Runaway black hole?

Galaxy NGC 1275. [Credit: NASA, ESA, and the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration]

Galaxy NGC 1275. [Credit: NASA, ESA, and the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration]

Sometimes theoretical science is stranger than science fiction

The most massive black hole ever measured may be an intergalactic hitchhiker that escaped from one galaxy before getting captured by another. If this scenario, laid out in a paper posted February 18 at arXiv.org, is proven correct, it would be the first time astronomers have definitively spotted a black hole that was expelled from its original galactic home.

A supermassive black hole lives in the center of just about every galaxy, including our own Milky Way. Most of the time, these black holes aren’t doing much, but when two galaxies collide — a rather common event — their black holes find each other and merge. Simulations involving relativity predict that under certain circumstances these merging black holes can be flung around and even kicked out of the merging galaxies. To understand how this can happen, we have to consider one of the fundamental properties of a black hole — its spin. Most (perhaps all) black holes are spinning, and this spin has a direction — you can think of black holes as kind of like spinning tops. If you bring two spinning black holes together, and their spins are going in the same direction, the pair coalesces into a single black hole inside of the merged galaxy as this simulation demonstrates:

The disks surrounding each of the black holes in the simulation are probably accretion disks — gaseous material rapidly spiraling down onto the black holes — that, in any case, show the direction of the black hole spin.

But what happens if you bring two spinning black holes together, and their spins are not going in the same direction? Turns out, this will cause the merging pair to flail around, sometimes with enough energy to kick the merged black hole out of the merged galaxy. The kicked black hole could carry gaseous material with it, if the material is gravitationally bound to the black hole, but the black hole would essentially wander the universe homeless.

The study above (authored by two of my long-time collaborators) proposes that the overly massive black hole residing in the relatively modest galaxy NGC 1277 was flung out of the neighboring galaxy, NGC 1275, long ago as it underwent a merger. Instead of wandering the universe homeless, however, the merged black hole pair found a new home in NGC 1277. What makes this scenario appealing is the close correlation between the masses of black holes and the masses of their host galaxies. Black holes are typically about 0.1% of the mass of their hosts, but NGC 1277 is a significant outlier from this relationship: this ho-hum galaxy hosts the most massive black hole ever observed, which weighs in at a stunning 14% of the galaxy’s mass. Adding to the appeal is the fact that NGC 1277 has a close galactic neighbor with a much greater mass, NGC 1275, a more likely original home for the excessively massive black hole.

Now that astrophysicists have a plausible theoretical explanation for NGC 1277’s outsized black hole, the search will be on for observational data supporting this idea.

8 thoughts on “Runaway black hole?

  1. Good question. IIRC, the maximum recoil occurs when the black hole spins are anti-aligned but in the same orbital plane. If they are perpendicular to one another, you still get a kick, just not the maximum.

  2. Is the kick typically sufficient to propel the black holes apart? What happens when the force isn’t enough? Have examples of such been found?

  3. It can be. Even though there are only two bodies orbiting each other, under certain conditions the black hole pair can act like a three-body system (which can be highly unstable) and kick one of the black holes out. This arises from the anisotropic way in which gravitational waves are emitted from the system.

    When the kick velocity isn’t sufficient to overcome the gravitational potential of the galaxy, you could in principle see an offset black hole in a galaxy. The black hole will travel away from the center until the gravitational potential of the galaxy brings it back. If the black hole is active (i.e. a quasar), you should be able to see an offset active nucleus.

    There are only a few plausible examples of kicked black holes. The strongest candidate is galaxy SDSS J092712.65+294344.0, but the giant elliptical galaxy M87 also hosts an offset nucleus that could have arisen from a black hole recoil. The reason we think there are so few examples is that dynamical friction (gravitational “drag” from stars in the galaxy) tends to slow the recoil and also dense gas surrounding black holes during mergers tends to align their spins.

  4. Thanks! I mostly just curious since every simulation I’ve seen have had the orbital planes lined up for easy examples.

    Wait, how does the dense gases change the black holes’ spin? I didn’t think the internal state was effected by anything less than the power of another black hole?

  5. The gas is in the form of an accretion disk with its own angular momentum. As the gas accretes onto the black holes, it torques them, and at least partially aligns their spins and orbital angular momentum with that of the disk.

  6. “affected” Sheesh, I need to proofread better.

    Thanks again.

    Of course now I’m going to ask dumb questions like how does gravity affect the spin inside the event horizon? My limited understanding was once something passed that horizon, information was lost, but this implies that gravity isn’t bound by that and a sufficient amount of information is changing the internal state. See what happens when laymen can post comments?

  7. Here’s a nifty little video that explains what’s (probably) going on inside a spinning black hole:

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