Results for "Joseph F. Hennawi"

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Self-Consistent Modeling of Reionization in Cosmological Hydrodynamical SimulationsJul 14 2016The ultraviolet background (UVB) emitted by quasars and galaxies governs the ionization and thermal state of the intergalactic medium (IGM), regulates the formation of high-redshift galaxies, and is thus a key quantity for modeling cosmic reionization. ... More
Observational Constraints on the Self Interacting Dark Matter Scenario and the Growth of Supermassive Black HolesAug 12 2001Dec 27 2001We consider the consequences of SIDM for a velocity dependent cross section per unit mass. Accretion of SIDM onto seed black holes can produce supermassive black holes that are too large for certain combinations of parameters,which is used to obtain a ... More
Shear Selected Cluster Cosmology I: Tomography and Optimal FilteringApr 19 2004Jan 31 2005We study the potential of weak lensing surveys to detect clusters of galaxies, using a fast Particle Mesh cosmological N-body simulation algorithm specifically tailored to investigate the statistics of these mass-selected clusters. In particular, we explore ... More
Quasars Probing Quasars IV: Joint Constraints on the Circumgalactic Medium from Absorption and EmissionMar 11 2013We have constructed a sample of 29 close projected quasar pairs where the background quasar spectrum reveals absorption from optically thick HI gas associated with the foreground quasar. These unique sightlines allow us to study the quasar circumgalactic ... More
Statistics of Quasars Multiply Imaged by Galaxy ClustersJun 08 2005We compute the expected number of quasars multiply imaged by cluster size dark halos for current wide field quasar surveys by carrying out a large ensemble of ray tracing simulations through clusters from a cosmological N-body simulation of the LCDM cosmology. ... More
Revealing the Warm and Hot Halo Baryons via Thomson Scattering of Quasar LightMar 05 2018Jun 23 2018The baryonic content and physical properties of the warm and hot ($10^5\lesssim T\lesssim 10^7$ K) phases of the circumgalactic medium (CGM) are poorly constrained, owing to the lack of observables probing the requisite range of temperature, spatial scale, ... More
Evolution of the AGN UV luminosity function from redshift 7.5Jul 25 2018Determinations of the UV luminosity function of AGN at high redshifts are important for constraining the AGN contribution to reionization and understanding the growth of supermassive black holes. Recent inferences of the luminosity function suffer from ... More
A New Method to Directly Measure the Jeans Scale of the Intergalactic Medium Using Close Quasar PairsMay 01 2013Although the baryons in the intergalactic medium (IGM) trace dark matter on Mpc scales, small-scale (~100 kpc) fluctuations are suppressed by pressure support, analogous to the classical Jeans argument. This Jeans filtering scale has fundamental cosmological ... More
Characterizing the Cluster Lens PopulationJun 08 2005We present a detailed investigation into which properties of CDM halos make them effective strong gravitational lenses. Strong lensing cross sections of 878 clusters from an N-body simulation are measured by ray tracing through 13,594 unique projections. ... More
Metal-Poor, Cool Gas in the Circumgalactic Medium of a z = 2.4 Star-Forming Galaxy: Direct Evidence for Cold Accretion?Jul 24 2013Aug 22 2013In our current galaxy formation paradigm, high-redshift galaxies are predominantly fuelled by accretion of cool, metal-poor gas from the intergalactic medium. Hydrodynamical simulations predict that this material should be observable in absorption against ... More
A Substantial Mass of Cool, Metal-Enriched Gas Surrounding the Progenitors of Modern-Day EllipticalsNov 26 2012The hosts of luminous z~2 quasars evolve into today's massive elliptical galaxies. Current theories predict that the circum-galactic medium (CGM) of these massive, dark-matter halos (M~10^12.5 Msun) should be dominated by a T~10^7 K virialized plasma. ... More
New Constraints on IGM Thermal Evolution from the Lyα Forest Power SpectrumAug 13 2018Dec 20 2018We determine the thermal evolution of the intergalactic medium (IGM) over $3\, \mathrm{Gyr}$ of cosmic time $1.8<z<5.4$ by comparing measurements of the Ly{\alpha} forest power spectrum to a suite of $\sim70$ hydrodynamical simulations. We conduct Bayesian ... More
Quasars Probing Quasars VII. The Pinnacle of the Cool Circumgalactic Medium Surrounds Massive z~2 GalaxiesSep 22 2014We survey the incidence and absorption strength of the metal-line transitions CII 1334 and CIV from the circumgalactic medium (CGM) surrounding z~2 quasars, which act as signposts for massive dark matter halos M_halo~10^12.5 Msun. On scales of the virial ... More
Quasars Probing Quasars IX. The Kinematics of the Circumgalactic Medium Surrounding z ~ 2 QuasarsMay 09 2017Mar 22 2018We examine the kinematics of the gas in the environments of galaxies hosting quasars at $z\sim2$. We employ 148 projected quasar pairs to study the circumgalactic gas of the foreground quasars in absorption. The sample selects foreground quasars with ... More
Modeling the Lyman-alpha Forest in Collisionless SimulationsFeb 25 2016Cosmological hydrodynamic simulations can accurately predict the properties of the intergalactic medium (IGM), but only under the condition of retaining high spatial resolution necessary to resolve density fluctuations in the IGM. This resolution constraint ... More
Evidence for Short $\sim 1$ Myr Lifetimes from the He II Proximity Zones of $z \sim 4$ QuasarsOct 08 2018The duration of quasar accretion episodes is a key quantity for distinguishing between models for the formation and growth of supermassive black holes, the evolution of quasars, and their potential feedback effects on their host galaxies. However, this ... More
Chemical constraints on the contribution of Population III stars to cosmic reionizationOct 02 2013Recent studies have highlighted that galaxies at z = 6-8 fall short of producing enough ionizing photons to reionize the IGM, and suggest that Population III stars could resolve this tension, because their harder spectra can produce ~10x more ionizing ... More
Beyond the Damping Tail: Cross-Correlating the Kinetic Sunyaev-Zel'dovich Effect with Cosmic ShearSep 11 2003Sep 16 2003Secondary anisotropies of the CMB have the potential to reveal intricate details about the history of our universe between the present and recombination epochs. However, because the CMB we observe is the projected sum of a multitude of effects, the interpretation ... More
Chemical enrichment of Damped Lyman Alpha systems as a direct constraint on Population III star formationJan 17 2013Jun 25 2013Damped Ly-alpha absorbers (DLAs) can be used to measure gas-phase metallicities at large cosmological lookback times with high precision. Relative abundances can still be measured accurately deep into the reionization epoch (z > 6) using transitions redward ... More
The Opacity of the Intergalactic Medium Measured Along Quasar Sightlines at $z\sim 6$Jul 11 2018Aug 14 2018We publicly release a new sample of $34$ medium resolution quasar spectra at $5.77\leq z_{\rm em}\leq6.54$ observed with the Echellette Spectrograph and Imager (ESI) on the Keck telescope. This quasar sample represents an ideal laboratory to study the ... More
Quasars Probing Quasars VIII. The Physical Properties of the Cool Circumgalactic Medium Surrounding z ~ 2-3 Massive Galaxies Hosting QuasarsOct 20 2015Aug 12 2016We characterize the physical properties of the cool T ~10^4 K circumgalactic medium 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 ... More
The SDSS-DR5 Survey for Proximate Damped Lya SystemsMar 22 2007Using the Sloan Digital Sky Survey, Data Release 5, we survey proximate damped Lya systems (PDLAs): absorption line systems with HI column density N(HI)> 2x10^20 cm^{-2} at velocity separation dv < 3000 km/s from their background quasar. These absorbers ... More
The Thermal Proximity Effect: A New Probe of He II Reionization History and the Quasar LifetimeNov 17 2016Despite decades of effort, the timing and duration of He II reionization, as well as its morphology and the properties of the quasars believed to drive it, are still not well constrained. In this paper we present a new method to study both He II reionization ... More
Early and Extended Helium Reionization Over More Than 600 Million Years of Cosmic TimeMay 28 2014Jun 01 2016We measure the effective optical depth of HeII Ly\alpha\ absorption \tau$_\mathsf{eff,HeII}$ at 2.3<z<3.5 in 17 UV-transmitting quasars observed with UV spectrographs on the Hubble Space Telescope (HST). The median \tau$_\mathsf{eff,HeII}$ values increase ... More
First Spectroscopic Study of a Young QuasarJun 14 2018The quasar lifetime $t_{\rm\,Q}$ is one of the most fundamental quantities for understanding quasar evolution and the growth of supermassive black holes (SMBHs), but remains uncertain by several orders of magnitude. In a recent study we uncovered a population ... More
Spatially Resolving the Kinematics of the <100 μas Quasar Broad Line Region using SpectroastrometryFeb 26 2015The broad line region (BLR) of luminous active galactic nuclei (AGN) is a prominent observational signature of the accretion flow around supermassive black holes, which can be used to measure their masses (M_BH) over cosmic history. Due to the <100 {\mu}as ... More
A z = 3 Lyman Alpha Blob Associated with a Damped Lyman Alpha System Proximate to its Background QuasarJul 14 2008We report on the discovery of a bright Lyman alpha blob associated with the z=3 quasar SDSSJ124020.91+145535.6 which is also coincident with strong damped Lyman alpha absorption from a foreground galaxy (a so-called proximate damped Lyman alpha system; ... More
Time-dependent behaviour of quasar proximity zones at $z \sim 6$Mar 29 2019Since the discovery of $z\sim 6$ quasars two decades ago, studies of their Ly$\alpha$-transparent proximity zones have largely focused on their utility as a probe of cosmic reionization. But even when in a highly ionized intergalactic medium, these zones ... More
A Novel Statistical Method for Measuring the Temperature-Density Relation in the IGM Using the $b$-$N_{\text{HI}}$ Distribution of Absorbers in the Ly$α$ ForestMar 28 2019We present a new method for determining the thermal state of the intergalactic medium based on Voigt profile decomposition of the Ly$\alpha$ forest. The distribution of Doppler parameter and column density ($b$-$N_{\text{HI}}$ distribution) is sensitive ... More
Protocluster Discovery in Tomographic Ly$α$ Forest Flux MapsDec 03 2014We present a new method of finding protoclusters using tomographic maps of Ly$\alpha$ Forest flux. We review our method of creating tomographic flux maps and discuss our new high performance implementation, which makes large reconstructions computationally ... More
Studying the Reionization Epoch with QSO Absorption LinesMar 12 2019Absorption signatures in the spectra of QSOs are one of our most powerful tools for studying galactic and intergalactic environments at high redshifts. With the discovery of QSOs out to z > 7, QSO absorption lines are now tracing the end stages of reionization ... More
A New Measurement of the Temperature Density Relation of the IGM From Voigt Profile FittingOct 02 2017Aug 16 2018We decompose the Lyman-{\alpha} (Ly{\alpha}) forest of an extensive sample of 74 high signal-to-noise ratio and high-resolution quasar spectra into a collection of Voigt profiles. Absorbers located near caustics in the peculiar velocity field have the ... More
Quasar Quartet Embedded in Giant Nebula Reveals Rare Massive Structure in Distant UniverseMay 14 2015All galaxies once passed through a hyperluminous quasar phase powered by accretion onto a supermassive black hole. But because these episodes are brief, quasars are rare objects typically separated by cosmological distances. In a survey for Lyman-alpha ... More
Observational Requirements for Lyman-alpha Forest Tomographic Mapping of Large-Scale Structure at z ~ 2Sep 05 2013May 15 2014The z > 2 Lyman-alpha (Lya) forest traces the underlying dark-matter distribution on large scales and, given sufficient sightlines, can be used to create 3D maps of large-scale structure. We examine the observational requirements to construct such maps ... More
Fitting Spectral Energy Distributions of AGN - A Markov Chain Monte Carlo ApproachJan 22 2014We present AGNfitter: a Markov Chain Monte Carlo algorithm developed to fit the spectral energy distributions (SEDs) of active galactic nuclei (AGN) with different physical models of AGN components. This code is well suited to determine in a robust way ... More
Does circumgalactic OVI trace low-pressure gas beyond the accretion shock? Clues from HI and low-ion absorption, line kinematics, and dust extinctionMar 14 2018May 25 2018Large OVI columns are observed around star-forming, low-redshift ~L* galaxies, with a dependence on impact parameter indicating that most O^5+ particles reside beyond half the halo virial radius (>~100 kpc). In order to constrain the nature of the gas ... More
Characterizing the Pressure Smoothing Scale of the Intergalactic MediumApr 01 2015Oct 13 2015The thermal state of the intergalactic medium (IGM) at z < 6 constrains the nature and timing of cosmic reionization events, but its inference from the Ly-alpha forest is degenerate with the 3-D structure of the IGM on ~100 kpc scales, where, analogous ... More
Modeling the HeII Transverse Proximity Effect: Constraints on Quasar Lifetime and ObscurationOct 12 2017Oct 27 2017The HeII transverse proximity effect - enhanced HeII Ly{\alpha} 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 ... More
Strong Clustering of Lyman Break Galaxies around Luminous Quasars at z~4Jan 04 2017In the standard picture of structure formation, the first massive galaxies are expected to form at the highest peaks of the density field, which constitute the cores of massive proto-clusters. Luminous quasars (QSOs) at z~4 are the most strongly clustered ... More
A cosmic web filament revealed in Lyman-alpha emission around a luminous high-redshift quasarJan 17 2014Simulations of structure formation in the Universe predict that galaxies are embedded in a "cosmic web", where the majority of baryons reside as rarefied and highly ionized gas. This material has been studied for decades in absorption against background ... More
The Evolution of the HeII-Ionizing Background at Redshifts 2.3<z<3.8 Inferred from a Statistical Sample of 24 HST/COS HeII Ly$α$ Absorption SpectraAug 15 2018We present new measurements of the large-scale effective optical depth of HeII Ly$\alpha$ absorption, $\tau_{\rm eff}$, at $2.54<z<3.86$ toward 16 HeII-transparent quasars observed with the Cosmic Origins Spectrograph on the Hubble Space Telescope, to ... More
Small-scale Intensity Mapping: Extended Halos as a Probe of the Ionizing Escape Fraction and Faint Galaxy Populations during ReionizationJul 24 2017We present a new method to quantify the value of the escape fraction of ionizing photons, and the existence of ultra-faint galaxies clustered around brighter objects during the epoch of cosmic reionization, using the diffuse Ly$\alpha$, continuum and ... More
Correcting CIV-Based Virial Black Hole MassesOct 27 2016Jan 20 2017The CIV broad emission line is visible in optical spectra to redshifts exceeding z~5. CIV has long been known to exhibit significant displacements to the blue and these `blueshifts' almost certainly signal the presence of strong outflows. As a consequence, ... More
A Universal Density Structure for Circum-Galactic GasApr 07 2016Jul 25 2016We develop a new method to constrain the physical conditions in the cool (~10^4 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 ... More
The Evolution of the HeII-Ionizing Background at Redshifts 2.3<z<3.8 Inferred from a Statistical Sample of 24 HST/COS HeII Ly$α$ Absorption SpectraAug 15 2018Apr 04 2019We present measurements of the large-scale (~40 comoving Mpc) effective optical depth of HeII Ly$\alpha$ absorption, $\tau_{\rm eff}$, at 2.54<z<3.86 toward 16 HeII-transparent quasars observed with the Cosmic Origins Spectrograph (COS) on the Hubble ... More
Mapping quasar light echoes in 3D with Lyα forest tomographyOct 11 2018The 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 Lyman-$\alpha$ (Ly$\alpha$) ... More
The Stacked Lyman-Alpha Emission Profile from the Circum-Galactic Medium of z~2 QuasarsApr 11 2016In the context of the FLASHLIGHT survey, we obtained deep narrow band images of 15 $z\sim2$ quasars with GMOS on Gemini-South in an effort to measure Ly$\alpha$ emission from circum- and inter-galactic gas on scales of hundreds of kpc from the central ... More
Constraining the dynamical importance of hot gas and radiation pressure in quasar outflows using emission line ratiosOct 26 2015Jan 24 2016Quasar feedback models often predict an expanding hot gas bubble which drives a galaxy-scale outflow. In many circumstances this hot gas radiates inefficiently and is therefore difficult to observe directly. We present an indirect method to detect the ... More
A New Method to Measure the Post-Reionization Ionizing Background from the Joint Distribution of Lyman-$α$ and Lyman-$β$ Forest TransmissionMar 29 2017The amplitude of the ionizing background that pervades the intergalactic medium (IGM) at the end of the epoch of reionization provides a valuable constraint on the emissivity of the sources which reionized the Universe. While measurements of the ionizing ... More
AGNfitter: A Bayesian MCMC approach to fitting spectral energy distributions of AGNJun 17 2016We present AGNfitter, a publicly available open-source algorithm implementing a fully Bayesian Markov Chain Monte Carlo method to fit the spectral energy distributions (SEDs) of active galactic nuclei (AGN) from the sub-mm to the UV, allowing one to robustly ... More
Statistical Detection of the HeII Transverse Proximity Effect: Evidence for Sustained Quasar Activity for $>$25 Million YearsJan 30 2017The HeII transverse proximity effect -- enhanced HeII Ly$\alpha$~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 HeII reionization. ... More
Implications of $z \sim 6$ Quasar Proximity Zones for the Epoch of Reionization and Quasar LifetimesMar 07 2017May 10 2017We study quasar proximity zones in the redshift range $5.77 \leq z \leq 6.54$ by homogeneously analyzing $34$ medium resolution spectra, encompassing both archival and newly obtained data, and exploiting recently updated systemic redshift and magnitude ... More
Coincident, 100 kpc-scale damped Lyman alpha absorption towards a binary QSO: how large are galaxies at z ~ 3?Apr 13 2007We report coincident damped Lyman alpha (DLA) and sub-DLA absorption at z = 2.66 and z = 2.94 towards the z ~ 3 13.8 arcsecond separation binary quasar SDSS 1116+4118 AB. At the redshifts of the absorbers, this angular separation corresponds to a proper ... More
The DEEP2 Galaxy Redshift Survey: Clustering of Quasars and Galaxies at z=1Jul 20 2006Sep 12 2006We present the clustering of DEEP2 galaxies at 0.7<z<1.4 around quasars identified using both the SDSS and DEEP2 surveys. We measure the two-point cross-correlation of a sample of 36 optically-selected, spectroscopically-identified quasars from the SDSS ... More
Deep HeII and CIV Spectroscopy of a Giant Lyman alpha Nebula: Dense Compact Gas Clumps in the Circumgalactic Medium of a z~2 QuasarApr 14 2015The recent discovery by Cantalupo et al. (2014) of the largest (~500 kpc) and luminous Ly-alpha 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 ... More
Clustering of Lyman-alpha Emitters Around Quasars at $z\sim4$Apr 11 2019The strong observed clustering of $z>3.5$ quasars indicates they are hosted by massive ($M_{\rm{halo}}\gtrsim10^{12}\,h^{-1}\,\rm{M_{\odot}}$) dark matter halos. Assuming quasars and galaxies trace the same large-scale structures, this should also manifest ... More
A VLT/FORS2 Narrowband Imaging Search for MgII Emission Around z ~ 0.7 GalaxiesMay 14 2019We perform a Very Large Telescope FOcal Reducer and low dispersion Spectrograph 2 (VLT/FORS2) narrowband imaging search around 5 star-forming galaxies at redshift z=0.67-0.69 in the Great Observatories Origins Deep Survey South (GOODS-S) field to constrain ... More
Statistics of Giant Arcs in Galaxy ClustersOct 11 2003We study the expected properties and statistics of giant arcs produced by galaxy clusters in a LambdaCDM universe and investigate how the characteristics of CDM clusters determine the properties of the arcs they generate. Due to the triaxiality and substructure ... More
Characterizing the Shapes of Galaxy Clusters Using Moments of the Gravitational Lensing ShearAug 03 2005Nov 07 2005We explore the use of the tangential component of weak lensing shear to characterize the ellipticity of clusters of galaxies. We introduce an ellipticity estimator, and quantify its properties for isolated clusters from LCDM N-body simulations. We compare ... More
Noise in strong lensing cosmographySep 01 2004Giant arcs in strong lensing galaxy clusters can provide a purely geometric determination of cosmological parameters, such as the dark energy density and equation of state. We investigate sources of noise in cosmography with giant arcs, focusing in particular ... More
The Redshift Distribution of Giant Arcs in the Sloan Giant Arcs SurveyOct 28 2010Jan 03 2011We measure the redshift distribution of a sample of 28 giant arcs discovered as a part of the Sloan Giant Arcs Survey (SGAS). Gemini/GMOS-North spectroscopy provides precise redshifts for 24 arcs, and "redshift desert" constraints for the remaining four. ... More
Two Lensed Lyman-alpha Emitting Galaxies at z~5Mar 01 2010Jul 22 2010We present observations of two strongly lensed $z\sim5$ Lyman-$\alpha$ Emitting (LAE) galaxies that were discovered in the Sloan Giant Arcs Survey (SGAS). We identify the two sources as SGAS J091541+382655, at $z=5.200$, and SGAS J134331+415455 at $z=4.994$. ... More
The spectral and environment properties of $z\sim2.0-2.5$ quasar pairsMay 08 2018Jun 06 2018We present the first results from our survey of intervening and proximate Lyman limit systems (LLSs) at $z$$\sim$2.0-2.5 using the Wide Field Camera 3 on-board the Hubble Space Telescope. The quasars in our sample are projected pairs with proper transverse ... More
Combined strong and weak lensing analysis of 28 clusters from the Sloan Giant Arcs SurveySep 12 2011Nov 22 2011We study the mass distribution of a sample of 28 galaxy clusters using strong and weak lensing observations. The clusters are selected via their strong lensing properties as part of the Sloan Giant Arcs Survey (SGAS) from the Sloan Digital Sky Survey ... More
A New Precision Measurement of the Small-Scale Line-of-Sight Power Spectrum of the Ly$α$ ForestSep 21 2017Dec 01 2017We present a new measurement of the Ly${\alpha}$ forest power spectrum at $1.8 < z < 3.4$ using 74 Keck/HIRES and VLT/UVES high-resolution, high-S/N quasar spectra. We developed a custom pipeline to measure the power spectrum and its uncertainty, which ... More
Gemini/GMOS Spectroscopy of 26 Strong Lensing Selected Galaxy Cluster CoresOct 13 2010Feb 19 2011We present results from a spectroscopic program targeting 26 strong lensing cluster cores that were visually identified in the Sloan Digital Sky Survey (SDSS) and the Second Red-Sequence Cluster Survey (RCS-2). The 26 galaxy cluster lenses span a redshift ... More
Dissecting the Gaseous Halos of z~2 Damped Ly$α$ Systems with Close Quasar PairsNov 21 2014We use spectroscopy of close pairs of quasars to study diffuse gas in the circumgalactic medium (CGM) surrounding a sample of 40 Damped Lya systems (DLAs). The primary sightline in each quasar pair probes an intervening DLA in the redshift range 1.6 < ... More
The UV-bright Quasar Survey (UVQS): DR1Feb 19 2016We present the first data release (DR1) from our UV-bright Quasar Survey (UVQS) for new $z \sim 1$ active galactic nuclei (AGN) across the sky. Using simple GALEX UV and WISE near-IR color selection criteria, we generated a list of 1450 primary candidates ... More
The Power Spectrum of the Lyman-$α$ Forest at z < 0.5Aug 16 2018We present new measurements of the flux power-spectrum P(k) of the z<0.5 H I Lyman-$\alpha$ forest spanning scales k ~ 0.001-0.1 s/km. These results were derived from 65 far ultraviolet quasar spectra (resolution R~18,000) observed with Cosmic Origin ... More
The Power Spectrum of the Lyman-$α$ Forest at z < 0.5Aug 16 2018Apr 09 2019We present new measurements of the flux power-spectrum P(k) of the $z<0.5$ HI Lyman-$\alpha$ forest spanning scales k ~ 0.001-0.1 s/km. These results were derived from 65 far ultraviolet quasar spectra (resolution R~18000) observed with the Cosmic Origin ... More
Discovery of Four Gravitationally Lensed Quasars from the Sloan Digital Sky SurveyAug 07 2007Nov 25 2007We present the discovery of four gravitationally lensed quasars selected from the spectroscopic quasar catalog of the Sloan Digital Sky Survey. We describe imaging and spectroscopic follow-up observations that support the lensing interpretation of the ... More
HST narrow-band search for extended Ly-alpha emission around two z>6 quasarsJul 09 2012We search for extended Ly-alpha emission around two z>6 quasars, SDSS J1030+0524 (z=6.309) and SDSS J1148+5251 (z=6.419) using WFC3 narrow-band filters on board the Hubble Space Telescope. For each quasar, we collected two deep, narrow-band images, one ... More
Two Lensed z~3 Lyman Break Galaxies Discovered in the SDSS Giant Arcs SurveyFeb 26 2010Mar 15 2010We report the discovery of two strongly-lensed z ~ 3 Lyman Break Galaxies (LBGs) discovered as u-band dropouts as part of the SDSS Giant Arcs Survey (SGAS). The first, SGAS J122651.3+215220 at z = 2.9233 is lensed by one of several sub-clusters, SDSS ... More
QSO Absorption Lines from QSOsMay 30 2006We present the results of a search for metal absorption lines in the spectra of background QSOs whose sightlines pass close to foreground QSOs. We detect MgII(2796,2803) absorption in Sloan Digital Sky Survey (SDSS) spectra of four z>1.5 QSOs whose lines ... More
The Sloan Digital Sky Survey Quasar Lens Search. I. Candidate Selection AlgorithmMay 22 2006We present an algorithm for selecting an uniform sample of gravitationally lensed quasar candidates from low-redshift (0.6<z<2.2) quasars brighter than i=19.1 that have been spectroscopically identified in the SDSS. Our algorithm uses morphological and ... More
A Simple Likelihood Method for Quasar Target SelectionApr 26 2011Sep 02 2011We present a new method for quasar target selection using photometric fluxes and a Bayesian probabilistic approach. For our purposes we target quasars using Sloan Digital Sky Survey (SDSS) photometry to a magnitude limit of g=22. The efficiency and completeness ... More
Measurement of the small-scale structure of the intergalactic medium using close quasar pairsApr 26 2017The distribution of diffuse gas in the intergalactic medium (IGM) imprints a series of hydrogen absorption lines on the spectra of distant background quasars known as the Lyman-$\alpha$ forest. Cosmological hydrodynamical simulations predict that IGM ... More
Inspiraling Halo Accretion Mapped in Lyman-$α$ Emission around a $z\sim3$ QuasarSep 24 2017In an effort to search for Ly$\alpha$ emission from circum- and intergalactic gas on scales of hundreds of kpc around $z\sim3$ quasars, and thus characterise the physical properties of the gas in emission, we have initiated an extensive fast-survey with ... More
Mapping the Lyman-Alpha Emission Around a z~6.6 QSO with MUSE: Extended Emission and a Companion at Close SeparationSep 18 2017We utilize the Multi Unit Spectroscopic Explorer (MUSE) on the Very Large Telescope (VLT) to search for extended Lyman-Alpha emission around the z~6.6 QSO J0305-3150. After carefully subtracting the point-spread-function, we reach a nominal 5-sigma surface ... More
Subaru Weak Lensing Measurements of Four Strong Lensing Clusters: Are Lensing Clusters Over-Concentrated?Jan 28 2009Apr 30 2009We derive radial mass profiles of four strong lensing selected clusters which show prominent giant arcs (Abell 1703, SDSS J1446+3032, SDSS J1531+3414, and SDSS J2111-0115), by combining detailed strong lens modeling with weak lensing shear measured from ... More
Photometric redshifts and quasar probabilities from a single, data-driven generative modelMay 19 2011Mar 21 2012We describe a technique for simultaneously classifying and estimating the redshift of quasars. It can separate quasars from stars in arbitrary redshift ranges, estimate full posterior distribution functions for the redshift, and naturally incorporate ... More
The End of Helium Reionization at z~2.7 Inferred from Cosmic Variance in HST/COS HeII Lyman Alpha Absorption SpectraMar 29 2011May 11 2011We report on the detection of strongly varying intergalactic HeII absorption in HST/COS spectra of two z~3 quasars. From our homogeneous analysis of the HeII absorption in these and three archival sightlines, we find a marked increase in the mean HeII ... More
Metal-enriched, sub-kiloparsec gas clumps in the circumgalactic medium of a faint z = 2.5 galaxyJun 17 2014Jun 03 2015We report the serendipitous detection of a 0.2 L$^*$, Lyman-$\alpha$ emitting galaxy at redshift 2.5 at an impact parameter of 50 kpc from a bright background QSO sightline. A high-resolution spectrum of the QSO reveals a partial Lyman-limit absorption ... More
Binary Quasars at High Redshift II: Sub-Mpc Clustering at z ~ 3-4Aug 26 2009We present measurements of the small-scale (0.1<~ r <~ 1 Mpc/h) quasar two-point correlation function at z>2.9, for a flux-limited (i<21) sample of 15 binary quasars compiled by Hennawi et al. (2009). The amplitude of the small-scale clustering increases ... More
Origin of Metals around Galaxies I: Catalogs of Metal-line Absorption Doublets from High-Resolution Quasar SpectraMay 01 2018Jun 23 2018We present the first paper of the series Origin of Metals around Galaxies (OMG) aimed to study the origin of the metals observed in the circumgalactic and intergalactic media. In this work we extract and build the catalogs of metal absorbers that will ... More
Quasars probing quasars X: The quasar pair spectral databaseApr 23 2018The 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" ... More
He II Proximity Effect and the Lifetime of QuasarsNov 11 2015Jun 02 2016The lifetime of quasars is fundamental for understanding the growth of supermassive black holes, and is an important ingredient in models of the reionization of the intergalactic medium. However, despite various attempts to determine quasar lifetimes, ... More
Think Outside the Color Box: Probabilistic Target Selection and the SDSS-XDQSO Quasar Targeting CatalogNov 29 2010Mar 01 2011We present the SDSS-XDQSO quasar targeting catalog for efficient flux-based quasar target selection down to the faint limit of the Sloan Digital Sky Survey (SDSS) catalog, even at medium redshifts (2.5 <~ z <~ 3) where the stellar contamination is significant. ... More
Predicting Quasar Continua Near Lyman-$α$ with Principal Component AnalysisJan 23 2018Measuring the proximity effect and the damping wing of intergalactic neutral hydrogen in quasar spectra during the epoch of reionization requires an estimate of the intrinsic continuum at rest-frame wavelengths $\lambda_{\rm rest}\sim1200$-$1260$ {\AA}. ... More
Discovery of a Fifth Image of the Large Separation Gravitationally Lensed Quasar SDSS J1004+4112Mar 15 2005We report the discovery of a fifth image in the large separation lensed quasar system SDSS J1004+4112. A faint point source located 0.2'' from the center of the brightest galaxy in the lensing cluster is detected in images taken with the Advanced Camera ... More
Quantitative Constraints on the Reionization History from the IGM Damping Wing Signature in Two Quasars at z > 7Feb 16 2018During reionization, neutral hydrogen in the intergalactic medium (IGM) imprints a damping wing absorption feature on the spectrum of high-redshift quasars. A detection of this signature provides compelling evidence for a significantly neutral Universe, ... More
Quasars Probing Quasars VI. Excess HI Absorption Within One Proper Mpc of z~2 QuasarsAug 28 2013With 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, ... More
A New Survey for Giant ArcsOct 03 2006Oct 09 2006We report on the first results of an imaging survey to detect strong gravitational lensing targeting the richest clusters selected from the photometric data of the Sloan Digital Sky Survey (SDSS) with follow-up deep imaging observations from the Wisconsin ... More
MASE: A New Data--Reduction Pipeline for the Magellan Echellette SpectrographOct 09 2009We introduce a data reduction package written in Interactive Data Language (IDL) for the Magellan Echellete Spectrograph (MAGE). MAGE is a medium-resolution (R ~4100), cross-dispersed, optical spectrograph, with coverage from ~3000-10000 Angstroms. The ... More
Shadow of a Colossus: A z=2.45 Galaxy Protocluster Detected in 3D Ly-a Forest Tomographic Mapping of the COSMOS FieldSep 09 2015Using moderate-resolution optical spectra from 58 background Lyman-break galaxies and quasars at $z\sim 2.3-3$ within a $11.5'\times13.5'$ area of the COSMOS field ($\sim 1200\,\mathrm{deg}^2$ projected area density or $\sim 2.4\,h^{-1}\,\mathrm{Mpc}$ ... More
400 pc imaging of a quasar host galaxy at z=6.6Mar 21 2019We report high spatial resolution (~0.076", 410pc) ALMA imaging of the dust continuum and the ionised carbon line [CII] in a luminous quasar host galaxy at z=6.6, 800 million years after the Big Bang. Based on previous studies, this galaxy hosts a ~1x10^9 ... More
A Survey of z>5.7 Quasars in the Sloan Digital Sky Survey III: Discovery of Five Additional QuasarsMay 07 2004We present the discovery of five new quasars at z>5.7, selected from the multicolor imaging data of the Sloan Digital Sky Survey (SDSS). Three of them, at redshifts 5.93, 6.07, and 6.22, were selected from ~1700 deg^2 of new SDSS Main Survey imaging in ... More
Binary Quasars at High Redshift I: 24 New Quasar Pairs at z ~ 3-4Aug 26 2009The clustering of quasars on small scales yields fundamental constraints on models of quasar evolution and the buildup of supermassive black holes. This paper describes the first systematic survey to discover high redshift binary quasars. Using color-selection ... More
Quasars Probing Quasars I: Optically Thick Absorbers Near Luminous QuasarsMar 28 2006Mar 29 2006With close pairs of quasars at different redshifts, a background quasar sightline can be used to study a foreground quasar's environment in absorption. We search 149 moderate resolution background quasar spectra, from Gemini, Keck, the MMT, and the SDSS ... More
400 pc Imaging of a Massive Quasar Host Galaxy at a Redshift of 6.6Mar 21 2019Apr 16 2019We report high spatial resolution (~0.076", 410pc) Atacama Large Millimeter/submillimeter Array imaging of the dust continuum and the ionized carbon line [CII] in a luminous quasar host galaxy at z=6.6, 800 million years after the big bang. Based on previous ... More