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/High Resolution Echelle Spectrometer spectrum of the bright background quasar in a projected pair with angular separation θ = 13.3 arcsec corresponding to R⊥ = 108 kpc at the redshift of the foreground quasar zfg = 2.4360 ± 0.0005, precisely determined from Gemini/Gemini Near-Infrared Spectrograph near-IR spectroscopy. Our echelle spectrum reveals optically thick gas (NH i ≈ 1019.7 cm−2) 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 +780 km s−1 relative to zfg; (2) the metallicity is nearly solar; (3) the temperature of the predominantly ionized gas is T≲ 20,000 K; (4) the electron density is ne ∼ 1 cm−3 indicating a characteristic size ∼10–100 pc for the absorbing “clouds”; (5) there is little (if any) warm gas 105 K ≲ T ≲ 106 K; (6) the gas is unlikely illuminated by the foreground quasar, implying anisotropic or intermittent emission. The mass of cold T ∼ 104 K 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 ∼ 100 kpc. 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.