Supermassive black holes are only active when they are feeding, and they don’t get a better meal than when their host galaxies are going through collisions. But while we have a general understanding of supermassive black holes and galaxy mergers, we are still missing some of the details.
A new study, published in the Monthly Notices of the Royal Astronomical Society, suggests that in the late stages of galaxy mergers, so much material is funneled towards the supermassive black holes at the center that they become enshrouded in gas and dust.
This is an intriguing finding. To fall into a black hole, the material cannot move too fast, so this gas and dust needs to slow down. Researchers have noticed that the combined gravitational effect of the merging galaxies can decelerate stuff and push it into a black hole at a more modest pace.
"The further along the merger is, the more enshrouded the AGN will be," lead author Claudio Ricci, from the Pontifical Catholic University of Chile, said in a statement. "Galaxies that are far along in the merging process are completely covered in a cocoon of gas and dust."
It is expected that a black hole wakes up late in the merger phase. When it begins to accumulate enough material, it starts feeding and becomes an active galactic nucleus (AGN), emitting a lot of X-rays. But if the AGN is shrouded, only the most powerful X-rays will make it through.
So the researchers used NASA’s NuSTAR – which has the ability to see the brightest X-rays – on 52 late-stage mergers and compared this data to observations by other X-ray telescopes like Swift, Chandra, and ESA’s Xmm-Newton, which focus on less powerful X-rays. If they see a signal with NuSTAR but not with the others, then the galaxy is heavily obscured.
They found that every target was shrouded in dust (although not all at the same level) and confirmed that it’s towards the end of a galaxy merger that black holes gain more mass.
"A supermassive black hole grows rapidly during these mergers," Ricci added. "The results further our understanding of the mysterious origins of the relationship between a black hole and its host galaxy."
Sometimes it’s a big galaxy cannibalizing a small galaxy and sometimes it’s two big galaxies smashing together. Regardless, galaxy collisions are one of the key mechanisms of galaxy evolution, changing galaxy shapes, starting large episodes of star formation, and keeping black holes well fed and active.