When two galaxies collide, their central supermassive black holes also smash together. But before they do, these galaxies often flicker on, absorbing huge quantities of gas and dust and producing what is called an active galactic nucleus (AGN). But not all mergers are created equal. In some such events, only one black hole becomes active, while in others, both do. University of Colorado Boulder researcher Scott Barrows and co-authors discovered that single activations seem to occur more often in mergers in which the galaxies are mismatched — or when one galaxy is huge and the other small.
This celestial firestorm is the blazing wreckage of a collision between two spiral galaxies NGC 4038 and NGC 4039, which are located about 62 million light-years from Earth. The two galaxies, whose bright yellow cores appear to the lower left and upper right of center, began their fateful confrontation a few hundred million years ago. The pair is nicknamed the Antennae Galaxies because of two long streamers of stars, gas, and dust that extend from the crash site. Wide-field images reveal the elongated ‘antennae’ formed during the initial impact, but this Hubble image concentrates on the heart of the galactic collision. Image credit: NASA / ESA / B. Whitmore, STScI.
“When lopsided galaxies join, the merger is less violent, and that leads to less gas and dust falling onto the black holes. And the less material you have falling onto the black holes, the less likely you are to have two of them become AGNs,” Dr. Barrows said.
He and his colleagues from the University of Colorado Boulder and Princeton University used data collected by NASA’s Chandra X-ray Observatory to systematically scan the night sky for the signatures of AGNs.
The astronomers spotted mergers in progress by looking for ‘offset galaxies,’ or galaxies with a single AGN that sits away from the center of the galaxy.
Such a lack of symmetry suggests that a second supermassive black hole, which hasn’t been turned on, might be hiding nearby.
The team next assembled a sample of 10 offset galaxies and compared that sample to galaxies with a pair of AGNs.
The results were stark: 9 out of the 10 galaxies with only one active black hole came from lopsided mergers, or cases in which one galaxy was more than four times the size of the other.
Two-thirds of the galaxies with two active black holes, in contrast, were experiencing clashes among near equals.
“When galaxies of roughly equal size meet, their black holes exert tremendous gravitational forces on each other,” Dr. Barrows said.
“Those forces, in turn, send clouds of gas and dust raining onto the black holes.”
“It’s these torques that extract energy from the gas and dust, allowing it to fall into the nucleus of the black hole.”
“In mismatched mergers, you simply have smaller forces exerted on the gas and dust in each galaxy.”
The scientists didn’t find any rhyme or reason to which black hole activated during a mismatched merger.
“In some cases, it was the bigger black hole. In other cases, the smaller one,” Dr. Barrows said.
“Next up, we will focus on how the smashing together of two black holes affects the galaxies themselves, including how they create and destroy stars.”
The researchers presented their findings June 7 at the 232nd Meeting of the American Astronomical Society in Denver, Colorado.
_____
Scott Barrows et al. 2018. Formation of Offset and Dual Active Galactic Nuclei. 232nd Meeting of the American Astronomical Society, abstract # 401.04
