Galaxy Collisions Dominate the Local Universe (Forwarded)

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FOR IMMEDIATE RELEASE: Tuesday, December 6, 2005

RELEASE NO: NOAO 05-11

Galaxy Collisions Dominate the Local Universe

More than half of the largest galaxies in the nearby universe have
collided and merged with another galaxy in the past two billion years,
according to a new study using hundreds of images from two of the
deepest sky surveys ever conducted.

The idea of large galaxies being assembled primarily by mergers rather
than evolving by themselves in isolation has grown to dominate
cosmological thinking. However, a troubling inconsistency within this
general theory has been that the most massive galaxies appear to be the
oldest, leaving minimal time since the Big Bang for the mergers to have
occurred.

"Our study found these common massive galaxies do form by mergers. It is
just that the mergers happen quickly, and the features that reveal the
mergers are very faint and therefore difficult to detect," says Pieter
van Dokkum of Yale University, lead author of the paper in the December
2005 issue of the Astronomical Journal.

The paper uses two recent deep surveys done with the National Science
Foundation's 4-meter telescopes at Kitt Peak National Observatory and
Cerro Tololo Inter-American Observatory, known as the NOAO Deep
Wide-Field Survey and the Multiwavelength Survey by Yale/Chile.
Together, these surveys covered an area of the sky 50 times larger than
the size of the full Moon.

"We needed data that are very deep over a very wide area to provide
statistically meaningful evidence," van Dokkum explains. "As happens so
often in science, fresh observations helped inform new conclusions."

Van Dokkum used images from the two surveys to look for telltale tidal
features around 126 nearby red galaxies, a color selection biased to
select the most massive galaxies in the local universe. These faint
tidal features turn out to be quite common, with 53 percent of the
galaxies showing tails, broad fans of stars trailing behind them or
other obvious asymmetries.

"This implies that there is a galaxy that has endured a major collision
and subsequent merger event for every single other 'normal' undisturbed
field galaxy," van Dokkum notes. "Remarkably, the collisions that
precede the mergers are still ongoing in many cases. This allows us to
study galaxies before, during, and after the collisions."

Though there are not many direct star-to-star encounters in this merger
process, such galaxy collisions can have profound effects on star
formation rates and the shape of the resulting galaxy.

These mergers do not resemble the spectacular mergers of blue spiral
galaxies that are featured in several popular Hubble Space Telescope
images. But these red galaxy mergers appear to be much more common.
Their ubiquity represents a direct confirmation of predictions by the
most common models for the formation of large-scale structure in the
Universe, with the added benefit of helping solve the apparent-age problem.

"In the past, people equated stellar age with the age of the galaxy,"
van Dokkum explains. "We have found that, though their stars are
generally old, the galaxies that result from these mergers are
relatively young."

It is not yet understood why the merging process does not lead to
enhanced star formation in the colliding galaxies. It may be that
massive black holes in the centers of the galaxies provide the energy to
heat or expel the gas that needs to be able to cool in order to form new
stars. Ongoing detailed study of the newly found mergers will provide
better insight into the roles that black holes play in the formation and
evolution of galaxies.

A series of images of different galaxies in this study that, taken
together, represent a time sequence of a typical red galaxy merger, is
available at
http://www.noao.edu/outreach/press/p...es/mergers.jpg

More information, including an animation of the mergers, is available
from Yale University,
http://www.astro.yale.edu/dokkum/mergers/

Based in Tucson, AZ, the National Optical Astronomy Observatory (NOAO)
consists of Kitt Peak National Observatory near Tucson, AZ, Cerro Tololo
Inter-American Observatory near La Serena, Chile, and the NOAO Gemini
Science Center. NOAO is operated by the Association of Universities for
Research in Astronomy (AURA) Inc., under a cooperative agreement with
the National Science Foundation.

IMAGE CAPTION:
[http://www.noao.edu/outreach/press/p...es/mergers.jpg (663KB)]
The panels show several of the newly found galaxy collisions in the
nearby universe, using the NOAO Deep Wide-Field Survey (NDWFS) and the
Multiwavelength Survey by Yale/Chile (MUSYC).

The collisions (occurring in different galaxy pairs) are seen in
different stages of the merger process, which taken together show the
sequence that occurs.

In (a) and (b) [top left and top right], the galaxies are still
separated, but huge tidal forces of gravity are already at work pulling
stars from the galaxies into enormous broad fans that stretch hundreds
of thousands of light-years in space.

In (c) and (d) [bottom left and bottom right], the colliding galaxies
have merged into single, larger galaxies. The violent past of these
galaxies can be inferred from the tidal "debris" that still surrounds
the newly formed galaxies.

You may also download a full resolution, 11MB TIFF file at
http://www.noao.edu/outreach/press/p...rs_highres.tif

Images (a), (b), and (d) are from the NDWFS; image (c) is from MUSYC.

Credit: P. van Dokkum/Yale University and NOAO/AURA/NSF