At the center of our galaxy lies colossal black hole with the mass of four million Suns, called Sagittarius A*. Scientists think spiral and elliptical galaxies like the Milky Way contain these beefed-up black holes at their cores, but what has been more difficult to quantify is the population of smaller black holes in the immediate vicinity of these central giants.
Longstanding theories suggest that there should be abundant small black holes—perhaps as many as 20,000—within a few light years of Sagittarius A*. This is both because small black holes likely gravitate toward their supermassive counterparts, but also due to the rich environment of dust and gas surrounding galactic cores, which provides fertile ground for the formation of huge, short-lived stars that collapse into black holes after they explode as supernovas.
Previous surveys aimed at rooting out these smaller objects have been unable to confirm the high numbers suggested in predictions. But this cosmic stalemate has now been upended with the publication of a new paper in Nature on Wednesday, led by Columbia University astrophysicist Chuck Hailey, co-director of the Columbia Astrophysics Lab.
Past teams had searched for bursts of X-ray emissions from black hole binaries, which are systems in which a black hole becomes gravitationally intertwined with another object like a star.
These bursts proved too rare to confirm theories about black hole populations near Sagittarius A* so instead, Hailey’s team focused on detecting binaries by searching for the dimmer X-ray signals that the systems emit during their dormant phases. To do this, the researchers scanned through 12 years of archival data captured by the Chandra X-ray Observatory telescope, and were able to find a dozen low-mass black holes coupled with stars within three light years of Sagittarius A*.
Given this confirmed population, the team calculated that there are likely several hundred of these binaries—and roughly 10,000 black holes with no binary partners—within a few light years of the Milky Way’s core. These black holes are thought to form a “density cusp,” meaning that the closer you get to Sagittarius A*, the denser the black hole population.
This information is useful not only for resolving outstanding theories about the abundance and distribution of black holes surrounding the Milky Way’s central bulge, but also for constraining predictions about gravitational waves. These newly detectable waves are produced by tumultuous events in the universe—like black holes colliding—and they yield different astronomical insights than light-emitting objects like stars or galaxies.
Over email, Hailey told me that it “would be a miracle” if scientists detected a gravitational wave that originated in our own galaxy, because capturing these waves is so rare. But, he said, “by getting solid numbers of black holes in the center of our galaxy, and the distribution of those black holes, which we have now observed, that information can be ‘spun’ by theorists into a deeper understanding” about the nature of gravitational wave events in other galaxies.
“After all, the Milky Way is an average type galaxy, so if we see lots of black holes snuggled up against the supermassive one here, we should see them in the centers of many other galaxies,” he told me. “So theorists will use our results to particularize their predictions of how many of these exotic binaries (and [gravitational wave] events) will happen in other galaxies so they can make much firmer predictions than they could otherwise make.”
Black holes, by their very nature, are difficult to detect and study. But with new techniques like those employed by Hailey and his co-authors, scientists are steadily untangling some of their most elusive mysteries.
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