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The high-velocity clouds (HVCs) in the outer Milky Way at 20° < l < 190° have similar spatial locations, metallicities, and kinematics. Moreover, their locations and kinematics are coincident with several extraplanar stellar streams. The HVC origins may be connected to the stellar streams, either stripped directly from them or precipitated by the aggregate dynamical roiling of the region by the stream progenitors. This paper suggests that these HVCs are 'misty' precipitation in the stream wakes based on the following observations. New high-resolution (2.6 km s−1) ultraviolet spectroscopy of the QSO H1821+643 resolves what appears to be a single HVC absorption cloud (at 7 km s−1 resolution) into five components with T ≲ 3 × 104 K. Photoionization models can explain the low-ionization components but require some depletion of refractory elements by dust, and model degeneracies allow a large range of metallicity. High-ionization absorption lines (Si  iv , C  iv , and O  vi) are kinematically aligned with the lower-ionization lines and cannot be easily explained with photoionization or equilibrium collisional ionization; these lines are best matched by non-equilibrium rapidly cooling models, i.e. condensing/precipitating gas, with high metallicity and a significant amount of H  i. Both the low- and high-ionization phases have low ratios of cooling time to freefall time and cooling time to sound-crossing time, which enables fragmentation and precipitation. The H1821+643 results are corroborated by spectroscopy of six other nearby targets that likewise show kinematically correlated low- and high-ionization absorption lines with evidence of dust depletion and rapid cooling. [ABSTRACT FROM AUTHOR]

Quasar absorption systems encode a wealth of information about the abundances, ionization structure, and physical conditions in intergalactic and circumgalactic media. Simple (often single-phase) photoionization models are frequently used to decode such data. Using five discrete absorbers from the COS Absorption Survey of Baryon Harbors (CASBaH) that exhibit a wide range of detected ions (e.g. Mg  ii , S  ii – S  vi , O  ii – O  vi , Ne  viii), we show several examples where single-phase ionization models cannot reproduce the full set of measured column densities. To explore models that can self-consistently explain the measurements and kinematic alignment of disparate ions, we develop a Bayesian multiphase ionization modelling framework that characterizes discrete phases by their unique physical conditions and also investigates variations in the shape of the UV flux field, metallicity, and relative abundances. Our models require at least two (but favour three) distinct ionization phases ranging from T ≈ 104 K photoionized gas to warm-hot phases at T ≲ 105.8 K. For some ions, an apparently single absorption 'component' includes contributions from more than one phase, and up to 30 per cent of the H  i is not from the lowest ionization phase. If we assume that all of the phases are photoionized, we cannot find solutions in thermal pressure equilibrium. By introducing hotter, collisionally ionized phases, however, we can achieve balanced pressures. The best models indicate moderate metallicities, often with subsolar N/ α , and, in two cases, ionizing flux fields that are softer and brighter than the fiducial Haardt & Madau UV background model. [ABSTRACT FROM AUTHOR]

We analyse the intracluster medium (ICM) and circumgalactic medium (CGM) in seven X-ray-detected galaxy clusters using spectra of background quasi-stellar objects (QSOs) (HSTCOS/ STIS), optical spectroscopy of the cluster galaxies (MMT/Hectospec and SDSS), and X-ray imaging/spectroscopy (XMM-Newton and Chandra). First, we report a very low covering fraction of HI absorption in the CGM of these cluster galaxies, fc = 25+25 -15 per cent, to stringent detection limits (N(H I) <1013 cm-2). As field galaxies have an HI covering fraction of ~100 per cent at similar radii, the dearth of CGM HI in our data indicates that the cluster environment has effectively stripped or overionized the gaseous haloes of these cluster galaxies. Secondly, we assess the contribution of warm-hot (105-106 K) gas to the ICM as traced by OVI and broad Ly α (BLA) absorption. Despite the high signal-to-noise ratio of our data, we do not detect OVI in any cluster, and we only detect BLA features in the QSO spectrum probing one cluster. We estimate that the total column density of warm-hot gas along this line of sight totals to ~3 per cent of that contained in the hot T > 107 K X-ray emitting phase. Residing at high relative velocities, these features may trace pre-shocked material outside the cluster. Comparing gaseous galaxy haloes from the low-density 'field' to galaxy groups and high-density clusters, we find that the CGM is progressively depleted of HI with increasing environmental density, and the CGM is most severely transformed in galaxy clusters. This CGM transformation may play a key role in environmental galaxy quenching. [ABSTRACT FROM AUTHOR]

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 and Δv = -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. [ABSTRACT FROM AUTHOR]

We report the discovery of large amounts of cold (T ~ 104 K), chemically young gas in an overdensity of galaxies at redshift z ≈ 1.6 located in the Great Observatories Origins Deep Survey southern field. The gas is identified thanks to the ultra-strong Mg II λ2800 absorption features it imprints onto the rest-frame UV spectra of galaxies in the background of the overdensity. There is no evidence that the optically thick gas is part of any massive galaxy (i.e., Mstar > 4 x 109 M⊙), but rather is associated with the overdensity; less massive and fainter galaxies (25.5 mag < z < 27.5 mag) have too large an impact parameter to be causing ultra-strong absorption systems, based on our knowledge of such systems. The lack of corresponding Fe II absorption features, not detected even in co-added spectra, suggests that the gas is chemically more pristine than the interstellar medium and outflows of star-forming galaxies at similar redshift, including the galaxies of the overdensity itself, and comparable to the most metal-poor stars in the Milky Way halo. A crude estimate of the projected covering factor of the high-column-density gas (NH ≳ 1020 cm-2) based on the observed fraction of galaxies with ultra-strong absorbers is CF ≈ 0.04. A broad, continuum absorption profile extending to the red of the interstellar Mg II absorption line by ≲2000 km s-1 is possibly detected in two independent co-added spectra of galaxies belonging to the overdensity, consistent with a large-scale infall motion of the gas onto the overdensity and its galaxies. Overall, these findings provide the first tentative evidence of accretion of cold, chemically young gas onto galaxies at high redshift, possibly feeding their star formation activity. We suggest the tact that the galaxies are members of a large structure, as opposed to field galaxies, might play a significant role in our ability to detect the accreting gas. [ABSTRACT FROM AUTHOR]

Serendipitous observations of a pair z = 0.37 interacting galaxies (one hosting a quasar) show a massive gaseous bridge of material connecting the two objects. This bridge is photoionized by the quasar (QSO), revealing gas along the entire projected 38 h70-1 kpc sightline connecting the two galaxies. The emission lines that result give an unprecedented opportunity to study the merger process at this redshift. We determine the kinematics, ionization parameter (log U ≈ -2.5 ± 0.03), column density (NH,⊥ ≈ 1021 cm-2), metallicity ([M/H] ≈ - 0.20 ± 0.15), and mass (≈108 M...) of the gaseous bridge. We simultaneously constrain properties of the QSO host (MDM > 8.8 × 1011 M...) and its companion galaxy (MDM > 2.1 × 1011 M...; M⋆ ~ 2 × 1010 M...; stellar burst age = 300-800 Myr; SFR ~6 M... yr-1; and metallicity 12 + log(O/H) = 8.64 ± 0.2). The general properties of this system match the standard paradigm of a galaxy-galaxy merger caught between first and second passages while one of the galaxies hosts an active quasar. The companion galaxy lies in the so-called green valley, with a stellar population consistent with a recent starburst triggered during the first passage of the merger and has no discernible active galactic nucleus activity. In addition to providing case studies of quasars associated with galaxy mergers, quasar/galaxy pairs with QSO-photoionized tidal bridges such as this one offer unique insights into the galaxy properties while also distinguishing an important and inadequately understood phase of galaxy evolution. [ABSTRACT FROM AUTHOR]