Slashdot Mirror
A Naked Black Hole Is Screaming Through the Universe (gizmodo.com)
New submitter PongoX11 writes: Millions of years ago, B3 1715+425 was just an ordinary supermassive black hole. It had a comfortable life, of devouring stars and belching deadly x-rays, at the center of its distant galaxy. Now, starless and alone, it's screaming through space at 2,000 kilometers per second -- and it may never stop. BC 1715+425's troubles began when its galaxy bumped up against another. This isn't all that unusual: in fact, astronomers believe that the largest galaxies in our universe formed during ancient mergers. Normally, when two galaxies collide, the supermassive black holes at their centers start to orbit one another, moving closer and closer together in an inescapable gravitational attraction. Eventually, those black holes can fuse, releasing a burst of energy as gravitational waves and completing the cosmic joining. Most of the time, this process seems to work out for all parties involved, judging from the fact that nearly all supermassive black holes reside at the center of galaxies, and nearly all galactic centers contain a supermassive black hole. But every now and then, something goes wrong and cosmic wreckage ensues. B3 1715+425, speeding away from the core of a bloated galactic merger 2 billion light years from Earth, is living proof of this. The working theory is that millions of years ago, B3 1715+425's galaxy passed through a much larger galaxy (one that had formed during many previous mergers) and got shredded to bits, a bit like a paper airplane flying into a hurricane. The leftovers include a faint galactic remnant, just 3,000 light years across, and the supermassive black hole itself, nearly naked and hemorrhaging ionized gas as it tears through the void. "We were looking for orbiting pairs of supermassive black holes, with one offset from the center of a galaxy, as telltale evidence of a previous galaxy merger," said James Condon, the astronomer at the National Radio Astronomy Observatory who led the study. "Instead, we found this black hole fleeing from the larger galaxy and leaving a trail of debris behind it."
"By definition, objects a long distance from each other are outside each other's escape velocity"
Excellent. So we don't need to worry about the Death Asteroid.
Galaxies typically form in gravitationally-bound clusters. and the super-massive black holes form at the centre of the galaxies; i.e. they co-move with the galaxies. If two galaxies from the same cluster merge, the holes at their cores do not have relative escape-velocity and end up gravitationally bound.
The interesting point in the current story is that the black hole has picked up much more velocity and escaped. I.e. it is exceptional.
I thought the same thing at first reading of the title but the wording involved has a subtle distinction. A "naked singularity" is a term which has been defined by physicists for a while and is, as you stated, a singularity in space-time which does not have an event horizon around it, or would be if such a thing can exist, which is currently undecided. The presence of an event horizon is what makes a black hole (things go in, nothing comes out [minus Hawking radiation, I know]). The term "naked black hole" seems to be a term just made up by the author of the linked article or somewhere back in the reporting chain and has no formal definition, as far as I know, but is highly misleading. I would say that whoever made up that term "naked black hole" is not very familiar with the field of gravitation or they would have avoided this confusing construction. Speaking of confusing, the last time I posted something involving General Relativity, I screwed it up; hopefully this time is better.