Characterizing the Circumgalactic Medium of Nearby Galaxies with HST/COS and HST/STIS Absorption-line Spectroscopy. II. Methods and Models^∗

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 ( ρ = 71 kpc), and the serendipitous subsample probes higher luminosity galaxies at z 0.2 with larger impact parameters ( ρ = 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 ) = 10.5 ± 0.3, or ∼ 30% of the total baryon reservoir of an L ≥ L * , star-forming galaxy.

The Ultraviolet Detection of Diffuse Gas in Galaxy Groups

“…In the fourth absorber (44739 / 0.11766) the O VI is very broad and symmetrical suggesting that a BLA is present and the absorber temperature is > 10 5 K (see Section 3.2). For the metal-free absorbers, no definitive ionization-state or temperature determination is possible, except for absorber 32123 / 0.16142, which has a very broad and symmetrical Lyα profile with a line width suggesting a temperature in CIE just barely exceeding 10 5 K. As discussed previously (Stocke et al 2013;Werk et al 2014;Keeney et al 2017), absorbers with no metal lines detectable in these spectra means only that the metallicities are low, typically 10% Solar metallicity, so that some metals may well be present in these six "metal-free" absorbers as well.…”

The fates of the circumgalactic medium in the FIRE simulations

“…also important for our understanding of different observed phenomena. Consider the massive CGM of low-redshift ∼ L galaxies, which have been the focus of comprehensive surveys with HST/COS (e.g., Werk et al 2013;Johnson et al 2015;Keeney et al 2017). If this CGM remains as gaseous halos until z ≈ 0, then it may contribute to observed highvelocity clouds (e.g.…”

On the Detectability of Visible-wavelength Line Emission from the Local Circumgalactic and Intergalactic Medium

“…Nicastro et al 2018;Shull et al 2012). In addition, there is a current debate over the total mass of gas in the cold (T ∼ 10 4 K) CGM of L ∼ L galaxies, between M cool CGM ∼ 3 × 10 10 M (Keeney et al 2017;Stocke et al 2013) and M cool CGM ∼ 9 × 10 10 M (Prochaska et al 2017;Werk et al 2014), which increases the uncertainty of the total cosmic baryon mass.…”