A glimpse at quasar host galaxy Far-UV emission using damped Lyα's as natural coronagraphs

In merger-driven models of massive galaxy evolution, the luminous quasar phase is expected to be accompanied by vigorous star formation in quasar host galaxies. In this paper, we use high column density damped Lyα (DLA) systems along quasar sight lines as natural coronagraphs to directly study the far-UV (FUV) radiation from the host galaxies of luminous background quasars. We have stacked the spectra of ∼2000 DLA systems (N _{H I}> 10^20.6cm^-2) with a median absorption redshift 〈z〉 = 2.6 selected from quasars observed in the SDSS-III Baryon Oscillation Spectroscopic Survey. We detect residual flux in the dark troughs of the composite DLA spectra. The level of this residual flux significantly exceeds systematic errors in the Sloan Digital Sky Survey fiber sky subtraction; furthermore, the residual flux is strongly correlated with the continuum luminosity of the background quasar, while uncorrelated with DLA column density or metallicity. We conclude that the flux could be associated with the average FUV radiation from the background quasar host galaxies (with medium redshift 〈z〉 = 3.1) that is not blocked by the intervening DLA. Assuming that all of the detected flux originates from quasar hosts, for the highest quasar luminosity bin (〈L〉 = 2.5 × 10¹³L _O), the host galaxy has an FUV intensity of 1.5 ± 0.2 × 10⁴⁰erg s^-1Å^-1; this corresponds to an unobscured UV star formation rate of 9 M _Oyr^-1.