Runaway Supermassive Black Hole

Many galaxies have a ‘supermassive black hole’ at their center. These range from hundreds of thousands to billions times the mass of our Sun.

I was surprised to read that astronomers have found evidence for a supermassive black hole shooting out of its host galaxy. They’ve seen a long thin feature—apparently a ‘wake’ of shocked gas and young stars—stretching 200,000 light years from the galaxy’s center and ending in a bright object that’s putting out 100 million times more power than our Sun. It

This is consistent with a supermassive black hole that was thrown out the galactic center at a speed of 1600 kilometers/second, which has been traveling for about 40 million years. This speed is faster than the galactic escape velocity!

But what could be muscular enough to throw a supermassive black hole around?

Only two possibilities are known.

One is another supermassive black hole. When two galaxies collide, their central black holes meet – and may start orbiting each other. If a third galaxy with its own supermassive black hole crashes in, one of the three black holes can get flung out.

That seems quite reasonable to me: galactic collisions are fairly common.

The other possibility is weirder.

When two black holes collide, they can emit gravitational radiation that’s beamed mainly in one direction… and this can give them a ‘kick’ in the opposite direction. I find this surprising in the first place. And it’s more surprising that this effect can be big enough to kick a black hole out of a galaxy! But that’s what some calculations say.

The picture above is from this paper:

• Pieter van Dokkum, Imad Pasha, Maria Luisa Buzzo, Stephanie LaMassa, Zili Shen, Michael A. Keim, Roberto Abraham, Charlie Conroy, Shany Danieli, Kaustav Mitra, Daisuke Nagai, Priyamvada Natarajan, Aaron J. Romanowsky, Grant Tremblay, C. Megan Urry and Frank C. van den Bosch, A candidate runaway supermassive black hole identified by shocks and star formation in its wake.

Abstract. The interaction of a runaway supermassive black hole (SMBH) with the circumgalactic medium (CGM) can lead to the formation of a wake of shocked gas and young stars behind it. Here we report the serendipitous discovery of an extremely narrow linear feature in HST/ACS images that may be an example of such a wake. The feature extends 62 kpc from the nucleus of a compact star-forming galaxy at z=0.964. Keck LRIS spectra show that the [OIII]/Hβ ratio varies from ~1 to ~10 along the feature, indicating a mixture of star formation and fast shocks. The feature terminates in a bright [OIII] knot with a luminosity of 1.9×10⁴¹ ergs/s. The stellar continuum colors vary along the feature, and are well-fit by a simple model that has a monotonically increasing age with distance from the tip. The line ratios, colors, and the overall morphology are consistent with an ejected SMBH moving through the CGM at high speed while triggering star formation. The best-fit time since ejection is ~39 Myr and the implied velocity is v~1600 km/s. The feature is not perfectly straight in the HST images, and we show that the amplitude of the observed spatial variations is consistent with the runaway SMBH interpretation. Opposite the primary wake is a fainter and shorter feature, marginally detected in [OIII] and the rest-frame far-ultraviolet. This feature may be shocked gas behind a binary SMBH that was ejected at the same time as the SMBH that produced the primary wake.

For more about the kick caused by gravitational waves, try this:

• Manuela Campanelli, Carlos O. Lousto, Yosef Zlochower and David Merritt, Maximum gravitational recoil, Phys. Rev. Lett. 98 (2007), 231102.