Astronomers uncover hidden black holes in merging galaxies

A new survey has uncovered eight sets of two or even three feeding supermassive black holes as galaxies smash together.
By | Published: June 28, 2019
These 15 images from the Sloan Digital Sky Survey show merging pairs of galaxies. A supermassive black hole lies in the brightest portion of each galaxy: its nucleus. Within this sample, astronomers have identified several pairs and two trios of actively feeding black holes.
Pfeifle et al., ApJ, Volume 875, Issue 2, i.d. 117, 2019
Astronomers know that nearly all large galaxies host a supermassive black hole millions or billions times the mass of our Sun in their center. Some, like the monster in the middle of the Milky Way, are quiescent, feeding from only sparse clouds of gas and dust in a small region of space around them. But some are active, sucking down vast wells of material. These active galactic nuclei, or AGN, are common throughout the universe, and astronomers have just found more — but this new set is special.

On June 28, Ryan Pfeifle of George Mason University presented a set of eight previously unknown dual or triple AGN at the annual meeting of the European Astronomical Society.  These are merging galaxies in which the supermassive black holes in each of the participating galaxies are all actively feeding on material from their surroundings. Pfeifle’s team found these pairs and trios using data from NASA’s Wide-Field Infrared Survey Explorer (WISE) telescope and Chandra X-ray Observatory, as well as the European Space Agency’s XMM-Newton X-ray telescope. Their work was published earlier this year in the April 23 edition of The Astrophysical Journal.

Hunting AGN

AGN are most likely to be found in circumstances where there is sufficient “food” available. One such circumstance is during a galaxy merger, when two or more galaxies smash together, a process that disrupts gas, dust and stars, often sending them toward the centers of the galaxies where they can become fodder for the black hole. In some cases, two or more chomping black holes can be spotted as the merger goes through its very last stages.

But this wealth of available food has a catch. Material streaming toward the center of a galaxy feeds the black hole, but it also cloaks the black hole in dense dust and gas. This absorbs optical light from the AGN, making it invisible to optical surveys looking for signs of active black holes. But infrared light can escape as reprocessed light — energy from the AGN that has been absorbed by the dust around it and then re-emitted by those dust grains at longer (infrared) wavelengths.

This is why using WISE was the key to finding these AGN. By searching in infrared light, the team uncovered AGN that would have been missed by other surveys. Pfeifle’s team selected objects that looked like AGN in the infrared data, then followed up by observing them in X-ray light, too. X-rays can shoot straight through gas and dust, and are a clear sign of a feeding black hole. By matching the two samples, the team could even determine whether the X-rays seemed to be coming from one or more sources: single, dual, or triple AGN.

SDSS J0849+1114 is a triple-AGN system. When viewed in infrared light with the Hubble Space Telescope (main image), three bright spots – AGN – can be seen. An X-ray image taken with Chandra appears in the upper right; the Sloan Digital Sky Survey image of this merger appears at lower left.
Pfeifle et al., ApJL, submitted

Twos and threes

Dual AGN are rare, and triple systems are rarer. This leads astronomers to ask why that might be. Maybe they’re just hard to spot, or maybe they don’t last very long. Maybe they really are just rare, and special circumstances are needed to get two or more supermassive black holes actively feeding during a merger. Learning the answer would tell astronomers a lot about how galaxies evolve, because mergers are one of the major ways in which galaxies grow and age, and they probably happen to most galaxies once or more throughout their lifetimes.

Also important is the fact that dual AGN are the precursors of supermassive black hole mergers, which, like smaller stellar-mass black hole mergers, produce gravitational waves that could be detected by the gravitational wave observatories now scanning the skies.

Two mergers in the sample, SDSS J130653.60+073518.1 and SDSS J084905.51+111447.2 (J1306+0735 and J084+1114 for short, respectively), are triple AGN candidates, meaning the team spotted X-rays coming from three separate sources as three galaxies come together. Triple AGN are even more interesting for gravitational-wave researchers, because three AGN circling each other make it more likely that at least two of those AGN will merge sooner than they would have if there were only two.

Triple systems also increase the chances that one of the AGN could end up kicked out of the galaxy altogether — a rare sight, but one astronomers have spotted before.

In a press release Pfeifle said the study shows the power of looking in infrared light for multiple AGN that are completely invisible to optical telescopes. Additionally, he said, the same technique will help astronomers find more triple AGN to learn how likely these systems are to occur and what they can teach us about our universe.

Are you ready to learn more about AGN? Check out this feature from the pages of Astronomy: Glimpsing the hearts of galaxies.