Astronomers Search for Quasars with a 'Kick' (Forwarded)

McDonald Observatory
University of Texas
Fort Davis, Texas

Contact:
Rebecca Johnson
ph: 512-475-6763 fax: 512-471-5060

Additional Contacts:

Dr. Greg Shields, 512-471-3000
Dr. Erin Bonning, 603-661-3669
Ms Sarah Salviander, 512-471-7460

29 May 2007

Astronomers Search for Quasars with a 'Kick'

HONOLULU, Hawaii -- A runaway black hole barreling out of a galaxy at more
than two billion miles per hour? Evidence of a quick getaway in the
aftermath of a massive intergalactic collision? That's just what astronomers
Erin Bonning of the Paris Observatory and Gregory Shields and Sarah
Salviander of The University of Texas at Austin have been searching for.
They are presenting the results of their search for supermassive speed
demons today at the 210th meeting of the American Astronomical Society in
Honolulu, Hawaii.

Recent theoretical predictions have shown that when two galaxies collide,
their central black holes sink to the center of the resulting galaxy and
begin to orbit each other while slowly emitting gravitational radiation.
Finally, they get so close that they collapse onto each other, creating one
giant black hole. Scientists have found that if the black holes are spinning
in a particular way when this happens, the final black hole will get a kick
and go flying away from the scene of the crime.

"The gravitational radiation shoots out to one side," Bonning said. "The
'kick' from this causes the black hole to recoil in the opposite direction
-- like a rocket."

Bonning, Shields, and Salviander have gone looking for these runaway black
holes in quasars -- active galaxies in which a glowing disk of hot gas
surrounds the black hole. Matter in the disk nearing the black hole is
heated by the violent orbital motion, causing it to give off copious amounts
of radiation -- a tell-tale signature of the omnivorous monster devouring
whatever ventures too close.

They have searched the publicly available data of the Sloan Digital Sky
Survey (SDSS) for quasars which show some evidence this kind of "kicked"
black hole. They looked at the spectra of about 2,600 quasars. If there were
a getaway disk moving through the galaxy, it would show up here.

"There are a couple of these quasars whose spectra raise suspicions,"
Shields said, though none show definitive evidence of being "kicked-out
quasars." It would be interesting for someone to take these "best cases" and
image them with Hubble Space Telescope, he said, to see if the quasar is
offset from its host galaxy.

Their results, which have been submitted to the journal Astrophysical
Journal Letters for publication, are important observational data for
scientists who simulate black hole mergers, since the strength of the kick
is related to the spins of the black holes.

"We didn't find nearly as many quasars with high velocity shifts as we
thought we would, looking at the theoretical predictions," Bonning said.
"And even those quasars that were 'shifty' didn't show any other evidence of
being absconding black holes. They were more likely to be stationary black
holes in the middle of a slightly more complicated quasar than usual."

However, Shields said, "It doesn't mean the calculations are wrong. It tells
you that the sequence of events when the galaxies collide and merge, and
their black holes spiral together, rarely -- if ever -- leads to the
conditions that give this kind of kick.

"In order to see the 'kick' effect, it requires a special alignment of the
merging galaxies. That's statistically rare. Less than one in ten of these
might get a kick of at least 1,000 kilometers per second. And if the merger
does not occur when the black hole is shining as a quasar, it will not be
visible." The maximum possible kick, he said, is 2,500 km/sec, which could
only occur when the two original black holes are of comparable size.

The search goes on. If it turns out that kicked black holes dragging
accretion disks cannot be found, that will also be an important result.
"Sometimes it's more exciting when you don't find something you're
expecting," Salviander said.

However, continuing to look for kicked black holes could have a huge payoff
-- the chance to see something spectacular.

The astronomers calculated that a merged black hole kicked out of a newly
merged galaxy would drag along a large part of its accretion disk. "This
would keep the black hole shining even as it wandered off from the center of
the galaxy," Shields said.

The black hole would drag away the inner part of its accretion disk most
strongly, Shields said. But outer parts of the disk would also follow. This
sets the stage for a magnificent collision in the future, as the lagging
portions of the original disk eventually catch up and crash onto the inner
part of the disk. (See illustration.)

"We're talking about millions of solar masses of matter crashing into the
accretion disk," Shields said. "There would be shock waves, and the disk
would be heated to millions of degrees, producing X-rays. It's a potentially
dramatic, but relatively short-lived event. It could be brighter than the
quasar itself -- one of the brightest X-ray events in the universe."

The crash would create an X-ray flare that lasts a thousand years, he said,
virtually the blink of an eye compared the quasar 's lifetime, which may be
tens of millions of years.

Dr. Bonning's work was funded by a Marie Curie Fellowship.

[NOTE: An image supporting this release is available at
http://mcdonaldobservatory.org/news/...age.php?id=106 ]