arXiv subjects

Donald G. York

Publications and source records attributed to Donald G. York.

At least 127 records · Page 7Linked to original sources

arXiv2007

Broad Absorption Line Variability in Repeat Quasar Observations from the Sloan Digital Sky Survey

We present a time-variability analysis of 29 broad absorption line quasars (BALQSOs) observed in two epochs by the Sloan Digital Sky Survey (SDSS). These spectra are selected from a larger sample of BALQSOs with multiple observations by virtue of exhibiting a broad CIV $λ$1549 absorption trough separated from the rest frame of the associated emission peak by more than 3600 km s$^{-1}$. Detached troughs facilitate higher precision variability measurements, since the measurement of the absorption in these objects is not complicated by variation in the emission line flux. We have undertaken a statistical analysis of these detached-trough BALQSO spectra to explore the relationships between BAL features that are seen to vary and the dynamics of emission from the quasar central engine. We have measured variability within our sample, which includes three strongly variable BALs. We have also verified that the statistical behavior of the overall sample agrees with current model predictions and previous studies of BAL variability. Specifically, we observe that the strongest BAL variability occurs among the smallest equivalent width features and at velocities exceeding 12,000 km s$^{-1}$, as predicted by recent disk-wind modeling.

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arXiv2007

The nature of damped Lyman alpha and sub-damped Lyman alpha absorbers

We present arguments based on the measured abundances in individual damped Lyman alpha systems (DLAs) and sub-damped Lyman alpha systems (sub-DLAs), and also the average abundances inferred in large samples of QSO absorption line systems, to suggest that the amount of dust in intervening QSO absorbers is small and is not responsible for missing many QSOs in magnitude limited QSO surveys. While we can not totally rule out a bimodal dust distribution with a population of very dusty, metal rich, absorbers which push the background QSOs below the observational threshold of current optical spectroscopic studies, based upon the current samples it appears that the metallicity in QSO absorbers decreases with increase in H I column densities beyond 10^{19} cm^{-2}. Thus the sub-DLA population is more metal rich than the DLAs, a trend which may possibly extend to the non-damped Lyman limit systems (NDLLS). Based on the recently discovered mass-metallicity relation for galaxies, we suggest that most sub-DLAs and possibly NDLLS, are associated with massive spiral/elliptical galaxies while most DLAs are associated with low mass galaxies. The sub-DLA galaxies will then contribute a larger fraction of total mass (stellar and ISM) and therefore metals, to the cosmic budget, specially at low redshifts, as compared to the DLAs.

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arXiv2006

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

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

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arXiv2006

Discovery of a Gravitationally Lensed Quasar from the Sloan Digital Sky Survey: SDSS J133222.62+034739.9

We report the discovery of the two-image gravitationally lensed quasar SDSS J133222.62+034739.9 (SDSS J1332+0347) with an image separation of Delta_theta=1.14". This system consists of a source quasar at z_s=1.445 and a lens galaxy at z_l=0.191. The agreement of the luminosity, ellipticity and position angle of the lens galaxy with those expected from lens model confirms the lensing hypothesis.

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arXiv2006

Two New Gravitationally Lensed Double Quasars from the Sloan Digital Sky Survey

We report the discoveries of the two-image gravitationally lensed quasars, SDSS J0746+4403 and SDSS J1406+6126, selected from the Sloan Digital Sky Survey (SDSS). SDSS J0746+4403, which will be included in our lens sample for statistics and cosmology, has a source redshift of z_s=2.00, an estimated lens redshift of z_l~0.3, and an image separation of 1.08". SDSS J1406+6126 has a source redshift of z_s=2.13, a spectroscopically measured lens redshift of z_l=0.27, and an image separation of 1.98". We find that the two quasar images of SDSS J1406+6126 have different intervening MgII absorption strengths, which are suggestive of large variations of absorbers on kpc scales. The positions and fluxes of both the lensed quasar systems are easily reproduced by simple mass models with reasonable parameter values. These objects bring to 18 the number of lensed quasars that have been discovered from the SDSS data.

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arXiv2006

A Large, Uniform Sample of X-ray Emitting AGN from the ROSAT All-Sky and Sloan Digital Sky Surveys: the Data Release 5 Sample

We describe further results of a program aimed to yield ~10^4 fully characterized optical identifications of ROSAT X-ray sources. Our program employs X-ray data from the ROSAT All-Sky Survey (RASS), and both optical imaging and spectroscopic data from the Sloan Digital Sky Survey (SDSS). RASS/SDSS data from 5740 deg^2 of sky spectroscopically covered in SDSS Data Release 5 (DR5) provide an expanded catalog of 7000 confirmed quasars and other AGN that are probable RASS identifications. Again in our expanded catalog, the identifications as X-ray sources are statistically secure, with only a few percent of the SDSS AGN likely to be randomly superposed on unrelated RASS X-ray sources. Most identifications continue to be quasars and Seyfert 1s with 15<m<21 and 0.01<z<4; but the total sample size has grown to include very substantial numbers of even quite rare AGN, e.g., now including several hundreds of candidate X-ray emitting BL Lacs and narrow-line Seyfert 1 galaxies. In addition to exploring rare subpopulations, such a large total sample may be useful when considering correlations between the X-ray and the optical, and may also serve as a resource list from which to select the "best" object (e.g., X-ray brightest AGN of a certain subclass, at a preferred redshift or luminosity) for follow-on X-ray spectral or alternate detailed studies.

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arXiv2006

Optical Emission Band Morphologies of the Red Rectangle

We present narrow-band images of the Red Rectangle (RR) nebula which reveal the distinct morphologies of this intriguing nebula in different optical emission bands. The morphology of the RR nebula in blue luminescence (BL) and extended red emission (ERE) are almost mutually exclusive. We also present the optical detection of the circum-binary disk of the RR in the light of the BL. The total intensities from the two optical band emissions (BL and ERE) when summed over the nebula are of comparable magnitude. Their spatial distributions with respect to the embedded illumination sources lead us to suggest that they may be attributed to different ionization stages of the same family of carriers.

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arXiv2006

Discovery of a Metal-Line Absorber Associated with a Local Dwarf Starburst Galaxy

We present optical and near-infrared images, H I 21 cm emission maps, optical spectroscopy, and Hubble Space Telescope/Space Telescope Imaging Spectrograph ultraviolet spectroscopy of the QSO/galaxy pair SBS 1122+594/IC 691. The QSO sight line lies at a position angle of 27 degrees from the minor axis of the nearby dwarf starburst galaxy IC 691 (cz_gal = 1204+-3 km/s, L_B ~ 0.09 L*, current star formation rate = 0.08-0.24 solar masses per year) and 33 kpc (6.6 arcmin) from its nucleus. We find that IC 691 has an H I mass of M_HI = (3.6+-0.1) x 10^8 solar masses and a dynamical mass of M_dyn = (3.1+-0.5) x 10^10 solar masses. The UV spectrum of SBS 1122+594 shows a metal-line (Ly-alpha + C IV) absorber near the redshift of IC 691 at cz_abs = 1110+-30 km/s. Since IC 691 is a dwarf starburst and the SBS 1122+594 sight line lies in the expected location for an outflowing wind, we propose that the best model for producing this metal-line absorber is a starburst wind from IC 691. We place consistent metallicity limits on IC 691 ([Z/Zsun] ~ -0.7) and the metal-line absorber ([Z/Zsun] < -0.3). We also find that the galaxy's escape velocity at the absorber location is v_esc = 80+-10 km/s and derive a wind velocity of v_w = 160+-50 km/s. Thus, the evidence suggests that IC 691 produces an unbound starburst wind that escapes from its gravitational potential to transport metals and energy to the surrounding intergalactic medium.

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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

QSO Absorption Lines from QSOs

We present the results of a search for metal absorption lines in the spectra of background QSOs whose sightlines pass close to foreground QSOs. We detect MgII(2796,2803) absorption in Sloan Digital Sky Survey (SDSS) spectra of four z>1.5 QSOs whose lines of sight pass within 26-98 kpc of lower redshift (z~0.5-1.5) QSOs. The 100% [4/4 pairs] detection of MgII in the background QSOs is clearly at odds with the incidence of associated (z_abs ~ z_em) systems -- absorbers which exist towards only a few percent of QSOs. Although the quality of our foreground QSO spectra is not as high as the SDSS data, absorption seen towards one of the background QSOs clearly does not show up at the same strength in the spectrum of the corresponding foreground QSO. This implies that the absorbing gas is distributed inhomogeneously around the QSO, presumably as a direct consequence of the anisotropic emission from the central AGN. We discuss possible origins for the MgII lines, including: absorption by gas from the foreground QSO host galaxy; companion galaxies fuelling the QSO through gravitational interactions; and tidal debris left by galaxy mergers or interactions which initiated the QSO activity. No single explanation is entirely satisfactory, and we may well be seeing a mixture of phenomena.

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arXiv2006

Elemental Abundance Measurements in Low-redshift Damped Lyman Alpha Absorbers

We present elemental abundance measurements for 9 damped Ly-alpha systems (DLAs) and 1 sub-DLA at 0.1 < z < 1.5 from recent observations with the Multiple Mirror Telescope. Most of these absorbers are found to be metal-poor, while 2 are found to have ~ 30-50 solar metallicities. Combining our data with other data from the literature, we find that the systems with higher [Zn/H] also have stronger depletion as measured by [Cr/Zn] and [Fe/Zn]. The relationship between the metallicity and H I column density is also investigated. Together with our previous MMT survey (Khare et al. 2004) we have discovered 2 of the 4 known absorbers at z < 1.5 that lie above (although near) the "obscuration threshold". This appears to be a result of selecting absorbers with strong metal lines in our sample. It would be interesting to find other similar systems by observing a larger sample and study how much such systems contribute to the cosmic budget of metals. Finally, an analysis of the N(H I)-weighted mean metallicity vs. redshift for our sample combined with data from the literature supports previous conclusions that the N(H I)-weighted mean global DLA metallicity rises slowly at best and falls short of solar levels by a factor of > 4 even at z=0.

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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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arXiv2006

Andromeda X, A New Dwarf Spheroidal Satellite of M31: Photometry

We report the discovery of Andromeda X, a new dwarf spheroidal satellite of M31, based on stellar photometry from the Sloan Digital Sky Survey (SDSS). Using follow-up imaging data we have estimated its distance and other physical properties. We find that Andromeda X has a dereddened central surface brightness of mu_V,0 ~ 26.7 mag arcsec^-2 and a total apparent magnitude of V_tot ~ 16.1, which at the derived distance modulus, (m - M)_0 ~ 24.12 - 24.34, yields an absolute magnitude of M_V ~ -8.1 +/- 0.5; these values are quite comparable to those of Andromeda IX, a previously-discovered low luminosity M31 satellite. The discoveries of Andromeda IX and Andromeda X suggest that such extremely faint satellites may be plentiful in the Local Group.

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arXiv2006

Average Extinction Curves and Relative Abundances for QSO Absorption Line Systems at 1 <= z_abs < 2

We have studied a sample of 809 Mg II absorption systems with 1.0 < z_abs < 1.86 in the spectra of SDSS 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 sub-samples, chosen on the basis of the line strengths and other absorber and QSO properties. Average extinction curves were obtained for the sub-samples 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 A feature is not present in the extinction curves, for any of the sub-samples. The extinction curves are similar to the SMC extinction curve with a rising UV extinction below 2200 A. 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 sub-samples. The column densities of several ions do not show such a correspondingly large variation. The depletion pattern is 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 N_H I \~ 10^{20} 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. 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.

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arXiv2005

SDSS J0806+2006 and SDSS J1353+1138: Two New Gravitationally Lensed Quasars from the Sloan Digital Sky Survey

We report the discoveries of two, two-image gravitationally lensed quasars selected from the Sloan Digital Sky Survey: SDSS J0806+2006 at z_s=1.540 and SDSS J1353+1138 at z_s=1.629 with image separations of 1.40" and 1.41" respectively. Spectroscopic and optical/near-infrared imaging follow-up observations show that the quasar images have identical redshifts and possess extended objects between the images that are likely to be lens galaxies at z_l~0.6 in SDSS J0806+2006 and z_l~0.3 in SDSS J1353+1138. The field of SDSS J0806+2006 contains several nearby galaxies that may significantly perturb the system, and SDSS J1353+1138 has an extra component near its Einstein ring that is probably a foreground star. Simple mass models with reasonable parameters reproduce the quasar positions and fluxes of both systems.

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arXiv2005

SDSSJ102111.02+491330.4: A Newly Discovered Gravitationally Lensed Quasar

We report follow-up observations of two gravitational lens candidates identified in the Sloan Digital Sky Survey (SDSS) dataset. We have confirmed that SDSS J102111.02+491330.4 is a previously unknown gravitationally lensed quasar. This lens system exhibits two images of a $z = 1.72$ quasar, with an image separation of $1{\farcs}14 \pm 0.04$. Optical and near-IR imaging of the system reveals the presence of the lensing galaxy between the two quasar images. Observations of SDSS J112012.12+671116.0 indicate that it is more likely a binary quasar than a gravitational lens. This system has two quasars at a redshift of $z = 1.49$, with an angular separation of $1{\farcs}49 \pm 0.02$. However, the two quasars have markedly different SEDs and no lens galaxy is apparent in optical and near-IR images of this system. We also present a list of 31 SDSS lens candidates which follow-up observations have confirmed are \textit{not} gravitational lenses.

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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

The Evolution of Damped Lyman-alpha Absorbers: Metallicities and Star Formation Rates

The damped Lyman-alpha (DLA) and sub-DLA quasar absorption lines provide powerful probes of the evolution of metals, gas, and stars in galaxies. One major obstacle in trying to understand the evolution of DLAs and sub-DLAs has been the small number of metallicity measurements at z < 1.5, an epoch spanning \~70 % of the cosmic history. In recent surveys with the Hubble Space Telescope and Multiple Mirror Telescope, we have doubled the DLA Zn sample at z < 1.5. Combining our results with those at higher redshifts from the literature, we find that the global mean metallicity of DLAs does not rise to the solar value at low redshifts. These surprising results appear to contradict the near-solar mean metallicity observed for nearby (z ~ 0) galaxies and the predictions of cosmic chemical evolution models based on the global star formation history. Finally, we discuss direct constraints on the star formation rates (SFRs) in the absorber galaxies from our deep Fabry-Perot Ly-alpha imaging study and other emission-line studies in the literature. A large fraction of the observed heavy-element quasar absorbers at 0 < z < 3.4 appear to have SFRs substantially below the global mean SFR, consistent with the low metallicities observed in the spectroscopic studies.

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