Runaway Black Hole Flees Behemoth Galaxy

Astronomers have spotted a supermassive black hole in a stripped-down galaxy racing away from a near-fatal close encounter in the center of a galaxy cluster.

It’s tempting to think of black holes as monsters, especially the supermassive variety. These beasts lurk at the centers of most galaxies and can be millions or even billions of times more massive than the Sun. When they eat, they feast — gobbling down gas then flinging radio-emitting jets out into intergalactic space.

So it’s a little odd that such a monster might be spotted running away, but that’s just what astronomers have found. In a recent search of nearby galaxies, Jim Condon (NRAO) and colleagues came upon a supermassive black hole stripped of most of its galaxy and fleeing from a big “Jabba the Hut”-type elliptical. The results have been accepted for publication in the Astrophysical Journal.

Predator Turned Prey

Condon started out trying to find supermassive black holes that didn’t live right in the centers of their galaxies. Galaxies are thought to grow in part by mergers with other galaxies. If that’s the case, the two black holes at the center of each galaxy would also have to unite, and that process might deliver a good kick to the resulting, larger black hole, shooting it off-center. Condon surveyed hundreds of nearby galaxies searching for this expected signature of galaxy mergers.

He didn’t find any. What he did find was even weirder — a galaxy on the run.

About 30,000 light-years from a brilliant, massive elliptical galaxy shone a surprisingly luminous source of radio waves. The source was too bright to be anything other than an accreting supermassive black hole. Yet it was much too far from the galaxy’s core to belong to it.

Condon and colleagues followed up with Hubble and Spitzer observations for a closer look. To their surprise, they found that the radio source sat in its own little galaxy, which has the mass of some 6 billion Suns — less than 1% of the Milky Way’s mass and atypically tiny to be hosting a supermassive black hole.

Hubble images also revealed a trail of ionized gas extending from the tiny galaxy to its much bigger sibling. The trail (and specific wavelengths of light that the trail emits) shows that the little galaxy is speeding away from the larger one at more than 2,000 kilometers per second (4.5 million mph). Whether it remains gravitationally bound or is heading out of the cluster toward intercluster space isn’t yet known.

A Close Encounter of the Galactic Kind

So what happened to this odd little galaxy? The scenario Condon’s team finds most likely is that the speedy galaxy was once a normal galaxy that fell into the gravitational well around the “Jabba the Hut” elliptical at the cluster’s center. It came very close to being destroyed, passing within 3,000 light-years of the elliptical’s core, but it managed to come out the other side of its slingshot pass. As it passed, gravitational tidal forces stripped away most of the galaxy’s stars and gas, but the very core of the galaxy — and its supermassive black hole — survived.

So now what? What’s left of the galaxy is still trailing debris and will eventually cease star formation. The supermassive black hole at its center may speed this process along if its radiation helps push gas out. In a billion years or so, the black hole probably will be invisible, wandering undetected through intergalactic space.

Watch Jim Condon explain the find:


Are there marauding black holes in the universe? Short answer: yes. For the long answer (and it's one you'll want to read), take a look at our special publication, Astronomy's 60 Greatest Mysteries.

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