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A SURVEY OF QUASAR HOST GALAXIES
Quasars reside in a variety of galaxies, from normal to highly disturbed. 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 Space Telescope images show examples of different home sites of all quasars. But all the sites must provide the fuel to power these unique light beacons. Astronomers believe that a quasar turns on when a massive black hole at the nucleus of a galaxy feeds on gas and stars. As the matter falls into the black hole, intense radiation is emitted. Eventually, the black hole will stop emitting radiation once it consumes all nearby matter. Then it needs debris from a collision of galaxies or another process to provide more fuel. The column of images on the left represents normal galaxies; the center, colliding galaxies; and the right, peculiar galaxies.
Top left: This image shows quasar PG 0052+251, which is 1.4 billion light-years from Earth, at the core of a normal spiral galaxy. Astronomers are surprised to find host galaxies, such as this one, that appear undisturbed by the strong quasar radiation.
Bottom left: Quasar PHL 909 is 1.5 billion light-years from Earth and lies at the core of an apparently normal elliptical galaxy.
Top center: The photo reveals evidence of a catastrophic collision between two galaxies traveling at about 1 million mph. The debris from this collision may be fueling quasar IRAS04505-2958, which is 3 billion light-years from Earth. Astronomers believe that a galaxy plunged vertically through the plane of a spiral galaxy, ripping out its core and leaving the spiral ring (at the bottom of the picture). The core lies in front of the quasar, the bright object in the center of the image. Surrounding the core are star-forming regions. The distance between the quasar and spiral ring is 15,000 light-years, which is one-seventh the diameter of our Milky Way. A foreground star lies just above the quasar.
Bottom center: Hubble has captured quasar PG 1012+008, located 1.6 billion light-years from Earth, merging with a bright galaxy (the object just below the quasar). The two objects are 31,000 light-years apart. The swirling wisps of dust and gas surrounding the quasar and galaxy provide strong evidence for an interaction between them. The compact galaxy on the left of the quasar also may be beginning to merge with the quasar.
Top right: Hubble has captured a tidal tail of dust and gas beneath quasar 0316-346, located 2.2 billion light-years from Earth. The peculiar-shaped tail suggests that the host galaxy has interacted with a passing galaxy that is not in the image.
Bottom right: Hubble has captured evidence of a dance between two merging galaxies. The galaxies may have orbited each other several times before merging, leaving distinct loops of glowing gas around quasar IRAS13218+0552. The quasar is 2 billion light-years from Earth. The elongated core in the center of the image may comprise the two nuclei of the merging galaxies.
Credits: John Bahcall (Institute for Advanced Study, Princeton) Mike Disney (University of Wales) and NASA