QSO extended emission-line regions

Extended emission-line regions (EELRs) on scales of a few tens of kpc are found around a substantial fraction of steep-spectrum radio-loud QSOs. In most cases, at least at low redshifts, the distribution of this gas seems to be uncorrelated with either the stellar distribution in the host galaxy or the radio structure. The origin of the gas and the physical processes that control its distribution are still uncertain. We review circumstantial evidence that high densities required to explain the strengths of certain emission lines in spectra of EELRs are due to transient shocks, and we present new Chandra images that show extended X-ray emission associated with two QSOs with known optical EELRs. These discrete X-ray clumps may also be evidence for shocks. We discuss observations that could potentially confirm a model in which EELRs are results of galaxy-wide superwinds that could originate either from starbursts in the host galaxy or from the QSO itself. Finally, we discuss in some detail the QSO 3C 48, which has one of the most luminous EELRs and for which we also have considerable knowledge about both the host galaxy and processes in the immediate vicinity of the QSO. High-velocity ionized gas near the nucleus, but apparently not associated with the radio jet, indicates an outflow that may be the origin of the EELR. (c) 2006 Elsevier B.V. All rights reserved.