# Characterizing the Circumgalactic Medium of Nearby Galaxies with HST/COS and HST/STIS Absorption-Line Spectroscopy: II. Methods and Models

## TL;DR

This paper details methods and models for analyzing the cool, photo-ionized circumgalactic medium of low-redshift galaxies using UV absorption-line spectroscopy, comparing results with previous surveys and estimating the CGM cool gas mass.

## Contribution

Introduces new methodologies for fitting H I absorption complexes and combines survey data to refine estimates of the CGM cool gas mass.

## Key findings

- No evidence for changing ionization or density with distance from galaxy
- Passive galaxies can host cool gas halos without star formation
- Estimated CGM cool gas mass is about 30% of galaxy baryons

## Abstract

We present basic data and modeling for a survey of the cool, photo-ionized Circum-Galactic Medium (CGM) of low-redshift galaxies using far-UV QSO absorption line probes. This survey consists of "targeted" and "serendipitous" CGM subsamples, originally described in Stocke et al. (2013, Paper 1). The targeted subsample probes low-luminosity, late-type galaxies at $z<0.02$ with small impact parameters ($\langle\rho\rangle = 71$ kpc), and the serendipitous subsample probes higher luminosity galaxies at $z\lesssim0.2$ with larger impact parameters ($\langle\rho\rangle = 222$ kpc). HST and FUSE UV spectroscopy of the absorbers and basic data for the associated galaxies, derived from ground-based imaging and spectroscopy, are presented. We find broad agreement with the COS-Halos results, but our sample shows no evidence for changing ionization parameter or hydrogen density with distance from the CGM host galaxy, probably because the COS-Halos survey probes the CGM at smaller impact parameters. We find at least two passive galaxies with H I and metal-line absorption, confirming the intriguing COS-Halos result that galaxies sometimes have cool gas halos despite no on-going star formation. Using a new methodology for fitting H I absorption complexes, we confirm the CGM cool gas mass of Paper 1, but this value is significantly smaller than found by the COS-Halos survey. We trace much of this difference to the specific values of the low-$z$ meta-galactic ionization rate assumed. After accounting for this difference, a best-value for the CGM cool gas mass is found by combining the results of both surveys to obtain $\log{(M/M_{\odot})}=10.5\pm0.3$, or ~30% of the total baryon reservoir of an $L \geq L^*$, star-forming galaxy.

---
Source: https://tomesphere.com/paper/1704.00235