arXiv subjects

Alexander S. Szalay

Publications and source records attributed to Alexander S. Szalay.

At least 91 records · Page 5Linked to original sources

arXiv2006

Spectral Energy Distributions and Multiwavelength Selection of Type 1 Quasars

We present an analysis of the mid-infrared (MIR) and optical properties of type 1 (broad-line) quasars detected by the Spitzer Space Telescope. The MIR color-redshift relation is characterized to z=3, with predictions to z=7. We demonstrate how combining MIR and optical colors can yield even more efficient selection of active galactic nuclei (AGN) than MIR or optical colors alone. Composite spectral energy distributions (SEDs) are constructed for 259 quasars with both Sloan Digital Sky Survey and Spitzer photometry, supplemented by near-IR, GALEX, VLA and ROSAT data where available. We discuss how the spectral diversity of quasars influences the determination of bolometric luminosities and accretion rates; assuming the mean SED can lead to errors as large as a factor of 2 for individual quasars. Finally, we show that careful consideration of the shape of the mean quasar SED and its redshift dependence leads to a lower estimate of the fraction of reddened/obscured AGNs missed by optical surveys as compared to estimates derived from a single mean MIR to optical flux ratio.

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arXiv2006

The SDSS Quasar Survey: Quasar Luminosity Function from Data Release Three

We determine the number counts and z=0-5 luminosity function for a well-defined, homogeneous sample of quasars from the Sloan Digital Sky Survey (SDSS). We conservatively define the most uniform statistical sample possible, consisting of 15,343 quasars within an effective area of 1622 deg^2 that was derived from a parent sample of 46,420 spectroscopically confirmed broad-line quasars in the 5282 deg^2 of imaging data from SDSS Data Release Three. The sample extends from i=15 to i=19.1 at z<3 and to i=20.2 for z>3. The number counts and luminosity function agree well with the results of the 2dF QSO Survey, but the SDSS data probe to much higher redshifts than does the 2dF sample. The number density of luminous quasars peaks between redshifts 2 and 3, although uncertainties in the selection function in this range do not allow us to determine the peak redshift more precisely. Our best fit model has a flatter bright end slope at high redshift than at low redshift. For z<2.4 the data are best fit by a redshift-independent slope of beta = -3.1 (Phi(L) propto L^beta). Above z=2.4 the slope flattens with redshift to beta=-2.37 at z=5. This slope change, which is significant at a >5-sigma level, must be accounted for in models of the evolution of accretion onto supermassive black holes.

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arXiv2005

Photometric Covariance in Multi-Band Surveys: Understanding the Photometric Error in the SDSS

In this paper we describe a detailed analysis of the photometric uncertainties present within the Sloan Digital Sky Survey (SDSS) imaging survey based on repeat observations of approximately 200 square degrees of the sky. We show that, for the standard SDSS aperture systems (petrocounts, counts_model, psfcounts and cmodel_counts), the errors generated by the SDSS photometric pipeline under-estimate the observed scatter in the individual bands. The degree of disagreement is a strong function of aperture and magnitude (ranging from 20% to more than a factor of 2). We also find that the photometry in the five optical bands can be highly correlated for both point sources and galaxies, although the correlation for point sources is almost entirely due to variable objects. Without correcting for this covariance the SDSS color errors could be in over-estimated by a factor of two to three. Combining these opposing effects, the SDSS errors on the colors differ from the observed color variation by approximately 10-20% for most apertures and magnitudes. We provide a prescription to correct the errors derived from the SDSS photometric pipeline as a function of magnitude and a semi-analytic method for generating the appropriate covariance between the different photometric passbands. Given the intrinsic nature of these correlations, we expect that all current and future multi-band surveys will also observe strongly covariant magnitudes. The ability of these surveys to complete their science goals is largely dependent on color-based target selection and photometric redshifts; these results show the importance of spending a significant fraction of early survey operations on re-imaging to empirically determine the photometric covariance of any observing/reduction pipeline.

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arXiv2005

Fourier Phase Analysis of SDSS Galaxies

We present a first analysis of the clustering of SDSS galaxies using the distribution function of the sum of Fourier phases. This statistic was recently proposed by one of authors as a new method to probe phase correlations of cosmological density fields. Since the Fourier phases are statistically independent of Fourier amplitudes, the phase statistic plays a complementary role to the conventional two-point statistics of galaxy clustering. In particular, we focus on the distribution functions of phase sum over three closed wavevectors as a function of the configurations of triangle wavevectors. We find that the observed distribution functions of phase sum are in good agreement with the lowest-order approximation from perturbation theory. For direct comparison with observations, we construct mock catalogs from N-body simulations taking account of several observational effects such as the survey geometry, the redshift distortion, and the discreteness due to the limited number of data. Indeed the observed phase correlations for the galaxies in the range of absolute magnitude -22<Mr<-18 agree well with those for Lambda CDM predictions with sigma8=0.9 evolved from Gaussian initial condition. This agreement implies that the galaxy biasing is approximately linear in redshift space. Instead, assuming that the galaxy biasing is described by a quadratic deterministic function at scale larger than 30Mpc/h, we can constrain the ratio of the quadratic biasing parameter b2 to the linear biasing parameter b1. We find that resulting b2/b1 is mainly dependent on sigma8 and that it is well fitted by b2/b1=0.54(+-0.06)-0.62(+-0.08)sigma8 in CDM models. Indeed, b2/b1 is nearly zero when sigma8=0.9.

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arXiv2005

SDSS J0246-0825: A New Gravitationally Lensed Quasar from the Sloan Digital Sky Survey

We report the discovery of a new two-image gravitationally lensed quasar, SDSS J024634.11-082536.2 (SDSS J0246-0825). This object was selected as a lensed quasar candidate from the Sloan Digital Sky Survey (SDSS) by the same algorithm that was used to discover other SDSS lensed quasars (e.g., SDSS J0924+0219). Multicolor imaging with the Magellan Consortium's Walter Baade 6.5-m telescope and the spectroscopic observations using the W. M. Keck Observatory's Keck II telescope confirm that SDSS J0246-0825 consists of two lensed images ($Δθ=$1\farcs04) of a source quasar at z=1.68. Imaging observations with the Keck telescope and the Hubble Space Telescope reveal an extended object between the two quasar components, which is likely to be a lensing galaxy of this system. From the absorption lines in the spectra of quasar components and the apparent magnitude of the galaxy, combined with the expected absolute magnitude from the Faber-Jackson relation, we estimate the redshift of the lensing galaxy to be z=0.724. A highly distorted ring is visible in the Hubble Space Telescope images, which is likely to be the lensed host galaxy of the source quasar. Simple mass modeling predicts the possibility that there is a small (faint) lensing object near the primary lensing galaxy.

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arXiv2005

Calibrating Photometric Redshifts of Luminous Red Galaxies

We discuss the construction of a photometric redshift catalogue of Luminous Red Galaxies (LRGs) from the Sloan Digital Sky Survey (SDSS), emphasizing the principal steps necessary for constructing such a catalogue -- (i) photometrically selecting the sample, (ii) measuring photometric redshifts and their error distributions, (iii) and estimating the true redshift distribution. We compare two photometric redshift algorithms for these data and find that they give comparable results. Calibrating against the SDSS and SDSS-2dF spectroscopic surveys, we find that the photometric redshift accuracy is $σ\sim 0.03$ for redshifts less than 0.55 and worsens at higher redshift ($\sim 0.06$). These errors are caused by photometric scatter, as well as systematic errors in the templates, filter curves, and photometric zeropoints. We also parametrize the photometric redshift error distribution with a sum of Gaussians, and use this model to deconvolve the errors from the measured photometric redshift distribution to estimate the true redshift distribution. We pay special attention to the stability of this deconvolution, regularizing the method with a prior on the smoothness of the true redshift distribution. The methods we develop are applicable to general photometric redshift surveys.

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arXiv2005

Large Scale Clustering of Sloan Digital Sky Survey Quasars: Impact of the Baryon Density and the Cosmological Constant

We report the first result of the clustering analysis of Sloan Digital Sky Survey (SDSS) quasars. We compute the two-point correlation function (2PCF) of SDSS quasars in redshift space at $8h^{-1}{\rm Mpc} < s < 500h^{-1}{\rm Mpc}$, with particular attention to its baryonic signature. Our sample consists of 19986 quasars extracted from the SDSS Data Release 4 (DR4). The redshift range of the sample is $0.72 \le z \le 2.24$ (the mean redshift is $\bar z = 1.46$) and the reddening-corrected $i$-band apparent magnitude range is $15.0 \le m_{i,{\rm rc}} \le 19.1$. Due to the relatively low number density of the quasar sample, the bump in the power spectrum due to the baryon density, $Ω_{\rm b}$, is not clearly visible. The effect of the baryon density is, however, to distort the overall shape of the 2PCF.The degree of distortion makes it an interesting alternate measure of the baryonic signature. Assuming a scale-independent linear bias and the spatially flat universe, i.e., $Ω_{\rm b} + Ω_{\rm d} + Ω_Λ=1$, where $Ω_{\rm d}$ and $Ω_Λ$ denote the density parameters of dark matter and the cosmological constant, we combine the observed quasar 2PCF and the predicted matter 2PCF to put constraints on $Ω_{\rm b}$ and $Ω_Λ$. Our result is fitted as $0.80- 2.8Ω_{\rm b} < Ω_Λ< 0.90 - 1.4Ω_{\rm b}$ at the 2$σ$ confidence level, which is consistent with results from other cosmological observations such as WMAP. (abridged)

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arXiv2005

When Database Systems Meet the Grid

We illustrate the benefits of combining database systems and Grid technologies for data-intensive applications. Using a cluster of SQL servers, we reimplemented an existing Grid application that finds galaxy clusters in a large astronomical database. The SQL implementation runs an order of magnitude faster than the earlier Tcl-C-file-based implementation. We discuss why and how Grid applications can take advantage of database systems.

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arXiv2005

Scientific Data Management in the Coming Decade

This is a thought piece on data-intensive science requirements for databases and science centers. It argues that peta-scale datasets will be housed by science centers that provide substantial storage and processing for scientists who access the data via smart notebooks. Next-generation science instruments and simulations will generate these peta-scale datasets. The need to publish and share data and the need for generic analysis and visualization tools will finally create a convergence on common metadata standards. Database systems will be judged by their support of these metadata standards and by their ability to manage and access peta-scale datasets. The procedural stream-of-bytes-file-centric approach to data analysis is both too cumbersome and too serial for such large datasets. Non-procedural query and analysis of schematized self-describing data is both easier to use and allows much more parallelism.

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arXiv2005

Where the Rubber Meets the Sky: Bridging the Gap between Databases and Science

Scientists in all domains face a data avalanche - both from better instruments and from improved simulations. We believe that computer science tools and computer scientists are in a position to help all the sciences by building tools and developing techniques to manage, analyze, and visualize peta-scale scientific information. This article is summarizes our experiences over the last seven years trying to bridge the gap between database technology and the needs of the astronomy community in building the World-Wide Telescope.

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arXiv2004

There Goes the Neighborhood: Relational Algebra for Spatial Data Search

We explored ways of doing spatial search within a relational database: (1) hierarchical triangular mesh (a tessellation of the sphere), (2) a zoned bucketing system, and (3) representing areas as disjunctive-normal form constraints. Each of these approaches has merits. They all allow efficient point-in-region queries. A relational representation for regions allows Boolean operations among them and allows quick tests for point-in-region, regions-containing-point, and region-overlap. The speed of these algorithms is much improved by a zone and multi-scale zone-pyramid scheme. The approach has the virtue that the zone mechanism works well on B-Trees native to all SQL systems and integrates naturally with current query optimizers - rather than requiring a new spatial access method and concomitant query optimizer extensions. Over the last 5 years, we have used these techniques extensively in our work on SkyServer.sdss.org, and SkyQuery.net.

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arXiv2004

Three-point Correlation Functions of SDSS Galaxies in Redshift Space: Morphology, Color, and Luminosity Dependence

We present measurements of the redshift--space three-point correlation function of galaxies in the Sloan Digital Sky Survey (SDSS). For the first time, we analyze the dependence of this statistic on galaxy morphology, color and luminosity. In order to control systematics due to selection effects, we used $r$--band, volume-limited samples of galaxies, constructed from the magnitude-limited SDSS data ($14.5<r<17.5$), and further divided the samples into two morphological types (early and late) or two color populations (red and blue). The three-point correlation function of SDSS galaxies follow the hierarchical relation well and the reduced three-point amplitudes in redshift--space are almost scale-independent ($Q_z=0.5\sim1.0$). In addition, their dependence on the morphology, color and luminosity is not statistically significant. Given the robust morphological, color and luminosity dependences of the two-point correlation function, this implies that galaxy biasing is complex on weakly non-linear to non-linear scales. We show that simple deterministic linear relation with the underlying mass could not explain our measurements on these scales.

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arXiv2004

The Clustering of AGN in the Sloan Digital Sky Survey

We present the two--point correlation function (2PCF) of narrow-line active galactic nuclei (AGN) selected within the First Data Release of the Sloan Digital Sky Survey. Using a sample of 13605 AGN in the redshift range 0.055 < z < 0.2, we find that the AGN auto--correlation function is consistent with the observed galaxy auto--correlation function on scales 0.2h^{-1}Mpc to >100h^{-1}Mpc. The AGN hosts trace an intermediate population of galaxies and are not detected in either the bluest (youngest) disk--dominated galaxies or many of the reddest (oldest) galaxies. We show that the AGN 2PCF is dependent on the luminosity of the narrow [OIII] emission line (L_{[OIII]}), with low L_{[OIII]} AGN having a higher clustering amplitude than high L_{[OIII]} AGN. This is consistent with lower activity AGN residing in more massive galaxies than higher activity AGN, and L_{[OIII]} providing a good indicator of the fueling rate. Using a model relating halo mass to black hole mass in cosmological simulations, we show that AGN hosted by ~ 10^{12} M_{odot} dark matter halos have a 2PCF that matches that of the observed sample. This mass scale implies a mean black hole mass for the sample of M_{BH} ~ 10^8 M_{odot}.

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arXiv2004

Galaxy-galaxy weak lensing in SDSS: intrinsic alignments and shear calibration errors

Galaxy-galaxy lensing has emerged as a powerful probe of the dark matter halos of galaxies, but is subject to contamination if intrinsically aligned satellites of the lens galaxy are used as part of the source sample. We present a measurement of this intrinsic shear using 200,747 lens galaxies from the Sloan Digital Sky Survey (SDSS) spectroscopic sample and a sample of satellites selected using photometric redshifts. The mean intrinsic shear at transverse separations of 30--446$h^{-1}$ kpc is constrained to be $-0.0062<Δγ<+0.0066$ (99.9 per cent confidence, including identified systematics), which limits contamination of the galaxy-galaxy lensing signal to at most $\sim 15$ per cent on these scales. We present these limits as a function of transverse separation and lens luminosity. We furthermore investigate shear calibration biases in the SDSS, which can also affect galaxy-galaxy lensing, and conclude that the shear amplitude is calibrated to better than 18 per cent. This includes noise-induced calibration biases in the ellipticity, which are small for the sample considered here, but which can be more important if low signal-to-noise or poorly resolved source galaxies are used.

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arXiv2004

Extending the SDSS Batch Query System to the National Virtual Observatory Grid

The Sloan Digital Sky Survey science database is approaching 2TB. While the vast majority of queries normally execute in seconds or minutes, this interactive execution time can be disproportionately increased by a small fraction of queries that take hours or days to run; either because they require non-index scans of the largest tables or because they request very large result sets. In response to this, we added a multi-queue job submission and tracking system. The transfer of very large result sets from queries over the network is another serious problem. Statistics suggested that much of this data transfer is unnecessary; users would prefer to store results locally in order to allow further cross matching and filtering. To allow local analysis, we implemented a system that gives users their own personal database (MyDB) at the portal site. Users may transfer data to their MyDB, and then perform further analysis before extracting it to their own machine. We intend to extend the MyDB and asynchronous query ideas to multiple NVO nodes. This implies development, in a distributed manner, of several features, which have been demonstrated for a single node in the SDSS Batch Query System (CasJobs). The generalization of asynchronous queries necessitates some form of MyDB storage as well as workflow tracking services on each node and coordination strategies among nodes.

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arXiv2004

The World Wide Telescope: An Archetype for Online Science

Most scientific data will never be directly examined by scientists; rather it will be put into online databases where it will be analyzed and summarized by computer programs. Scientists increasingly see their instruments through online scientific archives and analysis tools, rather than examining the raw data. Today this analysis is primarily driven by scientists asking queries, but scientific archives are becoming active databases that self-organize and recognize interesting and anomalous facts as data arrives. In some fields, data from many different archives can be cross-correlated to produce new insights. Astronomy presents an excellent example of these trends; and, federating Astronomy archives presents interesting challenges for computer scientists.

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arXiv2004

The Sloan Digital Sky Survey Science Archive: Migrating a Multi-Terabyte Astronomical Archive from Object to Relational DBMS

The Sloan Digital Sky Survey Science Archive is the first in a series of multi-Terabyte digital archives in Astronomy and other data-intensive sciences. To facilitate data mining in the SDSS archive, we adapted a commercial database engine and built specialized tools on top of it. Originally we chose an object-oriented database management system due to its data organization capabilities, platform independence, query performance and conceptual fit to the data. However, after using the object database for the first couple of years of the project, it soon began to fall short in terms of its query support and data mining performance. This was as much due to the inability of the database vendor to respond our demands for features and bug fixes as it was due to their failure to keep up with the rapid improvements in hardware performance, particularly faster RAID disk systems. In the end, we were forced to abandon the object database and migrate our data to a relational database. We describe below the technical issues that we faced with the object database and how and why we migrated to relational technology.

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arXiv2004

Palomar-QUEST: A case study in designing sky surveys in the VO era

The advent of wide-area multicolour synoptic sky surveys is leading to data sets unprecedented in size, complexity and data throughput. VO technology offers a way to exploit these to the full but requires changes in design philosophy. The Palomar-QUEST survey is a major new survey being undertaken by Caltech, Yale, JPL and Indiana University to repeatedly observe 1/3 of the sky (~15000 sq. deg. between -27 < Dec <27 in seven passbands. Utilising the 48-inch Oschin Schmidt Telescope at the Palomar Observatory with the 112-CCD QUEST camera covering the full 4 x 4 sq. deg. field of view, it will generate \~1TB of data per month. In this paper, we review the design of QUEST as a VO resource, a federated data set and an exemplar of VO standards.

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