The Formation of Quasars in Low-Luminosity Hosts via Galaxy Harassment

We have simulated disk galaxies undergoing continual bombardment by other galaxies in a rich cluster. "Galaxy harassment" leads to dramatic evolution of smaller disk galaxies and provides an extremely effective mechanism for fueling a central quasar. Within a few billion years after a small disk galaxy enters the cluster environment, up to 90% of its gas can be driven into the inner 500 pc. Up to half of the mass can be transferred in a burst lasting just 100-200 Myr. This transport of gas to the center of galaxy is far more efficient than any mechanism proposed before. Galaxy harassment was first proposed to explain the disturbance of blue galaxies in clusters seen at z ≳ 0.3, known as the "Butcher-Oemler effect." Quasars at the same redshifts lie in more clustered environments than those at lower redshifts. Recent Hubble Space Telescope observations find that roughly half of all observed quasar host galaxies are fainter than L*, with many of these less luminous hosts occurring at redshifts z ≳ 0.3. We examine five quasars that are claimed to have low-luminosity hosts and find that three are in rich clusters of galaxies; the fourth may be in a cluster, but the evidence for this is marginal. The environment of the fifth has not been studied.