Foreground Quasar Research Articles

DPQP: A Detection Pipeline for Quasar Pair Candidates Based on QSO Photometric Images and Spectra

Quasars have an important role in the studies of galaxy evolution and star formation. The rare close projection of two quasars in the sky allows us to study the environment and matter exchange around the foreground quasar (QSOfg) and the background quasar (QSObg). This paper proposes a pipeline DPQP for quasar pair (QP) candidates’ detection based on photometric images and the corresponding spectra. The pipeline consists of three main parts: a target source detector, a regressor, and a discriminator. In the first part, the target source detection network–YOLOv4 (TSD-YOLOv4) and the target source classification network (TSCNet) are used in sequence to detect quasars in SDSS photometric images. In the second part, a depth feature extraction network of quasar images (DE-QNet) is constructed to estimate the redshifts of quasars from photometric images. In the third part, a quasar pair score (Q-Score) metric is proposed based on the spectral analysis. The larger the Q-Score, the greater the possibility of two pairs being a quasar pair. The experimental results show that between redshift 1.0 and 4.0, the MAE of DE-QNet is 0.316, which is 16.1% lower than the existing method. Samples with |Δz| < 0.15 account for 77.1% of the test dataset. A new table with 1025 QP candidates is provided by traversing 50,000 SDSS photometric images.

The Study of the Circumgalactic Medium with Quasar Pairs

We have collected 10,025 foreground–background quasar pairs with projected distances d p < 500 kpc from the large quasar catalog of the Sloan Digital Sky Survey DR16Q. We investigate the properties of the Mg ii absorption lines with W r > 0.15 Å around foreground quasars, including both the line of sight (LOS; of foreground quasars) and transverse (TRA; perpendicular to the LOS) absorptions. Both the equivalent width (the correlation coefficient ρ = −0.915 and the probability P < 10−4 of no correlation) and incident rate (ρ = −0.964 and P < 10−6) of TRA Mg ii absorption lines are obviously anticorrelated with projected distance. The incident rate of TRA Mg ii absorption lines is obviously (>4σ) greater than that of LOS Mg ii absorption lines at projected distances d p < 200 kpc, while the TRA and LOS Mg ii both have similar (<3σ) incident rates at scales d p > 200 kpc. The anisotropic radiation from quasars would be the most possible interpretation for the anisotropic absorption around quasars. This could also indicate that the quasar radiation is not obviously impacting the gas halos of quasars at scales d p > 200 kpc.

CDFS-6664: A Candidate of Lyman-continuum Emission at z ∼ 3.8 Detected by the Hubble Deep UV Legacy Survey

Abstract We report the detection of Lyman continuum (LyC) emission from the galaxy, CDFS-6664, at z = 3.797 in a sample of Lyman break galaxies with detected [O iii] emission lines. The LyC emission is detected with a significance ∼5σ in the F336W band of the Hubble Deep UV Legacy Survey, corresponding to the 650–770 Å rest frame. The light centroid of the LyC emission is offset from the galaxy center by about 0.″2 (1.4 pkpc). The Hubble deep images at longer wavelengths show that the emission is unlikely provided by low-redshift interlopers. The photometric and spectroscopic data show that the possible contribution of an active galactic nucleus is quite low. Fitting the spectral energy distribution of this source to stellar population synthesis models, we find that the galaxy is young (∼50 Myr) and actively forming stars with a rate of 52.1 ± 4.9 M ⊙ yr−1. The significant star formation and the spatially offset LyC emission support a scenario where the ionizing photons escape from the low-density cavities in the ISM excavated by massive young stars. From the nebular model, we estimate the escape fraction of LyC photons to be 38% ± 7% and the corresponding intergalactic medium (IGM) transmission to be 60%, which deviates more than 3σ from the average transmission. The unusually high IGM transmission of LyC photons in CDFS-6664 may be related to a foreground type-2 quasar, CDF-202, at z = 3.7, with a projected separation of 1.′2 only. The quasar may have photoevaporated optically thick absorbers and enhance the transmission on the sightline of CDFS-6664.

The circum-galactic medium of quasars: transverse and line-of-sight absorptions

Quasar projected pairs (QPPs) can be used for investigating the circumgalactic medium of quasars through the study of intervening absorption lines in the spectrum of the background quasar (QSOB) that are at the same redshift of the foreground quasar (QSOF). Here we report on optical spectroscopy, gathered at Gran Telescopio Canarias, of 14 QPPs. In 7 cases we find MgII absorption lines associated with the foreground quasar. Only for two cases line-of-sight absorptions (LOS) are revealed. These new observations complement our previous study performed on other 30 QPPs. A brief discussion of the properties of the intervening absorption lines associated with the foreground quasar for the full dataset is reported.

Simba: the average properties of the circumgalactic medium of 2 ≤ z ≤ 3 quasars are determined primarily by stellar feedback

ABSTRACT We use the simba cosmological hydrodynamic simulation suite to explore the impact of feedback on the circumgalactic medium (CGM) and intergalactic medium (IGM) around 2 ≤ z ≤ 3 quasars. We identify quasars in simba as the most rapidly accreting black holes, and show that they are well matched in bolometric luminosity and correlation strength to real quasars. We extract Lyα absorption in spectra passing at different transverse distances ($10 \, \mathrm{kpc} \lesssim b \lesssim 10 \, \rm Mpc$) around those quasars, and compare to observations of the mean Lyα absorption profile. The observations are well reproduced, except within $100 \, \, \rm kpc$ from the foreground quasar, where simba overproduces absorption; this could potentially be mitigated by including ionization from the quasar itself. By comparing runs with different feedback modules activated, we find that (mechanical) AGN feedback has little impact on the surrounding CGM even around these most highly luminous black holes, while stellar feedback has a significant impact. By further investigating thermodynamic and kinematic properties of CGM gas, we find that stellar feedback, and not AGN feedback, is the primary physical driver in determining the average properties of the CGM around z ∼ 2–3 quasars. We also compare our results with previous works, and find that simba predicts much more absorption within $100 \, \rm kpc$ than the nyx and illustris simulations, showing that the Lyα absorption profile can be a powerful constraint on simulations. Instruments such as VLT-MUSE and upcoming surveys (e.g. WEAVE and DESI) promise to further improve such constraints.

Probing the Environment of High-z Quasars Using the Proximity Effect in Projected Quasar Pairs

Abstract We have used spectra of 181 projected quasar pairs at separations ≤1.5′ from the Sloan Digital Sky Survey Data Release 12 in the redshift range of 2.5–3.5 to probe the proximity regions of the foreground quasars. We study the proximity effect both in the longitudinal and in the transverse directions, by carrying out a comparison of the Lyα absorption lines originating from the vicinity of quasars to those originating from the general intergalactic medium at the same redshift. We found an enhancement in the transmitted flux within 4 Mpc to the quasar in the longitudinal direction. However, the trend is found to be reversed in the transverse direction. In the longitudinal direction, we derived an excess overdensity profile showing an excess up to r ≤ 5 Mpc after correcting for the quasar’s ionization, taking into account the effect of low spectral resolution. This excess overdensity profile matches with the average overdensity profile in the transverse direction without applying any correction for the effect of the quasar’s ionization. Among various possible interpretations, we found that the anisotropic obscuration of the quasar’s ionization seems to be the most probable explanation. This is also supported by the fact that all of our foreground quasars happen to be type 1 AGNs. Finally, we constrain the average quasar’s illumination along the transverse direction as compared to that along the longitudinal direction to be ≤27% (3σ confidence level).

Three- and two-point spatial correlations of intergalactic medium at z ∼ 2 using projected quasar triplets

ABSTRACT We present analysis of two- and three-point correlation functions of Lyα forest (at 2 ≤ z ≤ 2.5) using X-Shooter spectra of three background quasar triplets probing transverse separations of 0.5–1.6 pMpc. We present statistics based on transmitted flux and clouds identified using Voigt profile fitting. We show that the observed two-, three-point correlation functions and reduced three-point correlation (i.e. Q) are well reproduced by our simulations. We assign probabilities for realizing all the observed correlation properties simultaneously using our simulations. Our simulations suggest an increase in correlation amplitudes and Q with increasing NH i. We roughly see this trend in the observations too. We identify a concurrent gap of 17Å (i.e. 14.2 h−1 cMpc, one of the longest reported) wide along one of the triplets. Such gap is realized only in 14.2 per cent of our simulated sightlines and most of the time belongs to a void in the matter distribution. In the second triplet, we detect Damped Lyα systems (DLAs) along all three sightlines (with spatial separations 0.64 to 1.6 pMpc) within a narrow redshift interval (i.e. Δz = 0.088). Detection of a foreground quasar (∼1 pMpc from the triplet sightlines) and excess partial Lyman Limit systems (with NH i in the range $10^{16-17.2} \, \mathrm{cm}^{-2}$) around these DLAs suggest that we may be probing a large overdense region. We also report positive C iv–C iv correlations up to ∼500 km s−1 only in the longitudinal direction. Additionally, we conclude a positive C iv–Lyα correlations for higher NH i thresholds (${\gt}10^{15}\, \mathrm{cm}^{-2}$) up to a scale of ∼1000 km s−1 both in transverse and longitudinal directions.

Mapping Quasar Light Echoes in 3D with Lyα Forest Tomography

Abstract The intense radiation emitted by luminous quasars dramatically alters the ionization state of their surrounding IGM. This so-called proximity effect extends out to tens of Mpc, and manifests as large coherent regions of enhanced Lyα (Lyα) forest transmission in absorption spectra of background sightlines. Here, we present a novel method based on Lyα forest tomography, which is capable of mapping these quasar “light echoes” in three dimensions. Using a dense grid (10–100) of faint ( ) background galaxies as absorption probes, one can measure the ionization state of the IGM in the vicinity of a foreground quasar, yielding detailed information about the quasar’s radiative history and emission geometry. An end-to-end analysis—combining cosmological hydrodynamical simulations post-processed with a quasar emission model, realistic estimates of galaxy number densities, and instrument + telescope throughput—is conducted to explore the feasibility of detecting quasar light echoes. We present a new, fully Bayesian statistical method that allows one to reconstruct quasar light echoes from thousands of individual low-S/N transmission measurements. Armed with this tool, we undertake an exhaustive parameter study and show that light echoes can be convincingly detected for luminous (M 1450 &lt; −27.5 mag, corresponding to m 1450 &lt; 18.4 mag at ) quasars at redshifts 3 &lt; z QSO &lt; 5, and that a relative precision better than 20% on the quasar age can be achieved for individual objects in the expected range of ages between 1 and 100 Myr. The observational requirements are relatively modest: moderate-resolution (R ≳ 750), multiobject spectroscopy at a low signal-to-noise ratio (S/N &gt; 5) is sufficient, requiring three-hour integrations using existing instruments on 8 m class telescopes.

Modeling the He ii Transverse Proximity Effect: Constraints on Quasar Lifetime and Obscuration

Abstract The He ii transverse proximity effect—enhanced He ii Lyα transmission in a background sightline caused by the ionizing radiation of a foreground quasar—offers a unique opportunity to probe the emission properties of quasars, in particular the emission geometry (obscuration, beaming) and the quasar lifetime. Building on the foreground quasar survey published in Schmidt et al., we present a detailed model of the He ii transverse proximity effect, specifically designed to include light travel time effects, finite quasar ages, and quasar obscuration. We post-process outputs from a cosmological hydrodynamical simulation with a fluctuating He ii ultraviolet background model, with the added effect of the radiation from a single bright foreground quasar. We vary the age t age and obscured sky fractions Ωobsc of the foreground quasar, and explore the resulting effect on the He ii transverse proximity effect signal. Fluctuations in intergalactic medium density and the ultraviolet background, as well as the unknown orientation of the foreground quasar, result in a large variance of the He ii Lyα transmission along the background sightline. We develop a fully Bayesian statistical formalism to compare far-ultraviolet He ii Lyα transmission spectra of the background quasars to our models, and extract joint constraints on t age and Ωobsc for the six Schmidt et al. foreground quasars with the highest implied He ii photoionization rates. Our analysis suggests a bimodal distribution of quasar emission properties, whereby one foreground quasar, associated with a strong He ii transmission spike, is relatively old (22 Myr) and unobscured ( Ω obsc &lt; 35 % ), whereas three others are either younger than 10 Myr or highly obscured ( Ω obsc &gt; 70 % ).

Quasars Probing Quasars. X. The Quasar Pair Spectral Database

The rare close projection of two quasars on the sky provides the opportunity to study the host galaxy environment of a foreground quasar in absorption against the continuum emission of a background quasar. For over a decade the "Quasars probing quasars" series has utilized this technique to further the understanding of galaxy formation and evolution in the presence of a quasar at z>2, resolving scales as small as a galactic disc and from bound gas in the circumgalactic medium to the diffuse environs of intergalactic space. Presented here, is the public release of the quasar pair spectral database utilized in these studies. In addition to projected pairs at z>2, the database also includes quasar pair members at z<2, gravitational lens candidates and quasars closely separated in redshift that are useful for small-scale clustering studies. In total the database catalogs 5627 distinct objects, with 4083 lying within 5' of at least one other source. A spectral library contains 3582 optical and near-infrared spectra for 3028 of the cataloged sources. As well as reporting on 54 newly discovered quasar pairs, we outline the key contributions made by this series over the last ten years, summarize the imaging and spectroscopic data used for target selection, discuss the target selection methodologies, describe the database content and explore some avenues for future work. Full documentation for the spectral database, including download instructions, are supplied at the following URL http://specdb.readthedocs.io/en/latest/

A Fundamental Test for Galaxy Formation Models: Matching the Lyman-α Absorption Profiles of Galactic Halos Over Three Decades in Distance

Abstract Galaxy formation depends critically on the physical state of gas in the circumgalactic medium (CGM) and its interface with the intergalactic medium (IGM), determined by the complex interplay between inflow from the IGM and outflows from supernovae and/or AGN feedback. The average Lyα absorption profile around galactic halos represents a powerful tool to probe their gaseous environments. We compare predictions from Illustris and Nyx hydrodynamical simulations with the observed absorption around foreground quasars, damped Lyα systems, and Lyman-break galaxies. We show how large-scale BOSS and small-scale quasar pair measurements can be combined to precisely constrain the absorption profile over three decades in transverse distance . Far from galaxies, , the simulations converge to the same profile and provide a reasonable match to the observations. This asymptotic agreement arises because the ΛCDM model successfully describes the ambient IGM and represents a critical advantage of studying the mean absorption profile. However, significant differences between the simulations, and between simulations and observations, are present on scales , illustrating the challenges of accurately modeling and resolving galaxy formation physics. It is noteworthy that these differences are observed as far out as , indicating that the “sphere of influence” of galaxies could extend to approximately ∼7 times the halo virial radius. Current observations are very precise on these scales and can thus strongly discriminate between different galaxy formation models. We demonstrate that the Lyα absorption profile is primarily sensitive to the underlying temperature–density relationship of diffuse gas around galaxies, and argue that it thus provides a fundamental test of galaxy formation models.

Quasars Probing Quasars. IX. The Kinematics of the Circumgalactic Medium Surrounding z ∼ 2 Quasars

Abstract 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 ≲ ∣ 100 km s − 1 ∣ . 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 σ v ≈ 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.

Transverse and Longitudinal proximity effect

With close pairs (1.5arcmin) of quasars (QSOs), absorption in the spectra of a background quasar in the vicinity of a foreground quasar can be used to study the environment of the latter quasar at kpc-Mpc scales. For this we used a sample of 205 quasar pairs from the Sloan Digital Sky-Survey Data Release 12 (SDSS DR12) in the redshift range of 2.5 to 3.5 by studying their H I Ly-α absorption. We study the environment of QSOs both in the longitudinal as well as in the transverse direction by carrying out a statistical comparison of the Ly-α absorption lines in the quasar vicinity to that of the absorption lines caused by the inter-galactic medium (IGM). This comparison was done with IGM, matched in absorption redshift and signal-to-noise ratio (SNR) to that of the proximity region. In contrast to the measurements along the line-of-sight, the regions transverse to the quasars exhibit enhanced H I Ly-α absorption. This discrepancy can either be interpreted as due to an anisotropic emission from the quasars or as a consequence of their finite lifetime.

Circum-Galactic Medium in the Halo of Quasars

The properties of circum-galactic gas in the halo of quasar host galaxies are investigated analyzing Mg II 2800 and C IV 1540 absorption-line systems along the line of sight close to quasars. We used optical spectroscopy of closely aligned pairs of quasars (projected distance $\leq$ 200 kpc, but at very different redshift) obtained at the VLT and Gran Telescopio Canarias to investigate the distribution of the absorbing gas for a sample of quasars at z$\sim$1. Absorption systems of EW $\geq$ 0.3 $\rm{\AA}$ associated with the foreground quasars are revealed up to 200 kpc from the centre of the host galaxy, showing that the structure of the absorbing gas is patchy with a covering fraction quickly decreasing beyond 100 kpc. In this contribution we use optical and near-IR images obtained at VLT to investigate the relations between the properties of the circum-galactic medium of the host galaxies and of the large scale galaxy environments of the foreground quasars.

Statistical Detection of the He ii Transverse Proximity Effect: Evidence for Sustained Quasar Activity for &gt;25 Million Years

The reionization of helium at z~3 is the final phase transition of the intergalactic medium and supposed to be driven purely by quasars. The HeII transverse proximity effect - enhanced HeII transmission in a background sightline caused by the ionizing radiation of a foreground quasar - therefore offers a unique opportunity to probe the morphology of HeII reionization and to investigate the emission properties of quasars, e.g. ionizing emissivity, lifetime and beaming geometry. We use the most-recent HST/COS far-UV dataset of 22 HeII absorption spectra and conduct our own dedicated optical spectroscopic survey to find foreground quasars around these HeII sightlines. Based on a set of 66 foreground quasars, we perform the first statistical analysis of the HeII transverse proximity effect. Despite a large object-to-object variance, our stacking analysis reveals an excess in the average HeII transmission near the foreground quasars at 3 sigma significance. This statistical evidence for the transverse proximity effect is corroborated by a clear dependence of the signal strength on the inferred HeII ionization rate at the background sightline. Our detection places, based on the transverse light crossing time, a geometrical limit on the quasar lifetime of t_Q > 25 Myr. This evidence for sustained activity of luminous quasars is relevant for the morphology of HI and HeII reionization and helps to constrain AGN triggering mechanisms, accretion physics and models of black hole mass assembly. We show how future modeling of the transverse proximity effect can additionally constrain quasar emission geometries and e.g. clarify if the large observed object-to-object variance can be explained by current models of quasar obscuration.

Statistical Detection of the He ii Transverse Proximity Effect: Evidence for Sustained Quasar Activity for &gt;25 Million Years

Abstract 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.

QUASARS PROBING QUASARS. VIII. THE PHYSICAL PROPERTIES OF THE COOL CIRCUMGALACTIC MEDIUM SURROUNDING z ∼ 2–3 MASSIVE GALAXIES HOSTING QUASARS

ABSTRACT We characterize the physical properties of the cool T ∼ 104 K circumgalactic medium (CGM) surrounding z ∼ 2–3 quasar host galaxies, which are predicted to evolve into present-day massive ellipticals. Using a statistical sample of 14 quasar pairs with projected separation &lt;300 kpc and spectra of high dispersion and high signal-to-noise ratio, we find extreme kinematics with low metal ion lines typically spanning ≈500 km s−1, exceeding any previously studied galactic population. The CGM is significantly enriched, even beyond the virial radius, with a median metallicity [M/H] ≈ −0.6. The α/Fe abundance ratio is enhanced, suggesting that halo gas is primarily enriched by core-collapse supernovae. The projected cool gas mass within the virial radius is estimated to be 1.9 × 1011 M ⊙ (R ⊥/160 kpc)2, accounting for ≈1/3 of the baryonic budget of the galaxy halo. The ionization state of CGM gas increases with projected distance from the foreground quasars, contrary to expectation if the quasar dominates the ionizing radiation flux. However, we also found peculiarities not exhibited in the CGM of other galaxy populations. In one absorption system, we may be detecting unresolved fluorescent Lyα emission, and another system shows strong N v lines. Taken together, these anomalies suggest that transverse sightlines are—at least in some cases—possibly illuminated. We also discovered a peculiar case where detection of the C ii fine-structure line implies an electron density &gt;100 cm−3 and sub-parsec-scale gas clumps.

The extent of the Mg ii absorbing circumgalactic medium of quasars★

We investigate the extent and the properties of the Mg ii cool, low-density absorbing gas located in the halo and in the circumgalactic environment of quasars, using a sample of 31 projected quasar pairs with impact parameter pd < 200 kpc in the redshift range 0.5 ≲ z ≲ 1.6. In the transverse direction, we detect 18 Mg ii absorbers associated with the foreground quasars, while no absorption system originated by the gas surrounding the quasar itself is found along the line of sight. This suggests that the quasar emission induces an anisotropy in the absorbing gas distribution. Our observations indicate that the covering fraction (fC) of Mg ii absorption systems with rest-frame equivalent width Wr(λ2796) > 0.3 Å ranges from fC ∼ 1.0 at pd ≲ 65 kpc to fC ∼ 0.2 at pd ≳ 150 kpc, and appears to be higher than that for galaxies. Our findings support a scenario where the luminosity/mass of the host galaxies affect the extent and the richness of the absorbing Mg ii circumgalactic medium.

QUASARS PROBING QUASARS. VI. EXCESS H I ABSORPTION WITHIN ONE PROPER Mpc OFz∼ 2 QUASARS

With close pairs of quasars at different redshifts, a background quasar sightline can be used to study a foreground quasar's environment in absorption. We use a sample of 650 projected quasar pairs to study the HI Lya absorption transverse to luminous, z~2 quasars at proper separations of 30kpc < R < 1Mpc. In contrast to measurements along the line-of-sight, regions transverse to quasars exhibit enhanced HI Lya absorption and a larger variance than the ambient intergalactic medium, with increasing absorption and variance toward smaller scales. Analysis of composite spectra reveals excess absorption characterized by a Lya equivalent width profile W = 2.3A (R/100kpc)^-0.46. We also observe a high (~60%) covering factor of strong, optically thick HI absorbers (HI column log NHI > 17.3) at separations R<200kpc, which decreases to ~20% at R~1Mpc, but still represents a significant excess over the cosmic average. This excess of optically thick absorption can be described by a quasar-absorber cross-correlation function xi_QA(r) = (r/r_0)^gamma with a large correlation length r_0 = 12.5+2.7-1.4 Mpc/h (comoving) and gamma = 1.68+0.14-0.30. The HI absorption measured around quasars exceeds that of any previously studied population, consistent with quasars being hosted by massive dark matter halos Mhalo~10^12.5 Msun at z~2.5. The environments of these massive halos are highly biased towards producing optically thick gas, and may even dominate the cosmic abundance of Lyman limit systems and hence the intergalactic opacity to ionizing photons at z~2.5. The anisotropic absorption around quasars implies the transverse direction is much less likely to be illuminated by ionizing radiation than the line-of-sight, which we interpret in terms of the same obscuration effects frequently invoked in unified models of active galactic nuclei.

The Circumgalactic Medium of Quasars

AbstractI will argue that observations of the diffuse gas in the outskirts of quasar host galaxies, or the so called circumgalactic medium, are essential for understanding how luminous quasars evolve in a cosmological context. Such observations also provide a fruitful comparison to theory, because hydrodynamics at moderate overdensities is much easier to simulate than the complicated processes which trigger quasar activity. A novel technique will be introduced, whereby a foreground quasar can be studied in absorption against a background quasar, resolving scales as small as 30 kpc. This experiment reveals a rich absorption spectrum which contains a wealth of information about the physical conditions of diffuse gas around quasars. Hydrodynamical simulations of the massive dark matter halos which host luminous quasars under predict the amount of cool gas observed in quasar environs by a large factor, challenging our understanding of how massive galaxies form. I will also discuss a very sensitive search for Ly-alpha emission from the same gas which we study in absorption.