arXiv subjects

Patrick B. Hall

Publications and source records attributed to Patrick B. Hall.

At least 145 records · Page 8Linked to original sources

arXiv2004

A Lyman-alpha-only AGN from the Sloan Digital Sky Survey

The Sloan Digital Sky Survey has discovered a z=2.4917 radio-loud active galactic nucleus (AGN) with a luminous, variable, low-polarization UV continuum, H I two-photon emission, and a moderately broad Lyman-alpha line (FWHM = 1430 km/s) but without obvious metal-line emission. SDSS J113658.36+024220.1 does have associated metal-line absorption in three distinct, narrow systems spanning a velocity range of 2710 km/s. Despite certain spectral similarities, SDSS J1136+0242 is not a Lyman-break galaxy. Instead, the Ly-alpha and two-photon emission can be attributed to an extended, low-metallicity narrow-line region. The unpolarized continuum argues that we see SDSS J1136+0242 very close to the axis of any ionization cone present. We can conceive of two plausible explanations for why we see a strong UV continuum but no broad-line emission in this `face-on radio galaxy' model for SDSS J1136+0242: the continuum could be relativistically beamed synchrotron emission which swamps the broad-line emission; or, more likely, SDSS J1136+0242 could be similar to PG 1407+265, a quasar in which for some unknown reason the high-ionization emission lines are very broad, very weak, and highly blueshifted.

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arXiv2004

Microlensing of the Broad Emission Line Region in the Quadruple Lens SDSS J1004+4112

We present seven epochs of spectroscopy on the quadruply imaged quasar SDSS J1004+4112, spanning observed-frame time delays from 1 to 322 days. The spectra reveal differences in the emission lines between the lensed images. Specifically, component A showed a strong enhancement in the blue wings of several high-ionization lines relative to component B, which lasted at least 28 days (observed frame) then faded. Since the predicted time delay between A and B is <30 days, our time coverage suggests that the event was not intrinsic to the quasar. We attribute these variations to microlensing of part of the broad emission line region of the quasar, apparently resolving structure in the source plane on a scale of ~10^{16} cm at z=1.734. In addition, we observed smaller differences in the emission line profiles between components A and B that persisted throughout the time span, which may also be due to microlensing or millilensing. Further spectroscopic monitoring of this system holds considerable promise for resolving the structure of the broad emission line region in quasars.

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arXiv2004

SDSS J1335+0118: A New Two-Image Gravitational Lens

We report the discovery of the two-image gravitationally lensed quasar SDSS J1335+0118. The object was selected as a lens candidate from the Sloan Digital Sky Survey. The imaging and spectroscopic follow-up observations confirm that the system exhibits two gravitationally lensed images of a quasar at z=1.57. The image separation is 1.56''. We also detect an extended component between the two quasar images, likely the lensing galaxy. Preliminary mass modeling predicts the differential time delay Δt ~ 30 h^{-1} day assuming the redshift of the lens galaxy is 0.5.

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arXiv2003

SDSS Quasars and Dust Reddening

The Sloan Digital Sky Survey can be used to detect and characterize red and reddened quasars. In Richards et al. (2003), we showed that 6% of SDSS quasars have red colors consistent with significant dust reddening by an extinction curve similar to that of the Small Magellanic Cloud (SMC). We estimate that a further 10% of the luminous quasar population is missing from our magnitude-limited SDSS sample. More recent work (Hopkins et al., in preparation) confirms the conclusion that dust reddening is the primary explanation for SDSS quasars in the red tail of the color distribution. Fitting orthogonal first- and second-order polynomials to SDSS quasar photometry measures the slope and curvature of each object's UV/optical spectrum. The slope vs. curvature distribution is elongated along the axis predicted for SMC-like reddening, while the axes predicted for LMC- or MW-like reddening provide significantly poorer fits. Extension to longer wavelengths using a smaller sample of SDSS/2MASS matches confirms this result at high significance.

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arXiv2003

Constraints on Quasar Continuum, BELR, and BALR Physics from SDSS Composite Spectra

We review recent results on quasars from the SDSS as they relate to our understanding of the UV/optical continuum, the broad emission line region, and the broad absorption line region. The ensemble average colors of large numbers of quasars promise to provide constraints on the optical/UV continuum emission mechanism. High-ionization emission-line blueshifts and emission line properties as a function of optical/UV spectral index trace the structure of the broad emission line region. Statistical analysis of the broad absorption line quasar population suggests that they are more ubiquitous than one might otherwise think and are not likely to represent a completely distinct population of quasars, but that the BAL trough properties are a function of the underlying optical continuum and emission properties of the quasar.

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arXiv2003

An MHD-driven Disk Wind Outflow in SDSS J0300+0048?

The outflow in SDSS J030000.56+004828.0 has the highest low-ionization metal-line column density yet reported for a broad absorption line quasar. The absorption from different ions is also segregated as a function of velocity in a way that can only be explained by a disk wind outflow. Furthermore, the presence of the such large column densities of gas at the high observed outflow velocities may be incompatible with purely radiative acceleration. MHD contributions to the acceleration should be considered seriously.

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arXiv2003

Gemini-South + FLAMINGOS Demonstration Science: Near-Infrared Spectroscopy of the z=5.77 Quasar SDSS J083643.85+005453.3

We report an infrared 1-1.8 micron (J+H-bands), low-resolution (R=450) spectrogram of the highest-redshift radio-loud quasar currently known, SDSS J083643.85+005453.3, obtained during the spectroscopic commissioning run of the FLAMINGOS multi-object, near-infrared spectrograph at the 8m Gemini-South Observatory. These data show broad emission from both CIV 1549 and CIII] 1909, with strengths comparable to lower-redshift quasar composite spectra. The implication is that there is substantial enrichment of the quasar environment, even at times less than a billion years after the Big Bang. The redshift derived from these features is z = 5.774 +/- 0.003, more accurate and slightly lower than the z = 5.82 reported in the discovery paper based on the partially-absorbed Lyman-alpha emission line. The infrared continuum is significantly redder than lower-redshift quasar composites. Fitting the spectrum from 1.0 to 1.7 microns with a power law f(nu) ~ nu^(-alpha), the derived power law index is alpha = 1.55 compared to the average continuum spectral index = 0.44 derived from the first SDSS composite quasar. Assuming an SMC-like extinction curve, we infer a color excess of E(B-V) = 0.09 +/- 0.01 at the quasar redshift. Only approximately 6% of quasars in the optically-selected Sloan Digital Sky Survey show comparable levels of dust reddening.

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arXiv2003

Continuum and Emission-Line Properties of Broad Absorption Line Quasars

We investigate the continuum and emission-line properties of 224 broad absorption line quasars (BALQSOs) with 0.9<z<4.4 drawn from the Sloan Digital Sky Survey (SDSS) Early Data Release (EDR), which contains 3814 bona fide quasars. We find that low-ionization BALQSOs (LoBALs) are significantly reddened as compared to normal quasars, in agreement with previous work. High-ionization BALQSOs (HiBALs) are also more reddened than the average nonBALQSO. Assuming SMC-like dust reddening at the quasar redshift, the amount of reddening needed to explain HiBALs is E(B-V)~0.023 and LoBALs is E(B-V)~0.077 (compared to the ensemble average of the entire quasar sample). We find that there are differences in the emission-line properties between the average HiBAL, LoBAL, and nonBAL quasar. These differences, along with differences in the absorption line troughs, may be related to intrinsic quasar properties such as the slope of the intrinsic (unreddened) continuum; more extreme absorption properties are correlated with bluer intrinsic continua. Despite the differences among BALQSO sub-types and nonBALQSOs, BALQSOs appear to be drawn from the same parent population as nonBALQSOs when both are selected by their UV/optical properties. We find that the overall fraction of traditionally defined BALQSOs, after correcting for color-dependent selection effects due to different SEDs of BALQSO and nonBALQSOs, is 13.4+/-1.2% and shows no significant redshift dependence for 1.7<z<3.45. After a rough completeness correction for the effects of dust extinction, we find that approximately one in every six quasars is a BALQSO.

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arXiv2003

The Sloan Digital Sky Survey Quasar Catalog II. First Data Release

We present the second edition of the Sloan Digital Sky Survey (SDSS) Quasar Catalog. The catalog consists of 16713 objects in the SDSS First Data Release (DR1) that have luminosities larger than M_i=-22 (H_0=70 km/s, Omega_M=0.3, Omega_Lambda=0.7), have at least one emission line with FWHM larger than 1000 km/s, and have highly reliable redshifts. The area covered is ~1360 deg^2 and the redshifts range from 0.08 to 5.41, with a median value of 1.43. Each object has positions accurate to better than 0.2" rms per coordinate, five-band (ugriz) CCD-based photometry with typical accuracy of 0.03 mag, and information on the morphology and selection method, in addition to radio, near-infrared, and X-ray emission properties of the quasars, when available, from other large area surveys. Calibrated digital spectra covering 3800-9200 Angstroms at a spectral resolution 1800-2100, are available. This publication supersedes the first SDSS Quasar Catalog, which was based on material from the SDSS Early Data Release. A summary of corrections to current quasar databases is provided. The majority of the objects were found in SDSS commissioning data using a multicolor selection technique. Since the quasar selection algorithm was undergoing testing during the entire DR1 observational period, care must be taken when assembling samples for use in statistical studies. A total of 15786 objects (94%) in the catalog were discovered by the SDSS; 12,173 of which are reported here for the first time, including five quasars brighter than i=16.0 and 17 quasars with redshifts larger than 4.5.

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arXiv2003

A Large, Uniform Sample of X-ray Emitting AGN: Selection Approach and an Initial Catalog from the ROSAT All-Sky and Sloan Digital Sky Surveys

Many open questions in X-ray astronomy are limited by the relatively small number of objects in uniform optically-identified samples, especially when rare subclasses are considered, or subsets isolated to search for evolution or correlations between wavebands. We describe initial results of a program aimed to ultimately yield 10^4 X-ray source identifications--a sample about an order of magnitude larger than earlier efforts. The technique employs X-ray data from the ROSAT All-Sky Survey (RASS), and optical imaging and spectroscopic followup from the Sloan Digital Sky Survey (SDSS). Optical objects in the SDSS catalogs are automatically cross-correlated with RASS X-ray source positions; then priorities for follow-on SDSS optical spectra of candidate counterparts are automatically assigned using an algorithm based on the known fx/fopt ratios for various classes of X-ray emitters. SDSS parameters for optical morphology, magnitude, colors, plus FIRST radio data, serve as proxies for object class. Initial application of this approach to 1400 deg^2 of sky provides a catalog of 1200 spectroscopically confirmed quasars/AGN that are probable RASS identifications. Most of the IDs are new, and only a few percent of the AGN are likely to be random superpositions. The magnitude and redshift ranges of the counterparts extend over 15<m<21 and 0.03<z<3.6. Although most IDs are quasars and Sy 1s, a variety of other AGN subclasses are also sampled. Substantial numbers of rare AGN are found, including more than 130 narrow-line Seyfert 1s and 45 BL Lac candidates. These results already provide a sizeable set of new IDs, show utility of the sample in multi-waveband studies, and demonstrate the capability of the RASS/SDSS approach to efficiently proceed towards the largest homogeneously selected/observed sample of X-ray emitting AGN. Abridged Abstract

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arXiv2003

Mass and dust in the disk of a spiral lens galaxy

Gravitational lensing is a potentially important probe of spiral galaxy structure, but only a few cases of lensing by spiral galaxies are known. We present Hubble Space Telescope and Magellan observations of the two-image quasar PMN J2004-1349, revealing that the lens galaxy is a spiral galaxy. One of the quasar images passes through a spiral arm of the galaxy and suffers 3 magnitudes of V-band extinction. Using simple lens models, we show that the mass quadrupole is well-aligned with the observed galaxy disk. A more detailed model with components representing the bulge and disk gives a bulge-to-disk mass ratio of 0.16 +/- 0.05. The addition of a spherical dark halo, tailored to produce an overall flat rotation curve, does not change this conclusion.

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arXiv2003

SDSS J0903+5028: A New Gravitational Lens

We report the discovery of a new gravitationally lensed quasar from the Sloan Digital Sky Survey, SDSS J090334.92+502819.2. This object was targeted for SDSS spectroscopy as a Luminous Red Galaxy (LRG), but manual examination of the spectrum showed the presence of a quasar at z= 3.6 in addition to a red galaxy at z=0.388, and the SDSS image showed a second possible quasar image nearby. Follow-up imaging and spectroscopy confirmed the lensing hypothesis. In images taken at the ARC 3.5-meter telescope, two quasars are separated by 2.8 arc-seconds; the lensing galaxy is clearly seen and is blended with one of the quasar images. Spectroscopy taken at the Keck II telescope shows that the quasars have identical redshifts of z=3.6 and both show the presence of the same broad absorption line-like troughs. We present simple lens models which account for the geometry and magnifications. The lens galaxy lies near two groups of galaxies and may be a part of them. The models suggest that the groups may contribute considerable shear and may have a strong effect on the lens configuration.

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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

Red and Reddened Quasars in the Sloan Digital Sky Survey

We investigate the continuum and emission line properties of 4576 SDSS quasars as a function of their optical/UV SEDs. The optical/UV color distribution of our sample is roughly Gaussian, but with a red tail; we distinguish between 1) intrinsically blue (optically flat) quasars, 2) intrinsically red (optically steep) quasars, and 3) the 273 (6%) of our quasars whose continua are inconsistent with a single power-law and appear redder due to SMC-like dust reddening rather than synchrotron emission. The color distribution suggests that the population of moderately dust reddened broad-line quasars is smaller than that of unobscured quasars, but we estimate that a further 10% of the luminous quasar population is missing from the SDSS sample because of dust extinction with E(B-V)<0.5. We also investigate the emission and absorption line properties of these quasars as a function of color with regard to Boroson & Green type eigenvectors. Intrinsically red (optically steep) quasars tend to have narrower Balmer lines and weaker CIV, CIII], HeII and 3000A bump emission as compared with bluer (optically flatter) quasars. The change in strength of the 3000A bump appears to be dominated by the Balmer continuum and not by FeII emission. The dust reddened quasars have even narrower Balmer lines and weaker 3000A bumps, in addition to having considerably larger equivalent widths of [OII] and [OIII] emission. The fraction of broad absorption line quasars (BALQSOs) increases from ~3.4% for the bluest quasars to perhaps as large as 20% for the dust reddened quasars, but the intrinsic color distribution is affected by dust reddening. (abridged)

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arXiv2003

SDSS J092455.87+021924.9: an Interesting Gravitationally Lensed Quasar from the Sloan Digital Sky Survey

We report the discovery of a new gravitationally lensed quasar from the Sloan Digital Sky Survey, SDSS J092455.87+021924.9 (SDSS J0924+0219). This object was selected from among known SDSS quasars by an algorithm that was designed to select another known SDSS lensed quasar (SDSS 1226-0006A,B). Five separate components, three of which are unresolved, are identified in photometric follow-up observations obtained with the Magellan Consortium's 6.5m Walter Baade telescope at Las Campanas Observatory. Two of the unresolved components (designated A and B) are confirmed to be quasars with z=1.524; the velocity difference is less than 100 km sec^{-1} according to spectra taken with the W. M. Keck Observatory's Keck II telescope on Mauna Kea. A third stellar component, designated C, has the colors of a quasar with redshift similar to components A and B. The maximum separation of the point sources is 1.78". The other two sources, designated G and D, are resolved. Component G appears to be the best candidate for the lensing galaxy. Although component D is near the expected position of the fourth lensed component in a four image lens system, its properties are not consistent with being the image of a quasar at z~1.5. Nevertheless, the identical redshifts of components A and B and the presence of component C strongly suggest that this object is a gravitational lens. Our observations support the idea that a foreground object reddens the fourth lensed component and that another unmodeled effect (such as micro- or milli-lensing) demagnificates it, but we cannot rule out the possibility that SDSS0924+0219 is an example of the relatively rare class of ``three component'' lens systems.

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arXiv2003

The Incidence of Strong-Lensing Clusters in the Red-Sequence Cluster Survey

The incidence of giant arcs due to strong-lensing clusters of galaxies is known to be discrepant with current theoretical expectations. This result derives from a comparison of several cluster samples to predictions in the framework of the currently favored $Λ$CDM cosmology, and one possible explanation for the discrepancy is that this cosmological model is not correct. In this paper we discuss the incidence of giant arcs in the Red-Sequence Cluster Survey (RCS), which again shows significant disagreement with theoretical predictions. We briefly describe a total of eight strong lens systems, seven of which are discussed here for the first time. Based on the details of these systems, in particular on the ratio of single to multiple arc systems, we argue that it may be possible to explain this discrepancy in the currently favored cosmology, by modifying the details of the lenses themselves. Specifically, the high incidence of multiple arc systems and their overall high redshift suggests that a sub-population of the global cluster population is responsible for much of the observed lensing. The lack of lensing clusters at $z<0.64$ in the RCS indicates that a property associated with clusters at early times results in the boosted lensing cross sections; likely a combination of ellipticity and elongation along the line of sight, substructure, and changes in the cluster mass profiles is responsible. Cluster mass, which should evolve to globally higher values toward lower redshifts, is clearly not the most significant consideration for the formation of giant arcs.

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arXiv2003

Peculiar Broad Absorption Line Quasars found in DPOSS

With the recent release of large (i.e., > hundred million objects), well-calibrated photometric surveys, such as DPOSS, 2MASS, and SDSS, spectroscopic identification of important targets is no longer a simple issue. In order to enhance the returns from a spectroscopic survey, candidate sources are often preferentially selected to be of interest, such as brown dwarfs or high redshift quasars. This approach, while useful for targeted projects, risks missing new or unusual species. We have, as a result, taken the alternative path of spectroscopically identifying interesting sources with the sole criterion being that they are in low density areas of the g - r and r - i color-space defined by the DPOSS survey. In this paper, we present three peculiar broad absorption line quasars that were discovered during this spectroscopic survey, demonstrating the efficacy of this approach. PSS J0052+2405 is an Iron LoBAL quasar at a redshift z = 2.4512 with very broad absorption from many species. PSS J0141+3334 is a reddened LoBAL quasar at z = 3.005 with no obvious emission lines. PSS J1537+1227 is a Iron LoBAL at a redshift of z = 1.212 with strong narrow Mgii and Feii emission. Follow-up high resolution spectroscopy of these three quasars promises to improve our understanding of BAL quasars. The sensitivity of particular parameter spaces, in this case a two-color space, to the redshift of these three sources is dramatic, raising questions about traditional techniques of defining quasar populations for statistical analysis.

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arXiv2003

A Catalog of Broad Absorption Line Quasars from the Sloan Digital Sky Survey Early Data Release

We present a catalog of 224 broad absorption line quasars (BALQSOs) from the Sloan Digital Sky Survey's Early Data Release Quasar Catalog, including a relatively complete and homogeneous subsample of 131 BALQSOs. Since the identification of BALQSOs is subject to considerable systematic uncertainties, we attempt to create a complete sample of SDSS BALQSOs by combining the results of two automated selection algorithms and a by-eye classification scheme. One of these automated algorithms finds broad absorption line troughs by comparison with a composite quasar spectrum. We present the details of this algorithm and compare this method to that which uses a power-law fit to the continuum. The BALQSOs in our sample are further classified as high-ionization BALQSOs (HiBALs), low-ionization BALQSOs (LoBALs), and BALQSOs with excited iron absorption features (FeLoBALs); composite spectra of each type are presented. We further present a study of the properties of the BALQSOs in terms of the balnicity distribution, which rises with decreasing balnicity. This distribution of balnicities suggests that the fraction of quasars with intrinsic outflows may be significantly underestimated.

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