For the first time astronomers have glimpsed a long-predicted population of black holes lurking at the heart of the Milky Way.
Scientists already knew that our galaxy’s core holds a supermassive black hole weighing millions of times more than our sun and that this great beast is enveloped by a diverse entourage of lesser companions. Trapped in its gravitational clutches, run-of-the-mill stars whip around this gargantuan black hole like fireflies in a hurricane. So, too, do astrophysical exotica such as neutron stars and white dwarfs—the remnants left by normal stars when they die. Presumably black holes should be there as well, either born on the galactic center’s doorstep from the deaths of massive stars or arriving via migration from farther out.
Such black holes should each weigh 10 to 20 times more than our sun. That bulk would make them behave a bit like heavy pebbles outpacing fine silt to the watery bottom of a well, jostling through the lighter surrounding stars to reach stable orbits very close to the Milky Way’s core. Since the 1970s theorists studying this process have predicted a galactic center swarming with thousands of black holes bounded by an outer “cusp” beyond which the black holes’ numbers should plummet. But despite their predicted prevalence, these black holes are so dark and quiescent that they have been all but undetectable against the galactic center’s stellar splendor—at least, until now.
Using 12 years of archival data from NASA’s Chandra X-Ray Observatory, a team led by Columbia University astrophysicist Charles J. Hailey has found a dozen potential black holes within a few light-years of the Milky Way’s center, well within the gravitational reach of our galaxy’s supermassive black hole. The team speculates these must be the first observational signs of the long-theorized cusp. Based on the emissions and spatial distribution of these 12 systems, the team estimates 10,000 to 20,000 of these objects should be swirling around our galaxy’s core, mostly unseen. For perspective, apart from these newfound dozen scientists have identified only about 60 black holes in the entire Milky Way, and all but a few are far from the galactic center. The findings appeared in a paper in April 2018 in Nature.
The study appears to vindicate predictions from theorists such as Mark Morris, an astrophysicist at the University of California, Los Angeles, who in 1993 penned a key paper predicting tens of thousands of stellar-mass black holes would form a disk around the galactic center. Across the decades, other theorists tackling the problem have arrived at similar estimates. “There hasn’t ever been much controversy about this idea, because it’s just an inevitable consequence of simple Newtonian dynamics,” Morris says. “The only thing is, it has been really hard to prove.”
“Finding evidence for a large number of black holes at the center of the Milky Way confirms a fundamental and major prediction of galactic dynamics,” Hailey says. “These objects also provide a unique laboratory for learning about how big black holes interact with little ones because we can’t readily study these processes in other, more distant galaxies.”
Hailey and his team used Chandra data because black holes at the galactic center should be most visible via x-rays, produced when the black holes form a binary system with a low-mass star and feed on their captured companion.
Siphoned off by the black hole’s gravitational pull, the star’s outer layers will pile up outside the black hole’s maw in a spiraling, steadily glowing disk. The intense x-ray emissions from these disks would be exceedingly faint when viewed from Earth’s vicinity, sending only one photon apiece into Chandra’s optics every five or 10 minutes. These weak emissions would also be intermixed with many other x-ray sources from the galactic center. To pin down the nature of their dozen candidates, Hailey’s team plotted their spectral peaks and tracked their activity across time, finding patterns consistent with previous observations of binary black hole emissions elsewhere in the galaxy. The fact that there must then be tens of thousands of black holes at the galactic center stems from the notion that these objects would only very rarely be accompanied by a star to make them glow—most would remain isolated, invisible singletons.
Morris calls the work “exciting” but notes that because of the very low total numbers of photons used in the analysis, of the 12 putative black holes some might actually merely be statistical flukes produced by coincidentally timed emissions from other sources. Hailey, too, admits that of the dozen sources detected he feels certain only half are black holes—the remaining six, he says, display behavior that could also be explained as emissions from rapidly spinning neutron stars called millisecond pulsars.
Despite such uncertainty, Jordi Miralda-Escudé, an astrophysicist at the University of Barcelona unaffiliated with the work, says the results should have profound implications for future research. “A discovery like this will always have consequences that we cannot at present predict,” he says. “If confirmed, the existence of these black holes suggests similar concentrations should exist in the centers of most galaxies throughout the universe.” Such confirmations could come from perhaps another decade of additional Chandra observations or from studies by Chandra’s proposed successor, a space telescope called Lynx that nasa is studying for potential development and launch in the 2020s or 2030s.
Scientists studying gravitational waves would likely benefit the most from further studies of black holes hidden at the Milky Way’s core. Predicted by Albert Einstein more than a century ago, these elusive ripples in spacetime have only recently been observed, and the majority of detections to date have been traced to merging black holes billions of light-years away. Mysteriously, most of these black holes are inconveniently sized, appearing too large to have readily formed directly from dying massive stars. Alternative explanations posit that these anomalously massive black holes grew and merged in throngs of stars called globular clusters, but that process can easily require more time than the current age of the universe. “So how do you get these things?” Morris says. “Hundreds of papers have been written already speculating about this. But if you have clusters of black holes at the centers of galaxies, there are mechanisms by which some could rapidly grow, form binaries and merge with each other.”
Regardless of how scientists follow up this discovery, one way or another the result will be “pinning down the number of black holes in the center of a normal galaxy like the Milky Way,” Hailey says. “That will be invaluable, especially for researchers trying to calculate the nature and number of gravitational-wave events expected from galaxy cores. All the information astrophysicists need is right there, at the center of our galaxy.”