SHEEP: The Search for the High-Energy Extragalactic Population

We show that the mass of ionized gas in the broad-line regions (BLRs) of luminous quasars is at least several hundred M-, and probably of the order of 103-104 M-. BLR mass estimates in several existing textbooks suggest lower values but pertain to much less luminous Seyfert galaxies or include only a small fraction of the ionized/emitting volume of the BLR. The previous estimates also fail to include the large amounts of BLR gas that emit at low efficiency (in a given line) but that must be present based on reverberation and other studies. Very highly ionized gas as well as partially ionized and neutral gas lying behind the ionization zones are likely additional sources of mass within the BLR. The high masses found here imply that the chemical enrichment of the BLR cannot be controlled by mass ejection from one or a few stars. A significant stellar population in the host galaxies must be contributing. Simple scaling arguments based on normal galactic chemical enrichment and solar or higher BLR metallicities show that the minimum mass of the enriching stellar population is of the order of 10 times the BLR mass, or more than 104-105 M-. More realistic models of the chemical and dynamical evolution in galactic nuclei suggest that much larger, bulge-size stellar populations are involved.