Active galaxies are some of the brightest objects in the universe. These galaxies send massive jets of matter at speeds close to the speed of light, all powered by matter falling into the supermassive black hole in the center. While all galaxies appear to contain supermassive black holes, not all of them are active – our galaxy, the Milky Way, has a quiet black hole at its core. So what makes the difference? Why are some galaxies and their black holes throbbing while others are sitting quietly?

Obviously, taking a normal galaxy and converting it into an active galaxy is not as simple as hitting a light switch. Modeling suggested that the activation of the central black hole of the galaxy (and thus the galaxy as a whole) could be part of the galactic building process., Which occurs through the merging of smaller galaxies through collisions. These collisions can send out gas that flows into the galaxy, creating high enough densities in the core to activate the black hole. Now, another study suggests that these collisions could shut down the black hole again as well – it all depends on the specifics of the geometry.

Donut near the hole

To understand how this works, you need to understand the environment near a black hole. While a great deal of attention is paid to the bending attractiveness of reality at the black hole’s event horizon, in reality everything affecting the surrounding universe is located at a distance from this location. There, the black hole’s gravitational pull regulates any enlarged matter into a flat disk that feeds the material into the black hole called the accretion disk, with the black hole at its center. Moreover, a circular cake-shaped gaseous cloud fed more widely in this disk.

This organization is necessary to efficiently feed the material into the black hole. Collisions inside the fallout produce radiation that pushes all matter away. It is this active feeding that is thought to be essential to power the planes that create active galaxies.

To some extent, this is all self-regulating. Pushing enough gas toward the center of the galaxy, it will eventually form a donut (more technically an annular) and begin to feed black hole gas. Previous models have shown that galactic collisions are capable of doing this. All galaxies contain clouds of gas of varying densities around their disks. The disturbances caused by the collision have the ability to rearrange these clouds, sending some of them near the core of the galaxy, where matter can be captured by the gravitational pull of the black hole.

This does not happen in every case, however, a group of Japanese researchers – Yohei Miki, Masao Mori and Toshihiro Kawaguchi – were fascinated by a number of cases in which recent signs of collisions appeared in galaxies that were not active. In fact, some of these indicators had indications that the black hole had calmed down relatively recently, suggesting that collision and shutdown may be related.

Our closest large neighbor, the Andromeda galaxy M31, is inactive, and it also has a feature called the Giant Southern Stream that appears to be the remnants of a small galaxy it collided with. Because we understand the details of Andromeda better due to its proximity and model this collision in some detail, the researchers decided to look at the effect it might have on the central black hole.

Face to face upon collision

The modeling showed that if the dwarf galaxy entered the Andromeda center, it had the ability to disable the gas cake that fuels the black hole. Whether or not this is the case depends on the relative density of gas in a galaxy and gas in a donut. As long as the density is higher in the next galaxy, the torus should be disabled. Basically, the unregulated incoming gas will transfer some of its momentum to the gas orbiting the black hole, driving it away. The result is a messy mess that is unable to efficiently feed the black hole.

The researchers note that while the density of the gas alone is sufficient to choke a black hole, the engineering could also be important. If the incoming galaxy collides with the edge, its entire width will pass through the region of Andromeda’s central black hole, providing an even more tearing force. Conversely, if the density of the gas in the core of the galaxy is high enough, the incoming galaxy will not be able to disable it.

All of this is happening quickly, at least from the perspective of galaxies, and it takes about a million years to silence an active black hole.

After going beyond the conditions specified in Andromeda, the authors used averages of the number of small galaxies near large galaxies and the apparent frequency of collisions to calculate how often one of these collisions occurred. They estimate that a typical galaxy should occur roughly every hundred million years on average. This is just average. Some energetic galaxies can take longer before colliding with the correct configuration.

Models may also have a link to events close to home. Like Andromeda, the central black hole of the Milky Way is currently calm. The authors note that a space telescope called Gaia recently found evidence of a collision between a small galaxy and the central core of the Milky Way (a collision that created an unlikely structure calledGaia-Enceladus-SausageTherefore, it is possible that this event closed the central black hole of the Milky Way.

Natural AstronomyDOI: 2021. 10.1038 / s41550-020-01286-9 (About DOIs).