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Title: Blowing Star Formation Away in AGN Hosts (BAH) – I. First Observation of Warm Molecular Outflows with JWST MIRI
Authors: J.H. Costa-Souza et al.
First Author’s Institution: Federal University of Santa Maria
Status: Published in ApJ
One of the key questions in galaxy evolution is why big galaxies are so rare. We see lots of medium-sized galaxies (galaxies about the size of our Milky Way, with about 100 billion stars) but very few truly enormous ones (five or ten times bigger). We believe the answer to this question is feedback. As galaxies get bigger, physical processes occur that shut down further star formation. In the case of big galaxies, the most likely sources of feedback are active galactic nuclei: the supermassive black holes at the centers of galaxies. These objects can produce a truly astonishing amount of energy, and if some of that energy can act on the gas in the galaxy that is trying to form new stars (heating it up, moving it around, or expelling it from the galaxy entirely), it could interrupt star formation enough to explain why we don’t see many big galaxies.
In order to tell if this is happening, we need to study this star-forming gas. Stars are mainly formed from molecular hydrogen gas (H2). This gas comes in different phases: a cold (less than 100K) phase, a warm (100 to 1000K) phase, and a rare hot (more than 1000K) phase. It’s also not just enough to see that the gas is there — we need to see how it’s moving, in all three dimensions, because active galactic nucleus feedback can manifest itself as a large-scale outflow of gas. This is possible with specially designed instruments known as integral field units, which measure a full spectrum of light in each spatial pixel of an image. Provided the gas we’re trying to study emits some sort of spectral line, the integral field unit can measure its velocity towards or away from us using the Doppler shift. This type of analysis is known as galaxy kinematics.
In today’s article, the authors target specifically the warm phase of molecular gas, which thankfully emits a whole series of spectral lines. These lines are mostly rotational lines of the H2 molecule, and they emit in the mid-infrared, which is perfect for targeting with the Mid-Infrared Instrument (MIRI), an integral field unit on JWST. The authors are looking at one specific active galactic nucleus host: UGC 8782. The active galactic nucleus in this galaxy is a low-ionization nuclear emission region, or LINER (astrobites has a full guide on active galactic nucleus classification here), and it’s only about 200 megaparsecs (650 million light-years) away, which, by galaxy standards, is very close. From other optical and radio imaging, the authors figured that UGC 8782 had outflows in at least some phases of gas, which made it likely that it would have a warm molecular gas outflow as well.
What the authors found from their JWST observations was pretty much exactly what they expected: a large-scale outflow in the warm molecular gas of the galaxy. Figure 1 breaks down the emission into components that come from the main disk of the galaxy (which is behaving pretty normally — just doing regular rotation) and components that come from a large-scale outflow from the center of the galaxy. This is mostly apparent in the bulk velocity of the gas (the bottom-center panel), where there are negative velocities where the normal disk rotation has positive velocities, and in the velocity dispersion (the bottom-right panel), which is more than double the typical dispersion of the galaxy in the region where the outflow is occurring. This means that this gas has a lot more energy than the rest of the gas in the galaxy, another sign that it’s interacting with the active galactic nucleus. Not shown is a second outflow that is faster but smaller.
Figure 1: The kinematics of the warm molecular gas in UGC 8782. The gas is broken down into a disk component (top), tracing the regular rotation of the host galaxy, and an outflow component (bottom) of gas being pushed out of the center of the galaxy by the active galactic nucleus. The flux distribution (left), gas velocity (center), and velocity dispersion, or scatter in the velocity (right) are shown for each component. [Adapted from Costa-Souza et al. 2024]
Figure 2: The rate at which mass is being pushed out of the center of the galaxy by the active galactic nucleus (top) and the energy required to push that mass out (bottom), as a function of distance from the active galactic nucleus. Three phases of gas are shown: the warm molecular gas being studied in this article (light blue), the hotter molecular gas (orange), and the ionized gas (dark blue). [Costa-Souza et al. 2024]
An outflow this strong and powerful is fantastic evidence for feedback from an active galactic nucleus acting strongly on the star-forming gas in a galaxy, which means we’re one step closer to understanding the mystery of giant galaxies in the universe — it could be active galactic nucleus feedback causing them not to get made! Still, this is only one galaxy. We’ll have to study many others in the future to see how common this is, but JWST’s MIRI is more than up to the task.
Original astrobite edited by Storm Colloms.
About the author, Delaney Dunne:
I’m a PhD student at Caltech, where I study how galaxies form and evolve by mapping their molecular gas! I do this using COMAP, a radio-frequency line-intensity mapping experiment based in California’s Owens Valley.