One of the newly discovered black-hole pairs. On the left is the image from the Sloan Digital Sky Survey, showing just a single object. The image on the right, taken with the Keck telescope and adaptive optics, resolves the two active galactic nuclei, which are powered by massive black holes. |
Astronomers at the California Institute of
Technology, Univ. of Illinois at Urbana-Champaign, and Univ. of Hawaii
have discovered 16 close-knit pairs of supermassive black holes in merging
galaxies.
The discovery, based on observations done
at the W. M. Keck Observatory on Hawaii’s Mauna Kea, has been submitted for publication in the Astrophysical Journal.
These black-hole pairs, also called
binaries, are about a hundred to a thousand times closer together than most
that have been observed before, providing astronomers a glimpse into how these behemoths
and their host galaxies merge—a crucial part of understanding the evolution of
the universe. Although few similarly close pairs have been seen previously,
this is the largest population of such objects observed as the result of a
systematic search.
“This is a very nice confirmation of
theoretical predictions,” says S. George Djorgovski, professor of
astronomy, who will present the results at the conference. “These close
pairs are a missing link between the wide binary systems seen previously and
the merging black-hole pairs at even smaller separations that we believe must
be there.”
As the universe has evolved, galaxies have
collided and merged to form larger ones. Nearly every one—or perhaps all—of
these large galaxies contains a giant black hole at its center, with a mass
millions—or even billions—of times higher than the sun’s. Material such as
interstellar gas falls into the black hole, producing enough energy to outshine
galaxies composed of a hundred billion stars. The hot gas and black hole form
an active galactic nucleus, the brightest and most distant of which are called
quasars. The prodigious energy output of active galactic nuclei can affect the
evolution of galaxies themselves.
While galaxies merge, so should their
central black holes, producing an even more massive black hole in the nucleus
of the resulting galaxy. Such collisions are expected to generate bursts of
gravitational waves, which have yet to be detected. Some merging galaxies
should contain pairs of active nuclei, indicating the presence of supermassive
black holes on their way to coalescing. Until now, astronomers have generally
observed only widely separated pairs—binary quasars—which are typically
hundreds of thousands of light-years apart.
“If our understanding of structure
formation in the universe is correct, closer pairs of active nuclei must
exist,” adds Adam Myers, a research scientist at UIUC and one of the
coauthors. “However, they would be hard to discern in typical images blurred
by Earth’s atmosphere.”
The solution was to use Laser Guide Star
Adaptive Optics, a technique that enables astronomers to remove the atmospheric
blur and capture images as sharp as those taken from space. One such system is
deployed on the W. M. Keck Observatory’s 10-meter telescopes on Mauna Kea.
The astronomers selected their targets
using spectra of known galaxies from the Sloan Digital Sky Survey (SDSS). In
the SDSS images, the galaxies are unresolved, appearing as single objects
instead of binaries. To find potential pairs, the astronomers identified
targets with double sets of emission lines—a key feature that suggests the existence
of two active nuclei.
By using adaptive optics on Keck, the
astronomers were able to resolve close pairs of galactic nuclei, discovering 16
such binaries out of 50 targets. “The pairs we see are separated only by a
few thousands of light-years—and there are probably many more to be
found,” says Hai Fu, a Caltech postdoctoral scholar and the lead author of
the paper.
“Our results add to the growing
understanding of how galaxies and their central black holes evolve,” adds
Lin Yan, a staff scientist at Caltech and one of the coauthors of the study.
“These results illustrate the discovery
power of adaptive optics on large telescopes,” Djorgovski says. “With the
upcoming Thirty Meter Telescope, we’ll be able to push our observational
capabilities to see pairs with separations that are three times closer.”
In addition to Djorgovski, Fu, Myers, and
Yan, the team includes Alan Stockton from the Univ. of Hawaii
at Manoa. The work done at Caltech was supported by the National Science
Foundation and the Ajax Foundation.
