We present a systematic investigation of the circumgalactic medium (CGM) within projected distances d < 160 kpc of luminous red galaxies (LRGs). The sample comprises 16 intermediate-redshift (z = 0.21−0.55) LRGs of stellar mass M star > 10 11 M . Combining far-ultraviolet Cosmic Origin Spectrograph spectra from the Hubble Space Telescope and optical echelle spectra from the ground enables a detailed ionization analysis based on resolved component structures of a suite of absorption transitions, including the full H i Lyman series and various ionic metal transitions. By comparing the relative abundances of different ions in individually-matched components, we show that cool gas (T ∼ 10 4 K) density and metallicity can vary by more than a factor of ten in an LRG halo. Specifically, metal-poor absorbing components with < 1/10 solar metallicity are seen in 50% of the LRG halos, while gas with solar and super-solar metallicity is also common. These results indicate a complex multiphase structure and poor chemical mixing in these quiescent halos. We calculate the total surface mass density of cool gas, Σ cool , by applying the estimated ionization fraction corrections to the observed H i column densities. The radial profile of Σ cool is best-described by a projected Einasto profile of slope α = 1 and scale radius r s = 48 kpc. We find that typical LRGs at z ∼ 0.4 contain cool gas mass of M cool = (1 − 2) × 10 10 M at d < 160 kpc (or as much as M cool ≈ 4 × 10 10 M at d < 500 kpc), comparable to the cool CGM mass of star-forming galaxies. Furthermore, we show that high-ionization O vi and low-ionization absorption species exhibit distinct velocity profiles, highlighting their different physical origins. We discuss the implications of our findings for the origin and fate of cool gas in LRG halos.

We compare a sample of five high-resolution, high S/N Lyα forest spectra of bright 6 < z <∼ 6.5 QSOs aimed at spectrally resolving the last remaining transmission spikes at z > 5 with those obtained from mock absorption spectra from the Sherwood and Sherwood-Relics suites of hydrodynamical simulations of the intergalactic medium (IGM). We use a profile fitting procedure for the inverted transmitted flux, 1−F , similar to the widely used Voigt profile fitting of the transmitted flux F at lower redshifts, to characterise the transmission spikes that probe predominately underdense regions of the IGM. We are able to reproduce the width and height distributions of the transmission spikes, both with optically thin simulations of the post-reionization Universe using a homogeneous UV background and full radiative transfer simulations of a late reionization model. We find that the width of the fitted components of the simulated transmission spikes is very sensitive to the instantaneous temperature of the reionized IGM. The internal structures of the spikes are more prominent in low temperature models of the IGM. The width distribution of the observed transmission spikes, which require high spectral resolution (≤ 8 km s −1 ) to be resolved, is reproduced for optically thin simulations with a temperature at mean density of T 0 = (11000 ± 1600, 10500 ± 2100, 12000 ± 2200) K at z = (5.4, 5.6, 5.8). This is weakly dependent on the slope of the temperature-density relation, which is favored to be moderately steeper than isothermal. In the inhomogeneous, late reionization, full radiative transfer simulations where islands of neutral hydrogen persist to z ∼ 5.3, the width distribution of the observed transmission spikes is consistent with the range of T 0 caused by spatial fluctuations in the temperature-density relation.

We present multi-sightline absorption spectroscopy of cool gas around three lensing galaxies at z = 0.4 − 0.7. These lenses have half-light radii r e = 2.6 − 8 kpc and stellar masses of log M * /M = 10.9 − 11.4, and therefore resemble nearby passive elliptical galaxies. The lensed QSO sightlines presented here occur at projected distances of d = 3−15 kpc (or d ≈ 1−2 r e ) from the lensing galaxies, providing for the first time an opportunity to probe both interstellar gas at r ∼ r e and circumgalactic gas at larger radii r r e of these distant quiescent galaxies. We observe distinct gas absorption properties among different lenses and among sightlines of individual lenses. Specifically, while the quadruple lens for HE 0435−1223 shows no absorption features to very sensitive limits along all four sightlines, strong Mg II, Fe II, Mg I, and Ca II absorption transitions are detected along both sightlines near the double lens for HE 0047−1756, and in one of the two sightlines near the double lens for HE 1104−1805. The absorbers are resolved into 8 − 15 individual components with a line-of-sight velocity spread of ∆ v ≈ 300 − 600 km s −1 . The large ionic column densities, log N > ∼ 14, observed in two components suggest that these may be Lyman limit or damped Lyα absorbers with a significant neutral hydrogen fraction. The majority of the absorbing components exhibit a uniform super solar Fe/Mg ratio with a scatter of < 0.1 dex across the full ∆ v range. Given a predominantly old stellar population in these lensing galaxies, we argue that the observed large velocity width and Fe-rich abundance pattern can be explained by SNe Ia enriched gas at radius r ∼ r e . We show that additional spatial constraints in line-of-sight velocity and relative abundance ratios afforded by a multisightline approach provide a powerful tool to resolve the origin of chemically-enriched cool gas in massive halos.

We present a new Hubble Space Telescope (HST) Cosmic Origins Spectrograph (COS) absorption-line survey to study halo gas around 16 luminous red galaxies (LRGs) at z = 0.21 − 0.55. The LRGs are selected uniformly with stellar mass M star > 10 11 M and no prior knowledge of the presence/absence of any absorption features. Based on observations of the full Lyman series, we obtain accurate measurements of neutral hydrogen column density N (H I) and find that high-N (H I) gas is common in these massive quiescent halos with a median of log N (H I) = 16.6 at projected distances d < ∼ 160 kpc. We measure a mean covering fraction of optically-thick gas with log N (H I) > ∼ 17.2 of κ LLS = 0.44 +0.12 −0.11 at d < ∼ 160 kpc and κ LLS = 0.71 +0.11 −0.20 at d < ∼ 100 kpc. The line-of-sight velocity separations between the H I absorbing gas and LRGs are characterized by a mean and dispersion of v gas−gal = 29 km s −1 and σ v gas−gal = 171 km s −1 . Combining COS far-ultraviolet and ground-based echelle spectra provides an expanded spectral coverage for multiple ionic transitions, from low-ionization Mg II and Si II, to intermediate ionization Si III and C III, and to highionization O VI absorption lines. We find that intermediate ions probed by C III and Si III are the most prominent UV metal lines in LRG halos with a mean covering fraction of κ(C III) 0.1 = 0.75 +0.08 −0.13 for W r (977) 0.1Å at d < 160 kpc, comparable to what is seen for C III in L * and sub-L * star-forming and red galaxies but exceeding Mg II or O VI in quiescent halos. The COS-LRG survey shows that massive quiescent halos contain widespread chemically-enriched cool gas and that little distinction between LRG and star-forming halos is found in their H I and C III content.

This paper presents a survey of Mg II absorbing gas in the vicinity of 380 random galaxies, using 156 background quasi-stellar objects (QSOs) as absorption-line probes. The sample comprises 211 isolated (73 quiescent and 138 star-forming galaxies) and 43 non-isolated galaxies with sensitive constraints for both Mg II absorption and Hα emission. The projected distances span a range from d = 9 to 497 kpc, redshifts of the galaxies range from z = 0.10 to 0.48, and rest-frame absolute B-band magnitudes range from MB = −16.7 to −22.8. Our analysis shows that the rest-frame equivalent width of Mg II, Wr(2796), depends on halo radius (Rh), B-band luminosity(LB) and stellar mass (Mstar) of the host galaxies, and declines steeply with increasing d for isolated, star-forming galaxies. At the same time, Wr(2796) exhibits no clear trend for either isolated, quiescent galaxies or non-isolated galaxies. In addition, the covering fraction of Mg II absorbing gas 〈κ〉 is high with 〈κ〉 ≳ 60% at &lt;40 kpc for isolated galaxies and declines rapidly to 〈κ〉 ≈ 0 at d ≳ 100 kpc. Within the gaseous radius, the incidence of Mg II gas depends sensitively on both Mstar and the specific star formation rate inferred from Hα. Different from what is known for massive quiescent halos, the observed velocity dispersion of Mg II absorbing gas around star-forming galaxies is consistent with expectations from virial motion, which constrains individual clump mass to $m_{\rm cl} \gtrsim 10^5 \, \rm M_\odot$ and cool gas accretion rate of $\sim 0.7-2 \, M_\odot \, \rm yr^{-1}$. Finally, we find no strong azimuthal dependence of Mg II absorption for either star-forming or quiescent galaxies. Our results demonstrate that multiple parameters affect the properties of gaseous halos around galaxies and highlight the need of a homogeneous, absorption-blind sample for establishing a holistic description of chemically-enriched gas in the circumgalactic space.