Black holes act as galactic thermostats

The supermassive black holes at the centers of most medium to large size galaxies may drive a periodic self-regulating boom-bust cycle. This may be like a pendulum which swings only so far in one direction before it reaches a limit, turns around, and eventually reaches a limit in the other direction. In this case it would be interstellar gas acting like the pendulum. Starting from a very hot state the gas cools by emission of X rays, falls in towards the black hole, and a portion of it forms into an energetic active galactic nucleus (AGN) – which reheats gas further out until the AGN burns itself out and the cycle begins all over again. That is, at least, the scenario explored in more detail in research being reported this week.

Black holes act as galactic thermostats –

The supermassive black hole at the centre of a massive galaxy or galaxy cluster acts as a furnace, pumping heat into its surroundings. But astronomers have struggled to understand how a steady temperature is maintained throughout the whole galaxy when the black hole only appears to interact with nearby gas. Now, researchers in Canada and Australia believe the answer could be a feedback loop in which gravity causes gas to accumulate around the black hole until its density reaches a tipping point. Then, the gas rushes into the black hole, temporarily turning up the heat.

The same picture can also apply to entire clusters of galaxies, as various galaxies within the cluster that contain supermassive black holes go though their own individual cycles.

When gas around and inside a galaxy is extremely hot (as it would be in a young galaxy) it strongly emits X-rays and gradually cools off. Without a supermassive black hole in the center of the galaxy, the gas could eventually cool off enough to start forming stars throughout the galaxy. But if there is a sufficiently massive central black hole more gas is attracted to the center. If sufficient gas comes close enough to the black hole it flattens into an accretion disk and is heated again as gravitational potential energy is converted to kinetic energy. The result is an active galactic nucleus, the most energetic examples of which power quasars, the brightest long-lasting objects in the universe.

But an AGN doesn’t last forever. It can only be sustained as long as there is enough gas falling into it to fuel its enormous output of energy. An AGN typically lasts for less than about a hundred million years. When most of the nearby gas has been consumed, the AGN goes quiet again. The Milky Way has a central black hole (Sagittarius A*) that is relatively small (a little over 4 million solar masses). There is no AGN, which is fortunate for life on Earth. Paradoxically, in spite of their luminosity, it’s not necessarily easy to observe AGNs, which are usually surrounded by thick clouds of cooler dust. AGNs are often detected as radio galaxies, which have strong emissions at radio frequencies. One of the closest examples is Centaurus A, which is about 10 to 15 million light years distant.

AGNs were much more common in the early universe, when far less gas had condensed into stars. Estimates are that only about 1% of relatively nearby galaxies currently have an AGN. But just because a galaxy lacks an AGN now doesn’t mean that one couldn’t form in the future, even in our own galaxy, if the research described here is correct.

Further reading:

Investigating the properties of AGN feedback in hot atmospheres triggered by cooling-induced gravitational collapse


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