CLASSY X: Highlighting Differences Between Partial Covering and Semi-Analytic Modeling in the Estimate of Galactic Outflow Properties

TL;DR

This paper compares empirical and semi-analytical models for estimating galactic outflow properties from spectral data, revealing significant differences and uncertainties in the inferred gas column densities.

Contribution

It provides a systematic comparison between partial covering empirical methods and semi-analytical line transfer models in analyzing galactic outflows, highlighting potential biases.

Findings

Empirical models underestimate column densities in optically thick regimes.

Spectral resolution limits can hide dense gas components.

Semi-analytical models may overestimate densities if assumptions are incorrect.

Abstract

Feedback driven massive outflows play a crucial role in galaxy evolution by regulating star formation and influencing the dynamics of surrounding media. Extracting outflow properties from spectral lines is a notoriously difficult process for a number of reasons, including the possibility that a substantial fraction of the outflow is carried by dense gas in a very narrow range in velocity. This gas can hide in spectra with insufficient resolution. Empirically motivated analysis based on the Apparent Optical Depth method, commonly used in the literature, neglects the contribution of this gas, and may therefore underestimate the true gas column density. More complex semi-analytical line transfer (e.g., SALT) models, on the other hand, allow for the presence of this gas by modeling the radial density and velocity of the outflows as power laws. Here we compare the two approaches to quantify the uncertainties in the inferences of outflow properties based on 1-D "down-the-barrel" using the UV spectra of the CLASSY galaxy sample. We find that empirical modeling may significantly underestimate the column densities relative to SALT analysis, particularly in the optically thick regime. We use simulations to show that the main reason for this discrepancy is the presence of large amount of dense material at low velocities, which can be hidden by the finite spectral resolution of the data. The SALT models in turn could over-estimate the column densities if the assumed power laws of the density profiles strong are not a property of actual outflows.