Host Galaxies of Luminous Type 2 Quasars at z ~ 0.5

Abstract

We present deep Gemini GMOS optical spectroscopy of nine luminous quasars at redshifts z ~ 0.5, drawn from the SDSS type 2 quasar sample. Our targets were selected to have high intrinsic luminosities (M_V < -26 mag) as indicated by the [O III] 5007 A emission-line luminosity (L_[O III]). Our sample has a median black hole mass of ~ 10^8.8 M_sun inferred assuming the local M_BH-sigma_* relation and a median Eddington ratio of ~ 0.7, using stellar velocity dispersions sigma_* measured from the G band. We estimate the contamination of the stellar continuum from scattered quasar light based on the strength of broad H-beta, and provide an empirical calibration of the contamination as a function of L_[O III]; the scattered light fraction is ~ 30% of L_5100 for objects with L_[O III] = 10^9.5 L_sun. Population synthesis indicates that young post-starburst populations (< 0.1 Gyr) are prevalent in luminous type 2 quasars, in addition to a relatively old population (> 1 Gyr) which dominates the stellar mass. Broad emission complexes around He II 4686 A with luminosities up to 10^8.3 L_sun are unambiguously detected in three out of the nine targets, indicative of Wolf-Rayet populations. Population synthesis shows that ~ 5-Myr post-starburst populations contribute substantially to the luminosities (> 50% of L_5100) of all three objects with Wolf-Rayet detections. We find two objects with double cores and four with close companions. Our results may suggest that luminous type 2 quasars trace an early stage of galaxy interaction, perhaps responsible for both the quasar and the starburst activity.