Our Galaxy’s Center Might Hold Thousands of Black Holes

A new study has uncovered a dozen stellar-mass black holes within 3 light-years of the supermassive black hole at our galaxy’s core — and these might be just the tip of the iceberg.

X-ray binaries around galactic center (art)

This artist's rendering shows our galaxy's supermassive black hole surrounded by dust, gas, and 12 stellar-mass black holes. The inset shows that each black hole is paired with an ordinary star. A trickle of gas from the star feeds the black hole via an accretion disk, which emits an X-ray glow.
Columbia University

We’ve long known that a supermassive black hole with more than 4 million times the Sun’s mass lurks in our galactic center. Now, a study published in the April 5th Nature makes the case that the behemoth isn't alone. Potentially, 10,000 or so stellar-mass black holes might be keeping it company. The black hole population — if it’s real — would match theoretical predictions that lots of massive things ought to end up in our galaxy’s center.

Indeed, the Milky Way’s core is already a crowded place: More than 30 magnitudes’ worth of dust and gas block our view in visible light. The only way to peer into our galaxy’s enshrouded core is by going either very low (radio observations) or very high (X-rays or gamma rays). Charles Hailey (Columbia University) and colleagues chose to go high, basing their results on 12 days’ worth of observations that the Chandra X-ray Observatory collected over the past 12 years.

The team analyzed 92 sources that remain unresolved at X-ray wavelengths, so they look like points of light; 26 of these lie within 3 light-years of the supermassive black hole. For each of these sources, Chandra captured at least 100 photons over the 12 days of observations. (If that doesn't sound like a lot, that's because it's not — these are very faint sources!)

The astronomers then looked at how much radiation these sources emit at different energies: It’s a bit like putting light through a prism to see a rainbow, but the rainbow in this case is at X-ray wavelengths. And, surprisingly, the astronomers found that 12 of the 26 sources nearest the supermassive black hole tend to have “bluer” X-ray rainbows — that is, they’re relatively brighter at higher X-ray energies.

Most X-ray emitters in our galaxy’s center are white dwarfs that siphon gas off of ordinary stellar companions, radiating “redder” X-ray rainbows in the process (with more energy emitted at lower X-ray energies). But the new, “blue” X-ray sources appear to be binaries with something more massive — either neutron stars or black holes — made visible by the trickle of X-ray-emitting gas that feeds them.

X-ray sources in galactic center

A Chandra X-ray image of the galactic center is overlaid with circles around unresolved X-ray sources. Red circles indicate white dwarf binaries, which typically emit more low-energy X-rays, while cyan circles indicate likely black hole binaries, which emit relatively more high-energy X-rays. The yellow and green circle represent a region between 0.7 and 3 light-years from the black hole.
C. Hailey et al. / Nature

Hailey and colleagues argue that the sources don’t exhibit the outbursts characteristic of neutron star binaries, so they’re more likely to be black holes. Long-term monitoring of the galactic center has found nearly all the neutron star binaries by their outbursts, so it must be the black hole binaries that remain, quietly orbiting their stellar companions and feeding off just enough X-ray-emitting gas that we can (barely) see them.

If that’s the case, then these binary black holes would be the tip of an iceberg. Many more isolated black holes could exist in the galactic center, and we wouldn’t see them at all. How many depends on how these black holes came to be, a hotly debated question. If they formed right where they are, then there could be 10,000 — maybe even more! — black holes in the galaxy’s core*.

What’s perhaps most surprising is that these X-ray sources aren’t new; they’re all in the catalog of Chandra-discovered sources. “In some sense, the black hole binaries were hiding in plain sight,” Hailey says, “but weeding out the more prosaic sources and grappling with the X-ray emitting gas background takes a lot of time and energy, and the prospects for success were unclear. . . . It was such a compelling mystery, it was too tempting for us to resist.”

Maybe Less?

But — and this is a big but — it could be that not all of these sources are black holes. Moreover, they might not have formed in their current orbits. Astronomers have long been looking for quickly rotating neutron stars known as millisecond pulsars in the galactic center, which are widely thought to be captured from globular star clusters passing through the galactic center.

One of the reasons finding these pulsars is so important is that they could be to blame for the weirdly large amount of gamma rays that the Fermi telescope has observed radiating from the galactic center. While some astronomers have suggested that the signal might be the long-awaited signature of dark matter particles, millisecond pulsars present a less exotic (read: more easily accepted) option.

“That potential dark matter detection has driven people to do these really ambitious millisecond pulsar searches,” says Daryl Haggard (McGill University). “But they haven’t yielded anything so far.” It remains unclear whether that’s because they’re not there, or they’re just hard to find: Probing the galactic center at radio wavelengths is like looking for minnows in a turbulent and murky river; swirling streams of plasma often obscure the view.

Hailey and his team acknowledge that as many as half of their new-and-blue X-ray sources could be the sought-after millisecond pulsars. That would mean there would be fewer isolated black holes, maybe only several hundred instead of thousands. Even so, that’s still an awful lot of massive stellar remnants hiding in our galaxy’s center.

“In either case, it’s still interesting,” Haggard says, adding that future radio studies could help distinguish between black holes and neutron stars. Then we can start to get at the question of how these objects got there in the first place.


* In case you’re wondering: No, these black holes aren’t the dark matter we’ve been looking for. The amount of dark matter in the galaxy’s core would actually be quite small — most of it extends farther out than the Milky Way’s spiral-shaped disk. Also, since black holes used to be stars, they would have at one point been made of normal, baryonic particles, and we have good evidence that dark matter, whatever it is, isn’t baryonic.

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