January 17, 2005
New view of distant colliding galaxies captured
by Keck laser system
By Tim Stephens
For the first time, astronomers have been able to combine the
deepest optical images of the universe, obtained by the Hubble
Space Telescope, with equally sharp images in the near-infrared
part of the spectrum using a sophisticated new laser guide star
system for adaptive optics at the W. M. Keck Observatory in
The Keck laser creates a high-altitude
artificial star, allowing the adaptive optics system to
measure and correct for turbulence in the Earths
atmosphere. Images taken with adaptive optics have a resolution
at near-infrared wavelengths that is comparable to optical
images taken with the Hubble Space Telescope.
Image: Sarah Anderson
This galaxy contains two nuclei,
indicating a recent galaxy-galaxy collision. The right-hand
nucleus is slightly bluer than its partner. The galaxy
is an x-ray source and is 4.6 billion light years away.
This three color image combines optical light from the
Hubble space telescope with infrared light from the Keck
laser guide star system.Image: UCSC, UCLA, Keck
This galaxy is a 30-times stronger x-ray source than the
one above. It appears to be undergoing an even more violent
merger event, which funnels gas onto a massive black hole
in its core, producing x-rays. Black holes of this sort
have masses up to a billion times that of our Sun. The object
is 5.7 billion light-years away. Image:
UCSC, UCLA, Keck
The new observations, presented at the American Astronomical
Society (AAS) meeting in San Diego last week, reveal unprecedented
details of colliding galaxies with massive black holes at their
cores, seen at a distance of around 5 billion light-years.
Observing distant galaxies in the infrared range reveals older
populations of stars than can be seen at optical wavelengths,
and infrared light also penetrates clouds of interstellar dust
more readily than optical light.
The new infrared images of distant galaxies were obtained by
a team of researchers from UCSC, UCLA, and the Keck Observatory.
Jason Melbourne, a UCSC graduate student and lead author of
the study, said the initial findings include some surprises
and that researchers will continue to analyze the data in the
weeks to come.
"This is very exciting, because we have never been able
to achieve this level of spatial resolution in the infrared
before," Melbourne said.
In addition to Melbourne, the research team, led by David Koo
of UCSC and James Larkin of UCLA, includes Jennifer Lotz, Claire
Max, and Jerry Nelson at UCSC; Shelley Wright and Matthew Barczys
at UCLA; and Antonin H. Bouchez, Jason Chin, Scott Hartman,
Erik Johansson, Robert Lafon, David Le Mignant, Paul J. Stomski,
Douglas Summers, Marcos A. van Dam, and Peter L. Wizinowich
at Keck Observatory.
"For the first time now in these deepest images of the
universe we can cover all wavelengths of light from the optical
to the infrared with the same level of spatial resolution, which
allows us to observe detailed substructures in distant galaxies
and study their constituent stars with a precision we couldn't
possibly obtain otherwise," said Koo, a professor of astronomy
and astrophysics at UCSC.
The images were obtained by Wright and the Keck AO team during
testing of the laser guide star adaptive optics system on the
10-meter Keck II Telescope. They are the first science-quality
images of distant galaxies obtained with the new system. This
marks a major step for the Center for Adaptive Optics Treasury
Survey (CATS), which will use adaptive optics to observe a large
sample of faint, distant galaxies in the early universe, said
"We've worked very hard for several years taking data around
bright stars. But we have been very restricted in terms of the
number and types of objects that we can observe. Only with the
laser can we now reach the richest and most exciting targets,
especially those with beautiful optical images from the Hubble
Space Telescope," Larkin said.
Adaptive optics (AO) corrects for the blurring effect of the
atmosphere, which seriously degrades images seen by ground-based
telescopes. An AO system precisely measures this blurring and
corrects the image using a deformable mirror, applying corrections
hundreds of times per second. To measure the blurring, AO requires
a bright point-source of light in the telescope's field of view,
which can be created artificially by using a laser to excite
sodium atoms in the upper atmosphere, causing them to glow.
Without such a laser guide star, astronomers have had to rely
on bright stars ("natural guide stars"), which drastically
limits where AO can be used in the sky. Furthermore, natural
guide stars are too bright to allow observations of very faint,
distant galaxies in the same part of the sky, Koo said.
"The advent of the laser guide star at Keck has opened
up the sky for adaptive optics observations, and we can now
use Keck to focus on those fields where we already have wonderful,
deep optical images from the Hubble Space Telescope," Koo
Because the diameter of the Keck Telescope's mirror is four
times larger than Hubble's, it can obtain images four times
sharper than Hubble in the near-infrared now that the laser
guide star adaptive optics system is available to overcome the
blurring effects of the atmosphere.
The images being presented at the AAS meeting were obtained
in an area of the sky known as the GOODS-South field, where
deep observations have already been made by Hubble, the Chandra
X-ray Observatory, and other telescopes. There are six faint
galaxies in the images, including two x-ray sources identified
by Chandra. The x-ray emissions, combined with the disordered
morphology of these objects, suggested recent merger activity,
Melbourne said. Mergers can funnel large amounts of matter into
the center of a galaxy, and x-ray emissions from the galactic
center indicate the presence of a massive black hole that is
actively consuming matter.
"We are now fairly certain that we are seeing galaxies
that have undergone recent mergers," Melbourne said. "One
of these systems has a double nucleus, so you can actually see
the two nuclei of the merging galaxies. The other system is
highly disordered--it looks like a train wreck--and is a much
stronger x-ray source."
In addition to lighting up the galactic nucleus with x-ray emissions,
mergers also tend to trigger the formation of new stars by shocking
and compressing clouds of gas. So the researchers were surprised
to find that the system with a double nucleus is dominated by
relatively old stars and does not appear to be producing many
"If we are right about the merger scenario, then this merger
is occuring between two galaxies that had already formed most
of their stars billions of years before and did not have a lot
of gas left over to make new stars," Melbourne said.
If additional study shows that such objects are common further
back in time, these observations could help explain one of the
puzzles of galaxy formation. According to the prevailing theory
of hierarchical galaxy formation, large galaxies are built up
over billions of years through mergers between smaller galaxies.
Since mergers trigger star formation, it has been difficult
to explain the existence of very large galaxies that lack significant
populations of young stars.
"One idea is that you can have a so-called dry merger,
where two galaxies full of old stars but little gas merge without
forming many new stars. What we are seeing in this object is
consistent with a dry merger," Melbourne said. "Even
in a dry merger, there may still be enough gas to feed the black
hole, producing x-ray emissions, but not enough to yield a strong
burst of star formation."
Further observations at mid- to far-infrared wavelengths, expected
later this year from the Spitzer Space Telescope, may help confirm
this. The Spitzer data will provide a better indication of the
dust content of the galaxy, a crucial variable in interpreting
these observations, Melbourne said.
The laser guide star adaptive optics system was funded by the
W. M. Keck Foundation. The artificial laser guide star system
was developed and integrated in a partnership between the Lawrence
Livermore National Laboratory and the W. M. Keck Observatory.
The laser was integrated at Keck with the help of Dee Pennington,
Curtis Brown, and Pam Danforth.
The NIRC2 near-infrared camera was developed by the California
Institute of Technology, UCLA, and the Keck Observatory. The
Keck Observatory is operated as a scientific partnership among
Cal Tech, the University of California, and the National Aeronautics
and Space Administration.
This work has been supported by the Center for Adaptive Optics,
a National Science Foundation Science and Technology Center
managed by UCSC.
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