Resolving the Unresolved Galactic Winds in Multi-phase Models. I. Methodology and Application

TL;DR

This paper develops a method to fit galactic wind observations with a multiphase model, constraining key outflow parameters and revealing radial velocity trends, thereby improving understanding of galaxy evolution.

Contribution

It introduces a novel fitting approach for multiphase galactic winds, providing constraints on hot and cool outflow components from observational data.

Findings

Good fits achieved for most galaxies with tight constraints on cool-phase mass-loading.

Inferred cool-phase mass-loading factors are mostly around unity, with significant scatter.

Radial trends show increasing cool-phase velocity and variable mass-loading factors with radius.

Abstract

Galactic winds shape galaxy evolution; however, the outflowing gas is complex: it consists of multiple ionization phases, and its properties vary spatially. Therefore, methods that combine high-fidelity observations with state-of-the-art galactic-wind models are limited. Here we investigate methods for fitting the column density profiles derived from high-quality outflow observations with the multiphase, multiscale wind model from Fielding & Bryan 2022. We identify three key outflow parameters: the initial hot-phase mass-loading factor ($\eta_\text{ M,hot,0}$), the initial cool-phase mass-loading factor ($\eta_\text{ M,cool,0}$), and the initial cool-cloud mass. We obtain good fits for most galaxies, with tight constraints on $\eta_\text{ M,cool,0}$ and moderate constraints on the other two parameters. We find the inferred $\eta_\text{ M,cool,0}$ and $\eta_\text{ M,hot,0}$ are mostly of order unity, with significant scatter. The constraints on $\eta_\text{ M,hot,0}$ suggest that the interaction between the cool and hot phases allows us to indirectly constrain the properties of the hot wind from cool-outflow observations. The model also predicts various radial trends. First, for all galaxies, the cool-phase outflow velocity increases between $1-2$ times of the half-light radius, then reaches a plateau. Second, most galaxies exhibit increasing $\eta_\text{ M,cool}$ and decreasing $\eta_\text{ M,hot}$ with radius, with a few showing the reverse trends. These results are effective, model-conditional constraints, and are consistent with other recent multiphase simulations and observations. This highlights that the velocity-radius mapping encoded in UV absorption profiles enables recovery of outflow spatial structures from spatially integrated spectra. Our method paves the way for future broad parameter studies and guides updates of outflow simulations in future work.