Broad HI Absorbers as Metallicity-Independent Tracers of the Warm-Hot Intergalactic Medium

TL;DR

This study compiles and analyzes broad Ly alpha absorbers as metallicity-independent tracers of the warm-hot intergalactic medium, estimating their contribution to the universe's baryon content and comparing them with OVI absorbers.

Contribution

It provides a new catalog of reliable BLA candidates, compares different data analyses, and estimates the baryonic mass density of BLAs using Monte Carlo corrections.

Findings

BLA line frequency comparable to OVI absorbers

20-40% overlap between BLA and OVI absorbers

BLAs account for approximately 20% of baryons in the local universe

Abstract

Thermally broadened Ly alpha absorbers (BLAs) offer an alternative method to highly-ionized metal lines for tracing the WHIM. We compile a catalog of reliable BLA candidates along seven AGN sight lines from a larger set of Lya absorbers observed by HST/STIS. We compare our measurements based on independent reduction and analysis of the data to those published by other research groups. Purported BLAs are grouped into probable (15), possible (48) and non-BLA (56) categories. We infer a line frequency (dN/dz)_BLA=18+-11, comparable to observed OVI absorbers. There is significant overlap between BLA and OVI absorbers (20-40%) and we find that OVI detections in BLAs are found closer to galaxies than OVI non-detections. Based on 164 measured COG HI line measurements, we statistically correct the observed line widths via a Monte- Carlo simulation. Gas temperature and neutral fraction f(HI) are inferred from these statistically-corrected line widths and lead to a distribution of total hydrogen columns. We find Omega_BLA=(6.3+1.1-0.8)x10^-3. There are a number of critical systematic assumptions implicit in this calculation, and we discuss how each affects our results and those of previously published work. Taking our value, current OVI and BLA surveys can account for ~20% of the baryons in the local universe. Finally, we present new, high-S/N observations of several of the BLA candidate lines from Early Release Observations made by the Cosmic Origins Spectrograph on HST.