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We present observations of HI and O VI absorption systems proximate to a galaxy at zgal = 0.3529. The absorption was detected serendipitously in Cosmic Origins Spectrograph observations of the low-z QSO J0943+0531 (zqso = 0.564). The data show two separate clouds along the sightline at an impact parameter of 95 kpc from the galaxy. The first is likely low-metallicity gas falling onto the galaxy. This assessment is based on the high velocity offset of the cloud from the galaxy (Δv = 365 km s-1) and the weak metal line absorption, combined with photoionization modeling. The second cloud, with only a modest velocity separation from the galaxy (Δv = 85 km s-1), exhibits very strong O VI absorption qualitatively similar to O VI absorption seen in the Milky Way halo. Collisional ionization equilibrium models are ruled out by the metal line column density ratios. Photoionization modeling implies a length scale for the O VI cloud of ~0.1-1.2 Mpc, which indicates the absorbing gas most likely resides within the local filamentary structure. This system emphasizes that kinematic association alone is not sufficient to establish a physical connection to galaxies, even at small impact parameters and velocity separations. Observations such as these, connecting galaxies with their gaseous environments, are becoming increasingly important for understanding of galaxy evolution and provide constraints for cosmological simulations. [ABSTRACT FROM AUTHOR]

We present a search for outlying H II regions in the extended gaseous outskirts of nearby (D < 40 Mpc) galaxies and subsequent multi-slit spectroscopy used to obtain the H II region nebular oxygen abundances. The galaxies in our sample have extended H I disks and/or interaction-related H I features that extend well beyond their primary stellar components. We report oxygen abundance gradients out to 2.5 times the optical radius for these galaxies which span a range of morphologies and masses. We analyze the underlying stellar and neutral H I gas distributions in the vicinity of the H II regions to understand the physical processes that give rise to the observed metal distributions in galaxies. These measurements, for the first time, convincingly show flat abundance distributions out to large radii in a wide variety of systems and have broad implications for galaxy chemodynamical evolution. [ABSTRACT FROM AUTHOR]

We have serendipitously detected a strong O VI-bearing Lyman-limit system (LLS) at Zabs = 0.3558 toward the quasi-stellar object (QSO) J1009+0713 (Zem = 0.456) in our survey of low-redshift galaxy halos with the Hubble Space Telescope's (HST) Cosmic Origins Spectrograph. Its total rest-frame equivalent width of Wr = 835 ± 49 mA and column density of log N(O VI) = 15.0 are the highest for an intervening absorber yet detected in any low- redshift QSO sightline, with absorption spanning at least four major kinematic component groups over 400 km s-1 in its rest frame. HST/ Wide Field Camera 3 images of the galaxy field show that the absorber is associated with two galaxies lying at 14 and 46 kpc from the QSO line of sight. The absorber is kinematically complex and there are no less than nine individual Mg II components spanning 200 km s-1 in our Keck/HIRES optical data. The bulk of the absorbing gas traced by H I resides in two strong, blended component groups that possess a total log N(H I) 18-18.8, but most of the 0 vi is associated with two outlying components with log N(H I) = 14.8 and 16.5. The ion ratios and column densities of C, N, O, Mg, Si, S, and Fe, except the O VI, can be accommodated into a simple photoionization model in which diffuse, low-metallicity halo gas is exposed to a photoionizing field from stars in the nearby galaxies that propagates into the halo at 10% efficiency. In this model, the clouds have neutral fractions of ~ 1%-10% and thus total hydrogen column densities of log N(H) 19.5. Direct measurement of the gas metallicity is precluded by saturation of the main components of H I, but we constrain the metallicity firmly within the range 0.1-1 Z..., and photoionization modeling indirectly indicates a subsolar metallicity of 0.05-0.5 Z.... This highly ionized, multiphase, possibly low-metallicity halo gas resembles gas with similar properties in the Milky Way halo and other low-redshift LLS, suggesting that at least some other galaxies have their star formation fueled by metal-poor gas accreting from the intergalactic medium and ionized by the stars in the host galaxy. As observed in the Milky Way high-velocity clouds, the strong detected O VI is not consistent with the photoionization scenario but is consistent with general picture in which O VI arises in interface material surrounding the photoionized clouds or in a hotter, diffuse component of the halo. The appearance of strong O VI and nine Mg ii components in this system, and our review of similar systems in the literature, offer some support to this "interface" picture of high-velocity O VI: the total strength of the O VI shows a positive correlation with the number of detected components in the low-ionization gas. [ABSTRACT FROM AUTHOR]

Observational evidence shows that low-redshift galaxies are surrounded by extended haloes of multiphase gas, the so-called circumgalactic medium (CGM). To study the survival of relatively cool gas (T < 105 K) in the CGM, we performed a set of hydrodynamical simulations of cold (T = 104 K) neutral gas clouds travelling through a hot (T = 2 × 106 K) and low-density (n = 10-4 cm-3) coronal medium, typical of Milky Way-like galaxies at large galactocentric distances (~50-150 kpc).We explored the effects of different initial values of relative velocity and radius of the clouds. Our simulations were performed on a two-dimensional grid with constant mesh size (2 pc), and they include radiative cooling, photoionization heating and thermal conduction. We found that for large clouds (radii larger than 250 pc), the cool gas survives for very long time (larger than 250 Myr): despite that they are partially destroyed and fragmented into smaller cloudlets during their trajectory, the total mass of cool gas decreases at very low rates. We found that thermal conduction plays a significant role: its effect is to hinder formation of hydrodynamical instabilities at the cloud-corona interface, keeping the cloud compact and therefore more difficult to destroy. The distribution of column densities extracted from our simulations is compatible with those observed for low-temperature ions (e.g. Si II and Si III) and for high-temperature ions (O VI) once we take into account that OVI covers much more extended regions than the cool gas and, therefore, it is more likely to be detected along a generic line of sight. [ABSTRACT FROM AUTHOR]

Using high-resolution, high signal-to-noise ultraVIolet spectra of the zem = 0.9754 quasar PG1148+549 obtained with the Cosmic Origins Spectrograph (COS) on the Hubble Space Telescope, we study the physical conditions and abundances of Ne VIII+O VI absorption line systems at zabs = 0.68381, 0.70152, 0.72478. In addition to Ne VIII and OVI, absorption lines from multiple ionization stages of oxygen (O II, OIII, OIV) are detected and are well aligned with the more highly ionized species. We show that these absorbers are multiphase systems including hot gas (T ≈ 105.7 K) that produces Ne VIII and OVI, and the gas metallicity of the cool phase ranges from Z = 0.3 Z☉ to supersolar. The cool (≈104 K) phases have densities nH ≈ 10-4 cm-3 and small sizes (<4 kpc); these cool clouds are likely to expand and dissipate, and the Ne VIII may be within a transition layer between the cool gas and a surrounding, much hotter medium. The Ne VIII redshift density, dN/dz ~ 7+7-3, requires a large number of these clouds for everyL > 0.1L " galaxy and a large effectIVe absorption cross section (≳100 kpc), and indeed, we find a star-forming ~L≳ galaxy at the redshift of the zabs = 0.72478 system, at an impact parameter of 217 kpc. Multiphase absorbers like these Ne VIII systems are likely to be an important reservoir of baryons and metals in the circumgalactic media of galaxies. [ABSTRACT FROM AUTHOR]

The circumgalactic medium (CGM) is fed by galaxy outflows and accretion of intergalactic gas, but its mass, heavy element enrichment, and relation to galaxy properties are poorly constrained by observations. In a survey of the outskirts of 42 galaxies with the Cosmic Origins Spectrograph onboard the Hubble Space Telescope, we detected ubiquitous, large (150-kitoparsec) halos of ionized oxygen surrounding star-forming galaxies; we found much less ionized oxygen around galaxies with little or no star formation. This ionized CGM contains a substantial mass of heavy elements and gas, perhaps far exceeding the reservoirs of gas in the galaxies themselves. Our data indicate that it is a basic component of nearly all star-forming galaxies that is removed or transformed during the quenching of star formation and the transition to passive evolution. [ABSTRACT FROM AUTHOR]

Using Hubble Space Telescope (HST) Advanced Camera for Surveys/Wide Field Camera data we present the photometry and spatial distribution of resolved stellar populations in the outskirts of NGC 2915, a blue compact dwarf with an extended H I disc. These observations reveal an elliptical distribution of red giant branch stars, and a clumpy distribution of main-sequence stars that correlate with the H I gas distribution. We constrain the upper-end initial mass function (IMF) and determine the star formation law (SFL) in this field, using the observed main-sequence stars and an assumed constant star formation rate. Previously published Hα observations of the field, which show one faint H II region, are used to provide further constraints on the IMF. We find that the main-sequence luminosity function analysis alone results in a best-fitting IMF with a power-law slope α = -2.85 and upper-mass limit Mu=60M⊙However, if we assume that all Hα emission is confined to H II regions then the upper-mass limit is restricted to Mu≲20M⊙. For the luminosity function fit to be correct, we have to discount the Hα observations implying significant diffuse ionized gas or escaping ionizing photons. Combining the HST photometry with H I imaging, we find the SFL has a power-law index N = 1.53 ± 0.21. Applying these results to the entire outer H I disc indicates that it contributes 11-28 per cent of the total recent star formation in NGC 2915, depending on whether the IMF is constant within the disc or varies from the centre to the outer region. [ABSTRACT FROM AUTHOR]

We assess the metal content of the cool (~104 K) circumgalactic medium (CGM) about galaxies at z ≲ 1 using an Hi-selected sample of 28 Lyman limit systems (LLS; defined here as absorbers with 16.2 ≲ log NHI ≲ 18.5) observed in absorption against background QSOs by the Cosmic Origins Spectrograph on board the Hubble Space Telescope. The NHi 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 NHI 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. [ABSTRACT FROM AUTHOR]

We present Cosmic Origins Spectrograph (COS) measurements of metal abundances in eight 0.083 < zabs < 0.321 damped Lyα (DLA) and sub-DLA absorption systems serendipitously discovered in the COS--Halos survey. We find that these systems show a large range in metallicities, with -1.10 < [Z/H] < 0.31, similar to the spread found at higher redshifts. These low-redshift systems on average have subsolar metallicities, but do show a rise in metallicity over cosmic time when compared to higher-redshift systems. We find that the average sub-DLA metallicity is higher than the average DLA metallicity at all redshifts. Nitrogen is underabundant with respect to α-group elements in all but perhaps one of the absorbers. In some cases, [N/α] is significantly below the lowest nitrogen measurements in nearby galaxies. Systems for which depletion patterns can be studied show little, if any, depletion, which is characteristic of Milky Way halo-type gas. We also identify affiliated galaxies for three of the sub-DLAs using spectra obtained from a Keck/Low Resolution Imaging Spectrometer (LRIS). None of these sub-DLAs arise in the stellar disks of luminous galaxies; instead, these absorbers may exist in galaxy halos at impact parameters ranging from 38 to 92 kpc. Multiple galaxies are present near two of the sub-DLAs, and galaxy interactions may play a role in the dispersal of the gas. Many of these low-redshift absorbers exhibit simple kinematics, but one sub-DLA has a complicated mix of at least 13 components spread over 150 km s-1. We find three galaxies near this sub-DLA, which also suggests that galaxy interactions roil the gas. This study reinforces the view that DLAs have a variety of origins, and low-redshift studies are crucial for understanding absorber-galaxy connections. [ABSTRACT FROM AUTHOR]