Hubble Surveys the "Homes" of Quasars
Dramatic Hubble telescope pictures reveal that quasars live in a remarkable assortment of galaxies, many of which are violently colliding. This complicated picture suggests there may be a variety of mechanisms – some quite subtle – for "turning on" quasars, the universe's most energetic objects.
When seen through ground-based telescopes, these compact, enigmatic light sources resemble stars, yet they are billions of light-years away and several hundred billion times brighter than normal stars. The following Hubble snapshots offer examples of quasar home sites. Astronomers believe that a quasar turns on when a massive black hole at the center of a galaxy feeds on gas and stars.
Two teams of astronomers are releasing dramatic Hubble Space Telescope images today, which show that quasars live in a remarkable variety of galaxies, many of which are violently colliding. This complicated picture suggests there may be a variety of mechanisms - some quite subtle - for "turning on" quasars, the universe's most energetic objects.
The Hubble researchers are also intrigued by the fact that the quasars studied do not appear to have obviously damaged the galaxies in which they live. This could mean that quasars are relatively short lived phenomena which many galaxies, including the Milky Way, experienced long ago.
John Bahcall of the Institute for Advanced Study, Princeton, NJ, emphasizes that Hubble's clarity opens a complicated picture. "If we thought we had a complete theory of quasars before, now we know we don't," says Bahcall. "No coherent, single pattern of quasar behavior emerges. The basic assumption was that there was only one kind of host galaxy, or catastrophic event, which feeds a quasar. In reality we do not have a simple picture - we have a mess."
Mike Disney, University of Wales College, Cardiff, U.K., who is leader of the European team, says, "People had suspected that collisions might be an important mechanism for feeding black holes and generating the vast amounts of energy emitted by quasars. Now we know they are and we didn't know that before Hubble. This is a really exciting achievement."
Though a number of the images show collisions between pairs of galaxies which could trigger the birth of quasars, some pictures reveal apparently normal, undisturbed galaxies possessing quasars. "We were amazed by the beauty and clarity of the Hubble images, as well as the diversity of quasar environments," says Donald Schneider of Pennsylvania State University, University Park, PA.
Discovered only 33 years ago, quasars are among the most baffling objects in the universe because of their small size and prodigious energy output. Quasars are not much bigger than Earth's solar system but pour out 100 to 1,000 times as much light as an entire galaxy containing a hundred billion stars.
A super massive black hole, gobbling up stars, gas and dust, is theorized to be the "engine" powering a quasar. Most astronomers agree an active black hole is the only credible possibility that explains how quasars can be so compact, variable and powerful. Nevertheless, conclusive evidence has been elusive because quasars are so bright they mask any details of the "environment" where they live.
"These problems couldn't be solved without the Hubble Telescope," Disney said. "I gave up on studying quasars 20 years ago, because I realized we had to wait for a space telescope to provide a clear enough view for solving mysteries."
Observations by the European team, using the Wide Field Planetary Camera 2 (WFPC2) in high-resolution mode, reveal that quasars appear to be born in environments where two galaxies are interacting violently and probably colliding. "This had long been suspected as a mechanism for igniting a quasar but no one knew whether the idea was really right, before the Hubble," says Peter Boyce of the European team.
"In nearly every quasar we look at we clearly see one galaxy apparently swallowing another," Disney said. He selected three quasars known to be strong infrared emitters, suggesting that they might be in spiral galaxies, which typically contain an enormous amount of gas and dust. "When we image them with Hubble we see the most colossal smashups, where two giant spiral galaxies like our own Milky Way have crashed head on into one another and flung off pieces violently in all directions. Some of those bits seem to have finished up in the nucleus of one of the spirals where there is probably a giant black hole feeding on it."
Bahcall, Schneider and Sofia Kirkahos also used the WFPC2, but in wide-field mode, to survey 20 quasars. Bahcall finds about half of the quasars studied have host galaxies which look undisturbed. "Either the interacting companion is very close to the nucleus and below Hubble's resolution, or other mechanisms are at work in igniting quasars."
Both teams agree that Hubble images do show conclusively:
Further quasar research will be challenging because of the great distance and long time scales involved. "It's like having a few still shots of a football game and trying to decipher both the rules and the final score. It's very challenging, and great fun, but you are obviously open to making the most dramatic mistakes. We'll get there in the end but we may need a lot of Hubble pictures to be certain what is going on," Disney said.
Now that more is known about the environments in which quasars exist the teams emphasize astronomers must address even larger puzzles. Do most quasars flare up for a brief period of a galaxy's life (100 million years or less)? If so, then most galaxies, including our Milky Way, could be "burned out" quasars. If, alternatively, quasars are long-lived, it implies they are more rare. "This means a few extremely massive black holes formed very early in the universe," says Disney.
Astronomers also need to address a "chicken and egg" problem about the birth of quasars. Did the massive black holes form first and the galaxies formed around them, or did galaxies precede black holes, which quickly grew in their cores though stellar collision and merger?
Advanced instruments planned for Hubble should also help pin down more details. The Near Infrared Camera and Multi-Object Spectrometer (NICMOS), to be installed in 1997, and the Advanced Camera, to be installed in 1999, will have coronagraphic devices which will block out the glare of a quasar, allowing astronomers to see closer into a galaxy's nucleus. By viewing galactic structures in infrared light, the NICMOS should be able to provide important new details about the host galaxies of quasars.