arXiv subjects

Michael A. Strauss

Publications and source records attributed to Michael A. Strauss.

At least 217 records · Page 12Linked to original sources

arXiv2009

A Survey of z~6 Quasars in the SDSS Deep Stripe. II. Discovery of Six Quasars at z_{AB}>21

We present the discovery of six new quasars at z~6 selected from the Sloan Digital Sky Survey (SDSS) southern survey, a deep imaging survey obtained by repeatedly scanning a stripe along the celestial equator. The six quasars are about two magnitudes fainter than the luminous z~6 quasars found in the SDSS main survey and one magnitude fainter than the quasars reported in Paper I (Jiang et al. 2008). Four of them comprise a complete flux-limited sample at 21<z_AB<21.8 over an effective area of 195 deg^2. The other two quasars are fainter than z_AB=22 and are not part of the complete sample. The quasar luminosity function at z~6 is well described as a single power law Φ(L_{1450}) \propto L_{1450}^β over the luminosity range -28<M_{1450}<-25. The best-fitting slope βvaries from -2.6 to -3.1, depending on the quasar samples used, with a statistical error of 0.3-0.4. About 40% of the quasars discovered in the SDSS southern survey have very narrow Lya emission lines, which may indicate small black hole masses and high Eddington luminosity ratios, and therefore short black hole growth time scales for these faint quasars at early epochs.

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arXiv2009

Clustering of Low-Redshift (z <= 2.2) Quasars from the Sloan Digital Sky Survey

We present measurements of the quasar two-point correlation function, ξ_{Q}, over the redshift range z=0.3-2.2 based upon data from the SDSS. Using a homogeneous sample of 30,239 quasars with spectroscopic redshifts from the DR5 Quasar Catalogue, our study represents the largest sample used for this type of investigation to date. With this redshift range and an areal coverage of approx 4,000 deg^2, we sample over 25 h^-3 Gpc^3 (comoving) assuming the current LCDM cosmology. Over this redshift range, we find that the redshift-space correlation function, xi(s), is adequately fit by a single power-law, with s_{0}=5.95+/-0.45 h^-1 Mpc and γ_{s}=1.16+0.11-0.16 when fit over s=1-25 h^-1 Mpc. Using the projected correlation function we calculate the real-space correlation length, r_{0}=5.45+0.35-0.45 h^-1 Mpc and γ=1.90+0.04-0.03, over scales of rp=1-130 h^-1 Mpc. Dividing the sample into redshift slices, we find very little, if any, evidence for the evolution of quasar clustering, with the redshift-space correlation length staying roughly constant at s_{0} ~ 6-7 h^-1 Mpc at z<2.2 (and only increasing at redshifts greater than this). Comparing our clustering measurements to those reported for X-ray selected AGN at z=0.5-1, we find reasonable agreement in some cases but significantly lower correlation lengths in others. We find that the linear bias evolves from b~1.4 at z=0.5 to b~3 at z=2.2, with b(z=1.27)=2.06+/-0.03 for the full sample. We compare our data to analytical models and infer that quasars inhabit dark matter haloes of constant mass M ~2 x 10^12 h^-1 M_Sol from redshifts z~2.5 (the peak of quasar activity) to z~0. [ABRIDGED]

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arXiv2009

Five New High-Redshift Quasar Lenses from the Sloan Digital Sky Survey

We report the discovery of five gravitationally lensed quasars from the Sloan Digital Sky Survey (SDSS). All five systems are selected as two-image lensed quasar candidates from a sample of high-redshift (z>2.2) SDSS quasars. We confirmed their lensing nature with additional imaging and spectroscopic observations. The new systems are SDSS J0819+5356 (source redshift z_s=2.237, lens redshift z_l=0.294, and image separation θ=4.04"), SDSS J1254+2235 (z_s=3.626, θ=1.56"), SDSS J1258+1657 (z_s=2.702, θ=1.28"), SDSS J1339+1310 (z_s=2.243, θ=1.69"), and SDSS J1400+3134 (z_s=3.317, θ=1.74"). We estimate the lens redshifts of the latter four systems to be z_l=0.2-0.8 from the colors and magnitudes of the lensing galaxies. We find that the image configurations of all systems are well reproduced by standard mass models. Although these lenses will not be included in our statistical sample of z_s<2.2 lenses, they expand the number of lensed quasars which can be used for high-redshift galaxy and quasar studies.

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arXiv2009

X-ray Insights into the Nature of Weak Emission-Line Quasars at High Redshift

We present Chandra observations of nine high-redshift quasars (z=2.7-5.9) discovered by the Sloan Digital Sky Survey with weak or undetectable high-ionization emission lines in their UV spectra (WLQs). Adding archival X-ray observations of six additional sources of this class has enabled us to place the strongest constraints yet on the X-ray properties of this remarkable class of AGNs. Although our data cannot rule out the possibility that the emission lines are overwhelmed by a relativistically boosted continuum, as manifested by BL Lac objects, we find that WLQs are considerably weaker in the X-ray and radio bands than the majority of BL Lacs found at much lower redshifts. If WLQs are high-redshift BL Lacs, then it is difficult to explain the lack of a large parent population of X-ray and radio bright weak-lined sources at high redshift. We also consider the possibility that WLQs are quasars with extreme properties, and in particular that the emission lines are suppressed by high accretion rates. Using joint spectral fitting of the X-ray spectra of 11 WLQs we find that the mean photon index in the hard X-ray band is consistent with those observed in typical radio-quiet AGNs with no hint of an unusually steep hard-X-ray spectrum. This result poses a challenge to the hypothesis that WLQs have extremely high accretion rates, and we discuss additional observations required to test this idea.

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arXiv2008

Quasar Clustering from SDSS DR5: Dependences on Physical Properties

Using a homogenous sample of 38,208 quasars with a sky coverage of $4000 {\rm deg^2}$ drawn from the SDSS Data Release Five quasar catalog, we study the dependence of quasar clustering on luminosity, virial black hole mass, quasar color, and radio loudness. At $z<2.5$, quasar clustering depends weakly on luminosity and virial black hole mass, with typical uncertainty levels $\sim 10%$ for the measured correlation lengths. These weak dependences are consistent with models in which substantial scatter between quasar luminosity, virial black hole mass and the host dark matter halo mass has diluted any clustering difference, where halo mass is assumed to be the relevant quantity that best correlates with clustering strength. However, the most luminous and most massive quasars are more strongly clustered (at the $\sim 2σ$ level) than the remainder of the sample, which we attribute to the rapid increase of the bias factor at the high-mass end of host halos. We do not observe a strong dependence of clustering strength on quasar colors within our sample. On the other hand, radio-loud quasars are more strongly clustered than are radio-quiet quasars matched in redshift and optical luminosity (or virial black hole mass), consistent with local observations of radio galaxies and radio-loud type 2 AGN. Thus radio-loud quasars reside in more massive and denser environments in the biased halo clustering picture. Using the Sheth et al.(2001) formula for the linear halo bias, the estimated host halo mass for radio-loud quasars is $\sim 10^{13} h^{-1}M_\odot$, compared to $\sim 2\times 10^{12} h^{-1}M_\odot$ for radio-quiet quasar hosts at $z\sim 1.5$.

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arXiv2008

The 2dF-SDSS LRG and QSO Survey: The spectroscopic QSO catalogue

We present the final spectroscopic QSO catalogue from the 2dF-SDSS LRG and QSO (2SLAQ) Survey. This is a deep, 18<g<21.85 (extinction corrected), sample aimed at probing in detail the faint end of the broad line AGN luminosity distribution at z<2.6. The candidate QSOs were selected from SDSS photometry and observed spectroscopically with the 2dF spectrograph on the Anglo-Australian Telescope. This sample covers an area of 191.9 deg^2 and contains new spectra of 16326 objects, of which 8764 are QSOs, and 7623 are newly discovered (the remainder were previously identified by the 2QZ and SDSS surveys). The full QSO sample (including objects previously observed in the SDSS and 2QZ surveys) contains 12702 QSOs. The new 2SLAQ spectroscopic data set also contains 2343 Galactic stars, including 362 white dwarfs, and 2924 narrow emission line galaxies with a median redshift of z=0.22. We present detailed completeness estimates for the survey, based on modelling of QSO colours, including host galaxy contributions. This calculation shows that at g~21.85 QSO colours are significantly affected by the presence of a host galaxy up to redshift z~1 in the SDSS ugriz bands. In particular we see a significant reddening of the objects in g-i towards fainter g-band magnitudes. This reddening is consistent with the QSO host galaxies being dominated by a stellar population of age at least 2-3 Gyr. The full catalogue, including completeness estimates, is available on-line at http://www.2slaq.info/

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arXiv2008

Evidence for short-lived SN Ia progenitors

We use the VESPA algorithm and spectra from the Sloan Digital Sky Survey to investigate the star formation history of the host galaxies of 257 Type Ia supernovae. We find 5$σ$ evidence for a short-lived population of progenitors with lifetimes of less than 180 Myr, indicating a Type Ia supernova channel arising from stars in the mass range $\sim$3.5-8 $M_\odot$. As standardizeable candles, Type Ia supernovae play an important role in determining the expansion history of the Universe, but to be useful for future cosmological surveys, the peak luminosity needs to be free of uncorrected systematic effects at the level of 1-2%. If the different progenitor routes lead to supernovae with even moderately small differences in properties, then these need to be corrected for separately, or they could lead to a systematic bias in future supernovae surveys, as the prompt route is likely to increase in importance at high redshift. VESPA analysis of hosts could be a valuable tool in this, by identifying which progenitor route is most likely.

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arXiv2008

Mass models and environment of the new quadruply lensed quasar SDSS J1330+1810

We present the discovery of a new quadruply lensed quasar. The lens system, SDSS J1330+1810 at z_s=1.393, was identified as a lens candidate from the spectroscopic sample of the Sloan Digital Sky Survey. Optical and near-infrared images clearly show four quasar images with a maximum image separation of 1.76", as well as a bright lensing galaxy. We measure a redshift of the lensing galaxy of z_l=0.373 from absorption features in the spectrum. We find a foreground group of galaxies at z=0.31 centred ~120" southwest of the lens system. Simple mass models fit the data quite well, including the flux ratios between images, although the lens galaxy appears to be ~1 mag brighter than expected by the Faber-Jackson relation. Our mass modelling suggests that shear from nearby structure is affecting the lens potential.

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arXiv2008

Mid-infrared spectra of optically selected type 2 quasars

Type 2 quasars are luminous Active Galactic Nuclei whose central engines are seen through large amounts of gas and dust. We present Spitzer spectra of twelve type 2 quasars selected on the basis of their optical emission line properties. Within this sample, we find a surprising diversity of spectra, from those that are featureless to those showing strong PAH emission, deep silicate absorption at 10 micron, hydrocarbon absorption, high-ionization emission lines and H_2 rotational emission lines. About half of the objects in the sample are likely Compton-thick, including the two with the deepest Si absorption. The median star-formation luminosity of the objects in our sample measured from the strength of the PAH features is 5x10^11 L_sun, much higher than for field galaxies or for any other AGN sample, but similar to other samples of type 2 quasars. This suggests an evolutionary link between obscured quasars and peak star formation activity in the host galaxy. Despite the high level of star formation, the bolometric output is dominated by the quasar in all cases. For a given strength of 10 micron Si absorption, ULIRGs are significantly colder than are type 2 quasars (their F_nu[14.5 micron]/F_nu[27.5 micron] ratio is 0.5 dex lower), perhaps reflecting different obscuration geometries in these sources. We find that the appearance of the 10 micron feature (i.e., whether it shows in emission or in absorption) is well-correlated with the optical classification in type 1 and type 2 quasars, contrary to some models of clumpy obscuration. Furthermore, this correlation is significantly stronger in quasars (L_bol>10^45 erg/s) than it is in Seyfert galaxies (L_bol<<10^45 erg/s).

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arXiv2008

The shapes of galaxies in the Sloan Digital Sky Survey

We determine the underlying shapes of spiral and elliptical galaxies in the Sloan Digital Sky Survey Data Release 6 from the observed distribution of projected galaxy shapes, taking into account the effects of dust extinction and reddening. We assume that the underlying shapes of spirals and ellipticals are well approximated by triaxial ellipsoids. The elliptical galaxy data are consistent with oblate spheroids, with a correlation between luminosity and ellipticity: the mean values of minor to middle axis ratios are 0.41+-0.03 for Mr ~ -18 ellipticals, and 0.76+-0.04 for Mr ~-22.5 ellipticals. Ellipticals show almost no dependence of axial ratio on galaxy colour, implying a negligible dust optical depth. There is a strong variation of spiral galaxy shapes with colour indicating the presence of dust. The intrinsic shapes of spiral galaxies in the SDSS-DR6 are consistent with flat disks with a mean and dispersion of thickness to diameter ratio of (21+-2)%, and a face-on ellipticity, e, of ln(e)=-2.33+-0.79. Not including the effects of dust in the model leads to disks that are systematically rounder by up to 60%. More luminous spiral galaxies tend to have thicker and rounder disks than lower-luminosity spirals. Both elliptical and spiral galaxies tend to be rounder for larger galaxies. The marginalised value of the edge-on r-band dust extinction E_0 in spiral galaxies is E_0 ~ 0.45 magnitudes for galaxies of median colours, increasing to E_0=1 magnitudes for g-r>0.9 and E_0=1.9 for the luminous and most compact galaxies, with half-light radii <2kpc/h.

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arXiv2008

The SDSS Discovery of a Strongly Lensed Post-Starburst Galaxy at z=0.766

We present the first result of a survey for strong galaxy-galaxy lenses in Sloan Digital Sky Survey (SDSS) images. SDSS J082728.70+223256.4 was selected as a lensing candidate using selection criteria based on the color and positions of objects in the SDSS photometric catalog. Follow-up imaging and spectroscopy showed this object to be a lensing system. The lensing galaxy is an elliptical at z = 0.349 in a galaxy cluster. The lensed galaxy has the spectrum of a post-starburst galaxy at z = 0.766. The lensing galaxy has an estimated mass of $\sim 1.2 \times 10^{12} M_{\odot}$ and the corresponding mass to light ratio in the B-band is $\sim 26 M_{\odot}/L_{\odot}$ inside 1.1 effective radii of the lensing galaxy. Our study shows how catalogs drawn from multi-band surveys can be used to find strong galaxy-galaxy lenses having multiple lens images. Our strong lensing candidate selection based on photometry-only catalogs will be useful in future multi-band imaging surveys such as SNAP and LSST.

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arXiv2008

Biases in Virial Black Hole Masses: An SDSS Perspective

We compile black hole (BH) masses for $\sim 60,000$ quasars in the redshift range $0.1 \lesssim z \lesssim 4.5$ included in the Fifth Data Release of the Sloan Digital Sky Survey (SDSS), using virial BH mass estimators based on the \hbeta, \MgII, and \CIV emission lines. We find that: (1) within our sample, the widths of the three lines follow log-normal distributions, with means and dispersions that do not depend strongly on luminosity or redshift;(2) the \MgII- and \hbeta-estimated BH masses are consistent with one another; and (3) the \CIV BH mass estimator may be more severely affected by a disk wind component than the \MgII and \hbeta estimators, giving a positive bias in mass correlated with the \CIV-\MgII blueshift. Most SDSS quasars have virial BH masses in the range $10^8-10^9 M_\odot$. There is a clear upper mass limit of $\sim 10^{10} M_\odot$ for active BHs at $z \gtrsim 2$, decreasing at lower redshifts. Making the reasonable assumptions that the underlying BH mass distribution decreases with mass and that the Eddington ratio distribution at fixed BH mass has non-zero width, we show that the measured virial BH mass distribution and Eddington ratio distribution are subject to Malmquist bias. A radio quasar subsample (with $1.5\lesssim z\lesssim 2.3$) has mean virial BH mass larger by $\sim 0.12$ dex than the whole sample. A broad absorption line (BAL) quasar subsample (with $1.7\lesssim z\lesssim 2.2$) has identical virial mass distribution as the nonBAL sample, with no mean offset. (Abridged)

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arXiv2007

Do broad absorption line quasars live in different environments from ordinary quasars?

We select a sample of $\sim 4200$ traditionally defined broad absorption line quasars (BALQs) from the Fifth Data Release quasar catalog of the Sloan Digital Sky Survey. For a statistically homogeneous quasar sample with $1.7\le z\le 4.2$, the BAL quasar fraction is $\sim 14%$ and is almost constant with redshift. We measure the auto-correlation of non-BAL quasars (nonBALQs) and the cross-correlation of BALQs with nonBALQs using this statistically homogeneous sample, both in redshift space and using the projected correlation function. We find no significant difference between the clustering strengths of BALQs and nonBALQs. Assuming a power-law model for the real space correlation function $ξ(r)=(r/r_0)^{-1.8}$, the correlation length for nonBALQs is $r_0=7.6\pm 0.8 h^{-1}{\rm Mpc}$; for BALQs, the cross-correlation length is $r_0=7.4\pm 1.1 h^{-1}{\rm Mpc}$. Our clustering results suggest that BALQs live in similar large-scale environments as do nonBALQs.

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arXiv2007

SHARC-II 350 micron Observations of Thermal Emission from Warm Dust in z>=5 Quasars

We present observations of four z>= SDSS quasars at 350 micron with the SHARC-II bolometer camera on the Caltech Submillimeter Observatory. These are among the deepest observations that have been made by SHARC-II at 350 micron, and three quasars are detected at >=3 sigma significance, greatly increasing the sample of 350 micron (corresponds to rest frame wavelengths of <60 micron at z>=5), detected high-redshift quasars. The derived rest frame far-infrared (FIR) emission in the three detected sources is about five to ten times stronger than that expected from the average SED of the local quasars given the same 1450A luminosity. Combining the previous submillimeter and millimeter observations at longer wavelengths, the temperatures of the FIR-emitting warm dust from the three quasar detections are estimated to be in the range of 39 to 52 K. Additionally, the FIR-to-radio SEDs of the three 350 micron detections are consistent with the emission from typical star forming galaxies. The FIR luminosities are ~10^{13} L_solar and the dust masses are >= 10^{8}M_solar. These results confirm that huge amounts of warm dust can exist in the host galaxies of optically bright quasars as early as z~6. The universe is so young at these epochs (~1 Gyr) that a rapid dust formation mechanism is required. We estimate the size of the FIR dust emission region to be about a few kpc, and further provide a comparison of the SEDs among different kinds of dust emitting sources to investigate the dominant dust heating mechanism.

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arXiv2007

Discovery of Four Gravitationally Lensed Quasars from the Sloan Digital Sky Survey

We 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 following four quasars: SDSS J0832+0404 (image separation θ=1.98", source redshift z_s=1.115, lens redshift z_l=0.659); SDSS J1216+3529 (θ=1.49", z_s=2.012); SDSS J1322+1052 (θ=2.00", z_s=1.716); and SDSS J1524+4409 (θ=1.67", z_s=1.210, z_l=0.320). Each system has two lensed images. We find that the fainter image component of SDSS J0832+0404 is significantly redder than the brighter component, perhaps because of differential reddening by the lensing galaxy. The lens potential of SDSS J1216+3529 might be complicated by the presence of a secondary galaxy near the main lensing galaxy.

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arXiv2007

The Sloan Digital Sky Survey Quasar Lens Search. II. Statistical Lens Sample from the Third Data Release

We report the first results of our systematic search for strongly lensed quasars using the spectroscopically confirmed quasars in the Sloan Digital Sky Survey (SDSS). Among 46,420 quasars from the SDSS Data Release 3 (~4188 deg^2), we select a subsample of 22,683 quasars that are located at redshifts between 0.6 and 2.2 and are brighter than the Galactic extinction corrected i-band magnitude of 19.1. We identify 220 lens candidates from the quasar subsample, for which we conduct extensive and systematic follow-up observations in optical and near-infrared wavebands, in order to construct a complete lensed quasar sample at image separations between 1'' and 20'' and flux ratios of faint to bright lensed images larger than 10^{-0.5}. We construct a statistical sample of 11 lensed quasars. Ten of these are galaxy-scale lenses with small image separations (~1''-2'') and one is a large separation (15'') system which is produced by a massive cluster of galaxies, representing the first statistical sample of lensed quasars including both galaxy- and cluster-scale lenses. The Data Release 3 spectroscopic quasars contain an additional 11 lensed quasars outside the statistical sample.

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arXiv2007

The Sloan Digital Sky Survey Quasar Lens Search. III. Constraints on Dark Energy from the Third Data Release Quasar Lens Catalog

We present cosmological results from the statistics of lensed quasars in the Sloan Digital Sky Survey (SDSS) Quasar Lens Search. By taking proper account of the selection function, we compute the expected number of quasars lensed by early-type galaxies and their image separation distribution assuming a flat universe, which is then compared with 7 lenses found in the SDSS Data Release 3 to derive constraints on dark energy under strictly controlled criteria. For a cosmological constant model (w=-1) we obtain Ω_Λ=0.74^{+0.11}_{-0.15}(stat.)^{+0.13}_{-0.06}(syst.). Allowing w to be a free parameter we find Ω_M=0.26^{+0.07}_{-0.06}(stat.)^{+0.03}_{-0.05}(syst.) and w=-1.1\pm0.6(stat.)^{+0.3}_{-0.5}(syst.) when combined with the constraint from the measurement of baryon acoustic oscillations in the SDSS luminous red galaxy sample. Our results are in good agreement with earlier lensing constraints obtained using radio lenses, and provide additional confirmation of the presence of dark energy consistent with a cosmological constant, derived independently of type Ia supernovae.

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arXiv2007

A Survey of z~6 Quasars in the SDSS Deep Stripe: I. a Flux-Limited Sample at z_{AB}<21

We present the discovery of five quasars at z~6 selected from 260 deg^2 of the Sloan Digital Sky Survey (SDSS) southern survey, a deep imaging survey obtained by repeatedly scanning a stripe along the Celestial Equator. The five quasars with 20 =6.0 and =-25.8 is (5.0+/-2.1) x 10^{-9} Mpc^{-3} mag^{-1}. We model the bright-end quasar luminosity function (QLF) at z~6 as a power law Phi(L_{1450}) \propto L_{1450}^{beta}. The slope beta calculated from a combination of our sample and the luminous SDSS quasar sample is -3.1+/-0.4, significantly steeper than the slope of the QLF at z~4. Based on the derived QLF, we find that the quasar/AGN population cannot provide enough photons to ionize the intergalactic medium (IGM) at z~6 unless the IGM is very homogeneous and the luminosity (L*_{1450}) at which the QLF power law breaks is very low.

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