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 had 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"

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.

Copyright 2005 by Space Daily, Distributed United Press International

Related stories:

Caltech astronomers describe the bar scene at the beginning of the universe
Bars abound in spiral galaxies today, but this was not always the case. A group of 16 astronomers, led by Kartik Sheth of NASA's Spitzer Science Center at the California Institute of Technology, has found that bars tripled in number over the past seven billion years, indicating that spiral galaxies evolve in shape.
Lenses galore -- Hubble finds large sample of very distant galaxies
By using the gravitational magnification from six massive lensing galaxy clusters, the NASA/ESA Hubble Space Telescope has provided scientists with the largest sample of very distant galaxies seen to date. Some of the newly found magnified objects are dimmer than the faintest ones seen in the legendary Hubble Ultra Deep Field, which is usually considered the deepest image of the Universe.
Rare 'Star-Making Machine' Found in Distant Universe
Astronomers have uncovered an extreme stellar machine -- a galaxy in the very remote universe pumping out stars at a surprising rate of up to 4,000 per year. In comparison, our own Milky Way galaxy turns out an average of just 10 stars per year.
A Slimmer Milky Way Revealed by New Measurements
The Milky Way Galaxy has lost weight. A lot of weight. About a trillion Suns' worth, according to an international team of scientists from the Sloan Digital Sky Survey (SDSS-II), whose discovery has broad implications for our understanding of the Milky Way.
New Map Locates Metals in Millions of Milky Way Stars
An international team of scientists from the Sloan Digital Sky Survey (SDSS-II) has unveiled the most complete and detailed map yet of the chemical composition of our Galaxy.
Hubble's sweeping view of the Coma Galaxy Cluster
Hubble's Advanced Camera for Surveys has observed a large portion of the Coma Cluster, stretching across several million light-years. The entire cluster is more than 20 million light-years in diameter, is nearly spherical in shape and contains thousands of galaxies.
Hubble Space Telescope Spies Galaxy/Black Hole Evolution in Action
A set of 29 Hubble Space Telescope (HST) images of an exotic type of active galaxy known as a "post-starburst quasar" show that interactions and mergers drive both galaxy evolution and the growth of super-massive black holes at their centers.
Hubble Survey Finds Missing Matter, Probes Intergalactic Web
Although the universe contains billions of galaxies, only a small amount of its matter is locked up in these behemoths. Most of the universe's matter that was created during and just after the Big Bang must be found elsewhere.

News discussion:

Space & Earth science news