Cool Gas and Active Galaxies

By Shannon Murphy
Aug 11, 2015
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Reprinted from the Astronomy website...

In classic models of giant elliptical galaxies, the gas in the galaxy should cool, condense, and fall into the middle of the galaxy. As it does that, some of the gas will form into star forming regions. If that were the entire story, we would expect this cooling of gas and star formation at the center of the galaxy to happen relatively quickly. Instead, observations show that fewer stars form than expected, and star formation goes on for a lot longer than expected. So what is slowing down this star formation? A recent paper lead by Yuan Li and including Mateusz Ruszkowski goes a long way toward solving that puzzle.

At the center of the giant elliptical galaxy is a supermassive black hole (SMBH). As gas falls toward the black hole, some of it gets tangled in the magnetic field and blown out as a jet. This makes the center, or nucleus of the galaxy exceptionally bright, at least momentarily. When galaxies have such active cores, astronomers call it “active galactic nuclei" or AGN for short. Li and her colleges created a computer model to find out what effect the AGN has on the surrounding galaxy.

It turns out that the AGN, star formation, and cooling gas all interact in a self regulating way to prolong star formation. If the gas cools too much, more stars form and more gas falls toward the SMBH, which feeds the AGN jets. The jets heat the gas up, putting a stop to the gas falling into the SMBH and star formation. That in turn allows the gas to cool, starting the cycle over again. Recent observations from the Hubble space telescope and several ground-based telescopes match the model very well (you can read about that in a recent NASA press release.)

Li created several animations of the model results to help visualize what happens.

One of the animations shows the temperature of the gas in the inner region of the galaxy. Time is shown in Gyr (billions of years) At the beginning (at 270 million years), everything is orange and red and fairly even, indicating that the gas is generally hot. As soon as the simulation starts, the gas near the center immediately condenses and falls on the SMBH and triggers the jets, which are dark red, indicating they are hot. That stops the gas from cooling and falling into the SMBH, which causes the jets to shut off. When the jets shut off, some of the gas rapidly cools and condenses into clumps, which show up as green or blue. The cool clumps of course begin to fall onto the SMBH, which trigger the jets again, which stops the gas from falling it, which shuts off the jets, so the gas cools, and it quickly becomes very chaotic looking, until, at around 1.5 Gyr, it’s a turbulent mix of warm (orange and yellow) gas. This time, the gas condenses a little further from the core, so it is able to form bigger, more coherent clumps. The interaction with the jets this time causes filaments to form, which is exactly the start of structure observers see in the Perseus cluster. At this stage, the cool gas is able to form into a disk around the SMBH, which means it is stable enough to maintain its structure and feed the jets more evenly. As star formation eats up the cooling gas, it stops feeding the jet as well. Eventually the disc fades, then the gas is able to fall again, and the process starts over.

Another animation shows the same model, but it includes more of the galaxy, so you can see what is happening to the gas much farther from the nucleus.

Both of those animations show the temperature. In general, the gas tends to clump together where it’s cool, and spread out where it’s hot, but it is useful to actually plot the density. 
To view images: