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: Thermal Electrons in the Radio Afterglow of Relativistic Tidal Disruption Event ZTF22aaajecp/AT 2022cmc
Authors: Lauren Rhodes et al.
First Author’s Institution: McGill University
Status: Published in ApJ

Tidal disruption events (TDEs, see Figure 1), which occur when a star gets a little too close to a supermassive black hole (SMBH) and gets ripped apart, are a hot topic across the electromagnetic spectrum. During a TDE, the star can no longer hold itself together under the gravitational pull of the SMBH. Some bits of the star end up accreting onto the black hole, which can launch a temporary outflow, or, in especially dramatic cases, a relativistic jet.

Aside from being cool transients (as far as spectacular stellar deaths go, they are rivaled only by the supernova), TDEs are valuable to study for a variety of reasons. They likely play a role in galaxy evolution, SMBH growth, and even the life cycles of active galactic nuclei (AGN, in which material is constantly swirling onto the SMBH). Not to mention, they are a great tool for eyeballing the mass of the SMBH responsible for the TDE; if the black hole is any bigger than about 1 billion times the mass of the Sun, its event horizon — the point at which not even light can escape its gravity — extends too far, and a star will disappear across the event horizon before it ever gets ripped apart.

How to Spot a TDE

Usually, TDEs are discovered in the X-ray (after all, not many transients are powerful enough to generate an X-ray flare) or in the optical (thanks to high-cadence, wide-field surveys like the Zwicky Transient Facility and the Legacy Survey of Space and Time). However, radio is a particularly useful regime for TDE searches, especially in dusty host galaxies where we cannot see the optical TDE signatures at all; while dust obscures optical light, the radio is unaffected! There is just one catch: how exactly can we identify a TDE based on only its radio emission?

The radio light in a TDE is generated via synchrotron radiation — which is caused by fast moving electrons in strong magnetic fields — and originates in jets or shocks. Unfortunately, this is also the case in AGN, the primary extragalactic source of variable radio emission, and it can be difficult to distinguish between the two. Additionally, the amount of light we see from synchrotron radiation at a given frequency depends on how energy is distributed among the electrons in the source, which is unknown. Perhaps the simplest case — or at least the one you’ll learn about first in your undergraduate statistical mechanics course — is that the energy is distributed among the electrons thermally (left panel in Figure 2). However, most radio transients associated with black holes are assumed to have non-thermal electron energy distributions, usually where there are exponentially many more low-energy electrons than high-energy electrons (right panel in Figure 2).

As a further challenge, very few TDEs have had extensive follow-up observations in the radio, so we don’t have strong constraints on how most TDEs should behave, especially in the long term. Luckily, some astronomers are working to change this.

This includes today’s authors, who have focused on one particular TDE, ZTF22aaajecp/AT2022cmc (I’m sure everyone mispronounces her name), or 2022cmc for short. 2022cmc is a special TDE: it is one of only five TDEs suspected to have launched a relativistic jet, and it has the highest redshift (z ~ 1.2) of the five. It was observed first in the optical and then in the X-ray and radio. The X-ray emission was highly variable for 400 days post-TDE, and then disappeared, and the first 100 days of radio emission indicated there was a relativistic outflow. At the time, it was concluded that the jet shut off and the X-ray emission ceased. However, the story doesn’t end there; the authors of today’s article continued to observe 2022cmc in the radio for an additional 900 days at frequencies spanning about 1–100 GHz.

Our authors had two key observing strategies: wide frequency coverage and long duration. Measuring the change in brightness with observing frequency, or the spectral energy distribution (SED), is important for determining which processes are responsible for the radio emission, since thermal electrons and non-thermal electrons will produce different SEDs. On the other hand, measuring the change in brightness over time, or the light curve, is important for determining what is happening at the source, such as whether there is a jet and, if so, how long it lasts. Here, our authors combine light curves at six different frequencies (Figure 3) and SEDs at nine different epochs (Figure 4) to investigate both.

The Findings

Our authors find a couple of interesting results. First, in the time domain, they find the highest-frequency light curves fade with time, while the lower-frequency light curves increase initially, hit peaks at different times (after 300 days at 15.5 GHz, and after 800 days at 1.3 GHz), and then begin to decay. The spectral evolution is even more intriguing; the SED is always peaked, and the peak shifts to lower frequencies over time, which is typical for an expanding emission region. At frequencies below the peak, the SED doesn’t change much. However, the high-frequency side of the SED shows confusing behavior: early on, it has a very steep slope (the amount of light drops off sharply at higher frequencies), but it eventually flattens.

Investigations Begin

Next, our authors consider three possibilities to explain these observations. They first consider synchrotron radiation from only non-thermal electrons in a relativistic jet (Figure 5), which is the canonical model for a gamma-ray burst radio afterglow. Initially, the jet is so fast that all its radio light is relativistically beamed in one direction (like a flashlight), and we can only see the jet if it’s pointed directly at us. Eventually, the jet slows down enough that it is no longer beamed, and light is emitted in all directions (like a light bulb). At this point, we see the light fade over time, which is exactly what happened in the high-frequency light curves. However, there is one glaring problem: this model cannot possibly explain the observed long-lasting steep SED at high frequencies. So, our authors discard this theory.

Instead, our authors consider a jetted outflow that has non-thermal and thermal electrons both independently generating synchrotron emission¹, giving us the superposition of two radio spectra. This model also doesn’t fit the data well. The peak of the SED moves too slowly, a very dense local environment is required, and, once again, the authors cannot reconcile that steep high-frequency SED.

Lastly, our authors keep the thermal/non-thermal electrons combo and ditch the jet entirely, modeling a spherical outflow. Excitingly, this model best fits the observations, and, most importantly, can explain the mysterious high-frequency behavior. Initially, most of the radio emission originates in the thermal electron population, which produces more light at higher frequencies. As the outflow expands, the non-thermal electrons begin to dominate, and the SED transforms into the classic non-thermal synchrotron shape.

Next Steps

Ideally, our authors want to compare 2022cmc to other TDEs, but few TDEs have such thorough follow-up observations. They speculate that thermal electrons were important in the radio evolution of one of the few such TDEs, Swift J1644, and recommend we revisit its emission models. More broadly, they recommend two future priorities: first, we need extensive radio observations for larger samples of TDEs, especially around 100 GHz, where the thermal electrons contribute most to the SED; secondly, we should stop neglecting thermal electrons not just in TDE models, but in models of any black hole jets, including gamma-ray bursts and X-ray binaries. While this work addresses only one event, one thing is clear. For theorists and observers alike, there is a myriad of exciting work to be done, so it’s no wonder TDEs have garnered so much attention lately.

1. A convenient property of extragalactic synchrotron sources is that all their electrons radiate independently of one another, even if they are part of the same electron population. This is called incoherent emission. (Galactic sources are another story, but I’ll save that for another time.) ︎

Original astrobite edited by Flavia Pascal.

About the author, Chloe Klare:

I’m a PhD student in astronomy and astrophysics at Penn State (with a physics minor, so I get to use my semester spent in QFT for something!). I study active galactic nuclei (in the radio!), and I’m currently looking for baby synchrotron jets in active galactic nuclei.

The post Thermal Electrons Can Explain the Mysterious Radio Behavior of a Tidal Disruption Event appeared first on AAS Nova.

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: JWST Discovery of Warm Dust in the Circumgalactic Medium of the Makani Galaxy
Authors: Sylvain Veilleux et al.
First Author’s Institution: University of Maryland
Status: Published in ApJ

The Windy Galaxy

Billions of light-years away lies Makani, a record-breaking galaxy discovered in 2019 by researchers at the Keck Observatory atop Maunakea, Hawaiʻi. The galaxy shows evidence of a large merger event, where two galaxies collided to form a larger galaxy. This event triggered starburst activity, or several waves of intense star formation, that changed the makeup of not just Makani, but the region around it.

There are several processes that can create galactic winds. An active supermassive black hole at the center of the galaxy can slingshot particles out of the galaxy at close to the speed of light, or supernovae leftover from previous starburst activity can help to expel material from the galaxy. Makani, whose name in Hawaiian just so happens to mean “wind,” has both factors contributing to its galactic winds: massive amounts of star formation and a very active black hole in its center.

This contributes to some of the strongest galactic winds discovered to date, stretching approximately 100 kiloparsecs (330,000 light-years) beyond the galaxy, roughly ten times the extent of a typical galactic wind. These galactic winds remove gas from the galaxy and help to enrich the circumgalactic medium around the galaxy. There is still much to discover about how galactic winds change the makeup of such galaxies, and what exactly they take with them on their way out.

Earth’s Favorite Carcinogen: Polycyclic Aromatic Hydrocarbons

Polycyclic aromatic hydrocarbons (PAHs, pronounced P-A-Hs or “paws”) are small flat dust grains scattered into every corner of the universe, from the atmosphere of Earth in the form of exhaust and wildfire smoke to the circumgalactic medium (the mostly empty space between galaxies). These PAHs contribute to regulating the temperature of the interstellar medium, but astronomers have long wondered how they migrate across galaxies.

Even though they are smaller than a human hair, PAHs are still easily observable even in very distant galaxies.  These dust grains emit infrared light, and if there are enough PAHs in a region, they are bright enough to be observed with modern telescopes, such as JWST. By combining multiple observations across multiple filters, astronomers can calculate ratios that reveal information about the PAHs’ size, temperature, and charge, allowing reserachers to further discern properties about the regions in which these PAHs reside.

With redshifted galaxies like Makani, the light from PAHs is stretched beyond its usual wavelength, but fortunately for today’s authors, the light was stretched just enough to land known PAH features well within JWST filters, as seen in Figure 1. The authors were then able to use this information to track the distribution of these PAHs across Makani.

PAHs Gone Extragalactic

Using JWST, the authors scrutinized Makani for any signs of PAHs. They found that not only did PAHs exist within the galaxy, these molecules were found up to 30 kpc (~100,000 light-years) outside the galaxy. Makani’s strong galactic winds were ejecting the dust grains from the galaxy over an estimated timescale of a billion years.

Previously, astronomers had not expected to find high quantities of dust grains outside the protection of a galaxy. The journey across a galaxy is long and hot, with temperatures in the millions of degrees. A dust grain would likely not survive the trip, as it would be destroyed by high-energy photons. This suggests that the PAHs were somehow shielded from the hotter parts of the interstellar medium and circumgalactic medium in large enough volumes to make it beyond the edge of Makani.

This answered questions about how galaxies enrich the circumgalactic medium, but it also added several more. Since today’s authors were limited by the sensitivity and range of their instruments, they were unable to confirm if there were PAHs along Makani’s entire wind structure, or just the first 35 kpc (114,000 light-years) of it. They also would like to turn their sights towards the intergalactic medium to see if PAHs could make it even farther beyond the reach of their host galaxy.

Original astrobite edited by Chloe Klare.

About the author, Natalie Price:

As a first-year master’s student at Wesleyan University, I study how stellar winds interact with the local interstellar medium. Outside of the observatory, you can find me dancing, with my nose in a book, or running at non-relativistic speeds.

The post Hitchhiking PAHs Caught Catching a Ride on Galactic Winds appeared first on AAS Nova.

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: Unbreaking the Universe: MINERVA Measurements of Color Gradients in Massive Quiescent Galaxies Can Help Ease Too-Early Star Formation Tensions
Authors: Sam E. Cutler et al.
First Author’s Institution: Tufts University
Status: Published in ApJL

Universe Breakers

JWST launched in 2021, and one of its key goals was to observe the first galaxies as they formed. Early results from JWST found something surprising: extremely massive galaxies in the early universe that had “quenched,” or stopped making new stars, much faster than was expected. These early results led some to believe that our understanding of how quickly massive galaxies can form and quench was fundamentally wrong.

Before concluding that our understanding of the universe is broken, it’s important to find other reasons why we might see very massive galaxies in the early universe. One theory is that these galaxies are actually less massive than we think. To “weigh” a galaxy, astronomers first measure the amount of light being emitted by that galaxy. If all the light in a galaxy comes from stars (which should be true in the early universe, though active black holes can contribute a lot of light in some galaxies), you can infer the total number of stars by accounting for the average amount of light emitted per star of a certain mass; this is called the “mass-to-light ratio.” Since massive stars emit a lot more light than low-mass stars, and high-mass stars die out faster than low-mass stars, the mass-to-light ratio is higher for older stellar populations. To accurately “weigh” a galaxy, scientists therefore need both a good measurement of the total amount of light in the galaxy and how old its stellar population is. The authors of today’s article suggest that the uncertainty in these measurements might be leading to overestimates of galaxy masses and underestimates of galaxy star formation rates, possibly explaining the existence of the earliest massive quiescent (non-star-forming) galaxies.

The Impact of Color Gradients

Previous works about the massive quiescent galaxies in this study used spectroscopy to determine the galaxies’ masses and star formation rates. These spectra were taken with a “slit,” which introduces a source of error: if the slit is smaller than the image of the galaxy, some of the galaxy’s light will be missed in the spectrum. Astronomers usually correct for this by checking how much of the total light of a galaxy image is covered by the slit and scaling up the spectrum to account for the missing light, but this correction assumes that the spectrum is basically the same across the entire galaxy.

However, this assumption is rarely correct. In the local universe, star-forming spiral galaxies like the Milky Way tend to be made of a central “bulge” surrounded by a “disk” (see Figure 1). The bulge is no longer actively forming new stars, leading to a red-to-blue color gradient as you move out from the center of the galaxy. In astronomy, this is considered a “negative” color gradient. A slit that only covers the galaxy center would lead to an overall overestimate of the stellar mass and underestimate of the star formation rate. If the galaxies instead had a positive color gradient, with more star formation in the center of the galaxy, the mass would be underestimated and the star formation rate would be overestimated. Galaxies with both positive and negative color gradients have been found in the distant universe. By measuring the color gradient for each individual galaxy in their sample, the authors can correct the mass and star formation rate calculations from spectra taken with a slit spectrograph.

Results

Instead of using spectra, the authors of today’s article use imaging in different filters to measure color gradients for a sample of four galaxies in the early universe that had previously been found to be extremely massive and no longer forming stars. They use medium-band filters, or filters that let in only a small part of a galaxy’s spectrum, to produce a spectral energy distribution at each radius. This spectral energy distribution is less informative than a spectrum (think a handprint versus a fingerprint), but it’s detailed enough to model the likely stellar population at each radius to determine whether the galaxies have positive, negative, or no color gradients, allowing for better corrections to the properties measured from the galaxies’ centers. They also model the stellar population from the central part of the galaxy that would normally be covered by a slit, as well as from the actual spectrum, to test whether any differences in their results come from their choice of modeling techniques.

The authors find that three of the four galaxies have a negative color gradient in at least some areas. The fourth galaxy has a relatively flat color gradient, though this measurement is less certain due to the object’s redshift. When the authors plot the galaxies’ colors on a diagnostic diagram that separates red and old galaxies from young and blue galaxies, they find that three of the four galaxies have centers that appear red and old but outskirts that appear young and blue (Figure 2). This means previous measurements of the mass may have been overestimated, and previous measurements of the star formation rate may have been underestimated.

Next, the authors model the stellar ages across the galaxies using spectral energy distribution modeling. This technique compares the measured spectral energy distribution to libraries of stellar spectra, rules about how different dust or chemical composition impact galaxy appearance, and observational effects to predict the actual stellar makeup of a galaxy. In this case, the authors can’t know for sure whether the redder colors at galaxy centers come from an older stellar population, more dust, or changes in the chemical composition (the so-called “dust–age–metallicity degeneracy”). To account for this, the authors try modeling the stellar population two ways: once by assuming all change in color comes from a change in stellar age, and again by allowing the dust and chemical composition of the galaxy to change with radius. They find that if only the stellar age varies across the galaxy, age gradients in the galaxy stellar population can bring the estimated galaxy masses and star formation rates back toward agreement with models of galaxy evolution in the early universe, though not all the way. However, allowing dust and chemical composition to change across the face of the galaxy lead to higher stellar masses and less star formation, meaning the mystery may not yet be solved.

Is the Universe Broken?

To determine whether these galaxies still count as “universe breakers,” the authors use a cosmological model to predict the largest expected galaxy mass in the observed area at different periods in cosmic time. The previous results had less than a 0.3% chance of occurring under current cosmological models; with their updated numbers and stellar population models, the authors find that the observed galaxies sometimes have more than a 5% chance of occurring, though there is still tension. They point out that their analysis ignores processes that could help galaxies grow and quench in the early universe, like galaxy mergers; still, the existence of these massive galaxies with such low star formation rates remains a puzzle.

Original astrobite edited by Ansh Gupta.

About the author, Margaret Verrico:

I am a fourth-year graduate student at the University of Illinois Urbana-Champaign. I study the connection between supermassive black hole transients and their host galaxies. I am also an avid knitter and reader, and I am passionate about opening up STEM opportunities for people of all backgrounds.

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