arXiv subjects

Joseph F. Hennawi

Publications and source records attributed to Joseph F. Hennawi.

At least 235 records · Page 13Linked to original sources

arXiv2007

The SDSS-DR5 Survey for Proximate Damped Lya Systems

Using 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 are physically associated with their background quasars, and their statistics allow us to study quasar environments out to z~5. However, the large ionizing flux emitted by a quasar can ionize the neutral gas in a nearby galaxy possibly giving rise to a ``proximity effect'', analogous to the similar effect observed in the Lya forest. From a sample of 111 PDLAs, we measure the HI frequency distribution f(N), incidence, and gas mass density of the PDLAs near luminous quasars over the redshift interval z= 2.2 to 5. The incidence and mass density of PDLAs at z~3 is approximately twice that of intervening DLAs, but at z<2.5 and z>3.5 the f(N) distribution is consistent with the intervening population. We interpret the observed enhancement of PDLAs around quasars in terms of quasar-galaxy clustering, and compare the strength of the clustering signal to the expectation from independent measures of the respective clustering strengths of DLAs and quasars, as well as a complementary analysis of the clustering of absorbers around quasars in the transverse direction. We find that there are a factor of 5-10 fewer PDLAs around quasars than expected and interpret this result as evidence for the hypothesis that the ionizing flux from the quasars photoevaporates HI in nearby DLA galaxies, thus reducing their cross-section for DLA absorption. This constitues the first detection of a ``proximity effect'' for DLAs.

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arXiv2007

Clustering of High Redshift ($z\ge 2.9$) Quasars from the Sloan Digital Sky Survey

(Abridged) We study the two-point correlation function of a uniformly selected sample of 4,426 luminous optical quasars with redshift $2.9 \le z\le 5.4$ selected over 4041 deg$^2$ from the Fifth Data Release of the Sloan Digital Sky Survey. For a real-space correlation function of the form $ξ(r)=(r/r_0)^{-γ}$, the fitted parameters in comoving coordinates are $r_0 = 15.2 \pm 2.7 h^{-1}$ Mpc and $γ= 2.0 \pm 0.3$, over a scale range $4\le r_p\le 150 h^{-1}$ Mpc. Thus high-redshift quasars are appreciably more strongly clustered than their $z \approx 1.5$ counterparts, which have a comoving clustering length $r_0 \approx 6.5 h^{-1}$ Mpc. Dividing our sample into two redshift bins: $2.9\le z\le 3.5$ and $z\ge 3.5$, and assuming a power-law index $γ=2.0$, we find a correlation length of $r_0 = 16.9 \pm 1.7 h^{-1}$ Mpc for the former, and $r_0 = 24.3 \pm 2.4 h^{-1}$ Mpc for the latter. Following Martini & Weinberg, we relate the clustering strength and quasar number density to the quasar lifetimes and duty cycle. Using the Sheth & Tormen halo mass function, the quasar lifetime is estimated to lie in the range $4\sim 50$ Myr for quasars with $2.9\le z\le 3.5$; and $30\sim 600$ Myr for quasars with $z\ge 3.5$. The corresponding duty cycles are $0.004\sim 0.05$ for the lower redshift bin and $0.03\sim 0.6$ for the higher redshift bin. The minimum mass of halos in which these quasars reside is $2-3\times 10^{12}\ h^{-1}M_\odot$ for quasars with $2.9\le z\le 3.5$ and $4-6\times 10^{12}\ h^{-1}M_\odot$ for quasars with $z\ge 3.5$.

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arXiv2006

SDSS J1029+2623: A Gravitationally Lensed Quasar with an Image Separation of 22.5 Arcseconds

We report the discovery of a cluster-scale lensed quasar, SDSS J1029+2623, selected from the Sloan Digital Sky Survey. The lens system exhibits two lensed images of a quasar at z_s=2.197. The image separation of 22.5" makes it the largest separation lensed quasar discovered to date. The similarity of the optical spectra and the radio loudnesses of the two components support the lensing hypothesis. Images of the field show a cluster of galaxies at z_l~0.55 that is responsible for the large image separation. The lensed images and the cluster light center are not collinear, which implies that the lensing cluster has a complex structure.

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arXiv2006

A New Survey for Giant Arcs

We 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 Indiana Yale NOAO (WIYN) 3.5m telescope and the University of Hawaii 88-inch telescope (UH88). The clusters are selected from an area of 8000 deg^2 using the Red Cluster Sequence technique and span the redshift range 0.1 < z < 0.6, corresponding to a comoving cosmological volume of ~ 2 Gpc^3. Our imaging survey thus targets a volume more than an order of magnitude larger than any previous search. A total of 240 clusters were imaged of which 141 had sub-arcsecond image quality. Our survey has uncovered16 new lensing clusters with definite giant arcs, an additional 12 systems for which the lensing interpretation is very likely, and 9 possible lenses which contain shorter arclets or candidate arcs which are less certain and will require further observations to confirm their lensing origin. The number of new cluster lenses detected in this survey is likely > 30. Among these new systems are several of the most dramatic examples of strong gravitational lensing ever discovered with multiple bright arcs at large angular separation. These will likely become 'poster-child' gravitational lenses similar to Abell 1689 and CL0024+1654. The new lenses discovered in this survey will enable future sysetmatic studies of the statistics of strong lensing and its implications for cosmology and our structure formation paradigm.

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arXiv2006

Quasars Probing Quasars II: The Anisotropic Clustering of Optically Thick Absorbers around 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 used a sample of 17 Lyman limit systems with column density N_HI > 10^19 cm^-2 selected from 149 projected quasar pair sightlines, to investigate the clustering pattern of optically thick absorbers around luminous quasars at z ~ 2.5. Specifically, we measured the quasar-absorber correlation function in the transverse direction, and found a comoving correlation length of r_0=9.2_{+1.5}_{-1.7} Mpc/h (comoving) assuming a power law correlation function, with gamma=1.6. Applying this transverse clustering strength to the line-of-sight, would predict that ~ 15-50% of all quasars should show a N_HI > 10^19 cm^-2 absorber within a velocity window of v < 3000 km/s. This overpredicts the number of absorbers along the line-of-sight by a large factor, providing compelling evidence that the clustering pattern of optically thick absorbers around quasars is highly anisotropic. The most plausible explanationfor the anisotropy is that the transverse direction is less likely to be illuminated by ionizing photons than the line-of-sight, and that absorbers along the line-of-sight are being photoevaporated. A simple model for the photoevaporation of absorbers subject to the ionizing flux of a quasar is presented, and it is shown that absorbers with volume densities n_H < 0.1 cm^-3 will be photoevaporated if they lie within ~ 1 Mpc (proper) of a luminous quasar. Using this simple model, we illustrate how comparisons of the transverse and line-of-sight clustering around quasars can ultimately be used to constrain the distribution of gas in optically thick absorption line systems.

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arXiv2006

QSO Absorption Lines from QSOs

We 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 of sight pass within 26-98 kpc of lower redshift (z~0.5-1.5) QSOs. The 100% [4/4 pairs] detection of MgII in the background QSOs is clearly at odds with the incidence of associated (z_abs ~ z_em) systems -- absorbers which exist towards only a few percent of QSOs. Although the quality of our foreground QSO spectra is not as high as the SDSS data, absorption seen towards one of the background QSOs clearly does not show up at the same strength in the spectrum of the corresponding foreground QSO. This implies that the absorbing gas is distributed inhomogeneously around the QSO, presumably as a direct consequence of the anisotropic emission from the central AGN. We discuss possible origins for the MgII lines, including: absorption by gas from the foreground QSO host galaxy; companion galaxies fuelling the QSO through gravitational interactions; and tidal debris left by galaxy mergers or interactions which initiated the QSO activity. No single explanation is entirely satisfactory, and we may well be seeing a mixture of phenomena.

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arXiv2006

The Sloan Digital Sky Survey Quasar Lens Search. I. Candidate Selection Algorithm

We 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 color selections that are intended to identify small- and large-separation lenses, respectively. Our selection algorithm only relies on parameters that the SDSS standard image processing pipeline generates, allowing easy and fast selection of lens candidates. The algorithm has been tested against simulated SDSS images, which adopt distributions of field and quasar parameters taken from the real SDSS data as input. Furthermore, we take differential reddening into account. We find that our selection algorithm is almost complete down to separations of 1'' and flux ratios of 10^-0.5. The algorithm selects both double and quadruple lenses. At a separation of 2'', doubles and quads are selected with similar completeness, and above (below) 2'' the selection of quads is better (worse) than for doubles. Our morphological selection identifies a non-negligible fraction of single quasars: To remove these we fit images of candidates with a model of two point sources and reject those with unusually small image separations and/or large magnitude differences between the two point sources. We estimate the efficiency of our selection algorithm to be at least 8% at image separations smaller than 2'', comparable to that of radio surveys. The efficiency declines as the image separation increases, because of larger contamination from stars. We also present the magnification factor of lensed images as a function of the image separation, which is needed for accurate computation of magnification bias.

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arXiv2006

Quasars Probing Quasars I: Optically Thick Absorbers Near Luminous 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 search 149 moderate resolution background quasar spectra, from Gemini, Keck, the MMT, and the SDSS to survey Lyman Limit Systems (LLSs) and Damped Ly-alpha systems (DLAs) in the vicinity of 1.8 < z < 4.0 luminous foreground quasars. A sample of 27 new quasar-absorber pairs is uncovered with column densities, 17.2 < log (N_HI/cm^2) < 20.9, and transverse (proper) distances of 22 kpc/h < R < 1.7 Mpc/h, from the foreground quasars. If they emit isotropically, the implied ionizing photon fluxes are a factor of ~ 5-8000 times larger than the ambient extragalactic UV background over this range of distances. The observed probability of intercepting an absorber is very high for small separations: six out of eight projected sightlines with transverse separations R < 150 kpc/h have an absorber coincident with the foreground quasar, of which four have log N_HI > 10^19. The covering factor of log N_HI > 10^19 absorbers is thus ~ 50 % (4/8) on these small scales, whereas < 2% would have been expected at random. There are many cosmological applications of these new sightlines: they provide laboratories for studying fluorescent Ly-alpha recombination radiation from LLSs, constrain the environments, emission geometry, and radiative histories of quasars, and shed light on the physical nature of LLSs and DLAs.

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arXiv2006

The DEEP2 Galaxy Redshift Survey: Clustering of Galaxies as a Function of Luminosity at z=1

We measure the clustering of DEEP2 galaxies at z=1 as a function of luminosity on scales 0.1 Mpc/h to 20 Mpc/h. Drawing from a parent catalog of 25,000 galaxies at 0.7 L^*, have a steeper slope. The clustering scale-length, r_0, varies from 3.69 +/-0.14 for the faintest sample to 4.43 +/-0.14 for the brightest sample. The relative bias of galaxies as a function of L/L^* is steeper than the relation found locally for SDSS galaxies (Zehavi et al. 2005) over the luminosity range that we sample. The absolute bias of galaxies at z=1 is scale-dependent on scales r_p<1 Mpc/h, and rises most significantly on small scales for the brightest samples. For a concordance cosmology, the large-scale bias varies from 1.26 +/-0.04 to 1.54 +/-0.05 as a function of luminosity and implies that DEEP2 galaxies reside in dark matter halos with a minimum mass of ~1-3 10^12 h^-1 M_sun.

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arXiv2005

A Survey of z>5.7 Quasars in the Sloan Digital Sky Survey IV: Discovery of Seven Additional Quasars

We present the discovery of seven quasars at z>5.7, selected from ~2000 deg^2 of multicolor imaging data of the Sloan Digital Sky Survey (SDSS). The new quasars have redshifts z from 5.79 to 6.13. Five are selected as part of a complete flux-limited sample in the SDSS Northern Galactic Cap; two have larger photometric errors and are not part of the complete sample. One of the new quasars, SDSS J1335+3533 (z=5.93), exhibits no emission lines; the 3-sigma limit on the rest-frame equivalent width of Ly alpha+NV line is 5 A. It is the highest redshift lineless quasar known, and could be a gravitational lensed galaxy, a BL Lac object or a new type of quasar. Two new z>6 quasars, SDSS 1250+3130 (z=6.13) and SDSS J1137+3549 (z=6.01), show deep Gunn-Peterson absorption gaps in Ly alpha. These gaps are narrower the complete Gunn-Peterson absorption troughs observed among quasars at z>6.2 and do not have complete Ly beta absorption.

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arXiv2005

Characterizing the Cluster Lens Population

We 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. We measure concentrations, axis ratios, orientations, and the amount of substructure of each cluster, and compare the lensing weighted distribution of each quantity to that of the cluster population as a whole. The concentrations of lensing clusters are on average 34% larger than the typical cluster in the Universe. Despite this bias, the anomalously high concentrations (c >14) recently measured by several groups, appear to be inconsistent with the concentration distribution in our simulations, which predict < 2% of lensing clusters should have concentrations this high. No correlation is found between lensing cross section and the amount of substructure. We introduce several types of simplified dark matter halos, and use them to isolate which properties of CDM clusters make them effective lenses. Projections of halo substructure onto small radii and the large scale mass distribution of clusters do not significantly influence cross sections. The abundance of giant arcs is primarily determined by the mass distribution within an average overdensity of ~ 10,000. A multiple lens plane ray tracing algorithm is used to show that projections of large scale structure increase the giant arc abundance by a modest amount <7%. We revisit the question of whether there is an excess of giant arcs behind high redshift clusters in the RCS survey and find that the number of high redshift (z > 0.6) lenses is in good agreement with LCDM, although our simulations predict more low redshift (z < 0.6) lenses than were observed. (abridged)

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arXiv2005

Statistics of Quasars Multiply Imaged by Galaxy Clusters

We 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. Our calculation predicts ~ 4 quasar lenses with splittings theta > 10" in the SDSS spectroscopic quasar sample, consistent with the recent discovery of the wide separation lens SDSSJ1004+4112 which has theta=14.6". The SDSS faint photometric quasar survey will contain ~12 multiply imaged quasars with splittings theta > 10". Of these, ~ 2 will be lenses with separations theta > 30", and ~ 2 will be at high redshift (z ~ 4).

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arXiv2005

Binary Quasars in the Sloan Digital Sky Survey: Evidence for Excess Clustering on Small Scales

We present a sample of 218 new quasar pairs with proper transverse separations R_prop < 1 Mpc/h over the redshift range 0.5 < z < 3.0, discovered from an extensive follow up campaign to find companions around the Sloan Digital Sky Survey and 2dF Quasar Redshift Survey quasars. This sample includes 26 new binary quasars with separations R_prop < 50 kpc/h (theta < 10 arcseconds), more than doubling the number of such systems known. We define a statistical sample of binaries selected with homogeneous criteria and compute its selection function, taking into account sources of incompleteness. The first measurement of the quasar correlation function on scales 10 kpc/h < R_prop < 400 kpc/h is presented. For R_prop < 40 kpc/h, we detect an order of magnitude excess clustering over the expectation from the large scale R_prop > 3 Mpc/h quasar correlation function, extrapolated down as a power law to the separations probed by our binaries. The excess grows to ~ 30 at R_prop ~ 10 kpc/h, and provides compelling evidence that the quasar autocorrelation function gets progressively steeper on sub-Mpc scales. This small scale excess can likely be attributed to dissipative interaction events which trigger quasar activity in rich environments. Recent small scale measurements of galaxy clustering and quasar-galaxy clustering are reviewed and discussed in relation to our measurement of small scale quasar clustering.

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arXiv2005

Discovery of a Fifth Image of the Large Separation Gravitationally Lensed Quasar SDSS J1004+4112

We 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 for Surveys (ACS) and the Near Infrared Camera and Multi-Object Spectrometer (NICMOS) on the Hubble Space Telescope. The flux ratio between the point source and the brightest lensed component in the ACS image is similar to that in the NICMOS image. The location and brightness of the point source are consistent with lens model predictions for a lensed image. We therefore conclude that the point source is likely to be a fifth image of the source quasar. In addition, the NICMOS image reveals the lensed host galaxy of the source quasar, which can strongly constrain the structure of the lensing critical curves and thereby the mass distribution of the lensing cluster.

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arXiv2005

Shear Selected Cluster Cosmology I: Tomography and Optimal Filtering

We 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 the degree to which the radial positions of galaxy clusters can be determined tomographically, by using photometric redshifts of background source galaxies. We quantify the errors in the tomographic redshifts, and study their dependence on mass, redshift, detection significance, and filtering scheme. A Tomographic Matched Filtering (TMF) scheme, which combines tomography and matched filtering, is introduced, which optimally detects clusters of galaxies in weak lensing surveys. The TMF exploits the extra information provided by photometric redshifts of background source galaxies, neglected in previous studies, to optimally weight the sources. The efficacy and reliability of the TMF is investigated using a large ensemble of mock observations from our simulations and detailed comparisons are made to other filters. Using photometric redshift information with the TMF enhances the number of clusters detected with S/N > 4.5 by as much as 76%, and it increases the dynamic range of weak lensing searches for clusters, detecting more high redshift clusters and extending the mass sensitivity down to the scale of large groups. Furthermore, we find that coarsely binning source galaxies in as few as three redshift bins is sufficient to realize the gains of the TMF. Thus, the tomographic filtering techniques presented here can be applied to current ground based weak lensing data in as few as three bands. Cosmological applications of mass selected cluster samples are also discussed. (abridged)

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arXiv2004

Discovery of Two Gravitationally Lensed Quasars with Image Separations of 3 Arcseconds from the Sloan Digital Sky Survey

We report the discovery of two doubly-imaged quasars, SDSS J100128.61+502756.9 and SDSS J120629.65+433217.6, at redshifts of 1.838 and 1.789 and with image separations of 2.86'' and 2.90'', respectively. The objects were selected as lens candidates from the Sloan Digital Sky Survey (SDSS). Based on the identical nature of the spectra of the two quasars in each pair and the identification of the lens galaxies, we conclude that the objects are gravitational lenses. The lenses are complicated; in both systems there are several galaxies in the fields very close to the quasars, in addition to the lens galaxies themselves. The lens modeling implies that these nearby galaxies contribute significantly to the lens potentials. On larger scales, we have detected an enhancement in the galaxy density near SDSS J100128.61+502756.9. The number of lenses with image separation of ~3'' in the SDSS already exceeds the prediction of simple theoretical models based on the standard Lambda-dominated cosmology and observed velocity function of galaxies.

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arXiv2004

A Survey of z>5.7 Quasars in the Sloan Digital Sky Survey III: Discovery of Five Additional Quasars

We 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 the Northern Galactic Cap. An additional quasar, at redshift 5.85, was discovered by coadding the data obtained in the Fall Equatorial Stripe in the SDSS Southern Survey Region. The fifth object, at redshift 5.80, is selected from a non-standard SDSS scan in the Southern Galactic Cap outside the Main Survey area. The spectrum of SDSS J162331.81+311200.5 (z=6.22) shows a complete Gunn-Peterson trough at z_abs > 5.95, similar to the troughs detected in other three z>6.2 quasars known. We present a composite spectrum of the z>5.7 quasars discovered in the SDSS to date. The average emission line and continuum properties of z~6 quasars exhibit no significant evolution compared to those at low redshift. Using a complete sample of nine z>5.7 quasars, we find that the density of quasars with M_1450 < -26.7 at z~6 is (6+/-2) x 10^-10 per Mpc^3 consistent with our previous estimates. The luminosity distribution of the sample is fit with a power law luminosity function Psi(L) ~ L^(-3.2+/-0.7), somewhat steeper than but consistent with our previous estimates.

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arXiv2004

Microlensing of the Broad Emission Line Region in the Quadruple Lens SDSS J1004+4112

We present seven epochs of spectroscopy on the quadruply imaged quasar SDSS J1004+4112, spanning observed-frame time delays from 1 to 322 days. The spectra reveal differences in the emission lines between the lensed images. Specifically, component A showed a strong enhancement in the blue wings of several high-ionization lines relative to component B, which lasted at least 28 days (observed frame) then faded. Since the predicted time delay between A and B is <30 days, our time coverage suggests that the event was not intrinsic to the quasar. We attribute these variations to microlensing of part of the broad emission line region of the quasar, apparently resolving structure in the source plane on a scale of ~10^{16} cm at z=1.734. In addition, we observed smaller differences in the emission line profiles between components A and B that persisted throughout the time span, which may also be due to microlensing or millilensing. Further spectroscopic monitoring of this system holds considerable promise for resolving the structure of the broad emission line region in quasars.

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