WASHINGTON -- A comprehensive study of hundreds of galaxies observed
by the Keck telescopes in Hawaii and NASA's Hubble Space Telescope
has revealed an unexpected pattern of change that extends back 8
billion years, or more than half the age of the universe.
"Astronomers thought disk galaxies in the nearby universe had settled
into their present form by about 8 billion years ago, with little
additional development since," said Susan Kassin, an astronomer at
NASA's Goddard Space Flight Center in Greenbelt, Md., and the study's
lead researcher. "The trend we've observed instead shows the
opposite, that galaxies were steadily changing over this time
period."
Today, star-forming galaxies take the form of orderly disk-shaped
systems, such as the Andromeda Galaxy or the Milky Way, where
rotation dominates over other internal motions. The most distant blue
galaxies in the study tend to be very different, exhibiting
disorganized motions in multiple directions. There is a steady shift
toward greater organization to the present time as the disorganized
motions dissipate and rotation speeds increase. These galaxies are
gradually settling into well-behaved disks.
Blue galaxies -- their color indicates stars are forming within them -- show less disorganized motions and ever-faster rotation speeds the closer they are observed to the present. This trend holds true for galaxies of all masses, but the most massive systems always show the highest level of organization.
Researchers say the distant blue galaxies they studied are gradually transforming into rotating disk galaxies like our own Milky Way.
"Previous studies removed galaxies that did not look like the
well-ordered rotating disks now common in the universe today," said
co-author Benjamin Weiner, an astronomer at the University of Arizona
in Tucson. "By neglecting them, these studies examined only those
rare galaxies in the distant universe that are well-behaved and
concluded that galaxies didn't change."
Rather than limit their sample to certain galaxy types, the
researchers instead looked at all galaxies with emission lines bright
enough to be used for determining internal motions. Emission lines
are the discrete wavelengths of radiation characteristically emitted
by the gas within a galaxy. They are revealed when a galaxy's light
is separated into its component colors. These emission lines also
carry information about the galaxy's internal motions and distance.
The team studied a sample of 544 blue galaxies from the Deep
Extragalactic Evolutionary Probe 2 (DEEP2) Redshift Survey, a project
that employs Hubble and the twin 10-meter telescopes at the W. M.
Keck Observatory in Hawaii. Located between 2 billion and 8 billion
light-years away, the galaxies have stellar masses ranging from about
0.3 percent to 100 percent of the mass of our home galaxy.
A paper describing these findings will be published Oct. 20 in The
Astrophysical Journal.
The Milky Way galaxy must have gone through the same rough-and-tumble
evolution as the galaxies in the DEEP2 sample, and gradually settled
into its present state as the sun and solar system were being formed.
In the past 8 billion years, the number of mergers between galaxies
large and small has decreased sharply. So has the overall rate of
star formation and disruptions of supernova explosions associated
with star formation. Scientists speculate these factors may play a
role in creating the evolutionary trend they observe.
Now that astronomers see this pattern, they can adjust computer
simulations of galaxy evolution until these models are able to
replicate the observed trend. This will guide scientists to the
physical processes most responsible for it.
The DEEP2 survey is led by Lick Observatory at the University of
California at Santa Cruz in collaboration with the University of
California at Berkeley, the University of Hawaii at Manoa, Johns
Hopkins University in Baltimore, Md., the University of Chicago and
the California Institute of Technology in Pasadena.
The Hubble Space Telescope is a project of international cooperation
between NASA and the European Space Agency. NASA's Goddard Space
Flight Center in Greenbelt, Md., manages the telescope. The Space
Telescope Science Institute (STScI) in Baltimore, Md., conducts
Hubble science operations. STScI is operated by the Association of
Universities for Research in Astronomy, Inc. in Washington.
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