Courtesy of ALMA

A paper in The Astrophysical Journal Letters on Jan. 1 outlines the discovery of two active supermassive black holes at the center of a galaxy about 500 million light years away — which is close on the galactic scale.

The galaxy, UGC 4211, was formed by the merger of two different galaxies, each of which had a supermassive black hole —  black holes with masses hundreds to billions of times greater than that of our Sun — at its center. The two black holes then gravitated inwards and are now 750 light years away from each other, which is the closest separation of two supermassive black holes that we have conclusive evidence for. The black holes are also “active,” which means that they are currently feeding on the surrounding stellar material and growing. Such active supermassive black holes at the centers of galaxies are called active galactic nuclei, or AGN. 

“We know that over the billions of years that the universe has been evolving, galaxies have merged and continue to merge,” Meg Urry, an author of the paper, a professor of physics and astronomy and the director of the Yale Center for Astronomy and Astrophysics, said. “And, we also know that most galaxies, at least above a certain mass, have a supermassive black hole at their center. So, one of the questions that is not really solved yet is what happens to the two black holes in two galaxies when they merge?”

According to Urry, the plausible theory is that after the galaxy merger, the two black holes slowly sink to the center, form a binary — which are a pair of gravitationally-bound stellar objects in orbit around a common center of mass — and then eventually merge, emitting enormous amounts of gravitational waves in the process. Initially, the black holes sink due to friction with galactic gas and dust. Once the black holes get close to the center, however, the density of galactic material decreases and the pull of friction becomes negligible. If they get really close, they can lose energy and merge by emitting gravitational waves but it remains unclear how the black holes make the leap between friction and gravitational waves.

Such a merger takes millions of years to happen — the two black holes from this study are not expected to merge for at least another 200 million years. Based on this information, we should expect to see more instances of black holes orbiting each other. However, Urry said that such observations have been surprisingly few, which makes this discovery particularly significant.

Michael Koss, an astrophysicist at Eureka Scientific and the principal investigator of this research project, said that the discovery was a culmination of a ten-year endeavor to find AGNs in merged galaxies.

“The weird thing about doing astronomy is that you get a time machine but you only get to see one moment,” Koss said. “The entire timescale of these mergers is over a billion years. We don’t get to watch it play out for a particular system. So, either we can look at a bunch of systems or we can run simulations. And, when I was postdoc in Hawaii, there were these ideas based on simulations of galaxies that as two galaxies come close together, their black holes grow very quickly — become AGNs. So, if those simulations are true, then we should look for AGNs in mergers.”

Thus, Koss began looking for AGNs in galaxies that appeared to have been in mergers, and his team’s finding now provides observational evidence for the simulations.

For this study, Yale supplied telescope time at the Keck Observatory in Hawaii. First, Urry describes, the team used the near-infrared imager NIRC2 to survey the centers of galaxies that looked “disturbed,” which is an indicator of a merger. A large fraction of the observed galaxies had dual sources in the center — an exciting finding since the sources could potentially be two active black holes, but the results were not definitive. Thus, the team followed the survey with near-infrared spectrograph OSIRIS to confirm that in this particular galaxy, the sources were indeed a pair of gravitationally-bound active black holes.

OSIRIS proved two things: first, that the two sources had the same red-shift and thus are at the same distance from us; and second, that the two sources are not the same object distorted by gravitational lensing — the bending of light around mass — since the spectra of the two sources were not identical.  In this case, distortion from gravitational lensing could be caused by any cloud of gas and dust between the observing point and the source.

Keck was not the only telescope used for the research, however. The Very Large Telescope, the Atacama Large Millimeter/submillimeter Array and the Hubble Space Telescope all observed the same black hole pair in multiple wavelengths, which means that, unlike past discoveries, this one is highly unlikely to be a false positive.

Locating a supermassive black hole pair in such a close proximity suggests that pairs like this could be quite common in the universe, giving hope that the merging of two supermassive black holes in other systems could be observed and that scientists could detect the gravitational waves emitted from these events.

Since 2015, when the Laser Interferometer Gravitational Wave Observatory detected the first gravitational wave, many such detections have been made but they have all been gravitational waves from mergers of either stellar mass black holes — which are much smaller than their supermassive counterparts — or other astronomical objects, like neutron stars. Chiara M. F. Mingarelli, assistant professor at the University of Connecticut and gravitational-wave astrophysicist, explains that this is because detectors like LIGO can only pick up high-frequency gravitational waves but not the low-frequency ones that would be released when two supermassive black holes collide. These low frequency waves would give rise to a gravitational wave background.

Mingarelli’s current project involves trying to detect this background. Thus, she finds the dual black hole discovery particularly interesting because it has significant implications for the rate of supermassive black hole mergers and the intensity of gravitational wave background.

“This black hole pair is so nearby that either we got really lucky and found it because we are extremely lucky, or we found it because there are lots of them,” Mingarelli said. “We won’t know until we make more such observations, but this particular galaxy could be crucial to understanding the population statistics of black hole mergers and the gravitational wave background.”

The paper was published in Volume 942 of the Astrophysical Journal Letters.