Survey of 4,000 Galaxies Finds "Downsizing" on a Cosmic Scale (Forwarded)

Office of Public Affairs and Educational Outreach
National Optical Astronomy Observatory
Tucson, Arizona

For More Information:

Douglas Isbell
Office of Public Affairs and Educational Outreach
National Optical Astronomy Observatory
Phone: 520/318-8230

FOR IMMEDIATE RELEASE: Tuesday, August 30, 2005

RELEASE NO: NOAO 05-08

Survey of 4,000 Galaxies Finds "Downsizing" on a Cosmic Scale

A comprehensive survey of more than 4,000 elliptical and lenticular
galaxies in 93 nearby galaxy clusters has found a curious case of
galactic "downsizing."

Contrary to expectations, the largest, brightest galaxies in the census
consist almost exclusively of very old stars, with much of their stellar
populations having formed as long ago as 13 billion years. There appears
to be very little recent star formation in these galaxies, nor is there
strong evidence for recent ingestion of smaller, younger galaxies.

By contrast, the smaller, fainter galaxies studied by the NOAO
Fundamental Plane Survey are significantly younger -- their stars were
formed as little as four billion years ago, according to new results
from the survey team to be published in the September 10, 2005,
Astrophysical Journal.

These findings are based on a sample more than five times larger than
previous efforts. The results of the survey contrast sharply with
conventional hierarchical model of galaxy formation and evolution, where
large elliptical galaxies in the nearby universe formed by swallowing
smaller galaxies with young stars; this theory predicts that, on
average, the stars in the largest elliptical galaxies should be no older
than those in the smallest ones.

"This sample probes the largest and richest galaxy clusters in the
nearby universe, out to a distance of about a billion light-years from
Earth," says Jenica Nelan, lead author of the study. "Our analysis shows
that there is a clear relationship between mass and age in these red
galaxies, meaning that the stars in the biggest, oldest galaxies that we
studied formed early in the history of the Universe. On average, the
smaller galaxies have one-tenth the mass of the larger ones, and are
only about half their age."

"The term 'downsizing' essentially means that when the Universe was
young, the star formation activity occurred in large galaxies, but as
the Universe aged, the 'action' stopped in the larger galaxies, even as
it continued in smaller galaxies," says Michael Hudson of the University
of Waterloo, Ontario, Canada, principal investigator for the NOAO
Fundamental Plane Survey.

The new study is based on thousands of spectra obtained by the
Fundamental Plane Survey team over dozens of nights at the WIYN
3.5-meter telescope at Kitt Peak National Observatory, southwest of
Tucson, AZ, and the National Science Foundation's Blanco 4-meter
telescope at Cerro Tololo Inter-American Observatory, east of La Serena,
Chile. With some painstaking work, these spectra can reveal the average
age of the stars that make up a galaxy.

"Although we cannot directly see these galaxies as they were in the
past, their stars are a kind of 'fossil record' that can be used to
unearth their histories," Hudson explains. "It appears that the older
galaxies are much less of a 'melting pot' than had been thought, and
that their star formation activity turned off somehow while they were
being put together."

The evolutionary history of elliptical galaxies and lenticular galaxies
(which have a central bulge and a disk, but no evidence of spiral arms)
is not well understood. Their colors appear to be "redder" than typical
spiral galaxies. The largest ellipticals are the reddest of all, but
until this work it has not been clear whether this property results
primarily from being older in age, as the survey found, or from having a
higher proportion of heavy chemical elements (metallicity content).

"These so-called red galaxies contain the bulk of the stellar mass in
the nearby universe, but we know little about their formation and
evolution," says co-author Russell Smith of the University of Waterloo.
"It was thought that all of the red galaxies were made of stars that
formed very early, and are now quite old. Our results show that while
this is true for the large galaxies, the smaller ones formed their stars
comparatively recently in the history of the Universe. We predict that
as new surveys look deeper and hence further into the past, they should
see fewer faint red galaxies."

An image of galaxy cluster Abell 3266 taken by survey team members at
the Gemini South telescope as part of their follow-up work is available
above.

Lead author Jenica Nelan completed this work while earning her doctorate
at Dartmouth College; she is now an astronomer at Yale University.

Co-authors of this paper include Hudson and Smith of the University of
Waterloo; Gary Wegner of Dartmouth College; John R. Lucey, Stephen A. W.
Moore, and Stephen J. Quinney of the University of Durham, and Nicholas
B. Suntzeff of NOAO's Cerro Tololo Inter-American Observatory.

The Fundamental Plane Survey is one of 18 projects granted long-term
access to observing nights at the telescope of the National Optical
Astronomy Observatory (NOAO) under the NOAO Survey Program.

See here for more information:

http://www.noao.edu/gateway/surveys/programs.html
and
http://astro.uwaterloo.ca/~mjhudson/nfp

Based in Tucson, AZ, 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:
This image of galaxy cluster Abell 3266 was taken using the Gemini
Multi-Object Spectrograph (GMOS) instrument on the Gemini South
telescope on Cerro Pachón, Chile, in July 2005. The image was obtained
by the NOAO Fundamental Plane Survey team as part of follow-up
observations in support of their spectroscopic data from Kitt Peak
National Observatory and Cerro Tololo Inter-American Observatory.

Abell 3266 is located at a distance of 250 million light-years from
Earth. It is one of the most populous galaxy clusters in the nearby
universe, and one of 93 clusters that were the subject of an August 2005
NOAO press release. Galaxy clusters contain mostly red elliptical
galaxies, as seen in this view of the central region of Abell 3266.

Image Credit: Michael Hudson and Russell Smith (University of Waterloo)
and Gemini Observatory

Download:

* A3266.tif [2346x1458, 19.6 MB]
http://www.noao.edu/outreach/press/p...ages/A3266.tif
* A3266.jpg [2346x1458, 388 KB]
http://www.noao.edu/outreach/press/p...ages/A3266.jpg
* A3266_1200.jpg [1200x756 121 KB]
http://www.noao.edu/outreach/press/p...A3266_1200.jpg