Implications of the Large O VI Columns around Low-redshift &ITL&IT* Galaxies

Observations reveal massive amounts of O VI around star-forming L-* galaxies, with covering fractions of near unity extending to the host halo's virial radius. This O VI absorption is typically kinematically centered upon photoionized gas, with line widths that are suprathermal and kinematically offset from the galaxy. We discuss various scenarios and whether they could result in the observed phenomenology (cooling gas flows, boundary layers, shocks, virialized gas, photoionized clouds in thermal equilibrium). If predominantly collisionally ionized, as we argue is most probable, the O VI observations require that the circumgalactic medium (CGM) of L-* galaxies holds nearly all the associated baryons within a virial radius (similar to 10(11)M(circle dot)) and hosts massive flows of cooling gas with approximate to 30[nT/30 cm(-3)K] M-circle dot yr(-1), which must be largely prevented from accreting onto the host galaxy. Cooling and feedback energetics considerations require 10 < nT< 100 cm(-3)K for the warm and hot halo gases. We argue that virialized gas, boundary layers, hot winds, and shocks are unlikely to directly account for the bulk of the OVI. Furthermore, we show that there is a robust constraint on the number density of many of the photoionized similar to 10(4)K absorption systems that yields upper bounds in the range n<(0.1 - 3) x 10(-3)(Z/0.3) cm(-3), where Z is the metallicity, suggestive that the dominant pressure in some photoionized clouds is nonthermal. This constraint, which requires minimal ionization modeling, is in accord with the low densities inferred from more complex photoionization modeling. The large amount of cooling gas that is inferred could re-form these clouds in a fraction of the halo dynamical time, as some arguments require, and it requires much of the feedback energy available from supernovae and stellar winds to be dissipated in the CGM.