At left, in optical light, UGC 1382 appears to be a simple elliptical
galaxy. But spiral arms emerged when astronomers incorporated
ultraviolet and deep optical data (middle). Combining that with a view
of low-density hydrogen gas (shown in green at right), scientists
discovered that UGC 1382 is gigantic. › Full image and caption
About 250 million light-years away, there's a neighborhood of our
universe that astronomers had considered quiet and unremarkable. But
now, scientists have uncovered an enormous, bizarre galaxy possibly
formed from the parts of other galaxies.
A new study to be published in the Astrophysical Journal reveals the
secret of UGC 1382, a galaxy that had originally been thought to be old,
small and typical. Instead, scientists using data from NASA telescopes
and other observatories have discovered that the galaxy is 10 times
bigger than previously thought and, unlike most galaxies, its insides
are younger than its outsides, almost as if it had been built using
spare parts.
"This rare, 'Frankenstein' galaxy formed and is able to survive
because it lies in a quiet little suburban neighborhood of the universe,
where none of the hubbub of the more crowded parts can bother it," said
study co-author Mark Seibert of the Observatories of the Carnegie
Institution for Science, Pasadena, California. "It is so delicate that a
slight nudge from a neighbor would cause it to disintegrate."
Seibert and Lea Hagen, a graduate student at Pennsylvania State
University, University Park, came upon this galaxy by accident. They had
been looking for stars forming in run-of-the-mill elliptical galaxies,
which do not spin and are more three-dimensional and football-shaped
than flat disks. Astronomers originally thought that UGC 1382 was one of
those.
But while looking at images of galaxies in ultraviolet light through
data from NASA's Galaxy Evolution Explorer (GALEX), a behemoth began to
emerge from the darkness.
"We saw spiral arms extending far outside this galaxy, which no one
had noticed before, and which elliptical galaxies should not have," said
Hagen, who led the study. "That put us on an expedition to find out
what this galaxy is and how it formed."
Researchers then looked at data of the galaxy from other telescopes:
the Sloan Digital Sky Survey, the Two Micron All-Sky Survey (2MASS),
NASA's Wide-field Infrared Survey Explorer (WISE), the National Radio
Astronomy Observatory's Very Large Array and Carnegie's du Pont
Telescope at Las Campanas Observatory. After GALEX revealed previously
unseen structures to the astronomers, optical and infrared light
observations from the other telescopes allowed the researchers to build a
new model of this mysterious galaxy.
As it turns out, UGC 1382, at about 718,000 light-years across, is
more than seven times wider than the Milky Way. It is also one of the
three largest isolated disk galaxies ever discovered, according to the
study. This galaxy is a rotating disk of low-density gas. Stars don't
form here very quickly because the gas is so spread out.
But the biggest surprise was how the relative ages of the galaxy's
components appear backwards. In most galaxies, the innermost portion
forms first and contains the oldest stars. As the galaxy grows, its
outer, newer regions have the youngest stars. Not so with UGC 1382. By
combining observations from many different telescopes, astronomers were
able to piece together the historical record of when stars formed in
this galaxy -- and the result was bizarre.
"The center of UGC 1382 is actually younger than the spiral disk
surrounding it," Seibert said. "It's old on the outside and young on the
inside. This is like finding a tree whose inner growth rings are
younger than the outer rings."
The unique galactic structure may have resulted from separate
entities coming together, rather than a single entity that grew outward.
In other words, two parts of the galaxy seem to have evolved
independently before merging -- each with its own history.
At first, there was likely a group of small galaxies dominated by gas
and dark matter, which is an invisible substance that makes up about 27
percent of all matter and energy in the universe (our own matter is
only 5 percent). Later, a lenticular galaxy, a rotating disk without
spiral arms, would have formed nearby. At least 3 billion years ago, the
smaller galaxies may have fallen into orbit around the lenticular
galaxy, eventually settling into the wide disk seen today.
More galaxies like this may exist, but more research is needed to look for them.
"By understanding this galaxy, we can get clues to how galaxies form
on a larger scale, and uncover more galactic neighborhood surprises,"
Hagen said.
The GALEX mission, which ended in 2013 after more than a decade of
scanning the skies in ultraviolet light, was led by scientists at
Caltech in Pasadena, California. NASA's Jet Propulsion Laboratory, also
in Pasadena, managed the mission and built the science instrument. Data
for the 2MASS and WISE missions are archived at the Infrared Processing
and Analysis Center (IPAC) at Caltech. JPL is managed by Caltech for
NASA.
News Media Contact
Elizabeth Landau
Jet Propulsion Laboratory, Pasadena, Calif.
818-354-6425
elizabeth.landau@jpl.nasa.gov
Source: JPL-Caltech
