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 Sloan Digital Sky Survey type 2 quasar sample. Our targets were selected to have high intrinsic luminosities (M V < -26 mag) as indicated by the [O III] λ5007 emission-line luminosity (L [O III]). Our sample has a median black hole mass of 108.8 M ⊙ inferred assuming the local M BH-σ* relation and a median Eddington ratio of 0.7, using stellar velocity dispersions σ* measured from the G band. We estimate the contamination of the stellar continuum from scattered quasar light based on the strength of broad Hβ, 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] = 109.5 L ⊙. Population synthesis indicates that young poststarburst 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 λ4686 with luminosities up to 108.3 L ⊙ are unambiguously detected in three out of the nine targets, indicative of Wolf-Rayet (WR) populations. Population synthesis shows that 5 Myr poststarburst populations contribute substantially to the luminosities (>50% of L 5100) of all three objects with WR 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. © 2009. The American Astronomical Society. All rights reserved.