Active galaxies are some of the brightest objects in the Universe. These galaxies send out enormous jets of matter at near light speed, all powered by matter falling into the supermassive black hole at the center. While all galaxies seem to have supermassive black holes, not all of them are active—our own 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 active, while others sit quietly?
Obviously, taking a regular galaxy and turning it into an active one isn’t as simple as flicking a light switch. Modeling has suggested that activating a galaxy’s central black hole (and thus the galaxy as a whole) can be part of the galaxy-building process, which takes place through the merger of smaller galaxies through collisions. These collisions can send gas sloshing around inside the galaxy, creating high enough densities at the core to activate the black hole. Now, another study is suggesting that these collisions can shut the black hole down again as well—it all depends on the details of the geometry.
A donut near the hole
To understand how this works, you have to understand the environment near a black hole. While a great deal of attention is paid to the reality-bending gravity at the black hole’s event horizon, everything that actually affects the surrounding Universe takes place a bit of a distance from that location. There, the black hole’s gravity organizes any infalling matter into a flattened disk that feed matter into the black hole called an accretion disk, with the black hole at its center. Beyond that, a more diffuse, donut-shaped cloud of gas feeds into this disk.
This organization is needed to feed matter efficiently into the black hole. Collisions within the infalling material produces radiation that would otherwise drive all the material off. And it’s this efficient feeding that is thought to be necessary to power the jets that create active galaxies.
To a certain extent, all of this is self-organizing. Shove enough gas towards the center of the galaxy, and it will eventually form the donut (more technically a toroid) and start feeding the black hole gas. And earlier models had shown that galaxy collisions are capable of doing this. All galaxies have clouds of gas of various densities scattered around their disks. The disruptions 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 doesn’t happen in every case, however, and a group of Japanese researchers—Yohei Miki, Masao Mori, and Toshihiro Kawaguchi—were intrigued by a number of instances where signs of recent collisions were present in galaxies that weren’t active. In fact, some of these had indications that the black hole had quieted down relatively recently, suggesting that the collision and the shut down might be related.
Our nearest large neighbor, the M31 Andromeda galaxy, is inactive, and it also has a feature called the giant southern stream that appears to be the remains of a small galaxy that collided with it. Since we understand the details of Andromeda better due to its proximity and modeled that collision in some detail, the researchers decided to look into the effect it might have had on the central black hole.
Head on collision
The modeling showed that, if the dwarf galaxy runs into the center of Andromeda, it has the potential to disrupt the donut of gas that feeds the black hole. Whether it does or not depends on the relative densities of the gas in the galaxy and the gas in the donut. As long as the density is higher in the incoming galaxy, the torus should be disrupted. In essence, the disorganized incoming gas will transfer some of its momentum to the gas orbiting the black hole, driving it off. The result is a chaotic mess that isn’t capable of feeding the black hole efficiently.
The researchers note that, while the density of the gas alone is sufficient to choke off the black hole, geometry could matter as well. If the incoming galaxy strikes edge-on, then its entire width would pass through the area of Andromeda’s central black hole, providing a more disruptive force. In contrast, if the density of gas at the galactic core is high enough, then the incoming galaxy won’t be able to disrupt it.
All of this happens rather quickly, at least on galactic terms, taking only about a million years to silence the active black hole.
Going beyond the specific circumstances in Andromeda, the authors used averages of the number of small galaxies near large ones and the apparent frequency of collisions to calculate how often one of these collisions might occur. They estimate that one should happen to a typical galaxy roughly every hundred million years on average. That’s just an average, though; some active galaxies could go far longer before a collision with the right configuration took place.
The models may also have relevance for events closer to home. Like Andromeda, the Milky Way’s central black hole is quiet at the moment. And the authors note that a space-based telescope called Gaia recently found evidence of a collision between a small galaxy and the central core of the Milky Way (a collision that produced the improbably named structure the “Gaia–Enceladus–Sausage”). So, it’s possible that this event shut down the Milky Way’s central black hole.
Nature Astronomy, 2021. DOI: 10.1038/s41550-020-01286-9 (About DOIs).