How to turn off a quasar

Active galaxies are among the brightest objects in the Universe. These galaxies send out huge jets of matter at a speed close to light, all of which are fed by matter falling into the supermassive black hole in the center. While all galaxies appear to have supermassive black holes, not all of them are active – our own galaxy, the Milky Way, has a silent black hole at its center. So what makes the difference? Why are some galaxies and their black holes active, while others sit quietly?

Obviously, taking an ordinary galaxy and turning it into an active galaxy isn’t as easy as flipping a switch. Modeling has suggested that activating a galaxy’s central black hole (and therefore the galaxy as a whole) may be part of the galaxy-building process., which occurs by the fusion of smaller galaxies in collisions. These collisions can send gas tossed around inside the galaxy, creating densities high enough in the core to activate the black hole. Now another study suggests that these collisions may close the black hole as well – it all depends on the details of the geometry.

A donut near the hole

To understand how it works, you need to understand the environment near a black hole. While much attention is paid to gravity bending reality over the black hole’s event horizon, anything that actually affects the surrounding Universe takes place some distance from this location. There, the black hole’s gravity organizes all infallible matter into a flattened disc that feeds matter into the black hole called the accretion disc, with the black hole at its center. Beyond that, a more diffuse donut-shaped cloud of gas powers this disc.

This organization is necessary to efficiently feed matter into the black hole. Collisions in the infalling material produce radiation that would otherwise drive out all of the material. And it is this efficient power supply that is considered necessary to power the jets that create active galaxies.

To a certain extent, all of this is self-organizing. Push enough gas towards the center of the galaxy, and it will eventually form the donut (more technically a toroid) and start feeding black hole gas. And earlier models had shown that galaxy collisions were able to do this. All galaxies have gas clouds of different densities scattered around their disks. The disturbances caused by a collision have the potential to rearrange these clouds, and send some closer to the galactic core, where the material can be captured by the black hole’s gravity.

This does not happen in all cases, however, and a group of Japanese researchers – Yohei Miki, Masao Mori, and Toshihiro Kawaguchi – have been puzzled by a number of cases where signs of recent collisions were present in galaxies that didn were not active. In fact, some of them indicated that the black hole had calmed down relatively recently, suggesting that the collision and stopping may be related.

Our nearest large neighbor, galaxy M31 Andromeda, is inactive and also has a feature called the giant south flow which appears to be the remnants of a small galaxy that collided with it. Since we understand the details of Andromeda better due to its proximity and modeled this collision in detail, the researchers decided to examine the effect it might have had on the central black hole.

Frontal collision

Modeling has shown that if the dwarf galaxy collides with the center of Andromeda, it has the potential to disrupt the donut of gas that powers the black hole. Whether or not this is the case depends on the relative densities of gas in the galaxy and gas in the donut. As long as the density is higher in the incoming galaxy, the torus should be disturbed. In essence, the disorganized incoming gas will transfer some of its momentum to the gas orbiting the black hole, chasing it away. The result is a chaotic mess that is unable to effectively fuel the black hole.

The researchers note that while the density of the gas alone is enough to smother the black hole, the geometry could also matter. If the incoming galaxy hits the edge, then its entire width would pass through the area of ​​the central Andromeda black hole, providing a more disruptive force. On the other hand, if the density of gas at the galactic core is high enough, the incoming galaxy will not be able to disturb it.

All of this is happening fairly quickly, at least in galactic terms, taking only about a million years to silence the active black hole.

Beyond the circumstances specific to Andromeda, the authors used averages of the number of small galaxies close to large ones and the apparent frequency of collisions to calculate how often one of these collisions could occur. They estimate that one should arrive at a typical galaxy about every hundred million years on average. It’s just an average, however; some active galaxies could last much longer before a collision with the correct configuration takes place.

Templates may also be relevant for events closer to home. Like Andromeda, the central black hole in the Milky Way is calm for the time being. And the authors note that a space telescope called Gaia recently found evidence of a collision between a small galaxy and the central nucleus of the Milky Way (a collision that produced the structure improbably named the “Gaia-Enceladus-Sausage” ). It is therefore possible that this event closed the central black hole of the Milky Way.

Nature astronomy, 2021. DOI: 10.1038 / s41550-020-01286-9 (About DOIs).