2016MNRAS.457.3133C

We study the ionization structure of galactic outflows in 37 nearby, star-forming galaxies with the Cosmic Origins Spectrograph on the Hubble Space Telescope. We use the O i, Si ii, Si iii, and Si iv ultraviolet absorption lines to characterize the different ionization states of outflowing gas. We measure the equivalent widths, line widths, and outflow velocities of the four transitions, and find shallow scaling relations between them and galactic stellar mass and star formation rate. Regardless of the ionization potential, lines of similar strength have similar velocities and line widths, indicating that the four transitions can be modelled as a comoving phase. The Si equivalent width ratios (e.g. Si iv/Si ii) have low dispersion, and little variation with stellar mass; while ratios with O i and Si vary by a factor of 2 for a given stellar mass. Photoionization models reproduce these equivalent width ratios, while shock models under predict the relative amount of high ionization gas. The photoionization models constrain the ionization parameter (U) between -2.25 U)