Search

We report the serendipitous discovery of an object, UVQS J060819.93−715737.4, with a spectrum dominated by extremely intense, narrow C ii emission lines. The spectrum is similar to those of the very rare, late-type [WC11] low-mass Wolf–Rayet stars. Despite the recognition of these stars as a distinct class decades ago, there remains barely a handful of Galactic members, all of which are also planetary-nebula central stars. Although no obvious surrounding nebulosity is present in J0608, [O ii], [N ii], and [S ii] emission suggest the presence of an inconspicuous, low-excitation nebula. There is low-amplitude incoherent photometric variability on timescales of days to years, as well as numerous prominent P Cygni profiles, implying mass loss. There are indications of a binary companion. The star is located on the outskirts of the LMC, and the observed radial velocity (∼+250 km s−1) and proper motion strongly suggest membership. If indeed an LMC member, this is the first extragalactic late [WC] star, and the first with an accurately determined luminosity, as the Galactic examples are too distant for precise parallax determinations. A high-quality, broad-coverage spectrum of the prototype of the late [WC] class, CPD −56° 8032, is also presented. We discuss different excitation mechanisms capable of producing the great strength of the C ii emission. Numerous autoionizing levels of C ii are definitely populated by processes other than dielectronic recombination. Despite the spectacular emission spectra, observational selection makes objects such as these difficult to discover. Members of the [WC11] class may in fact be considerably more common than the handful of previously known late [WC] stars. [ABSTRACT FROM AUTHOR]

The MAMMOTH-I Nebula at redshift 2.3 is one of the largest known Lyα nebulae in the universe, spanning ∼440 kpc. Enormous Lyα nebulae like MAMMOTH-I typically trace the densest and most active regions of galaxy formation. Using sensitive low-surface-brightness observations of CO(1−0) with the Very Large Array, we trace the cold molecular gas in the inner 150 kpc of the MAMMOTH-I Nebula. CO is found in four regions that are associated with either galaxies or groups of galaxies that lie inside the nebula. In three of the regions, the CO stretches up to ∼30 kpc into the circumgalactic medium (CGM). In the centermost region, the CO has a very low velocity dispersion (FWHMCO ∼ 85 km s−1), indicating that this gas is dynamically cold. This dynamically cold gas coincides with diffuse rest-frame optical light in the CGM around a central group of galaxies, as discovered with the Hubble Space Telescope. We argue that this likely represents cooling of settled and enriched gas in the center of MAMMOTH-I. This implies that the dynamically cold gas in the CGM, rather than the obscured active galactic nucleus, marks the core of the potential well of this Lyα nebula. In total, the CO in the MAMMOTH-I Nebula traces a molecular gas mass of MH2 ∼ 1.4(αCO/3.6) × 1011M⊙, with roughly 50% of the CO(1−0) emission found in the CGM. Our results add to the increasing evidence that extended reservoirs of molecular gas exist in the CGM of massive high-z galaxies and protoclusters. [ABSTRACT FROM AUTHOR]

We identify a strong Lyα damping wing profile in the spectrum of the quasar P183+05 at z = 6.4386. Given the detection of several narrow metal absorption lines at z = 6.40392, the most likely explanation for the absorption profile is that it is due to a damped Lyα system. However, in order to match the data a contribution of an intergalactic medium 5%–38% neutral or additional weaker absorbers near the quasar is also required. The absorption system presented here is the most distant damped Lyα system currently known. We estimate an H i column density of 1020.68±0.25 cm−2, metallicity [O/H] = −2.92 ± 0.32, and relative chemical abundances of a system consistent with a low-mass galaxy during the first Gyr of the universe. This object is among the most metal-poor damped Lyα systems known and, even though it is observed only ∼850 Myr after the big bang, its relative abundances do not show signatures of chemical enrichment by Population III stars. [ABSTRACT FROM AUTHOR]

We present a novel absorption-line survey using 54 blue horizontal branch stars in the Milky Way halo as background sources for detecting gas flows at the disk–halo interface. Distance measurements to high-latitude (b > 60°) background stars at 3.1–13.4 kpc, combined with unprecedented spatial sampling and spectral resolution, allow us to examine the 3D spatial distribution and kinematics of gas flows near the disk. We detect absorption signatures of extraplanar Ca ii and Na i in Keck HIRES spectra and find that their column densities exhibit no trend with distance to the background sources, indicating that these clouds lie within 3.1 kpc of the disk. We calculate covering fractions of fCa ii = 63%, fNa i = 26%, and fH i = 52%, consistent with a picture of the circumgalactic medium (CGM) that includes multiphase clouds containing small clumps of cool gas within hotter, more diffuse gas. Our measurements constrain the scale of any substructure within these cool clouds to <0.5 kpc. Ca ii and Na i absorption features exhibit an intermediate-velocity (IV) component inflowing at velocities of −75 km s−1 −1 relative to the local standard of rest, consistent with previously studied H i structures in this region. We report the new detection of an inflow velocity gradient Δvz ∼ 6–9 km s−1 kpc−1 across the Galactic plane. These findings place constraints on the physical and kinematic properties of CGM gas flows through the disk–halo interface and support a galactic fountain model in which cold gas rains back onto the disk. [ABSTRACT FROM AUTHOR]

We perform a Very Large Telescope FOcal Reducer and low-dispersion Spectrograph 2 narrowband imaging search around five star-forming galaxies at redshift z = 0.67–0.69 in the Great Observatories Origins Deep Survey South field to constrain the radial extent of large-scale outflows traced by resonantly scattered Mg ii emission. The sample galaxies span star formation rates in the range and have stellar masses , and exhibit outflows traced by Mg ii absorption with velocities . These observations are uniquely sensitive, reaching surface brightness limits of 5.81 × 10−19 erg s−1 cm−2 arcsec2 per 1 arcsec2 aperture (at 5σ significance). We do not detect any extended emission around any of the sample galaxies, thus placing 5σ upper limits on the brightness of extended Mg ii emission of <6.51 × 10−19 erg s−1 cm−2 arcsec2 at projected distances R⊥ > 8–21 kpc. The imaging also resolves the Mg ii absorption observed toward each galaxy spatially, revealing approximately constant absorption strengths across the galaxy disks. In concert with radiative transfer models predicting the surface brightness of Mg ii emission for a variety of simple wind morphologies, our detection limits suggest that either (1) the extent of the Mg ii-emitting material in the outflows from these galaxies is limited to ≲20 kpc; or (2) the outflows are anisotropic and/or dusty. [ABSTRACT FROM AUTHOR]

We present Keck/Keck Cosmic Web Imager (KCWI) integral field spectrograph observations of the complex system surrounding SMM J02399−0136 (a lensed z = 2.8 sub-mm galaxy), including an associated Lyα nebula, a dust-obscured, broad-absorption-line quasar, and neighboring galaxies. At a 3σ surface brightness contour of 1.6 × 10−17 erg s−1 cm−2 arcsec−2, the Lyα nebula extends over 17 arcsec (≳140 physical kpc) and has a total Lyα luminosity of (uncorrected for lensing). The nebula exhibits a kinematic shear of over 100 pkpc with lowest velocities east of SMM J02399−0136 and increasing to the southwest. We also discover a bright, Lyα emitter, separated spatially and kinematically from the nebula, at a projected separation of ≈60 kpc from the quasar. This source has no clear central counterpart in deep Hubble Space Telescope imaging, giving an intrinsic Lyα rest-frame equivalent width greater than 312 Å (5σ). We argue that this “dark cloud” is illuminated by the quasar with a UV flux that is orders of magnitude brighter than the emission along our sightline. This result confirms statistical inferences that luminous quasars at z > 2 emit UV radiation anisotropically. Future KCWI observations of other lines, e.g., Lyβ, He ii, C iv, etc, and with polarimetry will further reveal the origin of the Lyα nebula and nature of the dark cloud. [ABSTRACT FROM AUTHOR]

We present measurements of the large-scale (≈40 comoving Mpc) effective optical depth of He ii Lyα absorption, , at 2.54 < z < 3.86 toward 16 He ii-transparent quasars observed with the Cosmic Origins Spectrograph on the Hubble Space Telescope, to characterize the ionization state of helium in the intergalactic medium (IGM). We provide the first statistical sample of measurements in six signal-to-noise ratio ≳3 He ii sightlines at z > 3.5, and study the redshift evolution and sightline-to-sightline variance of in 24 He ii sightlines. We confirm an increase of the median from ≃2 at z = 2.7 to at z > 3, and a scatter in that increases with redshift. The z > 3.5 He ii absorption is predominantly saturated, but isolated narrow (Δv < 650 km s−1) transmission spikes indicate patches of reionized helium. We compare our measurements to predictions for a range of UV background models applied to outputs of a large-volume (146 comoving Mpc)3 hydrodynamical simulation by forward-modeling our sample’s quality and size. At z > 2.74, the variance in significantly exceeds expectations for a spatially uniform UV background, but is consistent with a fluctuating radiation field sourced by variations in the quasar number density and the mean free path in the post-reionization IGM. We develop a method to infer the approximate median He ii photoionization rate of a fluctuating UV background from the median , finding a factor ≃5 decrease in between z ≃ 2.6 and z ≃ 3.1. At z ≃ 3.1, s−1 corresponds to a median He ii fraction of ≃2.5%, indicating that our data probe the tail end of He ii reionization. [ABSTRACT FROM AUTHOR]

We analyze the equivalent widths of H i () absorption from the inner (R < 160 kpc) circumgalactic medium (CGM) of 85 galaxies at z ∼ 0 with stellar masses M* ranging . Across three orders of magnitude in stellar mass, the CGM of present-day galaxies exhibits a very high covering fraction of cool hydrogen gas (fC = 87 ± 4%) indicating that the CGM is ubiquitous in modern, isolated galaxies. When H i is detected, its equivalent width declines with increasing radius regardless of the galaxy mass, but the scatter in this trend correlates closely with M*. Using the radial and stellar mass correlations, we construct a planar surface describing the cool CGM of modern galaxies: . The rms scatter around this bivariate relation is ∼0.2 dex. We interpret the explicit correlation between and M* to arise from the underlying dark matter halo mass (Mhalo), thereby suggesting a CGM fundamental plane between , R, and Mhalo. This correlation can be used to estimate the underlying dark matter halo mass from observations of saturated H i in the CGM of a modern galaxy. [ABSTRACT FROM AUTHOR]

We report a method of identifying candidate low-metallicity blue compact dwarf galaxies (BCDs) from the Sloan Digital Sky Survey (SDSS) imaging data, and present 3 m Lick Observatory and 10 m W.M. Keck Observatory optical spectroscopic observations of 94 new systems that have been discovered with this method. The candidate BCDs are selected from Data Release 12 (DR12) of SDSS on the basis of their photometric colors and morphologies. Using the Kast spectrometer on the 3 m telescope, we confirm that the candidate low-metallicity BCDs are emission-line galaxies, and we make metallicity estimates using the empirical R and S calibration methods. Follow-up observations on a subset of the lowest-metallicity systems are made at Keck using the Low Resolution Imaging Spectrometer, which allow for a direct measurement of the oxygen abundance. We determine that 45 of the reported BCDs are low-metallicity candidates with 12 + log(O/H) ≤ 7.65, including six systems which are either confirmed or projected to be among the lowest-metallicity galaxies known, at 1/30 of the solar oxygen abundance, or 12 + log(O/H) ∼ 7.20. [ABSTRACT FROM AUTHOR]

We examine the kinematics of the gas in the environments of galaxies hosting quasars at z ∼ 2. We employ 148 projected quasar pairs to study the circumgalactic gas of the foreground quasars in absorption. The sample selects foreground quasars with precise redshift measurements, using emission lines with precision ≲300 km s−1 and average offsets from the systemic redshift . We stack the background quasar spectra at the foreground quasar’s systemic redshift to study the mean absorption in C ii, C iv, and Mg ii. We find that the mean absorptions exhibit large velocity widths σv ≈ 300 km s−1. Further, the mean absorptions appear to be asymmetric about the systemic redshifts. The mean absorption centroids exhibit small redshift relative to the systemic δv ≈ +200 km s−1, with large intrinsic scatter in the centroid velocities of the individual absorption systems. We find the observed widths are consistent with gas in gravitational motion and Hubble flow. However, while the observation of large widths alone does not require galactic-scale outflows, the observed offsets suggest that the gas is on average outflowing from the galaxy. The observed offsets also suggest that the ionizing radiation from the foreground quasars is anisotropic and/or intermittent. [ABSTRACT FROM AUTHOR]

The He ii transverse proximity effect—enhanced He ii transmission in a background sightline caused by the ionizing radiation of a foreground quasar—offers a unique opportunity to probe the morphology of quasar-driven He ii reionization. We conduct a comprehensive spectroscopic survey to find quasars in the foreground of 22 background quasar sightlines with Hubble Space Telescope/COS He ii transmission spectra. With our two-tiered survey strategy, consisting of a deep pencil-beam survey and a shallow wide-field survey, we discover 131 new quasars, which we complement with known SDSS/BOSS quasars in our fields. Using a restricted sample of 66 foreground quasars with inferred He ii photoionization rates greater than the expected UV background at these redshifts () we perform the first statistical analysis of the He ii transverse proximity effect. Our results show qualitative evidence for a large object-to-object variance: among the four foreground quasars with the highest only one (previously known) quasar is associated with a significant He ii transmission spike. We perform a stacking analysis to average down these fluctuations, and detect an excess in the average He ii transmission near the foreground quasars at significance. This statistical evidence for the transverse proximity effect is corroborated by a clear dependence of the signal strength on . Our detection places a purely geometrical lower limit on the quasar lifetime of . Improved modeling would additionally constrain quasar obscuration and the mean free path of He ii-ionizing photons. [ABSTRACT FROM AUTHOR]

We present a deep search for 21 cm emission from the gaseous halo of Messier 31 as part of Project AMIGA, a large Hubble Space Telescope program to study the circumgalactic medium of the Andromeda galaxy. Our observations with the Robert C. Byrd Green Bank Telescope target sight lines to 48 background AGNs, more than half of which have been observed in the ultraviolet with the Cosmic Origins Spectrograph, with impact parameters (). We do not detect any 21 cm emission toward these AGNs to limits of cm−2 ( per 2 kpc-diameter beam). This column density corresponds to an optical depth of ∼2.5 at the Lyman limit; thus, our observations overlap with absorption line studies of Lyman limit systems at higher redshift. Our non-detections place a limit on the covering factor of such optically thick gas around M31 to (at 90% confidence) for . Although individual clouds have previously been found in the region between M31 and M33, the covering factor of strongly optically thick gas is quite small. Our upper limits on the covering factor are consistent with expectations from recent cosmological “zoom” simulations. Recent COS-Halos ultraviolet measurements of absorption about an ensemble of galaxies at show significantly higher covering factors within at the same , although the metal ion-to- ratios appear to be consistent with those seen in M31. [ABSTRACT FROM AUTHOR]

We present Atacama Large Millimeter/submillimeter Array (ALMA) 870 μm observations of 29 bright Herschel sources near high-redshift QSOs. The observations confirm that 20 of the Herschel sources are submillimeter-bright galaxies (SMGs) and identify 16 new SMG−QSO pairs that are useful to studies of the circumgalactic medium (CGM) of SMGs. Eight out of the 20 SMGs are blends of multiple 870 μm sources. The angular separations for six of the Herschel-QSO pairs are less than 10″, comparable to the sizes of the Herschel beam and the ALMA primary beam. We find that four of these six “pairs” are actually QSOs hosted by SMGs. No additional submillimeter companions are detected around these QSOs, and the rest-frame ultraviolet spectra of the QSOs show no evidence of significant reddening. Black hole accretion and star formation contribute almost equally in bolometric luminosity in these galaxies. The SMGs hosting QSOs show similar source sizes, dust surface densities, and star formation rate surface densities to those of other SMGs in the sample. We find that the black holes are growing ∼3× faster than the galaxies when compared to the present-day black hole/galaxy mass ratio, suggesting a QSO duty cycle of ≲30% in SMGs at . The remaining two Herschel-detected QSOs are undetected at 870 μm, but each has an SMG “companion” only 9″ and 12″ away (71 and 95 kpc at z = 3). They could be either merging or projected pairs. If the former, they would represent a rare class of “wet−dry” mergers. If the latter, the QSOs would, for the first time, probe the CGM of SMGs at impact parameters below 100 kpc. [ABSTRACT FROM AUTHOR]

We study winds in 12 X-ray AGN host galaxies at . We find, using the low-ionization Fe ii λ2586 absorption in the stacked spectra, that the probability distribution function (PDF) of the centroid-velocity shift in AGNs has 50th (median), 16th, and 84th percentiles of (−87, −251, +86) km s−1 respectively. The PDF of the velocity dispersion in AGNs has 50th (median), 84th, and 16th percentiles of (139, 253, 52) km s−1 respectively. The centroid velocity and the velocity dispersions are obtained from a two-component (ISM+wind) absorption-line model. The equivalent width PDF of the outflow in AGNs has 50th (median), 84th, and 16th percentiles of (0.4, 0.8, 0.1) Å. There is a strong ISM component in Fe ii absorption (with (1.2, 1.5, 0.8) Å, implying the presence of a substantial amount cold gas in the host galaxies. For comparison, star-forming and X-ray undetected galaxies at a similar redshift, matched roughly in stellar mass and galaxy inclination, have a centroid-velocity PDF with percentiles of (−74, −258, +90) km s−1, and a velocity-dispersion PDF with percentiles of (150, 259, 57) km s−1. Thus, winds in the AGN are similar to star formation-driven winds, and are too weak to escape and expel substantial cool gas from galaxies. Our sample doubles the previous sample of AGNs studied at and extends the analysis to . A joint reanalysis of the AGN sample and our sample yields consistent results to the measurements above. [ABSTRACT FROM AUTHOR]

We study quasar proximity zones in the redshift range by homogeneously analyzing 34 medium-resolution spectra, encompassing both archival and newly obtained data, and exploiting recently updated systemic redshift and magnitude measurements. Whereas previous studies found strong evolution of proximity zone sizes with redshift and argued that this provides evidence for a rapidly evolving intergalactic medium (IGM) neutral fraction during reionization, we measure a much shallower trend . We compare our measured proximity zone sizes to predictions from hydrodynamical simulations post-processed with one-dimensional radiative transfer and find good agreement between observations and theory, irrespective of the ionization state of the ambient IGM. This insensitivity to IGM ionization state has been previously noted, and results from the fact that the definition of proximity zone size as the first drop of the smoothed quasar spectrum below the 10% flux transmission level probes locations where the ionizing radiation from the quasar is an order of magnitude larger than the expected ultraviolet ionizing background that sets the neutral fraction of the IGM. Our analysis also uncovered three objects with exceptionally small proximity zones (two have proper Mpc), which constitute outliers from the observed distribution and are challenging to explain with our radiative transfer simulations. We consider various explanations for their origin, such as strong absorption line systems associated with the quasar or patchy reionization, but find that the most compelling scenario is that these quasars have been shining for ≲105 years. [ABSTRACT FROM AUTHOR]

Cosmological simulations suggest a strong correlation between high optical-depth Lyα absorbers, which arise from the intergalactic medium, and 3D mass overdensities on scales of 10–30 comoving Mpc. By examining the absorption spectra of ∼80,000 QSO sight lines over a volume of 0.1 Gpc3 in the Sloan Digital Sky Survey III (SDSS-III), we have identified an extreme overdensity, BOSS1441, which contains a rare group of strong Lyα absorbers at . This absorber group is associated with six QSOs at the same redshift on a 30 comoving Mpc scale. Using Mayall/MOSAIC narrowband and broadband imaging, we detect Lyα emitters (LAEs) down to and reveal a large-scale structure of LAEs in this field. Our follow-up LBT observations have spectroscopically confirmed 20 galaxies in the density peak. We show that BOSS1441 has an LAE overdensity of 10.8 ± 2.6 on a 15 comoving Mpc scale, which could collapse to a massive cluster with M ≳ 1015 at . This overdensity is among the most massive large-scale structures at discovered to date. [ABSTRACT FROM AUTHOR]

We analyze new far-ultraviolet spectra of 13 quasars from the COS-Halos survey that cover the H i Lyman limit of 14 circumgalactic medium (CGM) systems. These data yield precise estimates or more constraining limits than previous COS-Halos measurements on the H i column densities . We then apply a Monte-Carlo Markov chain approach on 32 systems from COS-Halos to estimate the metallicity of the cool ( K) CGM gas that gives rise to low-ionization state metal lines, under the assumption of photoionization equilibrium with the extragalactic UV background. The principle results are: (1) the CGM of field L* galaxies exhibits a declining H i surface density with impact parameter (at confidence), (2) the transmission of ionizing radiation through CGM gas alone is 70 ± 7%; (3) the metallicity distribution function of the cool CGM is unimodal with a median of and a 95% interval to ; the incidence of metal-poor () gas is low, implying any such gas discovered along quasar sightlines is typically unrelated to L* galaxies; (4) we find an unexpected increase in gas metallicity with declining (at confidence) and, therefore, also with increasing ; the high metallicity at large radii implies early enrichment; and (5) a non-parametric estimate of the cool CGM gas mass is , which together with new mass estimates for the hot CGM may resolve the galactic missing baryons problem. Future analyses of halo gas should focus on the underlying astrophysics governing the CGM, rather than processes that simply expel the medium from the halo. [ABSTRACT FROM AUTHOR]

Enormous Lyα nebulae (ELANe), unique tracers of galaxy density peaks, are predicted to lie at the nodes and intersections of cosmic filamentary structures. Previous successful searches for ELANe have focused on wide-field narrowband surveys or have targeted known sources such as ultraluminous quasi-stellar objects (QSOs) or radio galaxies. Utilizing groups of coherently strong Lyα absorptions, we have developed a new method to identify high-redshift galaxy overdensities and have identified an extremely massive overdensity, BOSS1441, at . In its density peak, we discover an ELAN that is associated with a relatively faint continuum. To date, this object has the highest diffuse Lyα nebular luminosity of erg s−1. Above the 2σ surface brightness limit of SB erg s−1 cm−2 arcsec−2, this nebula has an end-to-end spatial extent of 442 kpc. This radio-quiet source also has extended C iv and He ii emission on scales. Note that the Lyα, He ii, and C iv emissions all have double-peaked line profiles. Each velocity component has an FWHM of ≈700–1000 km s−1. We argue that this Lyα nebula could be powered by shocks due to an active galactic nucleus–driven outflow or photoionization by a strongly obscured source. [ABSTRACT FROM AUTHOR]

Modern cosmology predicts that a galaxy overdensity (e.g., protocluster) will be associated with a large intergalactic medium gas reservoir, which can be traced by Lyα forest absorption. We have undertaken a systematic study of the relation between Coherently Strong intergalactic Lyα Absorption systems (CoSLAs), which have the highest optical depth (τ) in the τ distribution, and mass overdensities on the scales of ∼10–20 h−1 comoving Mpc. On such large scales, our cosmological simulations show a strong correlation between the effective optical depth (τeff) of the CoSLAs and the three-dimensional mass overdensity. In spectra with moderate signal-to-noise ratio, however, the profiles of CoSLAs can be confused with individual high column density absorbers. For z > 2.6, where the corresponding Lyβ is redshifted to the optical, we have developed a selection technique to distinguish between these two alternatives. We have applied this technique to ∼6000 sight lines provided by Sloan Digital Sky Survey III quasar survey at z = 2.6–3.3 with a continuum-to-noise ratio greater than 8, and we present a sample of five CoSLA candidates with τeff on 15 h−1 Mpc greater than 4.5× the mean optical depth. At lower redshifts of z < 2.6, where the background quasar density is higher, the overdensity can be traced by intergalactic absorption groups using multiple sight lines with small angular separations. Our overdensity searches fully use the current and next generation of Lyα forest surveys, which cover a survey volume of >1 (h−1 Gpc)3. Systems traced by CoSLAs will yield a uniform sample of the most massive overdensities at z > 2 to provide stringent constraints to models of structure formation. [ABSTRACT FROM AUTHOR]

We present the first results from our KODIAQ Z survey aimed at determining the metallicity distribution and physical properties of the partial and full Lyman limit systems (pLLSs and LLSs; ), which are probed of the interface regions between the intergalactic medium (IGM) and galaxies. We study 31 H i-selected pLLSs and LLSs at observed with Keck/HIRES in absorption against background QSOs. We compare the column densities of metal ions to H i and use photoionization models to assess the metallicity. The metallicity distribution of the pLLSs/LLSs at is consistent with a unimodal distribution peaking at . The metallicity distribution of these absorbers therefore evolves markedly with z since at it is bimodal with peaks at and −0.3. There is a substantial fraction (25%–41%) of pLLSs/LLSs with metallicities well below those of damped Lyα absorbers (DLAs) at any studied z from to –4, implying reservoirs of metal-poor, cool, dense gas in the IGM/galaxy interface at all z. However, the gas probed by pLLSs and LLSs is rarely pristine, with a fraction of 3%–18% for pLLSs/LLSs with . We find C/α enhancement in several pLLSs and LLSs in the metallicity range , where C/α is 2–5 times larger than observed in Galactic metal-poor stars or high-redshift DLAs at similar metallicities. This is likely caused by preferential ejection of carbon from metal-poor galaxies into their surroundings. [ABSTRACT FROM AUTHOR]

The total contribution of diffuse halo gas to the galaxy baryon budget strongly depends on its dominant ionization state. In this paper, we address the physical conditions in the highly ionized circumgalactic medium (CGM) traced by absorption lines observed in COS-Halos spectra. We analyze the observed ionic column densities, absorption-line widths and relative velocities, along with the ratios of / for 39 fitted Voigt profile components of O vi. We compare these quantities with the predictions given by a wide range of ionization models. Photoionization models that include only extragalactic UV background radiation are ruled out; conservatively, the upper limits to / and measurements of imply unphysically large path lengths ≳100 kpc. Furthermore, very broad absorption (b > 40 km s−1) is a defining characteristic of the CGM of star-forming L* galaxies. We highlight two possible origins for the bulk of the observed : (1) highly structured gas clouds photoionized primarily by local high-energy sources or (2) gas radiatively cooling on large scales behind a supersonic wind. Approximately 20% of circumgalactic O vi does not align with any low-ionization state gas within ±50 km s−1 and is found only in halos with < 1012 . We suggest that this type of unmatched O vi absorption traces the hot corona itself at a characteristic temperature of K. We discuss the implications of these very distinct physical origins for the dynamical state, gas cooling rates, and total baryonic content of L* gaseous halos. [ABSTRACT FROM AUTHOR]

Using Hubble Space Telescope Cosmic Origins Spectrograph observations of 89 QSO sightlines through the Sloan Digital Sky Survey footprint, we study the relationships between C iv absorption systems and the properties of nearby galaxies, as well as the large-scale environment. To maintain sensitivity to very faint galaxies, we restrict our sample to , which defines a complete galaxy survey to or stellar mass . We report two principal findings. First, for galaxies with impact parameter , C iv detection strongly depends on the luminosity/stellar mass of the nearby galaxy. C iv is preferentially associated with galaxies with lower-mass galaxies rarely exhibit significant C iv absorption (covering fraction for 11 galaxies with ). Second, C iv detection within the population depends on environment. Using a fixed-aperture environmental density metric for galaxies with ρ < 160 kpc at , we find that (8/14) of galaxies in low-density regions (regions with fewer than seven galaxies within 1.5 Mpc) have affiliated C iv absorption; however, none (0/7) of the galaxies in denser regions show C iv. Similarly, the C iv detection rate is lower for galaxies residing in groups with dark matter halo masses of . In contrast to C iv, H i is pervasive in the circumgalactic medium without regard to mass or environment. These results indicate that C iv absorbers with trace the halos of galaxies but also reflect larger-scale environmental conditions. [ABSTRACT FROM AUTHOR]

We present an analysis of the metallicity distribution of the dense circumgalactic medium (CGM) of galaxies at 0.1 ≲ z ≲ 1.1 as probed by partial Lyman limit systems (pLLSs, 16.1 < log < 17.2) and LLSs (17.2 ≤ log < 17.7 in our sample). The new H i-selected sample, drawn from our Hubble Space Telescope COS G140L snapshot survey of 61 QSOs, has 20 pLLSs and 10 LLSs. Combined with our previous survey, we have a total of 44 pLLSs and 11 LLSs. We find that the metallicity distribution of the pLLSs is bimodal at z ≲ 1, with a minimum at [X/H] = −1. The low-metallicity peak comprises (57 ± 8)% of the pLLSs and is centered at [X/H] ≃ −1.87(1.3% solar metallicity), while the high-metallicity peak is centered at [X/H] ≃ −0.32 (48% solar metallicity). Although the sample of LLSs is still small, there is some evidence that the metallicity distributions of the LLSs and pLLSs are different, with a far lower fraction of very metal-poor ([X/H] < −1.4) LLSs than pLLSs. The fraction of LLSs with [X/H] < −1 is similar to that found in pLLSs (∼56%). However, higher H i column density absorbers (log > 19.0) show a much lower fraction of metal-poor gas; therefore, the metallicity distribution of gas in and around galaxies depends sensitively on NH i at z ≲ 1. We interpret the high-metallicity ([X/H] ≥ −1) pLLSs and LLSs as arising in outflows, recycling winds, and tidally stripped gas around galaxies. The low-metallicity pLLSs and LLSs imply that the CGM of z ≲ 1 galaxies hosts a substantial mass of cool, dense, low-metallicity gas that may ultimately accrete onto the galaxies. [ABSTRACT FROM AUTHOR]

A toy model is developed to understand how the spatial distribution of fluorescent emitters in the vicinity of bright quasars could be affected by the geometry of the quasar bi-conical radiation field and by its lifetime. The model is then applied to the distribution of high-equivalent-width Lyα emitters (with rest-frame equivalent widths above 100 Å, threshold used in, e.g., Trainor & Steidel) identified in a deep narrow-band 36 × 36 arcmin2 image centered on the luminous quasar Q0420–388. These emitters are found near the edge of the field and show some evidence of an azimuthal asymmetry on the sky of the type expected if the quasar is radiating in a bipolar cone. If these sources are being fluorescently illuminated by the quasar, the two most distant objects require a lifetime of at least 15 Myr for an opening angle of 60° or more, increasing to more than 40 Myr if the opening angle is reduced to a minimum of 30°. However, some other expected signatures of boosted fluorescence are not seen at the current survey limits, e.g., a fall off in Lyα brightness, or equivalent width, with distance. Furthermore, to have most of the Lyα emission of the two distant sources to be fluorescently boosted would require the quasar to have been significantly brighter in the past. This suggests that these particular sources may not be fluorescent, invalidating the above lifetime constraints. This would cast doubt on the use of this relatively low equivalent width threshold and thus also on the lifetime analysis in Trainor and Steidel. [ABSTRACT FROM AUTHOR]

We develop a new method to constrain the physical conditions in the cool (∼104 K) circumgalactic medium (CGM) from measurements of ionic column densities by assuming that the cool CGM spans a large range of gas densities and that small high-density clouds are hierarchically embedded in large low-density clouds. The new method combines the information available from different sightlines during the photoionization modeling, thus yielding tighter constraints on CGM properties compared to traditional methods that model each sightline individually. Applying this new technique to the COS Halos survey of low-redshift ∼L* galaxies, we find that we can reproduce all observed ion columns in all 44 galaxies in the sample, from the low ions to , with a single universal density structure for the cool CGM. The gas densities span the range ( is the cosmic mean), while the physical size of individual clouds scales as ∼ρ−1, from ≈35 kpc for the low-density clouds to ≈6 pc for the highest-density low-ion clouds. The deduced cloud sizes are too small for this density structure to be driven by self-gravity; thus, its physical origin is unclear. The implied cool CGM mass within the virial radius is (1.3 ± 0.4) × 1010 (∼1% of the halo mass), distributed rather uniformly over the 4 decades in density. The mean cool gas density profile scales as , where R is the distance from the galaxy center. We construct a 3D model of the cool CGM based on our results, which we argue provides a benchmark for the CGM structure in hydrodynamic simulations. Our results can be tested by measuring the coherence scales of different ions. [ABSTRACT FROM AUTHOR]

In the context of the FLASHLIGHT survey, we obtained deep narrowband images of 15 z ∼ 2 quasars with the Gemini Multi-object Spectrograph on Gemini South in an effort to measure Lyα emission from circum- and intergalactic gas on scales of hundreds of kpc from the central quasar. We do not detect bright giant Lyα nebulae (SB ∼ 10−17 erg s−1 cm−2 arcsec−2 at distances >50 kpc) around any of our sources, although we routinely (≃47%) detect smaller-scale <50 kpc Lyα emission at this surface brightness level emerging from either the extended narrow emission line regions powered by the quasars or by star formation in their host galaxies. We stack our 15 deep images to study the average extended Lyα surface brightness profile around z ∼ 2 quasars, carefully PSF-subtracting the unresolved emission component and paying close attention to sources of systematic error. Our analysis, which achieves an unprecedented depth, reveals a surface brightness of SBLyα ∼ 10−19 erg s−1 cm−2 arcsec−2 at ∼200 kpc, with a 2.3σ detection of Lyα emission at SB erg s−1 cm−2 arcsec−2 within an annulus spanning 50 kpc < R < 500 kpc from the quasars. Assuming that this Lyα emission is powered by fluorescence from highly ionized gas illuminated by the bright central quasar, we deduce an average volume density of nH = 0.6 × 10−2 cm−3 on these large scales. Our results are in broad agreement with the densities suggested by cosmological hydrodynamical simulations of massive (M ≃ 1012.5M⊙) quasar hosts; however, they indicate that the typical quasars at these redshifts are surrounded by gas that is a factor of ∼100 times less dense than the (∼1 cm−3) gas responsible for the giant bright Lyα nebulae around quasars recently discovered by our group. [ABSTRACT FROM AUTHOR]

We have collected near-infrared to X-ray data of 20 multi-epoch heavily reddened SDSS quasars to investigate the physical mechanism of reddening. Of these, J2317+0005 is found to be a UV cutoff quasar. Its continuum, which usually appears normal, decreases by a factor 3.5 at 3000 Å, compared to its more typical bright state during an interval of 23 days. During this sudden continuum cut-off the broad emission line fluxes do not change, perhaps due to the large size of the broad-line region (BLR), r 23/(1+z) days. The UV continuum may have suffered a dramatic drop out. However, there are some difficulties with this explanation. Another possibility is that the intrinsic continuum did not change but was temporarily blocked out, at least toward our line of sight. As indicated by X-ray observations, the continuum rapidly recovers after 42 days. A comparison of the bright state and dim states would imply an eclipse by a dusty cloud with a reddening curve having a remarkably sharp rise shortward of 3500 Å. Under the assumption of being eclipsed by a Keplerian dusty cloud, we characterized the cloud size with our observations, however, which is a little smaller than the 3000 Å continuum-emitting size inferred from accretion disk models. Therefore, we speculate that this is due to a rapid outflow or inflow with a dusty cloud passing through our line of sight to the center. [ABSTRACT FROM AUTHOR]

We measure the effective optical depth of He ii Lyα absorption at in 17 UV-transmitting quasars observed with UV spectrographs on the Hubble Space Telescope. The median values increase gradually from 1.95 at to at , but with a strong sightline-to-sightline variance. Many comoving Mpc regions of the intergalactic medium (IGM) remain transmissive (), and the gradual trend with redshift appears consistent with density evolution of a fully reionized IGM. These modest optical depths imply average He ii fractions of and He ii ionizing photon mean free paths of comoving Mpc at , thus requiring that a substantial volume of the helium in the universe was already doubly ionized at early times; this stands in conflict with current models of He ii reionization driven by luminous quasars. Along 10 sightlines we measure the coeval H i Lyα effective optical depths, allowing us to study the density dependence of at . We establish that the dependence of on increasing is significantly shallower than expected from simple models of an IGM reionized in He ii. This requires higher He ii photoionization rates in overdense regions or underdense regions being not in photoionization equilibrium. Moreover, there are very large fluctuations in at all which greatly exceed the expectations from these simple models. These data present a distinct challenge to scenarios of He ii reionization—an IGM where He ii appears to be predominantly ionized at , and with a radiation field strength that may be correlated with the density field, but exhibits large fluctuations at all densities. [ABSTRACT FROM AUTHOR]

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 (). Our statistical sample includes 463 quasars with spectral coverage spanning a total redshift path or an absorption path . Within this survey path, we identify 4 DLAs defined as absorbers with H i column density cm−2, which implies an incidence per absorption length at a median survey path redshift of z = 0.623. While our estimate of is lower than earlier estimates at from H i 21 cm emission studies, the results are consistent within the measurement uncertainties. Our data set is too small to properly sample the frequency distribution function , but the observed distribution agrees with previous estimates at . Adopting the shape of , we infer an H i mass density at of . This is significantly lower than previous estimates from targeted DLA surveys with the HST, but consistent with results from low-z H i 21 cm observations, and suggests that the neutral gas density of the universe has been decreasing over the past 10 Gyr. [ABSTRACT FROM AUTHOR]

Using moderate-resolution optical spectra from 58 background Lyman-break galaxies and quasars at within a 11.′5 × 13.′5 area of the COSMOS field ( projected area density or mean transverse separation), we reconstruct a 3D tomographic map of the foreground Lyα forest absorption at 2.2 < z < 2.5 with an effective smoothing scale of comoving. Comparing with 61 coeval galaxies with spectroscopic redshifts in the same volume, we find that the galaxy positions are clearly biased toward regions with enhanced intergalactic medium (IGM) absorption in the tomographic map. We find an extended IGM overdensity with deep absorption troughs at z = 2.45 associated with a recently discovered galaxy protocluster at the same redshift. Based on simulations matched to our data, we estimate the enclosed dark matter mass within this IGM overdensity to be , and argue based on this mass and absorption strength that it will form at least one z ∼ 0 galaxy cluster with , although its elongated nature suggests that it will likely collapse into two separate clusters. We also point out a compact overdensity of six MOSDEF galaxies at z = 2.30 within a radius and Δz ∼ 0.006, which does not appear to have a large associated IGM overdensity. These results demonstrate the potential of Lyα forest tomography on larger volumes to study galaxy properties as a function of environment, as well as revealing the large-scale IGM overdensities associated with protoclusters or other features of large-scale structure. [ABSTRACT FROM AUTHOR]

To investigate the evolution of metal-enriched gas over recent cosmic epochs as well as to characterize the diffuse, ionized, metal-enriched circumgalactic medium, we have conducted a blind survey for C iv absorption systems in 89 QSO sightlines observed with the Hubble Space Telescope Cosmic Origins Spectrograph. We have identified 42 absorbers at z < 0.16, comprising the largest uniform blind sample size to date in this redshift range. Our measurements indicate an increasing C iv absorber number density per comoving path length (= 7.5 ± 1.1) and modestly increasing mass density relative to the critical density of the universe (ΩC iv = 10.0 ± 1.5 × 10−8) from z ∼ 1.5 to the present epoch, consistent with predictions from cosmological hydrodynamical simulations. Furthermore, the data support a functional form for the column density distribution function that deviates from a single power law, also consistent with independent theoretical predictions. As the data also probe heavy element ions in addition to C iv at the same redshifts, we identify, measure, and search for correlations between column densities of these species where components appear to be aligned in velocity. Among these ion–ion correlations, we find evidence for tight correlations between C ii and Si ii, C ii and Si iii, and C iv and Si iv, suggesting that these pairs of species arise in similar ionization conditions. However, the evidence for correlations decreases as the difference in ionization potential increases. Finally, when controlling for observational bias, we find only marginal evidence for a correlation (86.8% likelihood) between the Doppler line width b(C iv) and column density N(C iv). [ABSTRACT FROM AUTHOR]

The recent discovery by Cantalupo et al. of the largest (∼500 kpc) luminous (L ≃ 1.43 × 1045 erg s−1) Lyα nebula associated with the quasar UM287 (z = 2.279) poses a great challenge to our current understanding of the astrophysics of the halos hosting massive z ∼ 2 galaxies. Either an enormous reservoir of cool gas is required M ≃ 1012M⊙, exceeding the expected baryonic mass available, or one must invoke extreme gas clumping factors not present in high-resolution cosmological simulations. However, observations of Lyα emission alone cannot distinguish between these two scenarios. We have obtained the deepest ever spectroscopic integrations in the He ii λ1640 and C iv λ1549 emission lines with the goal of detecting extended line emission, but detect neither line to a 3σ limiting SB ≃ 10−18 erg s−1 cm−2 arcsec−2. We construct simple models of the expected emission spectrum in the highly probable scenario that the nebula is powered by photoionization from the central hyper-luminous quasar. The non-detection of He ii implies that the nebular emission arises from a mass Mc ≲ 6.4 × 1010M⊙ of cool gas on ∼200 kpc scales, distributed in a population of remarkably dense (nH ≳ 3 cm−3) and compact (R ≲ 20 pc) clouds, which would clearly be unresolved by current cosmological simulations. Given the large gas motions suggested by the Lyα line (v ≃ 500 km s−1), it is unclear how these clouds survive without being disrupted by hydrodynamic instabilities. Our work serves as a benchmark for future deep integrations with current and planned wide-field IFU spectrographs such as MUSE, KCWI, and KMOS. Our observations and models suggest that a ≃10 hr exposure would likely detect ∼10 rest-frame UV/optical emission lines, opening up the possibility of conducting detailed photoionization modeling to infer the physical state of gas in the circumgalactic medium. [ABSTRACT FROM AUTHOR]

We use spectroscopy of close pairs of quasars to study diffuse gas in the circumgalactic medium (CGM) surrounding a sample of 40 damped Lyα systems (DLAs). The primary sightline in each quasar pair probes an intervening DLA in the redshift range , such that the second quasar sightline then probes Lyα, C ii, Si ii, and C iv absorption in the CGM transverse to the DLA to projected distances kpc. Analysis of the Lyα profiles in these CGM sightlines constrains the covering fraction (fC) of optically thick H i (having column density NH i ) to be ≳30% within kpc. Strong Si ii absorption with equivalent width Å occurs with an incidence within kpc, indicating that Si ii absorption associated with DLAs probes material within a physical distance kpc. However, we find that strong C iv λ1548 absorption is ubiquitous in these environments ( within kpc), and in addition exhibits a high degree of kinematic coherence on scales up to ∼175 kpc. We infer that this high-ionization material arises predominantly in large, quiescent structures extending beyond the scale of the DLA host dark matter halos rather than in ongoing galactic winds. The Lyα equivalent width in the DLA-CGM is anticorrelated with at confidence, suggesting that DLAs arise close to the centers of their host halos rather than on their outskirts. Finally, the average Lyα, and C ii and C iv equivalent widths measured as a function of are consistent with those measured around z ∼ 2 Lyman break galaxies. Assuming that DLAs trace a galaxy population at lower masses and luminosities, this finding implies that the absorption strength of cool circumgalactic material has a weak dependence on dark matter halo mass at . [ABSTRACT FROM AUTHOR]

We conduct a deep narrowband imaging survey of 13 Lyα blobs (LABs) located in the SSA22 proto-cluster at z ∼ 3.1 in the C iv and He ii emission lines in an effort to constrain the physical process powering the Lyα emission in LABs. Our observations probe down to unprecedented surface brightness (SB) limits of (2.1–3.4) × 10−18 erg s−1 cm−2 arcsec−2 per 1 arcsec2 aperture (5σ) for the He ii λ1640 and C iv λ1549 lines, respectively. We do not detect extended He ii and C iv emission in any of the LABs, placing strong upper limits on the He ii/Lyα and C iv/Lyα line ratios, of 0.11 and 0.16, for the brightest two LABs in the field. We conduct detailed photoionization modeling of the expected line ratios and find that, although our data constitute the deepest ever observations of these lines, they are still not deep enough to rule out a scenario where the Lyα emission is powered by the ionizing radiation from an obscured active galactic nucleus. Our models can accommodate He ii/Lyα and C iv/Lyα ratios as low as ≃0.05 and ≃0.07, respectively, implying that one needs to reach SB as low as (1–1.5) × 10−18 erg s−1 cm−2 arcsec−2 (at 5σ) in order to rule out a photoionization scenario. These depths will be achievable with the new generation of image-slicing integral field units such as the Multi Unit Spectroscopic Explorer (MUSE) on VLT and the Keck Cosmic Web Imager (KCWI). We also model the expected He ii/Lyα and C iv/Lyα in a different scenario, where Lyα emission is powered by shocks generated in a large-scale superwind, but find that our observational constraints can only be met for shock velocities vs ≳ 250 km s−1, which appear to be in conflict with recent observations of quiescent kinematics in LABs. [ABSTRACT FROM AUTHOR]