Quasars without clothes.

Are quasars born naked and only later in life cloaked in a starlit galaxy? New images taken with the Hubble Space Telescope suggest that some of the most brilliant of these light beacons may indeed emerge in isolation - an idea that appears to defy conventional wisdom about how a quasar forms.

According to a widely accepted model, massive black holes power quasars. As gas or stars venture close to a hole, the material heats up and radiates tremendous energy in a narrow beam of light - a quasar. But if infalling stars and gas provide the fuel for quasars, it would seem necessary that a "host" galaxy harboring this fuel surround each such beacon. Moreover, ground-based observations hint that the brightest quasars lie at the center of the brightest galaxies.

But when John N. Bahcall of the Institute for Advanced Study in Princeton, N.J., and his colleagues recently used a Hubble camera to search for galaxies around some particularly bright quasars, they came away nearly empty-handed. Among 15 bright, relatively nearby quasars in their survey, only four showed some evidence of a host galaxy. The host galaxies around the other 11 quasars are either too faint to be seen in the Hubble survey or simply don't exist.

"This is a giant leap backwards in our understanding of quasars," says Bahcall, who collaborated with Sofia Kirhakos of the Institute for Advanced Study and Donald P. Schneider of Pennsylvania State University in University Park.

"This is one of the most surprising results ever obtained with the Hubble Space Telescope," says cosmologist Jeremiah P. Ostriker of Princeton University.

The bright, seemingly naked quasars may in fact signal the birth pangs of galaxies. Bahcall speculates that at least some quasars form at the heart of budding galaxies that are initially too faint to observe. A quasar glows brightly because of the gas pulled into the region by gravity, he suggests. Later, as less material falls in and the quasar dims, the surrounding galaxy accumulates enough material to trigger star formation and shine. This model fits with ground-based observations that less luminous quasars do have host galaxies.

"This is purely conjecture . . . but the story hangs together if you assume that it's the beginning of quasar-galaxy evolution," Bahcall notes. The team, he adds, may simply have missed the host galaxies by not looking for faint enough objects. "We all expected the [galactic] environment to be [readily] visible."

Bahcall's team often found galaxies or remnants of galaxies in the same patch of sky as the quasars. Until researchers obtain spectra of these faint bodies, they won't know whether the galaxies actually reside at roughly the same distance as the quasars or just happen to lie along the same line of sight.

Though he regards the latter possibility as unlikely, Bahcall notes that if the galaxies and quasars don't reside at the same distance, it would have an enonnous impact on cosmology. In that case, he says, redshift - the lengthening, or reddening, of light emitted by faraway objects - may not indicate an object's distance. The notion of redshift as a measure of distance is central to Big Bang cosmology and the concept of an expanding universe.