Watch a Black Hole Get Evicted From a Galaxy

4 minute read

A couple of years ago, astronomer Michael Koss was searching the heavens for active galactic nuclei (AGN). In plain English, those are giant black holes, lurking in the cores of galaxies, which swallow matter so voraciously that the gas they gobble heats up to an incandescence visible billions of light-years away. And he wasn’t looking for just any AGN; he was looking for twin AGNs, which occur when two AGN-bearing galaxies merge into one.

Then, says Koss, “I found this thing.”

The thing was just a single spot of light, labeled SDSS1133, nestled in a dwarf galaxy called Markarian 177, located in the bowl of the Big Dipper, about 90 million light-years from Earth. It looked just like an AGN—except it wasn’t in the galaxy’s core. It was off-center by about 2,600 light-years. So maybe it wasn’t an AGN after all, but an unusual type of exploding star.

But when Koss went back to earlier observations, some made by NASA’s Swift satellite, the bright spot was there, at least as far back as the 1950’s. Since stars don’t usually take a half-century to explode, Koss and several colleagues were forced to consider a much stranger possibility: the mystery object could be an AGN after all, but one that was somehow booted from the center of its galaxy. Their report appears in the November 21 Monthly Notices of the Royal Astronomical Society.

You might imagine it would be touch difficult to boot a black hole anywhere, especially one that weighs millions of times as much as a star. There is, however, one thing that could do the job: a second black hole. Sort of. “We suspect we’re seeing the aftermath of a merger of two small galaxies and their central black holes,” said co-author Laura Blecha, of the University of Maryland, in a statement.

The idea goes like this. Astronomers know that galaxies that wander too close to each other get trapped by their mutual gravity, and merge into one; it happens all the time, in fact. Since virtually all galaxies have huge black holes inside, the new, combined galaxy ends up with twin black holes in their cores (some of which turn into the double AGN’s Koss was looking for in the first place).

But the black holes themselves can merge as well. When that happens, the cataclysm sends gravitational waves rippling across the universe. If the black holes have different masses and different spins, those waves can shoot out more powerfully in one direction than another—and that can kick the new, single black hole right out of the galaxy’s core. “That’s our most plausible case,” Koss says.

It’s not the only case, however: the scientists haven’t ruled out the idea that the bright spot is an exploding star after all. If so, the light seen in earlier images from the 1950s could have come from violent eruptions on the star, which culminated in an explosion back in 2001, when SDSS1133 brightened visibly. It’s not unheard of: a nearby star in our own galaxy, Eta Carinae, is erupting in what astronomers think could be a prelude to a full-fledged supernova explosion.

But SDSS1133 shines brightly in ultraviolet as well as visible light, even though the ultraviolet light from supernovas tends to fade quickly. Followup observations with the Hubble Space Telescope a year or so from now could clear up the question for good.

In the meantime, it’s natural to wonder whether SDSS1133 will eventually fly out of its host galaxy entirely and begin to roam the universe as a naked black hole. The answer, says Koss, is “it’s hard to say.” The galaxy itself is small, so it doesn’t have a lot of gravity to hold SDSS1133 back. But the original kick wasn’t all that hard, so the black hole might not have reached escape velocity.

In short, we’ll know one way or another whether SDSS is a black hole within the year. To learn whether it will escape Markarian 177—well, that’ll take a couple million.

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