Authors: J.H. Costa-Souza, Rogemar A. Riffel, Gabriel L. Souza-Oliveira, Nadia L. Zakamska, Marina Bianchin, Thaisa Storchi-Bergmann, Rogério Riffel
First Author’s Institution: Universidade Federal de Santa Maria, Santa Maria, Brazil
Status: Accepted to ApJ [open access]
One of the key questions in galaxy evolution is: why are big galaxies so rare? We see lots of medium-sized galaxies (galaxies about the size of our Milky Way, with about 100 billion stars), but only a 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 (AGN) – 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 which 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 any 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 100 K) phase, a warm (100 to 1000 K) phase, and a rare hot (more than 1000 K) 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 AGN feedback can manifest itself as a large-scale outflow of gas. This is possible with specially designed instruments known as Integral Field Units (IFUs), 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 IFU 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 paper, 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-IR instrument (MIRI), an IFU on the James Webb Space Telescope (JWST). The authors are looking at one specific AGN host: UCG 8782. The AGN in this galaxy is a Low-Ionization Nuclear Emission Region, or LINER (astrobites has a full guide on AGN classification here), and it’s only about 200 Mpc away (650 million light years), 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 AGN. Not shown is a second, faster but smaller, outflow.
This is great information – it’s good to know that these outflows are happening in the warm molecular gas phase! However, that’s not all that can be found from the H2 data from MIRI. The authors detect several different rotational H2 lines, which means that the different lines can be compared to determine the temperature of the gas, and to get a more accurate measurement of the mass of the gas. This information can then be combined to measure how fast the AGN is pushing mass out of the center of the galaxy (the ‘mass outflow rate’), and how much energy that requires (the ‘kinetic power’). The authors do this calculation for gas at different distances away from the AGN. The results are shown in Figure 2.
What the authors find is that the warm molecular gas is dominating the outflow both in terms of its mass outflow rate and its kinetic power. It’s so strong, it could push all the warm molecular gas available in the center of UGC 8782 away in only about a million years. Between 2-5% of the energy the AGN is outputting as light has to go into the molecular gas to create an outflow this powerful.
An outflow this strong and powerful is fantastic evidence for feedback from AGN 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 AGN 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.
Astrobite edited by Storm Colloms
Featured image credit: X-ray: NASA/CXC/PSU/K. Getman et al.; IRL NASA/JPL-Caltech/CfA/J. Wang et al. – NASA Image of the Day, Public Domain
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