Quasars Probing Quasars III: New Clues to Feedback, Quenching, and the   Physics of Massive Galaxy Formation
  Galaxies hosting z~2 quasars are the high-$z$ progenitors of today's massive `red-and-dead' galaxies. With close pairs of quasars at different redshifts, a background quasar can be used to study a foreground quasar's halo gas in absorption, providing a wealth of information about feedback, quenching, and the physics of massive galaxy formation. We present a Keck/HIRES spectrum of the bright background quasar in a projected pair with angular separation 13.3" corresponding to 108kpc at the redshift of the foreground quasar z_fg=2.4360 +/- 0.0005, precisely determined from Gemini/GNIRS near-IR spectroscopy. Our echelle spectrum reveals optically thick gas (NHI~10^19.7), coincident with the foreground quasar redshift. The ionic transitions of associated metal-lines reveal the following properties of the foreground quasar's halo: (1) the kinematics are extreme with absorption extending to +780km/s relative to z_fg; (2) the metallicity is nearly solar; (3) the temperature of the predominantly ionized gas is T<~20,000K; (4) the electron density is n_e~1 cm^-3 indicating a characteristic size ~10 - 100pc for the absorbing `clouds'; (7) there is a negligible amount of warm gas 10^5K < T < 10^6K; (8) the gas is unlikely illuminated by the foreground quasar, implying anisotropic or intermittent emission. The mass of cold T~10^4K gas implied by our observations is significant, amounting to a few percent of the total expected baryonic mass density of the foreground quasar's dark halo at r~100kpc. The origin of this material is still unclear, and we discuss several possibilities in the context of current models of feedback and massive galaxy formation. 
J. Xavier Prochaska (1) and Joseph F. Hennawi (2) ((1) UCO/Lick   Observatory, (2) UC Berkeley)