arXiv subjects

D. Schlegel

Publications and source records attributed to D. Schlegel.

211 records · Page 12Linked to original sources

arXiv2004

The Distribution of Quasars and Galaxies in Radio Color-Color and Morphology Diagrams

We positionally match the 6 cm GB6, 20 cm FIRST and NVSS, and 92 cm WENSS radio catalogs and find 16,500 matches in ~3,000 deg2 of sky. Using this unified radio database, we construct radio "color-magnitude-morphology" diagrams and find that they display a clear structure, rather than a random scatter. We propose a simple, yet powerful, method for morphological classification of radio sources based on FIRST and NVSS measurements. For a subset of matched sources, we find optical identifications using the SDSS Data Release 1 catalogs, and separate them into quasars and galaxies. Compact radio sources with flat radio spectra are dominated by quasars, while compact sources with steep spectra, and resolved radio sources, contain substantial numbers of both quasars and galaxies.

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arXiv2004

A Second Stellar Color Locus: a Bridge from White Dwarfs to M stars

We report the discovery of a locus of stars in the SDSS g-r vs. u-g color-color diagram that connects the colors of white dwarfs and M dwarfs. While its contrast with respect to the main stellar locus is only ~1:2300, this previously unrecognized feature includes 863 stars from the SDSS Data Release 1. The position and shape of the feature are in good agreement with predictions of a simple binary star model that consists of a white dwarf and an M dwarf, with the components' luminosity ratio controlling the position along this binary system locus. SDSS DR1 spectra for 47 of these objects strongly support this model. The absolute magnitude--color distribution inferred for the white dwarf component is in good agreement with the models of Bergeron et al. (1995).

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arXiv2004

Cosmological parameters from SDSS and WMAP

We measure cosmological parameters using the three-dimensional power spectrum P(k) from over 200,000 galaxies in the Sloan Digital Sky Survey (SDSS) in combination with WMAP and other data. Our results are consistent with a ``vanilla'' flat adiabatic Lambda-CDM model without tilt (n=1), running tilt, tensor modes or massive neutrinos. Adding SDSS information more than halves the WMAP-only error bars on some parameters, tightening 1 sigma constraints on the Hubble parameter from h~0.74+0.18-0.07 to h~0.70+0.04-0.03, on the matter density from Omega_m~0.25+/-0.10 to Omega_m~0.30+/-0.04 (1 sigma) and on neutrino masses from <11 eV to <0.6 eV (95%). SDSS helps even more when dropping prior assumptions about curvature, neutrinos, tensor modes and the equation of state. Our results are in substantial agreement with the joint analysis of WMAP and the 2dF Galaxy Redshift Survey, which is an impressive consistency check with independent redshift survey data and analysis techniques. In this paper, we place particular emphasis on clarifying the physical origin of the constraints, i.e., what we do and do not know when using different data sets and prior assumptions. For instance, dropping the assumption that space is perfectly flat, the WMAP-only constraint on the measured age of the Universe tightens from t0~16.3+2.3-1.8 Gyr to t0~14.1+1.0-0.9 Gyr by adding SDSS and SN Ia data. Including tensors, running tilt, neutrino mass and equation of state in the list of free parameters, many constraints are still quite weak, but future cosmological measurements from SDSS and other sources should allow these to be substantially tightened.

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arXiv2003

Counts of Low-redshift SDSS Quasar Candidates

We analyze the counts of low-redshift quasar candidates selected using nine-epoch SDSS imaging data. The co-added catalogs are more than 1 mag deeper than single-epoch SDSS data, and allow the selection of low-redshift quasar candidates using UV-excess and also variability techniques. The counts of selected candidates are robustly determined down to g=21.5. This is about 2 magnitudes deeper than the position of a change in the slope of the counts reported by Boyle et al. (1990, 2000) for a sample selected by UV-excess, and questioned by Hawkins & Veron (1995), who utilized a variability-selected sample. Using SDSS data, we confirm a change in the slope of the counts for both UV-excess and variability selected samples, providing strong support for the Boyle et al. results.

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arXiv2003

Quasar Radio Dichotomy: Two Peaks, or not Two Peaks, that is the Question

Recent claims by Ivezic et al. (2002) that the distribution of the radio-to-optical flux ratio, R, for quasars is bimodal (the so-called quasar radio dichotomy) were questioned on statistical grounds by Cirasuolo et al. (2003). We apply the approach suggested by Cirasuolo et al. to a sample of \~10,000 objects detected by SDSS and FIRST, and find support for the quasar radio dichotomy. The discrepancy between the claims by Cirasuolo et al. and the results presented here is most likely because 1) the ~100 times larger sample based on two homogeneous surveys that is used here allows a direct determination of the R distribution, rather than relying on indirect inferences based on Monte Carlo simulations of several heterogeneous surveys 2) the accurate SDSS colors and redshift information allow robust determination of the K-correction for R, which, if unaccounted for, introduces significant scatter that masks the intrinsic properties of the quasar R distribution.

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arXiv2003

Reaching to the Edge of the Milky Way Halo with SDSS

We discuss a sample of over 3000 candidate RR Lyrae stars selected by various methods using Sloan Digital Sky Survey data for about 1000 deg^2 of sky. These stars probe the halo structure out to ~100 kpc from the Galactic center. Their spatial and radial velocity distributions are very inhomogeneous, with the most prominent features tracing the Sgr dwarf tidal stream. Outside the Sgr dwarf tidal stream, the spatial distribution in the 5-60 kpc range of Galactocentric radius R is well described by an R^{-3} power law.

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arXiv2003

Halo Structure Traced by SDSS RR Lyrae

We discuss the density and radial velocity distributions of over 3000 candidate RR Lyrae stars selected by various methods using Sloan Digital Sky Survey data for about 1000 deg^2 of sky. This is more than 20 times larger sample than previously reported by SDSS (Ivezic et al. 2000), and includes candidate RR Lyrae stars out to the sample limit of 100 kpc. A cutoff in the radial distribution of halo RR Lyrae at ~50-60 kpc that was suggested by the early SDSS data appears to be a statistical anomaly confined to a small region (~100 deg^2). Despite the large increase in observed area, the most prominent features remain to be those associated with the Sgr dwarf tidal stream. We find multiple number density peaks along three lines of sight in the Sgr dwarf tidal stream plane, that may indicate several perigalactic passages of the Sgr dwarf galaxy.

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arXiv2003

Variability Studies with SDSS

The potential of the Sloan Digital Sky Survey for wide-field variability studies is illustrated using multi-epoch observations for 3,000,000 point sources observed in 700 deg2 of sky, with time spans ranging from 3 hours to 3 years. These repeated observations of the same sources demonstrate that SDSS delivers ~0.02 mag photometry with well behaved and understood errors. We show that quasars dominate optically faint (r > 18) point sources that are variable on time scales longer than a few months, while for shorter time scales, and at bright magnitudes, most variable sources are stars.

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arXiv2001

The Optical, Infrared and Radio Properties of Extragalactic Sources Observed by SDSS, 2MASS and FIRST Surveys

We positionally match sources observed by the Sloan Digital Sky Survey (SDSS), the Two Micron All Sky Survey (2MASS), and the Faint Images of the Radio Sky at Twenty-cm (FIRST) survey. Practically all 2MASS sources are matched to an SDSS source within 2 arcsec; ~11% of them are optically resolved galaxies and the rest are dominated by stars. About 1/3 of FIRST sources are matched to an SDSS source within 2 arcsec; ~80% of these are galaxies and the rest are dominated by quasars. Based on these results, we project that by the completion of these surveys the matched samples will include about 10^7 stars and 10^6 galaxies observed by both SDSS and 2MASS, and about 250,000 galaxies and 50,000 quasars observed by both SDSS and FIRST. Here we present a preliminary analysis of the optical, infrared and radio properties for the extragalactic sources from the matched samples. In particular, we find that the fraction of quasars with stellar colors missed by the SDSS spectroscopic survey is probably not larger than ~10%, and that the optical colors of radio-loud quasars are ~0.05 mag. redder (with 4-sigma significance) than the colors of radio-quiet quasars.

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arXiv2000

Shellflow. I. The Convergence of the Velocity Field at 6000 km/s

We present the first results from the Shellflow program, an all-sky Tully-Fisher (TF) peculiar velocity survey of 276 Sb-Sc galaxies with redshifts between 4500 and 7000 km/s. Shellflow was designed to minimize systematic errors between observing runs and between telescopes, thereby removing the possibility of a spurious bulk flow caused by data inhomogeneity. A fit to the data yields a bulk flow amplitude V_bulk = 70{+100}{-70} km/s (1 sigma error) with respect to the Cosmic Microwave Background, i.e., consistent with being at rest. At the 95% confidence level, the flow amplitude is < 300 km/s. Our results are insensitive to which Galactic extinction maps we use, and to the parameterization of the TF relation. The larger bulk motion found in analyses of the Mark III peculiar velocity catalog are thus likely to be due to non-uniformities between the subsamples making up Mark III. The absence of bulk flow is consistent with the study of Giovanelli and collaborators and flow field predictions from the observed distribution of IRAS galaxies.

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arXiv2000

Optical Spectroscopy of Supernova 1993J During Its First 2500 Days

We present 42 low-resolution spectra of Supernova (SN) 1993J, our complete collection from the Lick and Keck Observatories, from day 3 after explosion to day 2454, as well as one Keck high-dispersion spectrum from day 383. SN 1993J began as an apparent SN II, albeit an unusual one. After a few weeks, a dramatic transition took place, as prominent helium lines emerged in the spectrum. SN 1993J had metamorphosed from a SN II to a SN IIb. Nebular spectra of SN 1993J closely resemble those of SNe Ib and Ic, but with a persistent H_alpha line. At very late times, the H_alpha emission line dominated the spectrum, but with an unusual, box-like profile. This is interpreted as an indication of circumstellar interaction.

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arXiv1999

First Results from the Shellflow Survey

We present preliminary results from the Shellflow program, an all-sky Tully-Fisher (TF) survey of 297 Sb-Sc with redshifts between 4000 and 7500 km/s. The program was designed to ensure uniformity of the data between observing runs and between telescopes, thereby eliminating the possibility of a spurious bulk flow caused by data inhomogeneity. A simple bulk flow model suggests that the Shellflow galaxies are nearly at rest with respect to the cosmic microwave background (CMB). Taken at face value, this result suggests that most of the ~600 km/s motion of the Local Group with respect to the CMB is due to material within ~60 Mpc/h -- a striking confirmation of the gravitational instability picture and the notion of homogeneity in the Universe. Differences between our Shellflow ``null'' result and claims of larger bulk motions based on analyses of the MarkIII catalog are possibly due to small inhomogeneities between the sub-samples in MarkIII. Our preliminary Shellflow bulk velocity, V_bulk = 80+/-150 km/s in the CMB frame, must still be refined with Monte-Carlo simulations and tidal field analysis.

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arXiv1998

SHELLFLOW: The First Homogeneous All-Sky TF Survey at 6000 km/s

We present a new optical Tully-Fisher (TF) investigation for a complete, full-sky sample of 297 Sb-Sc spirals with redshifts between 4500 and 7000 km/s. The survey was specifically designed to provide uniform, well- calibrated data over both hemispheres. All previous TF surveys within the Supergalactic shell (cz < 6000 km/s) have relied on matching separate data sets in the Northern and Southern hemispheres and thus cannot attain full- sky homogeneity. Analyses of the cosmological dipole and peculiar velocities based on these studies have produced contradictory claims for the amplitude of the bulk flow and whether it is generated by internal or external mass fluctuations. With Shellflow, and further zero-point calibration of existing TF data sets, we expect a high-accuracy detection of the bulk flow amplitude and an unambiguous characterization of the tidal field at 6000 km/s. One of the key features of this study is not only its careful, all-sky sample selection but the independent duplication, by at least 2 of us, of all the data reductions. These reductions and a first flow analysis based on the Shellflow sample alone will be published in the Fall.

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