Implications of the Large O vi Columns around Low-redshift L∗ Galaxies

Abstract

Abstract 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). If collisionally ionized, as we argue is most probable, the O vi observations require that the circumgalactic medium (CGM) of L * galaxies holds nearly all of the associated baryons within a virial radius ( ) and hosts massive flows of cooling gas with , which must be largely prevented from accreting onto the host galaxy. Cooling and feedback energetics considerations require 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 O vi. Furthermore, we show that there is a robust constraint on the number density of many of the photoionized absorption systems that yields upper bounds in the range cm−3, suggesting that the dominant pressure in some photoionized clouds is nonthermal. This constraint is in accordance 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, and it requires much of the feedback energy available from supernovae to be dissipated in the CGM.