The circumgalactic medium (CGM) of late-type galaxies is characterized using UV spectroscopy of 11 targeted QSO/galaxy pairs at z 0.02 with the Hubble Space Telescope Cosmic Origins Spectrograph (COS) and ∼60 serendipitous absorber/galaxy pairs at z 0.2 with the Space Telescope Imaging Spectrograph. CGM warm cloud properties are derived, including volume filling factors of 3%-5%, cloud sizes of 0.1-30 kpc, masses of 10-10 8 M , and metallicities of ∼0.1-1 Z . Almost all warm CGM clouds within 0.5 R vir are metal-bearing and many have velocities consistent with being bound, "galactic fountain" clouds. For galaxies with L 0.1 L * , the total mass in these warm CGM clouds approaches 10 10 M , ∼10%-15% of the total baryons in massive spirals and comparable to the baryons in their parent galaxy disks. This leaves 50% of massive spiral-galaxy baryons "missing." Dwarfs (<0.1 L * ) have smaller area covering factors and warm CGM masses ( 5% baryon fraction), suggesting that many of their warm clouds escape. Constant warm cloud internal pressures as a function of impact parameter (P /k ∼ 10 cm −3 K) support the inference that previous COS detections of broad, shallow O vi and Lyα absorptions are of an extensive (∼400-600 kpc), hot (T ≈ 10 6 K), intra-cloud gas which is very massive ( 10 11 M ). While the warm CGM clouds cannot account for all the "missing baryons" in spirals, the hot intra-group gas can, and could account for ∼20% of the cosmic baryon census at z ∼ 0 if this hot gas is ubiquitous among spiral groups.

We use high-quality, medium-resolution Hubble Space Telescope/Cosmic Origins Spectrograph (HST/COS) observations of 82 UV-bright AGN at redshifts z AGN < 0.85 to construct the largest survey of the low-redshift intergalactic medium (IGM) to date: 5138 individual extragalactic absorption lines in H I and 25 different metal-ion species grouped into 2611 distinct redshift systems at z abs < 0.75 covering total redshift pathlengths ∆z HI = 21.7 and ∆z OVI = 14.5. Our semi-automated line-finding and measurement technique renders the catalog as objectively-defined as possible. The cumulative column-density distribution of H I systems can be parametrized dN (> N )/dz = C 14 (N/10 14 cm −2 ) −(β−1) , with C 14 = 25 ± 1 and β = 1.65 ± 0.02. This distribution is seen to evolve both in amplitude, C 14 ∝ (1 + z) 2.3±0.1 , and slope β(z) = 1.75 − 0.31 z for z ≤ 0.47. We observe metal lines in 418 systems, and find that the fraction of IGM absorbers detected in metals is strongly dependent on N HI . The distribution of O VI absorbers appear to evolve in the same sense as the Lyα forest. We calculate contributions to Ω b from different components of the low-z IGM and determine the Lyα decrement as a function of redshift. IGM absorbers are analyzed via a two-point correlation function in velocity space. We find substantial clustering of H I absorbers on scales of ∆v = 50 − 300 km s −1 with no significant clustering at ∆v 1000 km s −1 . Splitting the sample into strong and weak absorbers, we see that most of the clustering occurs in strong, N HI 10 13.5 cm −2 , metal-bearing IGM systems. The full catalog of absorption lines and fully-reduced spectra is available via the Mikulski Archive for Space Telescopes (MAST) as a high-level science product at

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In this paper, we use large-angle, nearby galaxy redshift surveys to investigate the relationship between the 81 low-redshift Lyα absorbers in our HST/GHRS survey and galaxies, superclusters, and voids. In a subsample of 46 Lyα absorbers located in regions where the February 8, 2000 CfA catalog is complete down to at least L * galaxies, the nearest galaxy neighbors range from 100h −1 70 kpc to > 10h −1 70 Mpc. Of these 46 absorbers, 8 are found in galaxy voids. After correcting for pathlength and sensitivity, we find that 22 ± 8% of the Lyα absorbers lie in voids, which requires that at least some low-column density absorbers are not extended halos of individual bright galaxies. The number density of these clouds yields a baryon fraction of 4.5 ± 1.5% in voids.The stronger Lyα absorbers (10 13.2−15.4 cm −2 ) cluster with galaxies more weakly than galaxies cluster with each other, while the weaker absorbers (10 12.4−13.2 cm −2 ) are more randomly distributed. The median distance from a low-z Lyα absorber in our sample to its nearest galaxy neighbor (∼ 500h −1 70 kpc) is twice the median distance between bright galaxies in the same survey volume. This makes any purposed "association" between these Lyα absorbers and individual galaxies problematic. The suggested correlation between Lyα absorber equivalent width (W) and nearest-galaxy impact parameter does not extend to W ≤ 200 mÅ, or to impact parameters > 200h −1 70 kpc. Instead, we find statistical support for the contention that absorbers align with large-scale filaments of galaxies. The pair of sightlines, 3C 273 and Q 1230+0115, separated by 0.9 • on the sky, provides an example of 8 absorbers and 7 galaxies aligned along a possible filamentary structure at least 20h −1 70 Mpc long. While some strong (W > 400 mÅ) Lyα absorbers may be gas in the extended gaseous halos of individual galaxies, much of the local Lyα "forest" appears to be associated with the large-scale structures of galaxies and some with voids.