Shining A Light On Galactic Outflows: Photo-Ionized Outflows

TL;DR

This study analyzes the ionization structure of galactic outflows in 37 nearby star-forming galaxies using ultraviolet absorption lines, revealing co-moving phases and constraining ionization parameters and metallicities.

Contribution

It provides new insights into the ionization states of outflows, demonstrating that multiple ionization lines can be modeled as a co-moving phase and constraining ionization parameters and metallicities.

Findings

Lines of similar strength have similar velocities and widths across ionization states.

Photo-ionization models successfully reproduce observed equivalent width ratios.

Mass outflow rates are sensitive to the ionization structure assumptions.

Abstract

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 modeled as a co-moving 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. Photo-ionization models reproduce these equivalent width ratios, while shock models under predict the relative amount of high ionization gas. The photo-ionization models constrain the ionization parameter (U) between -2.25 < log(U) < -1.5, and require that the outflow metallicities are greater than 0.5 Z$_\odot$. We derive ionization fractions for the transitions, and show that the range of ionization parameters and stellar metallicities leads to a factor of 1.15-10 variation in the ionization fractions. Historically, mass outflow rates are calculated by converting a column density measurement from a single metal ion into a total Hydrogen column density using an ionization fraction, thus mass outflow rates are sensitive to the assumed ionization structure of the outflow.