Probing Large Galaxy Halos at $z\sim0$ with Automated $\rm Ly \alpha$-Absorption Matching

Abstract

We present initial results from an ongoing large-scale study of the circumgalactic medium in the nearby Universe ($cz \leq 10,000$ km/s), using archival Cosmic Origins Spectrograph spectra of background quasi-stellar objects. This initial sample contains 33 sightlines chosen for their proximity to large galaxies (D > 25 kpc) and high signal-to-noise ratios (S/N > 10), yielding 48 Ly$\alpha$ absorption lines that we have paired with 33 unique galaxies, with 29 cases where multiple absorbers within a single sightline are paired with the same galaxy. We introduce a likelihood parameter to facilitate the matching of galaxies to absorption lines in a reproducible manner. We find the usual anti-correlation between Ly$\alpha$ equivalent width (EW) and impact parameter ($\rho$) when we normalize by galaxy virial radius ($R_{vir}$). Galaxies associated with a Ly$\alpha$ absorber are found to be more highly-inclined than galaxies in the survey volume at a >99% confidence level (equivalent to $\sim 3.6 \sigma$ for a normal distribution). In contrast with suggestions in other recent papers of a correlation with azimuth angle for Mg II absorption, we find no such correlation for Ly$\alpha$.