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D. G. York

Publications and source records attributed to D. G. York.

At least 55 records · Page 3Linked to original sources
arXiv2006

Extinction and metal column density of HI regions up to redshift z~2

We used the photometric database of the Sloan Digital Sky Survey (SDSS) to estimate the reddening of 13 SDSS quasars selected on the basis of the presence of zinc absorption lines in an intervening Damped Ly alpha (DLA) system. In 5 of these quasars the reddening is detected at ~2 sigma confidence level in two independent color indices of the SDSS ugriz photometric system. A detailed analysis of the data supports an origin of the reddening in the intervening absorbers. We used these rare measurements of extinction in DLA systems to probe the relation between extinction and metal column density in the interval of absorption redshift 0.7 </~ z </~ 2.0. We find that the mean extinction in the V band per atom of iron in the dust is remarkably similar to that found in interstellar clouds of the Milky Way. This result lends support to previous estimates of the dust obscuration effect in DLA systems based on a Milky Way extinction/metal column density relation. We propose a simple mechanism, based on dust grain destruction/accretion properties, which may explain the approximate constancy of the extinction per atom of iron in the dust.

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arXiv2006

The Most Metal-Rich Intervening Quasar Absorber Known

The metallicity in portions of high-redshift galaxies has been successfully measured thanks to the gas observed in absorption in the spectra of quasars, in the Damped Lyman-alpha systems (DLAs). Surprisingly, the global mean metallicity derived from DLAs is about 1/10th solar at 0<z<4 leading to the so-called ``missing-metals problem''. In this paper, we present high-resolution observations of a sub-DLA system at z_abs=0.716 with super-solar metallicity toward SDSS J1323-0021. This is the highest metallicity intervening quasar absorber currently known, and is only the second super-solar absorber known to date. We provide a detailed study of this unique object from VLT/UVES spectroscopy. We derive [Zn/H]=+0.61, [Fe/H]=-0.51, [Cr/H]=<-0.53, [Mn/H] = -0.37, and [Ti/H] = -0.61. Observations and photoionisation models using the CLOUDY software confirm that the gas in this sub-DLA is predominantly neutral and that the abundance pattern is probably significantly different from a Solar pattern. Fe/Zn and Ti/Zn vary among the main velocity components by factors of \~ 3 and ~ 35, respectively, indicating non-uniform dust depletion. Mn/Fe is super-solar in almost all components, and varies by a factor of ~ 3 among the dominant components. It would be interesting to observe more sub-DLA systems and determine whether they might contribute significantly toward the cosmic budget of metals.

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arXiv2005

A search for the most massive galaxies: Double Trouble?

We describe the results of a search for galaxies with large (> 350 km/s) velocity dispersions. The largest systems we have found appear to be the extremes of the early-type galaxy population: compared to other galaxies with similar luminosities, they have the largest velocity dispersions and the smallest sizes. However, they are not distant outliers from the Fundamental Plane and mass-to-light scaling relations defined by the bulk of the early-type galaxy population. They may host the most massive black holes in the Universe, and their abundance and properties can be used to constrain galaxy formation models. Clear outliers from the scaling relations tend to be objects in superposition (angular separations smaller than 1 arcsec), evidence for which comes sometimes from the spectra, sometimes from the images, and sometimes from both. The statistical properties of the superposed pairs, e.g., the distribution of pair separations and velocity dispersions, can be used to provide useful information about the expected distribution of image multiplicities, separations and flux ratios due to gravitational lensing by multiple lenses, and may also constrain models of their interaction rates.

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arXiv2005

A Fabry-Perot Imaging Search for Lyman-alpha Emission in Quasar Absorbers at z ~ 2.4

We have carried out a deep narrow-band imaging survey of six fields with heavy-element quasar absorption lines, using the Goddard Fabry-Perot (FP) system at the Apache Point Observatory (APO) 3.5-meter telescope. The aim of these observations was to search for redshifted Ly-$α$ emission from the galaxies underlying the absorbers at $z = 2.3-2.5$ and their companion galaxies. The 3 $σ$ sensitivity levels ranged between $1.9 \times 10^{-17}$ and $5.4 \times 10^{-17}$ erg s$^{-1}$ cm$^{-2}$ in observed-frame Ly-$α$ flux. No significant Ly-$α$ emitters were detected at $> 3 σ$ level. The absence of significant Ly-$α$ emission implies limits on the star formation rate (SFR) of 0.9-2.7 $M_{\odot}$ yr$^{-1}$ per 2-pixel x 2-pixel region, if no dust attenuation is assumed. We compare our results with those from other emission-line studies of absorber fields and with predictions for global average SFR based on the models of cosmic chemical evolution. Our limits are among the tightest existing constraints on Ly-$α$ emission from galaxies in absorber fields, but are consistent with many other studies. In the absence of dust attenuation, these studies suggest that SFRs in a large fraction of objects in the absorber fields may lie below the global mean SFR. However, it is possible that dust attenuation is responsible for the low emission line fluxes in some objects. It is also possible that the star-forming regions are compact and at smaller angular separations from the quasar than the width of our point spread function and, get lost in the quasar emission. We outline future observations that could help to distinguish between the various possibilities.

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arXiv2005

The Luminosity and Color Dependence of the Galaxy Correlation Function

We study the luminosity and color dependence of the galaxy 2-point correlation function in the Sloan Digital Sky Survey, starting from a sample of 200,000 galaxies over 2500 deg^2. We concentrate on the projected correlation function w(r_p), which is directly related to the real space ξ(r). The amplitude of w(r_p) grows continuously with luminosity, rising more steeply above the characteristic luminosity L_*. Redder galaxies exhibit a higher amplitude and steeper correlation function at all luminosities. The correlation amplitude of blue galaxies increases continuously with luminosity, but the luminosity dependence for red galaxies is less regular, with bright red galaxies more strongly clustered at large scales and faint red galaxies more strongly clustered at small scales. We interpret these results using halo occupation distribution (HOD) models assuming concordance cosmological parameters. For most samples, an HOD model with two adjustable parameters fits the w(r_p) data better than a power-law, explaining inflections at r_p ~ 1-3 Mpc/h as the transition between the 1-halo and 2-halo regimes of ξ(r). The implied minimum mass for a halo hosting a central galaxy above a luminosity threshold L grows as M_min ~ L at low luminosities and more steeply above L_*. The mass at which an average halo has one satellite galaxy brighter than L is M_1 ~ 23 M_min(L). These results imply a conditional luminosity function (at fixed halo mass) in which central galaxies lie far above a Schechter function extrapolation of the satellite population. HOD models nicely explain the joint luminosity-color dependence of w(r_p) in terms of the color fractions of central and satellite populations as a function of halo mass. The inferred HOD properties are in good qualitative agreement with theoretical predictions.

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arXiv2005

Evidence for the presence of dust in intervening QSO absorbers from the Sloan Digital Sky Survey

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 A feature which is present in the Milky Way dust extinction curve. The extinction curve resembles the SMC extinction curve. The observed Delta(g-i) excess for QSOs with strong absorption systems appears to be a result of the reddening due to dust in the intervening absorbers.

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arXiv2005

The Sloan Digital Sky Survey Quasar Catalog III. Third Data Release

We present the third edition of the Sloan Digital Sky Survey (SDSS) Quasar Catalog. The catalog consists of the 46,420 objects in the SDSS Third Data Release that have luminosities larger than M_i = -22 (in a cosmology with H_0 = 70 km/s/Mpc, Omega_M = 0.3, and Omega_Lambda = 0.7), have at least one emission line with FWHM larger than 1000 km/s or are unambiguously broad absorption line quasars, are fainter than i = 15.0, and have highly reliable redshifts. The area covered by the catalog is 4188 sq. deg. The quasar redshifts range from 0.08 to 5.41, with a median value of 1.47; the high-redshift sample includes 520 quasars at redshifts greater than four, of which 17 are at redshifts greater than five. For each object the catalog presents positions accurate to better than 0.2 arcsec. rms per coordinate, five-band (ugriz) CCD-based photometry with typical accuracy of 0.03 mag, and information on the morphology and selection method. The catalog also contains radio, near-infrared, and X-ray emission properties of the quasars, when available, from other large-area surveys. The calibrated digital spectra cover the wavelength region 3800--9200A at a spectral resolution about 2000; the spectra can be retrieved from the public database using the information provided in the catalog. A total of 44,221 objects in the catalog were discovered by the SDSS; 28,400 of the SDSS discoveries are reported here for the first time.

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arXiv2004

Deuterated molecular hydrogen in the Galactic ISM. New observations along seven translucent sightlines

We present column density measurements of the HD molecule in the interstellar gas toward 17 Galactic stars. The values for the seven most heavily reddened sightlines, with E(B-V) = 0.38-0.72, are derived from observations with the Far Ultraviolet Spectroscopic Explorer (FUSE). The other ten values are from a reanalysis of spectra obtained with Copernicus. In all cases, high-resolution ground-based observations of KI and/or the CH molecule were used to constrain the gas velocity structure and to correct for saturation effects. Comparisons of the column densities HD, CH, CN, and KI in these 17 sightlines indicate that HD is most tightly correlated with CH. Stringent lower limits to the interstellar D/H ratio, derived from the HD/2H2 ratio, range from 3.7 10^(-7) to 4.3 10^(-6). Our results also suggest that the HD/H2 ratio increases with the molecular fraction f(H2) and that the interstellar D/H ratio might be obtained from HD by probing clouds with f(H2) = 1. Finally, we note an apparent relationship between the molecular fractions of hydrogen and deuterium.

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arXiv2004

ROSAT-SDSS Galaxy Clusters Survey. I. The Catalog and the correlation of X-ray and optical properties

For a detailed comparison of the appearance of cluster of galaxies in X-rays and in the optical, we have compiled a comprehensive database of X-ray and optical properties of a sample of clusters based on the largest available X-ray and optical surveys: the ROSAT All Sky Survey (RASS) and the Sloan Digital Sky Survey (SDSS). The X-ray galaxy clusters of this RASS-SDSS catalog cover a wide range of masses, from groups of 10^{12.5} solar masses to massive clusters of 10^{15} solar masses in the redshift range from 0.002 to 0.45. The RASS-SDSS sample comprises all the X-ray selected objects already observed by the Sloan Digital Sky Survey (114 clusters). For each system we have uniformly determined the X-ray and optical properties. The total optical luminosity can be determined with a typical uncertainty of 20% with a result independent of the choice of local or global background subtraction. We found that the z band cluster luminosity determined within an aperture of 0.5 Mpc h_{70}^(-1) provides the best correlation with the X-ray luminosity with a scatter of about 60-70%. The scatter decreases to less than 40% if the correlation is limited to the bright X-ray clusters. The resulting correlation of L_X and L_{op} in the z and i bands shows a logarithmic slope of 0.38, a value not consistent with the assumption of a constant M/L. Consistency is found, however, for an increasing M/L with luminosity as suggested by other observations. We also investigated the correlation between L_{op} and the X-ray temperature obtaining the same result.

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arXiv2004

Unconventional AGN from the SDSS

We discuss some of the most unusual active galactic nuclei (AGN) discovered to date by the Sloan Digital Sky Survey (SDSS): the first broad absorption line quasar seen to exhibit He II absorption, several quasars with extremely strong, narrow UV Fe II emission, and an AGN with an unexplained and very strange continuum shape.

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arXiv2003

A Gravitationally Lensed Quasar with Quadruple Images Separated by 14.62 Arcseconds

Gravitational lensing is a powerful tool for the study of the distribution of dark matter in the Universe. The cold-dark-matter model of the formation of large-scale structures predicts the existence of quasars gravitationally lensed by concentrations of dark matter so massive that the quasar images would be split by over 7 arcsec. Numerous searches for large-separation lensed quasars have, however, been unsuccessful. All of the roughly 70 lensed quasars known, including the first lensed quasar discovered, have smaller separations that can be explained in terms of galaxy-scale concentrations of baryonic matter. Although gravitationally lensed galaxies with large separations are known, quasars are more useful cosmological probes because of the simplicity of the resulting lens systems. Here we report the discovery of a lensed quasar, SDSS J1004+4112, which has a maximum separation between the components of 14.62 arcsec. Such a large separation means that the lensing object must be dominated by dark matter. Our results are fully consistent with theoretical expectations based on the cold-dark-matter model.

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arXiv2003

Detection of Intergalactic HeII Absorption at Redshift 3.5

The large number of quasars found in the Sloan Digital Sky Survey has allowed searches for elusive, clear lines of sight towards HeII Ly-alpha absorption, a sensitive probe of the intergalactic medium. The few known systems indicate that HeII reionization occurs at z>3. We report the detection of a HeII Ly-alpha absorption edge in a quasar spectrum at z=3.50, the most distant such feature found to date. The candidate quasar was selected from a z~3 sample in the SDSS spectroscopic quasar survey and confirmed as part of an HST/STIS SNAP survey. We discuss the general characteristics of the absorption feature, as well as the probability for discovery of additional such objects.

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arXiv2003

The Deuterium-to-Hydrogen Ratio in a Low-Metallicity Cloud Falling onto the Milky Way

Using Far Ultraviolet Spectroscopic Explorer and Hubble Space Telescope observations of the QSO PG1259+593, we detect D I Lyman-series absorption in high velocity cloud Complex C, a low-metallicity gas cloud falling onto the Milky Way. This is the first detection of atomic deuterium in the local universe in a location other than the nearby regions of the Galactic disk. A new Westerbork Synthesis Radio Telescope (WSRT) interferometer map of the H I 21 cm emission toward PG1259+593 indicates that the sight line passes through a compact concentration of neutral gas in Complex C. We find D/H = (2.2+/-0.7)x10^-5, O/H = (8.0+/-2.5)x10^-5, and D/O = 0.28+/-0.12. The metallicity of Complex C gas toward PG1259+593 is approximately 1/6 solar, as inferred from the oxygen abundance [O/H] = -0.79 (+0.12, -0.16). While we cannot rule out a value of D/H similar to that found for the local ISM, we can confidently exclude values as low as those determined recently for extended sight lines in the Galactic disk. Combined with the sub-solar metallicity estimate and the low nitrogen abundance, this conclusion lends support to the hypothesis that Complex C is located outside the Milky Way, rather than inside in material recirculated between the Galactic disk and halo. The value of D/H for Complex C is consistent with the primordial abundance of deuterium inferred from recent Wilkinson Microwave Anisotropy Probe observations of the cosmic microwave background and simple chemical evolution models that predict the amount of deuterium astration as a function of metallicity. [Abbreviated abstract]

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arXiv2003

The Ensemble Photometric Variability of ~25000 Quasars in the Sloan Digital Sky Survey

Using a sample of over 25000 spectroscopically confirmed quasars from the Sloan Digital Sky Survey, we show how quasar variability in the rest frame optical/UV regime depends upon rest frame time lag, luminosity, rest wavelength, redshift, the presence of radio and X-ray emission, and the presence of broad absorption line systems. The time dependence of variability (the structure function) is well-fit by a single power law on timescales from days to years. There is an anti-correlation of variability amplitude with rest wavelength, and quasars are systematically bluer when brighter at all redshifts. There is a strong anti-correlation of variability with quasar luminosity. There is also a significant positive correlation of variability amplitude with redshift, indicating evolution of the quasar population or the variability mechanism. We parameterize all of these relationships. Quasars with RASS X-ray detections are significantly more variable (at optical/UV wavelengths) than those without, and radio loud quasars are marginally more variable than their radio weak counterparts. We find no significant difference in the variability of quasars with and without broad absorption line troughs. Models involving multiple discrete events or gravitational microlensing are unlikely by themselves to account for the data. So-called accretion disk instability models are promising, but more quantitative predictions are needed.

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arXiv2003

VLT+UVES Spectroscopy of the CaII LoBAL Quasar SDSS J030000.56+004828.0

We study high-resolution spectra of the `overlapping-trough' low-ionization broad absorption line (LoBAL) quasar SDSS J030000.56+004828.0. The CaII, MgII and MgI column densities in this object are the largest reported to date for any BAL outflow. The broad CaII absorption is mildly blended, but the blending can be disentangled to measure the CaII column density, which is large enough that the outflow must include a strong hydrogen ionization front. The outflow begins at a blueshift of ~1650 km/s from the systemic redshift. The lowest velocity BAL region produces strong CaII absorption but does not produce significant excited FeII absorption, while the higher velocity excited FeII absorption region produces very little CaII absorption. We have found that only a disk wind outflow can explain this segregation. Whether the outflow is smooth or clumpy, we conclude that the CaII BAL region has a density high enough to populate excited levels of FeII, but a temperature low enough to prevent them from being significantly populated. This requirement means the CaII BAL region has T<1000 K, and perhaps even T<550 K. This quasar also has an associated absorption line system (AAL) that exhibits partial covering, and therefore is likely located near the central engine. Its association with the BAL outflow is unclear. Blending of the AAL with the BAL trough shows that the spatial region covered by the BAL outflow can vary over velocity differences of ~1700 km/s.

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arXiv2003

An Initial Survey of White Dwarfs in the Sloan Digital Sky Survey

An initial assessment is made of white dwarf and hot subdwarf stars observed in the Sloan Digital Sky Survey. In a small area of sky (190 square degrees), observed much like the full survey will be, 269 white dwarfs and 56 hot subdwarfs are identified spectroscopically where only 44 white dwarfs and 5 hot subdwarfs were known previously. Most are ordinary DA (hydrogen atmosphere) and DB (helium) types. In addition, in the full survey to date, a number of WDs have been found with uncommon spectral types. Among these are blue DQ stars displaying lines of atomic carbon; red DQ stars showing molecular bands of C_2 with a wide variety of strengths; DZ stars where Ca and occasionally Mg, Na, and/or Fe lines are detected; and magnetic WDs with a wide range of magnetic field strengths in DA, DB, DQ, and (probably) DZ spectral types. Photometry alone allows identification of stars hotter than 12000 K, and the density of these stars for 15<g<20 is found to be ~2.2 deg^{-2} at Galactic latitudes 29-62 deg. Spectra are obtained for roughly half of these hot stars. The spectra show that, for 15<g<17, 40% of hot stars are WDs and the fraction of WDs rises to ~90% at g=20. The remainder are hot sdB and sdO stars.

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arXiv2003

A First Look at White Dwarf - M Dwarf Pairs in the Sloan Digital Sky Survey (SDSS)

We have identified 109 White Dwarf (WD) - M dwarf pairs in the Sloan Digital Sky Survey (SDSS) with g < 20th magnitude. For each system we determined the temperature of the WD primary and the spectral type of the M dwarf secondary. Using H-alpha emission as a proxy for the chromospheric activity level of the M dwarf, we investigated correlations between the activity level and properties of the system. Compared with field M dwarfs (Hawley et al. 1996), we see a slightly higher active fraction of early-type stars, with activity levels similar to the field. We have conducted followup observations at the ARC 3.5m telescope to obtain radial velocity information and to search for short period binaries, which may be on the verge of interacting. We report on one system with a 4.1 hour period, and several additional systems with significant velocity variations.

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