An article published in “The Astrophysical Journal” describes the discovery of the first pair of supermassive black holes orbiting each other at the center of the galaxy hosting them, called 0402+379. A team of astronomers used the Very Long Baseline Array (VLBA) radio telescope to locate the two black holes about 750 million light years from Earth. Their combined mass is about 15 billion times the Sun’s.
Today we know that most galaxies have a supermassive black hole in their center. When two galaxies merge, their supermassive black holes start attracting each other until they merge into a single, even more massive, black hole. All this was theorized long ago but no one had been able to find a pair of that kind yet.
The pair of supermassive black holes were found in the spiral galaxy 0402+379, discovered in 1995. Between 2003 and 2005, the VLBA radio telescope was used to find out that it has two nuclei instead of One, a sign of a galactic merger. Greg Taylor of the University of New Mexico (UNM), one of the authors of this new research, in 2006 concluded with a group of collaborators that the galaxy contained two supermassive black holes.
For this research, data collected during observations made between 2009 and 2015 were used and data collected during the observations from previous years were re-analyzed. Thanks to these data, the researchers found the two supermassive black holes estimating their combined mass and that the distance separating them is about 24 light years, really little in astronomical terms even though it’s a lot for two objects orbiting each other.
The estimate of the period taken by the two supermassive black holes to make a whole revolution around each other was estimated at 24,000 years. Karishma Bansal, the article’s first author, stated that it will take further observations in the next 3 or 4 years to confirm the motion and get a precise estimate of the orbital period. That’s because it’s so long that it takes a long time to notice any shift. In fact, the researchers have data covering more than a decade of observations yet still can’t notice the slightest curvature in their orbit.
It may take millions of years before we can see these two supermassive black holes merge. Such an event would be even more interesting today for its production of gravitational waves, one of the hottest subject of recent years for the opening of new frontiers of astronomy. Unfortunately in this case the waiting time is unfavorable.
This discovery represents the culmination of several years of observations with a very sophisticated instrument but in these cases it’s only a phase that will be used for further studies. Now researchers can look for other pairs of supermassive black holes and study more in greater details their influence on the galaxies that host them. Within a few billion years the Milky Way and Andromeda will be merging as well, creating a situation similar to that of the galaxy 0402+379.