Lyman Limit Systems Research Articles

The COS CGM Compendium. V. The Dichotomy of O vi Associated with Low- and High-metallicity Cool Gas at z < 1

We analyze the O vi content and kinematics for 126 H i-selected absorbers at 0.14 ≲ z ≲ 0.73 for which the metallicities of their cool photoionized phase have been determined. We separate the absorbers into 100 strong Lyα forest systems (SLFSs with 15 ≲ log N(H i) < 16.2) and 26 partial Lyman Limit systems (pLLSs with 16.2 ≤ log N(H i) ≤ 17.2). The sample is drawn from the COS CGM Compendium (CCC) and has O vi coverage in signal-to-noise ratio ≥ 8 Hubble Space Telescope/COS G130M/G160M QSO spectra, yielding a 2σ completeness level of logN (O vi) ≥ 13.6. The O vi detection rates differ substantially between low-metallicity (LM; [X/H] ≤ −1.4) and high-metallicity (HM; [X/H] > −1.4) SLFSs, with 20% and 60% detection rates, respectively. The O vi detection frequency for the HM and LM pLLSs is, however, similar at ∼60%. The SLFSs and pLLSs without detected O vi are consistent with the absorbing gas being in a single phase, while those with O vi trace multiphase gas. We show that the O vi velocity widths and column densities have different distributions in LM and HM gas. We find a strong correlation between O vi column density and metallicity. The strongest ( logN (O vi) ≳ 14) and broadest O vi absorbers are nearly always associated with HM absorbers, while weaker O vi absorbers are found in both LM and HM absorbers. From comparisons with galaxy-selected and blind O vi surveys, we conclude absorbers with logN (O vi) ≳ 14 most likely arise in the CGM of star-forming galaxies. Absorbers with weak O vi likely trace the extended CGM or intergalactic medium, while those without O vi likely originate in the intergalactic medium.

Lyman Limit System with O vi in the Circumgalactic Environment of a Pair of Galaxies

We report on the analysis of a multiphase Lyman limit system (LLS) at z = 0.39047 identified toward the background quasar FBQS J0209–0438. The O VI doublet lines associated with this absorber have a different profile from the low-ionization metals and H i. Ly α has a very broad H i (b ≈ 150 km s−1) component well-aligned with one of the O VI components. The Doppler b-parameters for the broad H i and O VI indicate gas with T = (0.8 − 2.0) × 106 K and a total hydrogen column density that is an order of magnitude larger than the cooler phase of gas responsible for the LLS. Observations by the Very Large Telescope MUSE show two moderately star-forming galaxies within ρ ≲ 105 kpc and ∣Δv∣ ≲ 130 km s−1 of the absorber, one of them a dwarf galaxy (M * ≈ 106 M ⊙) overlapping with the quasar point-spread function, and the other a larger galaxy (R 1/2 ≈ 4 kpc) with M * ≈ 3 × 1010 M ⊙ and M h ≈ 7 × 1011 M ⊙, and the dwarf galaxy within its virial radius. Although the absorption is aligned with the extended major axis of the larger galaxy, the line-of-sight velocity of the absorbing gas is inconsistent with corotating accretion. The metallicity inferred for the LLS is lower than the gas phase [O/H] of the two galaxies. The mixture of cool and warm/hot gas phases for the absorbing gas and its proximity and orientation to the galaxy pair points to the LLS being a high-velocity gas in the combined halo environment of both galaxies.

Helium reionization from empirical quasar luminosity functions before and after JWST

ABSTRACT Recently, models of the quasar luminosity function (QLF) rooted on large observational compilations have been produced that, unlike their predecessors, feature a smooth evolution with time. This bypasses the need to assume an ionizing emissivity evolution when simulating helium reionization with observations-based QLF, thus yielding more robust constraints. We combine one such QLF with a cosmological hydrodynamical simulation and 3D multifrequency radiative transfer. The simulated reionization history is consistently delayed in comparison to most other models in the literature. The predicted intergalactic medium temperature is larger than the observed one at $z \lesssim 3$. Through forward modelling of the He ii Lyman $\alpha$ forest, we show that our model produces an extended helium reionization and successfully matches the bulk of the observed effective optical depth distribution, although it overionizes the Universe at $z\lesssim 2.8$ as the effect of small-scale Lyman Limit Systems not being resolved. We thoroughly characterize transmission regions and dark gaps in He ii Lyman $\alpha$ forest sightlines. We quantify their sensitivity to the helium reionization, opening a new avenue for further observational studies of this epoch. Finally, we explore the implications for helium reionization of the large number of active galactic nuclei revealed at $z\gtrsim 5$ by JWST. We find that such modifications do not affect any observable at $z\le 4$, except in our most extreme model, indicating that the observed abundance of high-z AGNs does not bear consequences for helium reionization.

Impact of Helium ii Resonant Absorption on the Ultraviolet Background Modeled in Three Dimensions

We implement a treatment of helium ii absorption and reemission into the Technicolor Dawn cosmological simulations to study its impact on the metagalactic ultraviolet background (UVB) in three dimensions. By comparing simulations with and without He ii reprocessing, we show that it weakens the mean UVB by ∼3 dex from z = 10–5 between 3.5 and 4 Ryd, where the He ii Lyman series resonance occurs. In overdense regions, the overall UVB amplitude is higher, and the impact of He ii reprocessing is weaker, qualitatively indicating an early start to He ii reionization near galaxies. Comparing our simulations to two popular one-dimensional UV models, we find good agreement up to 3 Ryd at z = 5. At higher energies, our simulation shows significantly greater He ii absorption because it accounts for He ii arising both in diffuse regions and in Lyman limit systems. By contrast, the comparison models account only for He ii in Lyman limit systems, which are subdominant prior to the completion of He ii reionization. The H i and He ii reionization histories are nearly unaffected by He ii reprocessing, although the cosmic star formation rate density is altered by up to 4%. The cosmic mass density of C iv is reduced by ∼2 dex when He ii is accounted for, while Si iv, C ii, Mg ii, Si ii, and O i are unaffected.

GHOST commissioning science results III: Characterizing an iron-poor damped Lyman α system

Abstract The Gemini High-resolution Optical SpecTrograph (GHOST) is a new echelle spectrograph available on the Gemini-South telescope as of Semester 2024A. We present the first high resolution spectrum of the quasar J1449−1227 (redshift zem = 3.27) using data taken during the commissioning of GHOST. The observed quasar hosts an intervening iron-poor ([Fe/H] =−2.5) damped Lyman α (DLA) system at redshift z = 2.904. Taking advantage of the high spectral resolving power of GHOST (R ≈ 55000), we are able to accurately model the metal absorption lines of the metal-poor DLA and find a supersolar [Si/Fe], suggesting the DLA gas is in an early stage of chemical enrichment. Using simple ionization models, we find that the large range in the C iv/Si iv column density ratio of individual components within the DLA’s high ionization absorption profile can be reproduced by several metal-poor Lyman limit systems surrounding the low-ionization gas of the DLA. It is possible that this metal-poor DLA resides within a complex system of metal-poor galaxies or filaments with inflowing gas. The high spectral resolution, wavelength coverage and sensitivity of GHOST makes it an ideal spectrograph for characterizing the chemistry and kinematics of quasar absorption lines.

Cosmic Reionization on Computers: Statistics, Physical Properties, and Environments of Lyman Limit Systems at z ∼ 6

Lyman limit systems (LLSs) are dense hydrogen clouds with high enough H i column densities to absorb Lyman continuum photons emitted from distant quasars. Their high column densities imply an origin in dense environments; however, the statistics and distribution of LLSs at high redshifts still remain uncertain. In this paper, we use self-consistent radiative transfer cosmological simulations from the Cosmic Reionization on Computers (CROC) project to study the physical properties of LLSs at the tail end of cosmic reionization at z ∼ 6. We generate 3000 synthetic quasar sight lines to obtain a large number of LLS samples in the simulations. In addition, with the high physical fidelity and resolution of CROC, we are able to quantify the association between these LLS samples and nearby galaxies. Our results show that the fraction of LLSs spatially associated with nearby galaxies increases with H i column density. Moreover, we find that LLSs that are not near any galaxy typically reside in filamentary structures connecting neighboring galaxies in the intergalactic medium (IGM). This quantification of the distribution and association of LLSs to large-scale structure informs our understanding of the IGM–galaxy connection during the “Epoch of Reionization,” and provides a theoretical basis for interpreting future observations.

A halo model for cosmological Lyman-limit systems

ABSTRACT We present an analytical model for cosmological Lyman-limit systems (lls) that successfully reproduces the observed evolution of the mean free path (λeff) of ionizing photons. The evolution of the co-moving mean free path is predominantly a consequence of the changing meta galactic photoionization rate and the increase with cosmic time of the minimum mass below which haloes lose their gas due to photoheating. In the model, Lyman-limit absorption is caused by highly ionized gas in the outskirt of dark matter haloes. We exploit the association with haloes to compute statistical properties of λeff and of the bias, b, of lls. The latter increases from b ∼ 1.5 → 2.6 from redshifts z = 2 → 6. Combined with the rapid increase with redshift of the bias of the haloes that host a quasar, the model predicts a rapid drop in the value of λeff when measured in quasar spectra from z = 5 → 6, whereas the actual value of λeff falls more smoothly. We derive an expression for the effective optical depth due to Lyman limit absorption as a function of wavelength and show that it depends sensitively on the poorly constrained number density of lls as a function of column density. The optical depth drops below unity for all wavelengths below a redshift of ∼2.5 which is therefore the epoch when the Universe first became transparent to ionizing photons.

The Bimodal Absorption System Imaging Campaign (BASIC). I. A Dual Population of Low-metallicity Absorbers at z < 1

The bimodal absorption system imaging campaign (BASIC) aims to characterize the galaxy environments of a sample of 36 H i-selected partial Lyman limit systems (pLLSs) and Lyman limit systems (LLSs) in 23 QSO fields at z ≲ 1. These pLLSs/LLSs provide a unique sample of absorbers with unbiased and well-constrained metallicities, allowing us to explore the origins of metal-rich and low-metallicity circumgalactic medium (CGM) at z < 1. Here we present Keck/KCWI and Very Large Telescope/MUSE observations of 11 of these QSO fields (19 pLLSs) that we combine with Hubble Space Telescope/Advanced Camera for Surveys imaging to identify and characterize the absorber-associated galaxies at 0.16 ≲ z ≲ 0.84. We find 23 unique absorber-associated galaxies, with an average of one associated galaxy per absorber. For seven absorbers, all with <10% solar metallicities, we find no associated galaxies with within ρ/R vir and ∣Δv∣/v esc ≤ 1.5 with respect to the absorber. We do not find any strong correlations between the metallicities or H i column densities of the gas and most of the galaxy properties, except for the stellar mass of the galaxies: the low-metallicity ([X/H] ≤ −1.4) systems have a probability of for having a host galaxy with within ρ/R vir ≤ 1.5, while the higher metallicity absorbers have a probability of . This implies metal-enriched pLLSs/LLSs at z < 1 are typically associated with the CGM of galaxies with , whereas low-metallicity pLLSs/LLSs are found in more diverse locations, with one population arising in the CGM of galaxies and another more broadly distributed in overdense regions of the universe. Using absorbers not associated with galaxies, we estimate the unweighted geometric mean metallicity of the intergalactic medium to be [X/H] ≲ −2.1 at z < 1, which is lower than previously estimated.

Weighing cosmic structures with clusters of galaxies and the intergalactic medium

ABSTRACT We present an analysis aimed at combining cosmological constraints from number counts of galaxy clusters identified through the Sunyaev–Zeldovich effect, obtained with the South Pole Telescope (SPT), and from Lyman α spectra obtained with the MIKE/HIRES and X-shooter spectrographs. The SPT cluster analysis relies on mass calibration based on weak lensing measurements, while the Lyman α analysis is built over mock spectra extracted from hydrodynamical simulations. The resulting constraints exhibit a tension (∼3.3σ) between the low σ8 values preferred by the low-redshift cluster data, $\sigma _8=0.74 ^{+0.03}_{-0.04}$, and the higher one preferred by the high-redshift Lyman α data, $\sigma _8=0.91 ^{+0.03}_{-0.03}$. We present a detailed analysis to understand the origin of this tension and to establish whether it arises from systematic uncertainties related to the assumptions underlying the analyses of cluster counts and/or Lyman α forest. We found this tension to be robust with respect to the choice of modelling of the IGM, even when including possible systematics from unaccounted sub-Damped Lyman α (DLA) and Lyman-limit systems (LLS) in the Lyman α data. We conclude that to solve this tension would require a large bias on the cluster mass estimate, or large unaccounted errors on the Lyman α mean fluxes. Our results have important implications for future analyses based on cluster number counts from future large photometric surveys (e.g. Euclid and LSST) and on larger samples of high-redshift quasar spectra (e.g. DESI and WEAVE surveys). If confirmed at the much higher statistical significance reachable by such surveys, this tension could represent a significant challenge for the standard ΛCDM paradigm.

Discovery of three new near-pristine absorption clouds at z = 2.6–4.4

ABSTRACT We report the discovery of three new ‘near-pristine’ Lyman limit systems (LLSs), with metallicities ≈1/1000 solar, at redshifts 2.6, 3.8, and 4.0, with a targeted survey at the Keck Observatory. High-resolution echelle spectra of eight candidates yielded precise column densities of hydrogen and weak, but clearly detected, metal lines in seven LLSs; we previously reported the one remaining, apparently metal-free LLS, to have metallicity &amp;lt;1/10 000 solar. Robust photoionization modelling provides metallicities [Si/H] = −3.05 to −2.94, with 0.26 dex uncertainties (95 per cent confidence) for three LLSs, and $\textrm {[Si/H]} \gtrsim -2.5$ for the remaining four. Previous simulations suggest that near-pristine LLSs could be the remnants of PopIII supernovae, so comparing their detailed metal abundances with nucleosynthetic yields from supernovae models is an important goal. Unfortunately, at most two different metals were detected in each new system, despite their neutral hydrogen column densities ($10^{19.2}\textrm {--}10^{19.4}\, \textrm {cm}^{-2}$) being two orders of magnitude larger than the two previous, serendipitously discovered near-pristine LLSs. Nevertheless, the success of this first targeted survey for near-pristine systems demonstrates the prospect that a much larger, future survey could identify clear observational signatures of PopIII stars. With a well-understood selection function, such a survey would also yield the number density of near-pristine absorbers that, via comparison to future simulations, could reveal the origin(s) of these rare systems.

Damped Lyα Absorbers in Star-forming Galaxies at z < 0.15 Detected with the Hubble Space Telescope and Implications for Galactic Evolution

We report Hubble Space Telescope Cosmic Origins Spectrograph spectroscopy of 10 quasars with foreground star-forming galaxies at 0.02 < z < 0.14 within impact parameters of ∼1–7 kpc. We detect damped/sub-damped Lyα (DLA/sub-DLA) absorption in 100% of cases where no higher-redshift Lyman-limit systems extinguish the flux at the expected wavelength of Lyα absorption, obtaining the largest targeted sample of DLA/sub-DLAs in low-redshift galaxies. We present absorption measurements of neutral hydrogen and metals. Additionally, we present Green Bank Telescope 21 cm emission measurements for five of the galaxies (including two detections). Combining our sample with the literature, we construct a sample of 117 galaxies associated with DLA/sub-DLAs spanning 0 < z < 4.4, and examine trends between gas and stellar properties, and with redshift. The H i column density is anticorrelated with impact parameter and stellar mass. More massive galaxies appear to have gas-rich regions out to larger distances. The specific star formation rate (sSFR) of absorbing galaxies increases with redshift and decreases with M*, consistent with evolution of the star formation main sequence (SFMS). However, ∼20% of absorbing galaxies lie below the SFMS, indicating that some DLA/sub-DLAs trace galaxies with longer-than-typical gas-depletion timescales. Most DLA/sub-DLA galaxies with 21 cm emission have higher H i masses than typical galaxies with comparable M*. High M HI/M* ratios and high sSFRs in DLA/sub-DLA galaxies with M* < 109 M ⊙ suggest these galaxies may be gas-rich because of recent gas accretion rather than inefficient star formation. Our study demonstrates the power of absorption and emission studies of DLA/sub-DLA galaxies for extending galactic evolution studies to previously under-explored regimes of low M* and low SFR.

Effects of Small-scale Absorption Systems on Neutral Islands during the Late Epoch of Reionization

Abstract The reionization process is expected to be prolonged by the small-scale absorbers (SSAs) of ionizing photons, which have been seen as Lyman-limit systems in quasar absorption line observations. We use a set of semi-numerical simulations to investigate the effects of absorption systems on the reionization process, especially their impacts on the neutral islands during the late epoch of reionization (EoR). Three models are studied, i.e., the extreme case of no-SSA model with a high level of ionizing background, the moderate-SSA model with a relatively high level of ionizing background, and the dense-SSA model with a low level of ionizing background. We find that while the characteristic scale of neutral regions decreases during the early and middle stages of reionization, it stays nearly unchanged at about 10 co-moving Mpc during the late stage for the no-SSA and moderate-SSA models. However, in the case of weak ionizing background in the dense-SSA model, the characteristic island scale shows obvious evolution, as large islands break into many small ones that are slowly ionized. The evolutionary behavior of neutral islands during the late EoR thus provides us with a novel way to constrain the abundance of SSAs. We discuss the 21 cm observation with the upcoming Square Kilometre Array. The different models can be distinguished by either the 21 cm imaging or the 21 cm power spectrum measurements.

Thermal Instabilities and Shattering in the High-redshift WHIM: Convergence Criteria and Implications for Low-metallicity Strong H i Absorbers

Abstract Using a novel suite of cosmological simulations zooming in on a megaparsec-scale intergalactic sheet (pancake) at z ∼ (3–5), we conduct an in-depth study of the thermal properties and H i content of the warm-hot intergalactic medium (WHIM) at those redshifts. The simulations span nearly three orders of magnitude in gas cell mass, ∼(7.7 × 106–1.5 × 104)M ⊙, one of the highest-resolution simulations of such a large patch of the intergalactic medium (IGM) to date. At z ∼ 5, a strong accretion shock develops around the pancake. Gas in the postshock region proceeds to cool rapidly, triggering thermal instabilities and generating a multiphase medium. We find the mass, morphology, and distribution of H i in the WHIM to all be unconverged, even at our highest resolution. Interestingly, the lack of convergence is more severe for the less-dense, metal-poor intrapancake medium (IPM) in between filaments and far outside galaxies. With increased resolution, the IPM develops a shattered structure with most of the H i in kiloparsec-scale clouds. From our lowest-to-highest resolution, the covering fraction of metal-poor (Z &lt; 10−3 Z ⊙) Lyman-limit systems (N H I &gt; 1017.2cm−2) in the z ∼ 4 IPM increases from ∼(3–15)%, while that of metal-poor damped Lyα absorbers (N H I &gt; 1020cm−2) increases from ∼(0.2–0.6)%, with no sign of convergence. We find that a necessary condition for the formation of a multiphase shattered structure is resolving the cooling length, l cool = c s t cool, at T ∼ 105 K. If this is unresolved, gas “piles up” at T ≲ 105 K and further cooling becomes very inefficient. We conclude that state-of-the-art cosmological simulations are still unable to resolve the multiphase structure of the WHIM, with potentially far-reaching implications.

The Cosmic Ultraviolet Baryon Survey (CUBS) – IV. The complex multiphase circumgalactic medium as revealed by partial Lyman limit systems

ABSTRACT We present a detailed study of two partial Lyman limit systems (pLLSs) of neutral hydrogen column density $N_\mathrm{H\, I}\approx (1-3)\times 10^{16}\, \mathrm{cm}^{-2}$ discovered at $z$ = 0.5 in the Cosmic Ultraviolet Baryon Survey (CUBS). Available far-ultraviolet spectra from the Hubble Space Telescope Cosmic Origins Spectrograph and optical echelle spectra from MIKE on the Magellan Telescopes enable a comprehensive ionization analysis of diffuse circumgalactic gas based on resolved kinematics and abundance ratios of atomic species spanning five different ionization stages. These data provide unambiguous evidence of kinematically aligned multiphase gas that masquerades as a single-phase structure and can only be resolved by simultaneous accounting of the full range of observed ionic species. Both systems are resolved into multiple components with inferred α-element abundance varying from [α/H] ≈−0.8 to near solar and densities spanning over two decades from log nH/cm−3 ≈ −2.2 to &amp;lt;−4.3. Available deep galaxy survey data from the CUBS program taken with VLT/MUSE, Magellan/LDSS3-C and Magellan/IMACS reveal that the $z$ = 0.47 system is located 55 kpc from a star-forming galaxy with prominent Balmer absorption of stellar mass ${{M_{\rm star}}}\approx 2\times 10^{10}\, {{M_{\odot}}}$, while the $z$ = 0.54 system resides in an overdense environment of 11 galaxies within 750 kpc in projected distance, with the most massive being a luminous red galaxy of ${{M_{\rm star}}}\approx 2\times 10^{11}\, {{M_{\odot}}}$ at 375 kpc. The study of these two pLLSs adds to an emerging picture of the complex, multiphase circumgalactic gas that varies in chemical abundances and density on small spatial scales in diverse galaxy environments. The inhomogeneous nature of metal enrichment and density revealed in observations must be taken into account in theoretical models of diffuse halo gas.

Discovery of a Fast Iron Low-ionization Outflow in the Early Evolution of the Nearby Tidal Disruption Event AT 2019qiz

Abstract We report the results of ultraviolet (UV) and optical photometric and spectroscopic analysis of the tidal disruption event (TDE) AT 2019qiz. Our follow-up observations started &lt;10 days after the source began to brighten in the optical and lasted for a period of six months. Our late-time host-dominated spectrum indicates that the host galaxy likely harbors a weak active galactic nucleus. The initial Hubble Space Telescope (HST) spectrum of AT 2019qiz exhibits an iron and low-ionization broad absorption line (FeLoBAL) system that is seen for the first time in a TDE. This spectrum also bears a striking resemblance to that of Gaia16apd, a superluminous supernova. Our observations provide insights into the outflow properties in TDEs and show evidence for a connection between TDEs and engine-powered supernovae at early phases, as originally suggested by Metzger &amp; Stone. In a time frame of 50 days, the UV spectra of AT 2019qiz started to resemble those of previous TDEs with only high-ionization broad absorption lines. The change in UV spectral signatures is accompanied by a decrease in the outflow velocity, which began at 15,000 km s−1 and decelerated to ∼10,000 km s−1. A similar evolution in the Hα emission-line width further supports the speculation that the broad Balmer emission lines are formed in TDE outflows. In addition, we detect narrow absorption features on top of the FeLoBAL signatures in the early HST UV spectrum of AT 2019qiz. The measured H i column density corresponds to a Lyman-limit system, whereas the metal absorption lines (such as N v, C iv, Fe ii, and Mg ii) are likely probing the circumnuclear gas and interstellar medium in the host galaxy.

The Cosmic Ultraviolet Baryon Survey (CUBS) – III. Physical properties and elemental abundances of Lyman-limit systems at z &amp;lt; 1

ABSTRACT We present a systematic investigation of physical conditions and elemental abundances in four optically thick Lyman-limit systems (LLSs) at z = 0.36–0.6 discovered within the Cosmic Ultraviolet Baryon Survey (CUBS). Because intervening LLSs at z &amp;lt; 1 suppress far-UV (ultraviolet) light from background QSOs, an unbiased search of these absorbers requires a near-UV-selected QSO sample, as achieved by CUBS. CUBS LLSs exhibit multicomponent kinematic structure and a complex mix of multiphase gas, with associated metal transitions from multiple ionization states such as C ii, C iii, N iii, Mg ii, Si ii, Si iii, O ii, O iii, O vi, and Fe ii absorption that span several hundred km s−1 in line-of-sight velocity. Specifically, higher column density components (log N(H i)/cm−2≳ 16) in all four absorbers comprise dynamically cool gas with $\langle T \rangle =(2\pm 1) \times 10^4\,$K and modest non-thermal broadening of $\langle b_\mathrm{nt} \rangle =5\pm 3\,$km s−1. The high quality of the QSO absorption spectra allows us to infer the physical conditions of the gas, using a detailed ionization modelling that takes into account the resolved component structures of H i and metal transitions. The range of inferred gas densities indicates that these absorbers consist of spatially compact clouds with a median line-of-sight thickness of $160^{+140}_{-50}$ pc. While obtaining robust metallicity constraints for the low density, highly ionized phase remains challenging due to the uncertain $N\mathrm{(H\, {\small I})}$, we demonstrate that the cool-phase gas in LLSs has a median metallicity of $\mathrm{[\alpha /H]_{1/2}}=-0.7^{+0.1}_{-0.2}$, with a 16–84 percentile range of [α/H] = (−1.3, −0.1). Furthermore, the wide range of inferred elemental abundance ratios ([C/α], [N/α], and [Fe/α]) indicate a diversity of chemical enrichment histories. Combining the absorption data with deep galaxy survey data characterizing the galaxy environment of these absorbers, we discuss the physical connection between star-forming regions in galaxies and diffuse gas associated with optically thick absorption systems in the z &amp;lt; 1 circumgalactic medium.

A partial Lyman limit system tracing intragroup gas at z ≈ 0.8 towards HE 1003 + 0149

We present an analysis of the galaxy environment and physical properties of a partial Lyman limit system at z = 0.83718 with HI and metal line components closely separated in redshift space ($|\Delta v| \approx 400$ km/s) towards the background quasar HE1003+0149. The HST/COS far-ultraviolet spectrum provides coverage of lines of oxygen ions from OI to OV. Comparison of observed spectral lines with synthetic profiles generated from Bayesian ionization modeling reveals the presence of two distinct gas phases in the absorbing medium. The low-ionization phase of the absorber has sub-solar metallicities (1/10-th solar) with indications of [C/O] < 0 in each of the components. The OIV and OV trace a more diffuse higher-ionization medium with predicted HI column densities that are $\approx 2$ dex lower. The quasar field observed with VLT/MUSE reveals three dwarf galaxies with stellar masses of $M^* \sim 10^{8} - 10^{9}$ M$_\odot$, and with star formation rates of $\approx 0.5 - 1$ M$_\odot$ yr$^{-1}$, at projected separations of $\rho/R_{\mathrm{vir}} \approx 1.8 - 3.0$ from the absorber. Over a wider field with projected proper separation of $\leq 5$ Mpc and radial velocity offset of $|\Delta v| \leq 1000$ km/s from the absorber, 21 more galaxies are identified in the $VLT$/VIMOS and Magellan deep galaxy redshift surveys, with 8 of them within $1$ Mpc and $500$ km/s, consistent with the line of sight penetrating a group of galaxies. The absorber presumably traces multiple phases of cool ($T \sim 10^4$ K) photoionized intragroup medium. The inferred [C/O] < 0 hints at preferential enrichment from core-collapse supernovae, with such gas displaced from one or more of the nearby galaxies, and confined to the group medium.

The Distribution and Evolution of Quasar Proximity Zone Sizes

Abstract In this paper, we study the sizes of quasar proximity zones with synthetic quasar absorption spectra obtained by postprocessing a Cosmic Reionization On Computers (CROC) simulation. CROC simulations have both relatively large box sizes and high spatial resolution, allowing us to resolve Lyman limit systems (LLSs), which are crucial for modeling the quasar absorption spectra. We find that before reionization, most quasar proximity zone sizes grow steadily for ∼10 Myr, while after reionization, they grow rapidly but only for ∼0.1 Myr. We also find a slow growth of R obs with decreasing turn-on redshift. In addition, we find that ∼1%–2% of old quasars (30 Myr old) display extremely small proximity zone sizes (&lt;1 proper Mpc), the vast majority of which are due to the occurrence of a damped Lyα absorber (DLA) or an LLS along the line of sight. These DLAs and LLSs are contaminated with metal, which offers a way to distinguish them from the normal proximity zones of young quasars.

Constraining the cosmic UV background at z &amp;gt; 3 with MUSE Lyman-α emission observations

ABSTRACT The intensity of the Cosmic UV background (UVB), coming from all sources of ionizing photons such as star-forming galaxies and quasars, determines the thermal evolution and ionization state of the intergalactic medium (IGM) and is, therefore, a critical ingredient for models of cosmic structure formation. Most of the previous estimates are based on the comparison between observed and simulated Lyman-α forest. We present the results of an independent method to constrain the product of the UVB photoionization rate and the covering fraction of Lyman limit systems (LLSs) by searching for the fluorescent Lyman-α emission produced by self-shielded clouds. Because the expected surface brightness is well below current sensitivity limits for direct imaging, we developed a new method based on 3D stacking of the IGM around Lyman-α emitting galaxies (LAEs) between 2.9 &amp;lt; z &amp;lt; 6.6 using deep MUSE observations. Combining our results with covering fractions of LLSs obtained from mock cubes extracted from the EAGLE simulation, we obtain new and independent constraints on the UVB at z &amp;gt; 3 that are consistent with previous measurements, with a preference for relatively low UVB intensities at z = 3, and which suggest a non-monotonic decrease of ΓH i with increasing redshift between 3 &amp;lt; z &amp;lt; 5. This could suggest a possible tension between some UVB models and current observations which however require deeper and wider observations in Lyman-α emission and absorption to be confirmed. Assuming instead a value of UVB from current models, our results constrain the covering fraction of LLSs at 3 &amp;lt; z &amp;lt; 4.5 to be less than 25 per cent within 150 kpc from LAEs.

Pair lines of sight observations of multiphase gas bearing O vi in a galaxy environment

ABSTRACT Using HST/COS spectra of the twin quasar lines of sight Q 0107–025A &amp; Q 0107–025B, we report on the physical properties, chemical abundances, and transverse sizes of a multiphase medium in a galaxy field at z = 0.399. The angular separation between the quasars corresponds to a physical separation of 520 kpc at the absorber redshift. The absorber towards Q 0107–025B is a partial Lyman limit system (pLLS) with $\log N({\mathrm{H}}{\small I})/\hbox{cm$^{-2}$}\approx 16.8$. The H i column density in the absorber along the other sightline is ≈ 2 orders of magnitude lower. The O vi along both sightlines have comparable column densities and broad b-values (b &amp;gt; 30 km s−1) whereas the low ionization lines are considerably narrower. The low ionization gas is inconsistent with the O vi when modelled assuming photoionization from the same phase. In both lines of sight, O vi and the broad H i coinciding, are best explained through collisional ionization in a cooling plasma with solar metallicity. Ionization models infer 1/10th solar metallicity for the pLLS and solar metallicity for the lower column density absorber along the other sightline. Within ± 250 km s−1 and 2 Mpc of projected distance from the sightlines 12 galaxies are identified, of which five are within 500 kpc. The twin sightlines are at normalized impact parameters of ρ ∼ 1.1Rvir, and ρ ∼ 0.8Rvir from a M* ∼ 1010.7 M⊙, L ∼ 0.07L*, and star formation rate (SFR) &amp;lt; 0.1 M⊙ yr−1 galaxy, potentially probing its CGM (circumgalactic medium). The next closest in normalized separation are a dwarf galaxy with M* ∼ 108.7 M⊙, and SFR ∼ 0.06 M⊙ yr−1, and an intermediate mass galaxy with M* ∼ 1010.0 M⊙, and SFR ∼ 3 M⊙ yr−1. Along both sightlines, O vi could be either tracing narrow transition temperature zones at the interface of low ionization gas and the hot halo of nearest galaxy, or a more spread-out warm component that could be gas bound to the circumgalactic halo or the intragroup medium. The latter scenarios lead to a warm gas mass limit of M ≳ 4.5 × 109 M⊙.