Top: A map of the hydrogen emission from DLA2222-0946. The mapped
region, which covers only a portion of the galaxy, is about 5,000
parsecs (or 16,300 light-years) across. The position of the background
quasar is marked by a "Q". Bottom: The corresponding map of the movement
of the gas in the galaxy. Red means moving away from us; blue is moving
towards us. Large image
Mauna Kea, Hawaii – University of
Hawaii at Manoa astronomer Regina Jorgenson has obtained the first image
that shows the structure of a normal galaxy in the early universe as
captured by the W. M. Keck Observatory. The results were presented at
the winter American Astronomical Society meeting being held this week
near Washington, DC.
The galaxy, called DLA2222-0946, is so faint
that it is virtually invisible at all but a few specific wavelengths.
It
is a member of a class of galaxies thought to be the progenitors of
spiral galaxies like our own Milky Way.
These galaxies are known
to contain most of the neutral gas that is the fuel for star formation,
so they are an important tool for understanding star and galaxy
formation and evolution. Discovered and classified over 30 years ago,
they have been notoriously difficult to see directly.
Dr.
Jorgenson, an NSF Astronomy and Astrophysics Postdoctoral Fellow at the
University of Hawaii's Institute for Astronomy, worked with Dr. Arthur
Wolfe of the University of California, San Diego. They used the advanced
technologies of the W. M. Keck Observatory on Mauna Kea to obtain the
first-ever spatially resolved images of a galaxy of this type.
The galaxy was detected with the 10-meter, Keck I telescope fitted with OSIRIS and the Laser Guide Star Adaptive Optics system.
DLA2222-0946
was initially detected not by its own light, but by absorbing some of
the light of an even more distant quasar. Galaxies detected in this way
are called damped Lyman-alpha systems, or DLAs, based on the specific
color of light they absorb due to their copious reservoirs of hydrogen
gas.
While thousands of DLAs are now known thanks to the large
Sloan Digitized Sky Survey (SDSS), their detection in absorption tells
us only about the small part of the galaxy pierced by the background
quasar’s light. This is akin to trying to map a fog bank from a single
headlight shining through it.
A full understanding of the distant galaxy requires a direct detection, which had eluded astronomers until now.
“These
galaxies are extraordinary for being ordinary—they represent normal
types of galaxies, rather than the brightest, extreme, and most rapidly
star-forming galaxies that are typically observed at these redshifts,”
Dr. Jorgenson explained. “But this normalcy makes them nearly impossible
to detect directly from the light they give off because first, that
emission is relatively weak, and second, the bright background quasar
used to find the galaxy hampers the detection of fainter foreground
emission from the galaxy itself.”
The galaxy is located at a
redshift of 2.354, which corresponds to a time when the universe was
about 20 percent of its current age, about 10.8 billion years ago. This
time in the universe’s history was a key period of galaxy formation, and
hence observing typical galaxies from this time will potentially
provide great insight into the relevant physical processes.
Determining
exactly how galaxies such as these, which are essentially massive
reservoirs of neutral gas, turn that gas into stars is a key missing
piece of the star and galaxy formation puzzle.
A preprint of the paper, which will be published in the Astrophysical Journal, can be found at the arXiv website at: http://arxiv.org/abs/1311.0045.
OSIRIS,
the OH-Suppressing Infrared Imaging Spectrograph, is an "integral field
spectrograph." The instrument works behind the adaptive optics system,
and uses an array of lenslets to sample a small rectangular patch of the
sky at resolutions approaching the diffraction limit of the 10-meter
Keck Telescope. OSIRIS records an infrared spectrum at each point within
the patch in a single exposure, greatly enhancing its efficiency and
precision when observing small objects such as distant galaxies. It is
used to characterize the dynamics and composition of early stages of
galaxy formation.
The W. M. Keck Observatory operates the largest,
most scientifically productive telescopes on Earth. The two, 10-meter
optical/infrared telescopes on the summit of Mauna Kea on the Island of
Hawaii feature a suite of advanced instruments including imagers,
multi-object spectrographs, high-resolution spectrographs,
integral-field spectroscopy and world-leading laser guide star adaptive
optics systems. The Observatory is a private 501(c) 3 non-profit
organization and a scientific partnership of the California Institute of
Technology, the University of California and NASA.
Source: W. M. Keck Observatory
