January 20, 1997

Keck Telescopes find galaxies of normal stars surrounding quasars

Many quasars, the most luminous objects in the universe, are swaddled by galaxies containing ordinary stars that lie at the same distances from earth as the quasars themselves, according to a new study that used the Keck I and Keck II Telescopes.

The observations also provide tantalizing evidence that galaxy collisions fuel the prodigious energy outputs of at least some quasars, presumably by hurling fresh stellar debris into the gorges of gigantic black holes at the cores of the galaxies.

UCSC astronomers took the first detailed spectra of the nebulous blobs that lurk faintly around most quasars found to date. The spectra, which reveal the compositions and distances of light-emitting objects, clearly point to galaxies full of normal stars around the quasars. Features in the spectra indicate that the stars are mature, with ages of at least 1 billion to perhaps 10 billion years.

Most astronomers believed that quasars reside within typical galaxies, not alone or bathed in some unusual nonstellar material. Indeed, recent images from the Hubble Space Telescope revealed numerous quasars shining forth from blobs resembling galaxies. However, researchers needed spectra to confirm that the objects were galaxies, and that the galaxies and quasars were the same distances from earth. Keck's light-gathering power made that analysis possible.

"The evidence is extraordinarily strong that quasars live in host galaxies, just as we expected," says Joseph Miller, leader of the observing team and director of UCO/Lick Observatory. "This is consistent with the hypothesis that quasars are natural features of massive galaxies. They are direct consequences of how galaxies evolve, not strange aberrations or freaks of nature."

The spectra of a few "companion" blobs close to the host galaxies show that the companions also are populations of stars at the same distances. These stars most likely form the cores of galaxies in the throes of being gravitationally rent by collisions, Miller says.

"Our spectra suggest that galaxy merging has taken place," he says. "Like throwing gasoline onto a fire, this process may make quasars burst forth with new energy by transferring mass into the centers of the host galaxies. Quasars may turn on and off or change their appearances depending on how each merger evolves."

Miller presented his research on January 13 at the American Astronomical Society meeting. His coworkers were former UCSC graduate student Hien Tran, now a postdoctoral researcher at Lawrence Livermore National Laboratory, and current graduate student Andrew Sheinis.

Miller's team conducted its study on the Keck I Telescope in August 1996 and on the Keck II Telescope in December 1996. In both cases, they used the Low-Resolution Imaging Spectrograph (LRIS), designed by astronomers at Caltech. They examined spectra of the nebulosities around ten quasars, with an average exposure time of about two hours for each spectrum.

The quasars in the study, says Miller, are among the closest and brightest quasars in the sky. Their redshifts--a measure of how quickly they are receding--vary from 15 to 30 percent of the speed of light (z Å .15 to .35). This corresponds roughly to a distance of between 1.5 and 3 billion light-years from earth. Miller's group plans to expand its study to include both a larger sample of "nearby" quasars and ones that have larger redshifts. Such research may allow the team to unveil relationships between the natures of the quasars and the galaxies in which they dwell.

When they displayed their recent Hubble Space Telescope images of quasars, astronomers led by John Bahcall of the Institute for Advanced Study in Princeton, N.J., and Mike Disney of the University of Wales said that many of the purported host galaxies appeared to be in the midst of collisions or to have suffered recent violence. Others, however, looked quiescent. Some weak quasars, Miller notes, may get their fuel from the mass shed by normal stars as they evolve. "Bars" and instabilities in the structures of some galaxies also may propel material toward their cores, but that process is poorly understood.

The Keck Observatory, a partnership of UC, Caltech, and NASA, is located at the summit of the dormant Mauna Kea volcano on the Big Island of Hawaii. Its twin 400-inch telescopes, Keck I and Keck II, began routine observations in November 1993 and October 1996, respectively. The W. M. Keck Foundation of Los Angeles granted more than $130 million toward Keck I and Keck II--about 80 percent of the total construction costs.

UC Observatories/Lick Observatory, headquartered at UCSC, oversees both the Lick Observatory east of San Jose, California, and the UC share of the Keck Observatory.

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