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.

New research uses distant sources to investigate a question close to home: how the Milky Way replenishes its supply of star-forming gas.

Dynamic Galaxies

In photographs, galaxies often appear static and isolated, surrounded by nothing but the blankness of space. This picture is deceiving: galaxies across the universe are dynamic, constantly compressing gas into new stars, expelling gas through supernovae and galactic winds, and replenishing their star forming material by channeling gas from cosmic filaments, accreting clouds from their galactic halos, or collecting previously expelled gas as it rains back down.

This interplay of inflowing and outflowing gas has important implications for a galaxy’s ability to form stars; without a way to resupply, galaxies would soon run out of star-forming gas. In a research article published this week, astronomers investigate one channel through which our own galaxy maintains its supply of star-formation fuel.

Quasars Lend a Hand

To learn more about how the Milky Way collects and incorporates new star-forming material, a team led by Hannah Bish (Space Telescope Science Institute; University of Washington) studied the low-velocity gas flowing into and out of our galaxy at the disk–halo interface. This interface is a transitional region in which gas from the Milky Way’s expansive, diffuse halo is incorporated into the disk, where it can condense into stars.

To study gas at the disk–halo interface, Bish’s team enlisted the help of some of the universe’s most luminous objects: quasars. A quasar is a bright, compact galactic center powered by accretion of gas onto a supermassive black hole. When the light from these distant, brilliant beacons passes through gas clouds like those enveloping the Milky Way, the composition, temperature, and velocity of the gas clouds leaves a spectral fingerprint on the light from the quasar.

Complex Inflows and Outflows

The team used spectra of 132 quasars and ground-based measurements of neutral hydrogen to identify and disentangle the motions of gases with different temperatures and compositions. Along 67% of the lines of sight, Bish and collaborators found evidence for both inflowing and outflowing gas, suggesting that these processes occur simultaneously.

They also found differences between inflows in different locations within the galaxy as well as for gas of different temperatures; in the Milky Way’s southern hemisphere, gas of all temperatures and coming from all directions flows inward at 5–10 km/s. In the northern hemisphere, things are more complicated: cooler gas flows inward at 5–10 km/s, while warmer gas streams toward our galaxy much faster. This trend also varies with location, appearing only in certain directions.

Bish’s team explored these results using a simple model of the Milky Way, which they found could largely reproduce the behavior in the southern hemisphere, but not the more complicated findings from the northern hemisphere. They suggested that the difference in behavior between the cooler and warmer gas could be evidence that accretion in the northern hemisphere occurs in patches, and that warm inflowing gas is piling up and cooling where it collides with denser gas at the disk–halo interface.

Looking forward, Bish’s team anticipates that high-resolution models — and more quasar observations to constrain those models — will be key to uncovering the causes of this curious behavior.

Citation

“Differential Accretion of Ionized Low-Velocity Gas at the Milky Way’s Disk–Halo Interface,” Hannah V. Bish et al 2026 ApJ 997 230. doi:10.3847/1538-4357/ae2741

The post Fueling Up: How Does the Milky Way Get Its Star-Forming Gas? appeared first on AAS Nova.