We assess the metal content of the cool (∼10 4 K) circumgalactic medium (CGM) about galaxies at z 1 using an H I-selected sample of 28 Lyman limit systems (LLS, defined here as absorbers with 16.2 log N H I 18.5) observed in absorption against background QSOs by the Cosmic Origins Spectrograph on-board the Hubble Space Telescope. The N H I selection avoids metallicity biases inherent in many previous studies of the low-redshift CGM. We compare the column densities of weakly ionized metal species (e.g., O II, Si II, Mg II) to N H I in the strongest H I component of each absorber. We find that the metallicity distribution of the LLS (and hence the cool CGM) is bimodal with metal-poor and metal-rich branches peaking at [X/H] −1.6 and −0.3 (or about 2.5% and 50% solar metallicities). The cool CGM probed by these LLS is predominantly ionized. The metal-rich branch of the population likely traces winds, recycled outflows, and tidally stripped gas; the metal-poor branch has properties consistent with cold accretion streams thought to be a major source of fresh gas for star forming galaxies. Both branches have a nearly equal number of absorbers. Our results thus demonstrate there is a significant mass of previously-undiscovered cold metal-poor gas and confirm the presence of metal enriched gas in the CGM of z 1 galaxies.

We present a survey for optically thick Lyman limit absorbers at z < 2.6 using archival Hubble Space Telescope observations with the Faint Object Spectrograph and Space Telescope Imaging Spectrograph. We identify 206 Lyman limit systems (LLSs) increasing the number of catalogued LLSs at z < 2.6 by a factor of ∼10. We compile a statistical sample of 50 τ LLS ≥ 2 LLSs drawn from 249 QSO sight lines that avoid known targeting biases. The incidence of such LLSs per unit redshift, l(z) = dn/dz, at these redshifts is well described by a single power law, l(z) ∝ (1 + z) γ , with γ =1.33 ± 0.61 at z < 2.6, or with γ = 1.83 ± 0.21 over the redshift range 0.2 ≤ z ≤ 4.9. The incidence of LLSs per absorption distance, l(X), decreases by a factor of ∼1.5 over the ∼0.6 Gyr from z = 4.9 to 3.5; l(X) evolves much more slowly at low redshifts, decreasing by a similar factor over the ∼8 Gyr from z = 2.6 to 0.25. We show that the column density distribution function, f (N HI ), at low redshift is not well fitted by a single power law index (f (N HI ) ∝ N −β HI ) over the column density range 13 ≤ log N HI ≤ 22 or log N HI ≥ 17.2. While low and high redshift f (N HI ) distributions are consistent for log N HI > 19.0, there is some evidence that f (N HI ) evolves with z for log N HI 17.7, possibly due to the evolution of the UV background and galactic feedback. Assuming LLSs are associated with individual galaxies, we show that the physical cross section of the optically thick envelopes of galaxies decreased by a factor of ∼9 from z ∼5 to 2 and has remained relatively constant since that time. We argue that a significant fraction of the observed population of LLSs arises in the circumgalactic gas of sub-L * galaxies.

We present UV and optical observations from the Cosmic Origins Spectrograph on the Hubble Space Telescope and Keck of a z = 0.27395 Lyman limit system (LLS) seen in absorption against the QSO PG1630+377. We detect H I absorption with log N (H I) = 17.06 ± 0.05 as well as Mg II, C III, Si III, and O VI in this system. The column densities are readily explained if this is a multi-phase system, with the intermediate and low ions arising in a very low metallicity ([Mg/H] = −1.71 ± 0.06) photoionized gas. We identify via Keck spectroscopy and Large Binocular Telescope imaging a 0.3 L * star-forming galaxy projected 37 kpc from the QSO at nearly identical redshift (z = 0.27406, ∆v = −26 km s −1 ) with near solar metallicity ([O/H] = −0.20 ± 0.15). The presence of very low metallicity gas in the proximity of a near-solar metallicity, sub-L * galaxy strongly suggests that the LLS probes gas infalling onto the galaxy. A search of the literature reveals that such low metallicity LLSs are not uncommon. We found that 50% (4/8) of the well-studied z 1 LLSs have metallicities similar to the present system and show sub-L * galaxies with ρ < 100 kpc in those fields where redshifts have been surveyed. We argue that the properties of these primitive LLSs and their host galaxies are consistent with those of cold mode accretion streams seen in galaxy simulations. 7 Cold here implies the gas is not heated to the virial temperature; however, the temperature of the gas (∼ 10 4 K) is well above that of the cold material in the disk of the galaxy.

We use the Hubble Space Telescope (HST) archive of ultraviolet (UV) quasar spectroscopy to conduct the first blind survey for damped Lyα absorbers (DLAs) at low redshift (

Project AMIGA (Absorption Maps In the Gas of Andromeda) is a survey of the circumgalactic medium (CGM) of Andromeda (M31, R vir ;300 kpc) along 43 QSO sightlines at impact parameters 25 R569 kpc (25 at RR vir). We use ultraviolet absorption measurements of Si II, Si III, Si IV, C II, and C IV from the Hubble Space Telescope/Cosmic Origins Spectrograph and O VI from the Far Ultraviolet Spectroscopic Explorer to provide an unparalleled look at how the physical conditions and metals are distributed in the CGM of M31. We find that Si III and O VI have a covering factor near unity for R1.2 R vir and 1.9 R vir , respectively, demonstrating that M31 has a very extended ∼10 4-10 5.5 K ionized CGM. The metal and baryon masses of the 10 4-10 5.5 K CGM gas within R vir are 10 8 and 4×10 10 (Z/0.3 Z e) −1 M e , respectively. There is not much azimuthal variation in the column densities or kinematics, but there is with R. The CGM gas at R0.5 R vir is more dynamic and has more complicated, multiphase structures than at larger radii, perhaps a result of more direct impact of galactic feedback in the inner regions of the CGM. Several absorbers are projected spatially and kinematically close to M31 dwarf satellites, but we show that those are unlikely to give rise to the observed absorption. Cosmological zoom simulations of ∼L * galaxies have O VI extending well beyond R vir as observed for M31 but do not reproduce well the radial column density profiles of the lower ions. However, some similar trends are also observed, such as the lower ions showing a larger dispersion in column density and stronger dependence on R than higher ions. Based on our findings, it is likely that the Milky Way has a ∼10 4-10 5.5 K CGM as extended as for M31 and their CGM (especially the warm-hot gas probed by O VI) are overlapping.