01/10/1995 08:00:00

Astronomers Count Galaxies in Deepest Infared Images of the Sky

PASADENA—Caltech astronomers have counted galaxies to a limit of about 24th magnitude, the faintest ever counted in infrared light. Observing five small patches of sky with the 10-meter telescope at the W. M. Keck Observatory in Hawaii, the scientists found that the numbers of galaxies continued to rise with increasing faintness, a result that agrees well with models in which the universe is "open" and will continue to expand forever.

These observations will be presented by a consortium of Caltech astronomers on January 10, 1995, to the American Astronomical Society (AAS) meeting in Tucson, Arizona. Early results appeared in the January 1, 1995, issue of Astrophysical Journal Letters, and these and additional results were presented at the AAS meeting.

Working on several nights spread from late April to October of last year, the astronomers made their observations in the K-band, a range of wavelengths in the infrared, centered near 2.2 microns, or 2,200 nanometers. Astronomers count galaxies in near-infrared wavelengths to minimize both the effects of dimming caused by dust, and the effects of large variations in brightness due to star formation, two problems that are common to galaxy counts in visible light.

"These preliminary results are consistent with cosmological models that include a low-density, open universe, and little galactic evolution over the past several billion years," explained S. George Djorgovski, coauthor of the study and an associate professor of astronomy at Caltech. Most current observations point toward an open universe, though some scientists still maintain that the universe has a higher density, and is exactly balanced at the point between being open and closed.

Counting galaxies as a function of their brightness is a classic cosmological test developed in the 1920s by astronomer Edwin Hubble. These counts shed light on two important questions. First, what is the "global" geometry of the universe? That is, how dense is it, how fast is it expanding, and will it ultimately expand forever, or collapse upon itself? And second, do galaxies evolve, and if so, in what ways and how fast do they do so?

The galaxy counts to be presented are about five times deeper than the deepest published K-band images to date, meaning that it recorded objects five times fainter than the best earlier studies. The numbers of galaxies seen imply a cumulative density over the entire "surface" of the sky of more than 20 billion galaxies down to 24th magnitude in the K-band, which roughly corresponds to 29th magnitude in the more commonly used blue light.

This study nicely complements results from the Hubble Space Telescope (HST), which has sharper vision and provides better information on the morphology of galaxies. But the greater light-gathering power of the Keck Telescope enables scientists to see deeper into space than the HST can, and to obtain redshifts for the faintest galaxies. Instruments mounted on the Keck also allow it to observe in near-infrared wavelengths, something the Hubble is not yet able to do.

The density of the universe is a major factor that affects how many galaxies can be seen. If the universe has a high density, then it must be expanding relatively slowly, due to its own gravitational pull upon itself. Therefore it would have a smaller volume and contain fewer galaxies to be counted. However, the galaxies would be closer and therefore appear brighter. A low density, on the other hand, would imply a faster expansion rate, a larger volume, and more numerous galaxies, which would appear fainter.

Evolution of galaxies also plays a role in the number that are visible. It's believed that most galaxies were brighter in the past and have dimmed as they evolved. The brightest stars have the shortest lives, so a young galaxy will have more of these brilliant lights still burning, while in an older galaxy most of them will have burned out. Further complicating the picture, galaxies tend to collide and merge over time, which both makes them brighter, so that more should be visible, and reduces their numbers, so that fewer should be seen.

Detailed cosmological models eventually will help disentangle these complicated, sometimes contradictory effects. The results to be presented in Tucson are the first step in a longer study, and alone are not sufficient for a full resolution of the puzzle. But they are consistent with open-universe models in which galaxies evolve very slowly or not at all, that is, in which the brightness of galaxies does not fade significantly over time.

In the next step of the study, astronomers will use the powerful Keck telescope to measure the redshifts of many of these faint galaxies, to determine how far away they are. The redshift is an effect seen in rapidly receding sources of light, where the spectral lines of such sources move toward longer wavelengths, or toward the red end of the visible spectrum. The faster the source is speeding away from us, the greater the redshift, and the farther away the source must be. Redshift measurements will provide new information about distance, which when combined with the counts presented here, will help determine more accurately the large-scale density of the universe.

The authors of the study are professors S. George Djorgovski, B. Thomas Soifer, Gerry Neugebauer, Roger Blandford, and Judith Cohen, Member of the Professional Staff Keith Matthews, Research Fellow Ian Smail, and graduate students James Larkin, Michael Pahre, Julia Smith, and David Hogg, all from Caltech; Dr. Wendy Harrison from the W. M. Keck Observatory; and Professor Jerry Nelson from the Lick Observatory. 

Written by John Avery