Column Density, Kinematics, and Thermal State of Metal-bearing Gas within the Virial Radius of z ∼ 2 Star-forming Galaxies in the Keck Baryonic Structure Survey

Abstract

We present results from the Keck Baryonic Structure Survey (KBSS) including the first detailed measurements of the column densities, kinematics, and internal energy of metal-bearing gas within the virial radius (35–100 physical kpc) of eight ~L* galaxies at z ~ 2. From our full sample of 130 metal-bearing absorbers, we infer that halo gas is kinematically complex when viewed in singly, doubly, and triply ionized species. Broad O vi and C iv absorbers are detected at velocities similar to the lower-ionization gas but with a very different kinematic structure, indicating that the circumgalactic medium (CGM) is multiphase. There is a high covering fraction of metal-bearing gas within 100 kpc, including highly ionized gas such as O vi; however, observations of a single galaxy probed by a lensed background QSO suggest the size of metal-bearing clouds is small (<400 pc for all but the O vi-bearing gas). The mass in metals found within the halo is substantial, equivalent to ≳25% of the metal mass within the interstellar medium. The gas kinematics unambiguously show that 70% of galaxies with detected metal absorption have some unbound metal-enriched gas, suggesting galactic winds may commonly eject gas from halos at z ~ 2. When modeled assuming that ions with different ionization potentials can originate within a single gaseous structure, significant thermal broadening is detected in CGM absorbers that dominates the internal energy of the gas. Some 40% of the detected gas has temperatures in the range 10^(4.5-5.5) K where cooling times are short, suggesting the CGM is dynamic, with constant heating or cooling to produce this short-lived thermal phase.