It was previously thought that collisions between galaxies would necessarily add to the activity of the massive black holes at their centers. However, researchers have performed the most accurate simulations of a range of collision scenarios and have found that some collisions can reduce the activity of their central black holes. The reason is that certain head-on collisions may in fact clear the galactic nuclei of the matter which would otherwise fuel the black holes contained within.
When you think about gargantuan phenomena such as the collision of galaxies, it might be tempting to imagine it as some sort of cosmic cataclysm, with stars crashing and exploding, and destruction on an epic scale. But actually it is closer to a pair of clouds combining, usually a larger one absorbing a smaller one. It’s unlikely any stars within them would collide themselves. But that said, when galaxies collide, the consequences can be enormous.
Galaxies collide in different ways. Sometimes a small galaxy will collide with the outer part of a larger one and either pass through or merge, in either case exchanging a lot of stars along the way. But galaxies can also collide head-on, where the smaller of the two will be torn apart by overpowering tidal forces of the larger one. It’s in this scenario that something very interesting can happen within the galactic nucleus.
“At the heart of most galaxies lies a massive black hole, or MBH,” said Research Associate Yohei Miki from the University of Tokyo. “For as long as astronomers have explored galactic collisions, it has been assumed that a collision would always provide fuel for an MBH in the form of matter within the nucleus. And that this fuel would feed the MBH, significantly increasing its activity, which we would see as ultraviolet and X-ray light amongst other things. However, we now have good reason to believe that this sequence of events is not inevitable and that in fact the exact opposite might sometimes be true.”
Papers:
Yohei Miki, Masao Mori and Toshihiro Kawaguchi, "Destruction of the
central black hole gas reservoir through head-on galaxy collisions," Nature Astronomy: January 25, 2021, doi:10.1038/s41550-020-01286-9.
Link (Publication)
Source: The University of Tokyo/Press Release
