Column Density Profiles of Cold Clouds Driven by Galactic Outflows

TL;DR

This study uses hydrodynamic simulations to analyze the ionization and column density profiles of cold clouds in galactic outflows, revealing the influence of Mach number and conduction, and highlighting the need for non-equilibrium ionization models.

Contribution

The paper introduces a new analysis tool and provides detailed ion column density profiles from simulations, improving interpretation of galactic outflow observations.

Findings

Higher Mach numbers lead to more compact clouds.

Conduction efficiency affects cloud density and size.

Ionization equilibrium models overpredict certain ion column densities.

Abstract

Absorption line studies are essential to understanding the origin, nature, and impact of starburst-driven galactic outflows. Such studies have revealed a multiphase medium with a number of poorly-understood features leading to a need to study the ionization mechanism of this gas. To better interpret these observations, we make use of a suite of adaptive mesh refinement hydrodynamic simulations of cold, atomic clouds driven by hot, supersonic outflows, including the effect of radiative cooling, thermal conduction, and an ionizing background characteristic of a starbursting galaxy. Using a new analysis tool, Trident, we estimate the equilibrium column density distributions for ten different ions: H I, Mg II, C II, C III, C IV, Si III, Si IV, N V, O VI, and Ne VIII. These are fit to model profiles with two parameters describing the maximum column density and coverage, and for each ion we provide a table of these fit parameters, along with average velocities and line widths. Our results are most sensitive to Mach number and conduction efficiency, with higher Mach numbers and more efficient conduction leading to more compact, high column density clouds. We use our results to interpret down-the-barrel observations of outflows and find that the adopted ionization equilibrium model overpredicts column densities of ions such as Si IV and does not adequately capture the observed trends for N V and O VI, implying the presence of strong non equilibrium ionization effects.