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Why the Hell Is This Enormous Black
Hole Streaking Across Space?
Ryan F. Mandelbaum
Friday 3:20pm · Filed to: COOL STUFF
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Image: Chiaberge et Al
We don’t understand quasars all that well, but are pretty certain that these
incredibly bright lights belong in the centers of galaxies. So it looked a
little weird when astronomers spotted quasar 3C 186 thirty six thousand
light years away from the center of its galaxy, seemingly trying to escape.
Given their brightness and location in galactic centers, it’s likely that
quasars are supermassive black holes, and that the bright light comes from
the friction of gas and dust orbiting the central mass at incredibly high
speeds. An international team of astronomers spotted this strange galactic
escape eight billion light years away with the Hubble Space Telescope,
Sloan Digital Sky Survey, and Chandra X-Ray Observatory. They think the
quasar could have received a kick from gravitational waves, when it was
created in a violent collision between black holes three billions of time the
mass of our own Sun.
But that explanation didn’t sit quite right with Grant Tremblay,
astrophysicist at Yale and one of the paper’s authors. At first, Tremblay
and his colleagues thought the galaxy and its quasar had just recently
merged with another one. “But mergers that happen recently look like
total trainwreck messes,” he told Gizmodo. “This is a really regular
looking galaxy, which means if there was a major merger, it would have
had to have happened several billi0n years ago.” In other words, it would
have taken time after the merger for the galaxy to re-assume its current
elliptical shape.
Furthermore, black holes want to return to the center of their galaxies
through a process called dynamical friction, said Tremblay. This quasar
seems to be rushing away at around 2000 kilometers per second (4.5
million miles per hour) instead.
A more obvious photo of the black hole escaping - the black circle is the galactic center (Image: Chiaberge
et al)
So, why would a bright quasar like 3C 186 run away from such a good
lookin’ galaxy? The paper, which will be published next week in the
journal Astronomy & Astrophysics, proposes one of the most violent
collisions possible. First, the idea goes, two supermassive black holes spun
around each other and fused, their combined mass creating the three
billion solar mass quasar 3C 186. Any leftover momentum from the
spinning would turn into gravitational waves, ripples that travel through
and change the shape of spacetime itself, which physicists first observed
last year. Models show that resulting gravitational wave ‘kick’ could
account for the black hole’s speedy escape.
That’s just one explanation, of course, said Tremblay. There could be
others: Maybe the quasar is speeding away from a much messier galaxy
that only recently collided with another, and our imaging has just made it
look like it’s in good shape, for example. Some explanations Tremblay and
company have ruled out, like the possibility that the quasar is just sitting
in front of the galaxy but not at all associated with it. It would be silly to
think we know what’s going on for sure. “Nature is weird,” he said. “There
are a lot of galaxies in the universe. From a pure dynamical friction
argument, a black hole should sink to the center of its host galaxy. But that
doesn’t mean its offset can only be from a gravitational wave recoil.”
Others thought the gravitational wave explanation seemed pretty good. “I
tend to agree with their conclusion,” Lionel London, a research associate
at Cardiff University who models gravitational waves, told Gizmodo in a
Skype message. “The inspiral of two black holes... causes the entire twoblack hole system to wiggle around, and just before merger, there’s a little
bit of wiggle left, which manifests as a kicked final black hole.”
Scientists are continuing to study this quasar to confirm their gravitational
wave-based explanation. But until then, good luck out there 3C 186. That
galaxy wasn’t good enough for you anyway.
[Astronomy & Astrophysics via HubbleSite.org]
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