars.

The Many Uses of a Light Curve

One of the main ways that exoplanet scientists search for planets is with a technique called the transit method. Scientists monitor the brightness of a star, looking for a dip in that brightness. This dip can be caused by a planet passing, or “transiting,” in front of the star as seen from Earth. However, this graph of the brightness of the star — called a light curve — can also be used to detect other celestial happenings. Astronomers have used light curves to study binary star systems, supernovae, and — in the case of today’s article — starspots!

A Light Curve of Interest

The authors of today’s article studied the light curve of a star called KOI-340, where KOI stands for Kepler Object of Interest. KOIs are stars that were observed by the Kepler space telescope and are suspected to host exoplanets. This particular KOI is a subgiant star, which means that it is brighter and larger than a normal main-sequence star, but it is not as bright or as large as a giant star. KOI-340 also hosts a smaller, colder M-dwarf companion, making it a binary star system.

The light curve of KOI-340 was of interest to the authors because the depth of the planet’s transit is shallower than it should be if the star did not have any starspots. They therefore used a modeling code called STarSPot (STSP), which is publicly available on github, to model 36 transits of the planet orbiting KOI-340 to show evidence of starspots on the surface of the star. They found that the average radius of the starspots is roughly 10% the radius of the star, which would make these starspots some of the largest ever recorded (Figure 1).

histogram showing starspot sizes relative to the size of the sun and KOI-340

Figure 1: This plot shows the radius distribution for the starspots on KOI-340 (grey), compared with the distributions for the sunpots when the Sun was at solar maximum (red) and at solar minimum (blue) over the same duration as the Kepler mission (four years). The black dashed lines show the size of the smallest starspot, found in Transit 19, and the main starspot, found in Transit 21. The dashed red line shows the size of the largest sunspot ever detected. [Schutte et al. 2022]

The authors even found a starspot as large as 16% of the radius of the star (Figure 2)!

modeling of a planet transiting in front of a large starspot

Figure 2: In today’s article, the authors modeled 36 transits of the planet orbiting KOI-340 to show evidence of starspots on the surface of the star. The top of this plot shows the location of the starspot found in Transit 21. The bottom of this plot shows the light curve with the fit from STSP (red line), compared to the fit if KOI-340 did not have any starspots (cyan line). The residuals are shown as blue points below the light curve. [Schutte et al. 2022]

In a different transit, the authors found an additional dip in their fit, indicating the presence of a bright spot on KOI-340 followed by a dark spot (Figure 3).

modeling of a planet transiting in front of a large starspot and a bright spot

Figure 3: Similar to Figure 2, the top of this plot shows the locations of the starspot and the bright spot found in Transit 11 (the black and red circles, respectively). The bottom of this plot shows the light curve with the fit from STSP, compared to the fit if KOI-340 did not have any starspots. The residuals are shown as blue points below the light curve. [Schutte et al. 2022]

This work shows that starspots are not simply a nuisance to exoplanet scientists; they are also a window into the life and evolution of the star itself. For example, the authors of today’s article were able to conclude that the increased activity on KOI-340 as compared to the Sun is likely due to KOI-340’s faster rotation and/or the increasing size of its convection zone as it evolves from a main-sequence star into a red giant star.

In general, today’s authors show us how multifaceted starspots can be, as well as the splendor of polka-dotted stars!

Original astrobite edited by Maryum Sayeed.

About the author, Catherine Clark:

Catherine Clark is a PhD candidate at Northern Arizona University and Lowell Observatory. Her research focuses on the smallest, coldest, faintest stars, and she uses high-resolution imaging techniques to look for them in multi-star systems. She is also working on a Graduate Certificate in Science Communication. Previously she attended the University of Michigan, where she studied Astronomy & Astrophysics, as well as Spanish. Outside of research, she enjoys spending time outdoors hiking and photographing, and spending time indoors playing games and playing with her cats.

Hubble Space Telescope image of the dwarf spiral galaxy NGC 5949

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: Wandering Black Hole Candidates in Dwarf Galaxies at VLBI Resolution
Authors: Andrew J. Sargent et al.
First Author’s Institution: United States Naval Observatory and The George Washington University
Status: Published in ApJ

How do you make a black hole billions of times the mass of the Sun? Even for the planet-building Magratheans, this seems like a tall order. Plenty of mechanisms have been proposed to explain the formation of these supermassive black holes found at the centers of most galaxies. Some involve the mergers of “seeds” — massive black holes weighing in at merely hundreds to hundreds of thousands of solar masses. A simple way to test these theories is to search for relic massive black holes, and low-mass dwarf galaxies are excellent targets. Since dwarf galaxies haven’t undergone many mergers, any massive black holes they harbor should have avoided being gobbled up by growing supermassive black holes.

Today’s article studies 13 possible massive black hole candidates in dwarf galaxies, some of which may have wandered to the edges of their hosts. What’s up with that — and are they really massive black holes? Let’s dive in!

Here’s a question: since supermassive black holes are usually found near the center of their galaxies, why might we expect some massive black holes in dwarf galaxies to lie further out? The answer has to do with gravity: since dwarf galaxies are much less massive than the galaxies that host supermassive black holes, their gravitational potential is lower, making it easier for massive black holes to “wander” away from their centers. This means that if you see a radio source that appears far to the side of a dwarf galaxy’s center, it could be an massive black hole — or it could be an accreting supermassive black hole (an active galactic nucleus) in a galaxy far, far away that by chance simply happens to lie behind the dwarf galaxy. These unwanted interlopers can pose a challenge for identifying massive black holes.

Another issue with finding massive black holes is that they’re faint. While massive black holes go through periods of accretion like supermassive black holes, their low masses mean that they don’t accrete as quickly, reducing their luminosities. By the early 2000s, only two accreting black holes had been found in dwarf galaxies. Fortunately, this changed with the advent of sky surveys like the now famous Sloan Digital Sky Survey (SDSS), which has been running since 2000 and has amassed detections of close to a billion unique sources.

The 13 massive black hole candidates, shown in Figure 1, were assembled in an article from 2020 by some of the same astronomers who authored today’s article. In the 2020 article, the team sifted through 43,707 low-mass dwarf galaxies from SDSS, looking for sources that had been detected at radio frequencies by the Very Large Array. After keeping the matches and eliminating the radio sources that were background active galactic nuclei or could be explained by processes related to star formation, the team ended up with 13 massive black hole candidates, many of which aren’t aligned with the centers of their host galaxies.

optical images of the 13 dwarf galaxies in the sample

Figure 1: The 13 dwarf galaxies hosting possible massive black hole candidates, as seen by the Dark Energy Camera Legacy Survey at optical wavelengths. The red crosses show the location of the compact radio sources that may be massive black holes. While some appear close to their host’s center, others are significantly farther away. [Reines et al. 2020]

In this more recent article, the authors performed follow-up observations using the Very Long Baseline Array (VLBA). The VLBA uses radio telescopes thousands of kilometers apart to reach high angular resolution and allow astronomers to see fine details. Unfortunately, the VLBA was only able to detect four of the 13 candidates — and those four, because of their luminosity and position, seemed most likely to be active galactic nuclei in galaxies far beyond the dwarfs the team was targeting. The detected candidates are shown in Figure 2.

radio emission detected from four sources in the sample

Figure 2: The four sources the team was able to detect with the VLBA. Here, S is flux density, a quantity that describes the intensity of radio emission. As these sources are actually background active galactic nuclei rather than massive black holes in the targeted dwarf galaxies, the physical scales in the lower right are inaccurate. [Sargent et al. 2022]

This seems like an enormous problem! Only four detections, all of which appear to be imposters? Fortunately, the situation isn’t as dire as it might seem. While the VLBA is good at resolving sources on small scales in the configuration the team used, it may not resolve large-scale sources — and the radio emission from accreting massive black holes might be in the form of larger structures like radio lobes, rather than central point sources.

Multiwavelength observations confirmed that two of the remaining nine candidates are likely accreting supermassive black holes near the center of their host galaxies, but the other seven remain unknown. Five of those seven candidates are too bright to be from star formation and, based on their positions, could be either more background active galactic nucleus interlopers or, tantalizingly, wandering massive black holes.

Where do we go next? Follow-up observations at other wavelengths could be useful. The group suggests the Hubble Space Telescope in particular as a means of figuring out what those seven sources truly are. Given the difficulties involved in detecting massive black holes, even one more could prove valuable as astronomers try to understand the formation of the largest black holes in the universe.

Original astrobite edited by Suchitra Narayanan.

About the author, Graham Doskoch:

I’m a graduate student at West Virginia University, pursuing a PhD in radio astronomy. My research focuses on pulsars and efforts to use them to detect gravitational waves as part of pulsar timing arrays like NANOGrav and the IPTA. I love running, hiking, reading, and just enjoying nature.

composite ultraviolet and infrared image of the triangulum galaxy

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

Title: The Panchromatic Hubble Andromeda Treasury: Triangulum Extended Region (PHATTER) II. The Spatially Resolved Recent Star Formation History of M33
Authors: Margaret Lazzarini et al.
First Author’s Institution: California Institute of Technology
Status: Published in ApJ

The Panchromatic Hubble Andromeda Treasury (PHAT) team has already done the impossible. Led by Professor Julianne Dalcanton (read our interview with her from #AAS233 here!), PHAT completely revolutionized observational astronomy by imaging over 117 million stars in the disk of the Andromeda Galaxy, otherwise known as Messier 31. Imaging Messier 31 took two weeks of Hubble Space Telescope time, which is a remarkable achievement considering many observational astronomers are lucky to get even a few precious hours on Hubble!

Now, the PHAT team is ready for round two. They have moved on to Messier 31’s neighbor and the third most massive galaxy in our Local Group: the Triangulum Galaxy, or Messier 33. And of course, this observing program wouldn’t be complete without a new, catchy acronym: the Panchromatic Hubble Andromeda Treasury: Triangulum Extended Region, or “PHATTER.” Studying Messier 33 in addition to Messier 31 is beneficial because Messier 33 has had more star formation overall and can therefore provide more insight into a new parameter space previously unexplored in Messier 31. Messier 33 also has a lower stellar surface density (i.e., lower star-to-area ratio), so resolving individual stars is much easier in Messier 33 than in Messier 31. The PHATTER team has generously made their data publicly available, providing photometry (i.e., the measured flux from astronomical objects) for over 22 million stars covering 38 square kiloparsecs (about 400 million square light-years) of Messier 33.

This article, the second in the PHATTER series (where the first described the observations and photometry), measured the star formation history of Messier 33. Measuring the star formation history of a galaxy can provide crucial information about the astrophysical phenomena that shape galaxy formation, such as how the structure of a galaxy changes over time.

To measure star formation rates of galaxies, astronomers have historically used two different methods. The first method involves studying ultraviolet emission from massive young stars. Because young stars primarily emit at ultraviolet wavelengths, ultraviolet flux is often used as a tracer for star formation within the last 200 million years. The second method involves studying H-alpha emission, which occurs when the electron in a hydrogen atom falls from the third energy level to the second. H-alpha emission often indicates that hydrogen is being ionized, usually by young O stars, and this emission traces star formation within the last 5 million years. However, both of these techniques are limited by dust extinction, which can be difficult to correct for.

The authors of today’s article use a novel method referred to as “CMD-based modeling” to measure the star formation history of Messier 33. The basic premise of this technique is that if you have high-accuracy photometry, you can use color–magnitude diagrams (CMDs, the observer’s version of the H-R diagram, where instead of plotting luminosity vs. temperature, you plot magnitude vs. color) to infer the star formation rates throughout history that would have produced a given observed population of stars. For example, younger stars spend less time in a given color–magnitude diagram zone than older red giant branch stars, and this information can be used to interpret the observed color–magnitude distribution of stars in a galaxy. Another useful benefit of the CMD-based modeling technique is that it simultaneously fits for the dust extinction, unlike the ultraviolet or H-alpha methods.

To measure the star formation history in bins across the face of Messier 33, the authors split their photometry into ~2,000 regions, each of which contained 4,000 stars on average. To measure the star formation history, the team fit color–magnitude diagrams in each region using the MATCH software, which finds the combination of stellar populations that best produces the observed color–magnitude diagram. Using this software, the authors were able to reconstruct Messier 33’s star formation history by measuring the star formation rate in ~50-million-year bins, up until 630 million years ago. While the CMD-based method requires high-resolution photometry, you can study the star formation rate throughout history, whereas the ultraviolet and H-alpha techniques only measure recent star formation.

The Structure of Messier 33

Detailed star formation histories can be used to measure how a galaxy’s stellar structure has changed over time. Messier 33 has typically been characterized as a flocculent spiral galaxy, meaning its spiral arms are less defined than those of a grand design spiral galaxy like Messier 101 (see Figure 1 for a comparison of the two). However, by studying the star formation rate throughout Messier 33’s history (as opposed to just the recent star formation), the authors were able to reconstruct the evolution of Messier 33’s spiral structure using the measured star formation rate in ~50-million-year time bins.

The authors found that while Messier 33 does indeed have flocculent spiral structure that formed about 79 million years ago, it previously had two distinct spiral arms. In short, the younger stellar populations (younger than 80 million years old) present as a flocculent spiral structure and the older stellar populations are primarily present in two distinct spiral arms.

In Figure 2, you can clearly see the split between these two stellar populations. The authors also clearly detect a bar in Messier 33 that is older than about 79 million years, which is significant because there has been a lot of recent debate in the literature about whether Messier 33 has a bar. The detection of bars in galaxies has strong implications for the galaxy formation history; bars force a lot of gas towards the galaxy’s center, fueling new star formation, building central bulges of stars, and feeding massive black holes. In particular for Messier 33, a small bar could explain discrepancies between models and observed gas velocities in the inner disk. The authors suggest that more modeling should be done to explain why the younger stellar populations did not form in a bar, whereas the older stellar populations did.

plots of Messier 33's star-formation rate during two time periods

Figure 2: The spiral structure clearly evolves from 79–631 million years ago to 0–79 million years ago, indicating a transition in the spiral structure of Messier 33 around 79 million years ago from a two-armed barred spiral structure (right) to the more flocculent spiral structure we observe today. [Lazzarini et al. 2022]

Finally, the authors compared their global star formation rate (which has units of solar masses per year and measures the total mass of stars being added to the galaxy each year) to that measured by the conventional methods using ultraviolet and H-alpha emission. The author found their measured value was about 1.6 times larger than the ultraviolet/H-alpha measurement, indicating that ultraviolet/H-alpha measurements may not capture the full star formation rate of a galaxy. In the future, the authors plan to extend this analysis by focusing on measuring the age gradient of Messier 33’s spiral arms and bar.

Original astrobite edited by Isabella Trierweiler.

About the author, Abby Lee:

I am a graduate student at UChicago, where I study cosmic distance scales and the Hubble tension. Outside of astronomy, I like to play soccer, run, and learn about fashion design!

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