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

TL;DR

This study investigates the physical conditions of the highly ionized circumgalactic medium in star-forming galaxies using COS-Halos data, revealing complex ionization sources and the significance of hot coronae in galaxy halos.

Contribution

It provides new insights into the origins and ionization mechanisms of OVI in the CGM, challenging simple photoionization models and highlighting the role of cooling and hot gas.

Findings

Photoionization models with only UV background are ruled out.

Broad OVI absorption is characteristic of star-forming galaxy halos.

Unmatched OVI traces hot coronae at ~10^5.5 K.

Abstract

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 OVI 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 NV/OVI for 39 fitted Voigt profile components of OVI. 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 NV/OVI and measurements of N$_{\rm OVI}$ imply unphysically large path lengths > 100 kpc. Furthermore, very broad OVI 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 OVI: (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 OVI does not align with any low-ionization state gas within $\pm$50 km s$^{-1}$ and is found only in halos with M$_{\rm halo}$ < 10$^{12}$ M$_{\odot}$. We suggest that this type of unmatched OVI 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.