A Glimpse at Quasar Host Galaxy Far-UV Emission, Using DLAs 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 Lyman Alpha (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 $\sim$2,000 DLA systems (N_HI>10^{20.6} cm^{-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 SDSS 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 all of the
detected flux originates from quasar hosts, for the highest quasar luminosity
bin (< L >= 2.5x 10^{13} L_sun), the host galaxy has a FUV intensity of 1.5 +/-
0.2 x 10^{40} erg s^{-1} A^{-1}; this corresponds to an unobscured UV star
formation rate of 9 M_sun/yr.
