Y. AlSayyad

Lowell Observatory, Flagstaff, Arizona, United States

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Publications (21)51.28 Total impact

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    ABSTRACT: We present the fiducial main-sequence stellar locus traced by 10 photometric colours observed by Sloan Digital Sky Survey (SDSS), Two Micron All Sky Survey (2MASS), and Wide-field Infrared Survey Explorer (WISE). Median colours are determined using 1052 793 stars with r-band extinction less than 0.125. We use this locus to measure the dust extinction curve relative to the r band, which is consistent with previous measurements in the SDSS and 2MASS bands. The WISE band extinction coefficients are larger than predicted by standard extinction models. Using 13 lines of sight, we find variations in the extinction curve in H, Ks, and WISE bandpasses. Relative extinction decreases towards Galactic anticentre, in agreement with prior studies. Relative extinction increases with Galactic latitude, in contrast to previous observations. This indicates a universal mid-IR extinction law does not exist due to variations in dust grain size and chemistry with Galactocentric position. A preliminary search for outliers due to warm circumstellar dust is also presented, using stars with high signal-to-noise ratio in the W3 band. We find 199 such outliers, identified by excess emission in Ks - W3. Inspection of SDSS images for these outliers reveals a large number of contaminants due to nearby galaxies. Six sources appear to be genuine dust candidates, yielding a fraction of systems with infrared excess of 0.12 ± 0.05 per cent.
    Monthly Notices of the Royal Astronomical Society 05/2014; 440(4). DOI:10.1093/mnras/stu466 · 5.23 Impact Factor
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    ABSTRACT: We present the fiducial main sequence stellar locus traced by 10 photometric colors observed by SDSS, 2MASS, and WISE. Median colors are determined using 1,052,793 stars with r-band extinction less than 0.125. We use this locus to measure the dust extinction curve relative to the r-band, which is consistent with previous measurements in the SDSS and 2MASS bands. The WISE band extinction coefficients are larger than predicted by standard extinction models. Using 13 lines of sight, we find variations in the extinction curve in H, Ks, and WISE bandpasses. Relative extinction decreases towards Galactic anti-center, in agreement with prior studies. Relative extinction increases with Galactic latitude, in contrast to previous observations. This indicates a universal mid-IR extinction law does not exist due to variations in dust grain size and chemistry with Galactocentric position. A preliminary search for outliers due to warm circumstellar dust is also presented, using stars with high signal-to-noise in the W3-band. We find 199 such outliers, identified by excess emission in Ks-W3. Inspection of SDSS images for these outliers reveals a large number of contaminants due to nearby galaxies. Six sources appear to be genuine dust candidates, yielding a fraction of systems with infrared excess of 0.12$\pm$0.05%.
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    ABSTRACT: We measure the two-point cross-correlation function of C IV absorber systems and quasars, using spectroscopic data from the Sloan Digital Sky Survey III Baryon Oscillation Spectroscopic Survey (BOSS; Data Release 9). The 19,701 quasars and 6149 C IV "moderate" absorbers, 0.28 Å < rest-frame equivalent width (EW) < 5 Å, in our study cover a redshift range of 2.1 < z < 2.5 over 3300 deg2 and represent a factor of two increase in sample size over previous investigations. We find a correlation scale length and slope of the redshift-space cross-correlation function of s 0 = 8.46 ± 1.24 Mpc, γ = 1.68 ± 0.19, in the redshift-space range 10 < s < 100 Mpc. We find a projected cross-correlation function of C IV absorption systems and quasars of r 0 = 7.76 ± 2.80 Mpc, γ = 1.74 ± 0.21. We measure the combined quasar and C IV bias to be b QSOb C IV = 8.81 ± 2.28. Using an estimate of b QSO from the quasar auto-correlation function we find b C IV = 2.38 ± 0.62. This b C IV implies that EW > 0.28 Å C IV absorbers at z ~ 2.3 are typically found in dark matter halos that have masses ≥1011.3-1013.4M ☉ at that redshift. The complete BOSS sample will triple the number of both quasars and absorption systems and increase the power of this cross-correlation measurement by a factor of two.
    The Astrophysical Journal 05/2013; 768(1):38. DOI:10.1088/0004-637X/768/1/38 · 6.28 Impact Factor
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    ABSTRACT: The statistics of peak counts in reconstructed shear maps contain information beyond the power spectrum, and can improve cosmological constraints from measurements of the power spectrum alone if systematic errors can be controlled. We study the effect of galaxy shape measurement errors on predicted cosmological constraints from the statistics of shear peak counts with the Large Synoptic Survey Telescope (LSST). We use the LSST image simulator in combination with cosmological N-body simulations to model realistic shear maps for different cosmological models. We include both galaxy shape noise and, for the first time, measurement errors on galaxy shapes. We find that the measurement errors considered have relatively little impact on the constraining power of shear peak counts for LSST.
    The Astrophysical Journal 01/2013; 774(1). DOI:10.1088/0004-637X/774/1/49 · 6.28 Impact Factor
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    ABSTRACT: The complete 10-yr survey from the Large Synoptic Survey Telescope (LSST) will image ˜20 000 deg2 of the sky in six filter bands every few nights, bringing the final survey depth to r ˜ 27.5, with over four billion well-measured galaxies. To take full advantage of this unprecedented statistical power, the systematic errors associated with weak lensing measurements need to be controlled to a level similar to the statistical errors. This work is the first attempt to quantitatively estimate the absolute level and statistical properties of the systematic errors on weak lensing shear measurements due to the most important physical effects in the LSST system via high-fidelity ray-tracing simulations. We identify and isolate the different sources of algorithm-independent, additive systematic errors on shear measurements for LSST and predict their impact on the final cosmic shear measurements using conventional weak lensing analysis techniques. We find that the main source of the errors comes from an inability to adequately characterize the atmospheric point spread function due to its high-frequency spatial variation on angular scales smaller than ˜10 arcmin in the single short exposures, which propagates into a spurious shear correlation function at the 10-4-10-3 level on these scales. With the large multi-epoch data set that will be acquired by LSST, the stochastic errors average out, bringing the final spurious shear correlation function to a level very close to the statistical errors. Our results imply that the cosmological constraints from LSST will not be severely limited by these algorithm-independent, additive systematic effects.
    Monthly Notices of the Royal Astronomical Society 01/2013; 428(3):2695-2713. DOI:10.1093/mnras/sts223 · 5.23 Impact Factor
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    ABSTRACT: The complete 10-year survey from the Large Synoptic Survey Telescope (LSST) will image $\sim$ 20,000 square degrees of sky in six filter bands every few nights, bringing the final survey depth to $r\sim27.5$, with over 4 billion well measured galaxies. To take full advantage of this unprecedented statistical power, the systematic errors associated with weak lensing measurements need to be controlled to a level similar to the statistical errors. This work is the first attempt to quantitatively estimate the absolute level and statistical properties of the systematic errors on weak lensing shear measurements due to the most important physical effects in the LSST system via high fidelity ray-tracing simulations. We identify and isolate the different sources of algorithm-independent, \textit{additive} systematic errors on shear measurements for LSST and predict their impact on the final cosmic shear measurements using conventional weak lensing analysis techniques. We find that the main source of the errors comes from an inability to adequately characterise the atmospheric point spread function (PSF) due to its high frequency spatial variation on angular scales smaller than $\sim10'$ in the single short exposures, which propagates into a spurious shear correlation function at the $10^{-4}$--$10^{-3}$ level on these scales. With the large multi-epoch dataset that will be acquired by LSST, the stochastic errors average out, bringing the final spurious shear correlation function to a level very close to the statistical errors. Our results imply that the cosmological constraints from LSST will not be severely limited by these algorithm-independent, additive systematic effects.
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    ABSTRACT: A main science goal for the Large Synoptic Survey Telescope (LSST) is to measure the cosmic shear signal from weak lensing to extreme accuracy. One difficulty, however, is that with the short exposure time ($\simeq$15 seconds) proposed, the spatial variation of the Point Spread Function (PSF) shapes may be dominated by the atmosphere, in addition to optics errors. While optics errors mainly cause the PSF to vary on angular scales similar or larger than a single CCD sensor, the atmosphere generates stochastic structures on a wide range of angular scales. It thus becomes a challenge to infer the multi-scale, complex atmospheric PSF patterns by interpolating the sparsely sampled stars in the field. In this paper we present a new method, PSFent, for interpolating the PSF shape parameters, based on reconstructing underlying shape parameter maps with a multi-scale maximum entropy algorithm. We demonstrate, using images from the LSST Photon Simulator, the performance of our approach relative to a 5th-order polynomial fit (representing the current standard) and a simple boxcar smoothing technique. Quantitatively, PSFent predicts more accurate PSF models in all scenarios and the residual PSF errors are spatially less correlated. This improvement in PSF interpolation leads to a factor of 3.5 lower systematic errors in the shear power spectrum on scales smaller than $\sim13'$, compared to polynomial fitting. We estimate that with PSFent and for stellar densities greater than $\simeq1/{\rm arcmin}^{2}$, the spurious shear correlation from PSF interpolation, after combining a complete 10-year dataset from LSST, is lower than the corresponding statistical uncertainties on the cosmic shear power spectrum, even under a conservative scenario.
    Monthly Notices of the Royal Astronomical Society 06/2012; DOI:10.1111/j.1365-2966.2012.22134.x · 5.23 Impact Factor
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    ABSTRACT: By selecting astrometric and photometric data from the Sloan Digital Sky Survey (SDSS), the L{\'e}pine & Shara Proper Motion North Catalog (LSPM-North), the Two Micron All Sky Survey (2MASS), and the USNO-B1.0 catalog, we use a succession of methods to isolate white dwarf candidates for follow-up spectroscopy. Our methods include: reduced proper motion diagram cuts, color cuts, and atmospheric model adherence. We present spectroscopy of 26 white dwarfs obtained from the CTIO 4m and APO 3.5m telescopes. Additionally, we confirm 28 white dwarfs with spectra available in the SDSS DR7 database but unpublished elsewhere, presenting a total of 54 WDs. We label one of these as a recovered WD while the remaining 53 are new discoveries. We determine physical parameters and estimate distances based on atmospheric model analyses. Three new white dwarfs are modeled to lie within 25 pc. Two additional white dwarfs are confirmed to be metal-polluted (DAZ). Follow-up time series photometry confirms another object to be a pulsating ZZ Ceti white dwarf.
    The Astronomical Journal 03/2012; 143(4). DOI:10.1088/0004-6256/143/4/103 · 4.97 Impact Factor
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    ABSTRACT: We present a preliminary characterization of the Galactic stellar populations from the Wide-field Infrared Space Explorer (WISE) Preliminary Data Release. We trace the main sequence color locus in 16 dimensions using a matched sample between IRAS, SDSS and 2MASS for several million stars. This exquisite characterization of "normal” stars enables efficient and robust searches for non-main-sequence stars. For example, the excellent faint limit of WISE yields a sample of AGB candidates using the 4.6, 12, and 22 micron bandpasses that exceeds the IRAS AGB catalog by up to an order of magnitude. Our preliminary analysis suggests that the WISE database will be a goldmine for studies of Galactic stellar populations.
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    ABSTRACT: SDSS-III BOSS provides homogeneous sample of 150,000 quasar spectra during the course of survey. The spectra give important information about the intervening absorbing material as well as about the quasars themselves. We can study the clustering properties of the intervening CIV absorbers found in the quasar spectra by cross-correlating the CIV absorber sample with the well-understood sample of quasars. Measuring the linear bias for the CIV absorber systems in a large homogeneous survey allows us to constrain the origin of these CIV absorber systems. It will also provide the constraints for the feedback processes in galaxy formation theory. We study the cross correlation in the range 2.1 < z < 2.6 as the abundance of CIV absorbers and quasars overlap at these redshift. We present the results from a CIV absorber clustering study from the first 2 years of BOSS data.
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    ABSTRACT: Foreground absorption features observed in the spectra of distant quasars provide insight into the evolution of elemental abundances in galaxies and the intergalactic medium as well as the growth of large-scale structure from high redshift to the present. Through the development of an automated pipeline to extract and identify absorption lines in SDSS-III BOSS quasar spectra, we have compiled the largest sample of metal absorbers to date. Using these data, we present new high-precision measurements of the redshift evolution of the number densities and equivalent width distributions of Mg II and C IV absorbers. These measurements sample a wider redshift range than previously attainable and provide new insights into the evolution of the gas content of galaxy haloes from z 5.
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    ABSTRACT: When simulating the distribution of sources across the night sky, querying for stationary objects, such as galaxies, is relatively simple. For moving objects, such as near earth objects (NEOs) and main belt asteroids (MBAs), this becomes increasing more complex. Each family of solar system objects has a range of abundances and speeds through ra/dec space. For example, MBAs are plentiful ( 107) but move slowly (< 1 deg/day), and NEOs are rare ( 105) but can move up to 70 deg/day. How do we optimally store and query all families of moving objects? We describe performance results and experiences using different methods, such as storing bounding boxes for the trajectories, and spatial abstraction tools, such as MSSQL geospatial support and SkyServer's HTM index and library of spatial constructs. We apply these results to simulations of the data flow from the Large Synoptic Survey Telescope with the goal of querying simulated catalogs quickly for a list of objects that would appear in the LSST's circular aperture at a given pointing and epoch.
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    ABSTRACT: The new generation of telescopes under construction return to the same area of the sky with sufficient frequency to enable tracking of moving objects such as asteroids, near-earth objects, and comets [4,5]. To detect these moving objects, one image may be subtracted from another (separated by several days or weeks) to differentiate variable and moving sources from the dense background of stars and galaxies. Moving sources may then be identified by querying against a database of expected positions of known asteroids. At a high-level, this task maps onto executing the query: “Return all known asteroids that are expected to be located within a given region at a given time.” We consider the problem of querying for asteroids in a specified interval in space and time, specifically as applied to populating the simulations of the data flow from the Large Synoptic Survey Telescope (LSST).
    Scientific and Statistical Database Management - 23rd International Conference, SSDBM 2011, Portland, OR, USA, July 20-22, 2011. Proceedings; 01/2011
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    ABSTRACT: We search for new white dwarfs by compiling data from multiple surveys including the Sloan Digital Sky Survey (SDSS), the Two Micron All Sky Survey (2MASS), the Lepine & Shara Proper Motion North (LSPM-North) catalog, and USNO-B catalog. By initially filtering via reduced proper motion diagrams, color cuts, and atmospheric model adherence, white dwarf candidates are isolated for followup spectroscopy. Subdwarf contaminates were observed to persist through the initial filtering process solely in the modeled Teff regime < 7000 K. To mitigate this contamination effect, linear discriminant analysis (LDA) was utilized in further cool white dwarf candidate searches. We present spectroscopy of 17 white dwarfs taken with the CTIO 4m and the APO 3.5m telescopes, while 30 additional white dwarf spectra were found in the SDSS database. These spectra represent a range of spectral types and Teff and serve to investigate the merit of LDA as a tool for contaminant identification and elimination. Our LDA process aided in the discovery of two cool white dwarfs with preliminary distance estimates of 13.6 pc and 15.7 pc. This research was supported by the 2010 CTIO REU program. Based on observations obtained with the Apache Point Observatory 3.5-meter telescope, which is owned and operated by the Astrophysical Research Consortium.
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    ABSTRACT: We analyze the cross-correlation of Mg II (λ2796, 2803) quasar absorption systems with luminous red galaxies (LRGs) from the Fifth Data Release (DR5) of the Sloan Digital Sky Survey. The absorption line sample consists of 2705 unambiguously intervening Mg II absorption systems, detected at a 4σ level, covering a redshift range (0.36 ≤z abs≤ 0.8) and a rest equivalent-width range of 0.8 Å ≤ W λ2796 r ≤ 5.0 Å. We cross-correlate these absorbers with 1,495,604 LRGs with accurate photometric redshifts in the same redshift range and examine the relationship of Mg II equivalent width and clustering amplitude. We confirm with high precision a previously reported weak anticorrelation of equivalent width and the dark-matter halo mass, measuring dark-matter halo masses of Mg II absorbers to be log Mh (M ☉ h –1) = 11.29 ± 0.36 0.62 for the Wr ≥ 1.4 Å sample and log Mh (M ☉ h –1) = 12.70 ± 0.53 1.16 for absorbers with 0.8 Å ≤Wr < 1.4 Å. These measurements agree with previous reported values within the stated errors. Additionally, we investigate the significance of a number of potential sources of bias inherent in absorber-LRG cross-correlation measurements, including absorber velocity distributions and weak lensing of background quasars, which we determine is capable of producing a 20%-30% bias in angular cross-correlation measurements on scales less than 2'. We measure the Mg II-LRG cross-correlation for 719 absorption systems with v < 60, 000 km s–1 in the quasar rest frame and find that these absorbers typically reside in dark-matter halos that are ~10-100 times more massive than those hosting unambiguously intervening Mg II absorbers. Furthermore, we find evidence for evolution of the redshift number density, ∂N/∂z, with 2σ significance for the strongest (W λ2796 r 2.0 Å) absorbers in the DR5 sample. This width-dependent ∂N/∂z evolution does not significantly affect the recovered equivalent width-halo mass anticorrelation and adds to existing evidence that the strongest Mg II absorption systems are correlated with an evolving population of field galaxies at these redshifts, while the nonevolving ∂N/∂z of the weakest absorbers more closely resembles the LRG population.
    The Astrophysical Journal 05/2009; 698(1):819. DOI:10.1088/0004-637X/698/1/819 · 6.28 Impact Factor
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    ABSTRACT: We studied a sample of 415 associated (zabs ~ zem; relative velocity with respect to QSO in units of c, β < 0.01) Mg II absorption systems with 1.0 ≤ zabs ≤ 1.86, in the spectra of SDSS DR3 QSOs, to determine the dust content and ionization state in the absorbers. We also compared these properties to those of a similarly selected sample of 809 intervening systems (β > 0.01), so as to understand their origin. Normalized, composite spectra were derived for absorption line measurements, for the full sample and for several subsamples, chosen on the basis of the line strengths and other absorber and QSO properties. From these, and from the equivalent widths in individual spectra, we conclude that the associated Mg II absorbers have higher ionization (higher ratios of the strengths of C IV and Mg II lines), than the intervening absorbers. The ionization decreases with increasing β. Average extinction curves were obtained for the subsamples by comparing their geometric mean QSO spectra with those of matching (in zem and i magnitude) samples of QSOs without absorption lines. There is clear evidence for SMC-like dust attenuation in these systems; the 2175 Å absorption feature is absent. The extinction is almost twice that observed in intervening systems. We reconfirm that QSOs with nonzero FIRST radio flux are intrinsically redder than the QSOs with no detection in the FIRST survey. The incidence of associated Mg II systems in radio-detected QSOs is 1.7 times that in radio-undetected QSOs. The associated absorbers in radio-detected QSOs cause 3 times more reddening than those in radio-undetected QSOs. This excess reddening is correlated with the strength of Mg II absorption, possibly suggesting an intrinsic nature for the associated absorbers in radio-detected QSOs.
    The Astrophysical Journal 12/2008; 679(1):239. DOI:10.1086/587122 · 6.28 Impact Factor
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    ABSTRACT: We have studied a sample of 415 associated (z_ab z_em; relative velocity with respect to QSO <3000km/s) Mg II absorption systems with 1.0<=z_ab<=1.86, in the spectra of SDSS DR3 QSOs, to determine the dust content and ionization state in the absorbers. We studied the dependence of these properties on the properties of the QSOs and also, compared the properties with those of a similarly selected sample of 809 intervening systems (apparent relative velocity with respect to the QSO of >3000km/s), so as to understand their origin. From the analysis of the composite spectra, as well as from the comparison of measured equivalent widths in individual spectra, we conclude that the associated Mg II absorbers have higher apparent ionization, measured by the strength of the C IV absorption lines compared to the Mg II absorption lines, than the intervening absorbers. The ionization so measured appears to be related to apparent ejection velocity, being lower as the apparent ejection velocity is more and more positive. There is clear evidence, from the composite spectra, for SMC like dust attenuation in these systems; the 2175AA absorption feature is not present. The extinction is almost twice that observed in the similarly selected sample of intervening systems. We reconfirm that QSOs with non-zero FIRST radio flux are intrinsically redder than the QSOs with no detection in the FIRST survey. The incidence of associated Mg II systems in QSOs with non-zero FIRST radio flux is 1.7 times that in the QSOs with no detection in the FIRST survey. The associated absorbers in radio-detected QSOs which comprise about 12% of our sample, cause 3 times more reddening than the associated absorbers in radio-undetected QSOs. This excess reddening possibly suggests an intrinsic nature for the associated absorbers in radio-detected QSOs.
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    ABSTRACT: We have studied a sample of 809 Mg ii absorption systems with 1.0 ≤zabs≤ 1.86 in the spectra of Sloan Digital Sky Survey quasi-stellar objects (QSOs), with the aim of understanding the nature and abundance of the dust and the chemical abundances in the intervening absorbers. Normalized, composite spectra were derived, for abundance measurements, for the full sample and several subsamples, chosen on the basis of the line strengths and other absorber and QSO properties. Average extinction curves were obtained for the subsamples by comparing their geometric mean spectra with those of matching samples of QSOs without absorbers in their spectra. There is clear evidence for the presence of dust in the intervening absorbers. The 2175-Å feature is not present in the extinction curves, for any of the subsamples. The extinction curves are similar to the Small Magellanic Cloud (SMC) extinction curve with a rising ultraviolet (UV) extinction below 2200 Å. The absorber rest-frame colour excess, E(B−V), derived from the extinction curves, depends on the absorber properties and ranges from <0.001 to 0.085 for various subsamples. The column densities of Mg ii, Al ii, Si ii, Ca ii, Ti ii, Cr ii, Mn ii, Fe ii, Co ii, Ni ii and Zn ii do not show such a correspondingly large variation. The overall depletions in the high E(B−V) samples are consistent with those found for individual damped Lyman systems, the depletion pattern being similar to halo clouds in the Galaxy. Assuming an SMC gas-to-dust ratio, we find a trend of increasing abundance with decreasing extinction; systems with NH I∼ 1020 cm−2 show solar abundance of Zn. The large velocity spread of strong Mg ii systems seems to be mimicked by weak lines of other elements. The ionization of the absorbers, in general appears to be low: the ratio of the column densities of Al iii to Al ii is always less than 1/2. QSOs with absorbers are, in general, at least three times as likely to have highly reddened spectra as compared to QSOs without any absorption systems in their spectra.
    Monthly Notices of the Royal Astronomical Society 03/2006; 367(3):945 - 978. DOI:10.1111/j.1365-2966.2005.10018.x · 5.52 Impact Factor
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    ABSTRACT: We find evidence for dust in the intervening QSO absorbers from the spectra of QSOs in the Sloan Digital Sky Survey Data Release 1. No evidence is found for the 2175 Å feature which is present in the Milky Way dust extinction curve.
    Proceedings of the International Astronomical Union 02/2005; 1(C199):427 - 429. DOI:10.1017/S1743921305002978
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    ABSTRACT: The spectra of the Sloan Digital Sky Survey (SDSS) are being used to construct a catalogue of QSO absorption lines, for use in studies of abundances, relevant radiation fields, number counts as a function of redshift, and other matters, including the evolution of these parameters. The catalogue includes intervening, associated, and BAL absorbers, in order to allow a clearer definition of the relationships between these three classes. We describe the motivation for and the data products of the project to build the SDSS QSO absorption line catalogue.
    Proceedings of the International Astronomical Union 02/2005; 1:58 - 64. DOI:10.1017/S1743921305002437

Publication Stats

202 Citations
51.28 Total Impact Points

Institutions

  • 2014
    • Lowell Observatory
      Flagstaff, Arizona, United States
  • 2011–2014
    • University of Washington Seattle
      • Department of Astronomy
      Seattle, Washington, United States
  • 2005–2009
    • University of Chicago
      • • Enrico Fermi Institute
      • • Department of Astronomy and Astrophysics
      Chicago, IL, United States
  • 2008
    • University of Illinois, Urbana-Champaign
      • Department of Astronomy
      Urbana, Illinois, United States