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Galaxies Colliding All Around Us Galaxies Colliding All Around Us
http://www.universetoday.com/am/publish/noao_local_universe.html

Summary - The history of our nearby Universe has been dominated by galactic
collisions. More than half of the nearby galaxies have collided other galaxies
in the last 2 billion year according to data from two comprehensive sky surveys.
By processing 126 galaxies in the NOAO Deep Wide-Field Survey and the
Multiwavelength Survey by Yale/Chile, researchers have found that 53% of
galaxies have evidence of long tails of stars trailing away from them; the
result of a recent galactic collision.
Full Story -
Newly found galaxy collisions in the nearby universe. Image credit: NOAO.
Click to enlarge
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 here.
More information, including an animation of the mergers, is available from Yale
University.

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.

Original Source: NOAO News Release



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