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