The COS-Halos Survey: Origins of the Highly Ionized Circumgalactic Medium of Star-Forming Galaxies

The total contribution of diffuse halo gas to the galaxy baryon budget strongly depends on its dominant ionization state. In this paper, we address the physical conditions in the highly ionized circumgalactic medium (CGM) traced by {{O}} {{VI}} absorption lines observed in COS-Halos spectra. We analyze the observed ionic column densities, absorption-line widths and relative velocities, along with the ratios of {{N}} {{V}}/{{O}} {{VI}} for 39 fitted Voigt profile components of O VI. We compare these quantities with the predictions given by a wide range of ionization models. Photoionization models that include only extragalactic UV background radiation are ruled out; conservatively, the upper limits to {{N}} {{V}}/{{O}} {{VI}} and measurements of {N}_{{O}{{VI}}} imply unphysically large path lengths ≳100 kpc. Furthermore, very broad {{O}} {{VI}} absorption (b > 40 km s^-1) is a defining characteristic of the CGM of star-forming L* galaxies. We highlight two possible origins for the bulk of the observed {{O}} {{VI}}: (1) highly structured gas clouds photoionized primarily by local high-energy sources or (2) gas radiatively cooling on large scales behind a supersonic wind. Approximately 20% of circumgalactic O VI does not align with any low-ionization state gas within ±50 km s^-1 and is found only in halos with {M}_{halo} < 10¹² {M}_⊙ . We suggest that this type of unmatched O VI absorption traces the hot corona itself at a characteristic temperature of {10}^5.5 K. We discuss the implications of these very distinct physical origins for the dynamical state, gas cooling rates, and total baryonic content of L* gaseous halos.