arXiv subjects

D. J. Schlegel

Publications and source records attributed to D. J. Schlegel.

At least 55 records · Page 3Linked to original sources

arXiv2004

The Origin of the Mass--Metallicity Relation: Insights from 53,000 Star-Forming Galaxies in the SDSS

We utilize Sloan Digital Sky Survey imaging and spectroscopy of ~53,000 star-forming galaxies at z~0.1 to study the relation between stellar mass and gas-phase metallicity. We derive gas-phase oxygen abundances and stellar masses using new techniques which make use of the latest stellar evolutionary synthesis and photoionization models. We find a tight (+/-0.1 dex) correlation between stellar mass and metallicity spanning over 3 orders of magnitude in stellar mass and a factor of 10 in metallicity. The relation is relatively steep from 10^{8.5} - 10^{10.5} M_sun, in good accord with known trends between luminosity and metallicity, but flattens above 10^{10.5} M_sun. We use indirect estimates of the gas mass based on the H-alpha luminosity to compare our data to predictions from simple closed box chemical evolution models. We show that metal loss is strongly anti-correlated with baryonic mass, with low mass dwarf galaxies being 5 times more metal-depleted than L* galaxies at z~0.1. Evidence for metal depletion is not confined to dwarf galaxies, but is found in galaxies with masses as high as 10^{10} M_sun. We interpret this as strong evidence both of the ubiquity of galactic winds and of their effectiveness in removing metals from galaxy potential wells.

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arXiv2004

A Catalog of Spectroscopically Identified White Dwarf Stars in the First Data Release of the Sloan Digital Sky Survey

We present the full spectroscopic white dwarf and hot subdwarf sample from the SDSS first data release, DR1. We find 2551 white dwarf stars of various types, 240 hot subdwarf stars, and an additional 144 objects we have identified as uncertain white dwarf stars. Of the white dwarf stars, 1888 are non-magnetic DA types and 171, non-magnetic DBs. The remaining (492) objects consist of all different types of white dwarf stars: DO, DQ, DC, DH, DZ, hybrid stars like DAB, etc., and those with non-degenerate companions. We fit the DA and DB spectra with a grid of models to determine the Teff and log(g) for each object. For all objects, we provide coordinates, proper motions, SDSS photometric magnitudes, and enough information to retrieve the spectrum/image from the SDSS public database. This catalog nearly doubles the known sample of spectroscopically-identified white dwarf stars. In the DR1 imaged area of the sky, we increase the known sample of white dwarf stars by a factor of 8.5. We also comment on several particularly interesting objects in this sample.

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arXiv2003

Relationship between environment and the broad-band optical properties of galaxies in the SDSS

We examine the relationship between environment and the luminosities, surface brightnesses, colors, and profile shapes of luminous galaxies in the Sloan Digital Sky Survey (SDSS). For the SDSS sample, galaxy color is the galaxy property most predictive of the local environment. Galaxy color and luminosity jointly comprise the most predictive pair of properties. At fixed luminosity and color, density is not closely related to surface brightness or to Sersic index -- the parameter in this study that astronomers most often associate with morphology. In the text, we discuss what measureable residual relationships exist, generally finding that at red colors and fixed luminosity, the mean density decreases at the highest surface brightnesses and Sersic indices. In general, these results suggest that the morphological properties of galaxies are less closely related to galaxy environment than are their masses and star-formation histories.

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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

Candidate Type II Quasars from the Sloan Digital Sky Survey: I. Selection and Optical Properties of a Sample at 0.3<Z<0.83

Type II quasars are the long-sought luminous analogs of type II (narrow emission line) Seyfert galaxies, suggested by unification models of active galactic nuclei (AGN) and postulated to account for an appreciable fraction of the cosmic hard X-ray background. We present a sample of 291 type II AGN at redshifts 0.3<Z<0.83 from the spectroscopic data of the Sloan Digital Sky Survey. These objects have narrow (FWHM<2000 km/s), high equivalent width emission lines with high-ionization line ratios. We describe the selection procedure and discuss the optical properties of the sample. About 50% of the objects have [OIII] λ_air 5007 line luminosities in the range 3\times 10^8-10^10 L_Sun, comparable to those of luminous (-27<M_B<-23) quasars; this, along with other evidence, suggests that the objects in the luminous subsample are type II quasars.

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arXiv2003

The velocity dispersion function of early-type galaxies

The distribution of early-type galaxy velocity dispersions, phi(sigma), is measured using a sample drawn from the SDSS database. Its shape differs significantly from that which one obtains by simply using the mean correlation between luminosity, L, and velocity dispersion, sigma, to transform the luminosity function into a velocity function: ignoring the scatter around the mean sigma-L relation is a bad approximation. An estimate of the contribution from late-type galaxies is also made, which suggests that phi(sigma) is dominated by early-type galaxies at velocities larger than ~ 200 km/s.

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arXiv2003

Stellar and Dynamical Masses of Ellipticals in the Sloan Digital Sky Survey

We study the variation of the dark matter mass fraction of elliptical galaxies as a function of their luminosity, stellar mass, and size using a sample of 29,469 elliptical galaxies culled from the Sloan Digital Sky Survey. We model ellipticals as a stellar Hernquist profile embedded in an adiabatically compressed dark matter halo. This model allows us to estimate a dynamical mass ($M_{dynm}$) at the half-light radius from the velocity dispersion of the spectra, and to compare these to the stellar mass estimates ($M_{*}$) from Kauffmann et al (2003). We find that $M_{*}/L$ is independent of luminosity, while $M_{dynm}/L$ increases with luminosity, implying that the dark matter fraction increases with luminosity. We also observe that at a fixed luminosity or stellar mass, the dark matter fraction increases with increasing galaxy size or, equivalently, increases with decreasing surface brightness: high surface brightness galaxies show almost no evidence for dark matter, while in low surface brightness galaxies, the dark matter exceeds the stellar mass at the half light radius. We relate this to the fundamental plane of elliptical galaxies, suggesting that the tilt of this plane from simple virial predictions is due to the dark matter in galaxies. We find that a simple model where galaxies are embedded in dark matter halos and have a star formation efficiency independent of their surface brightness explains these trends. We estimate the virial mass of ellipticals as being approximately 7-30 times their stellar mass, with the lower limit suggesting almost all of the gas within the virial radius is converted into stars.

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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

Early-type galaxies in the SDSS. I. The sample

A sample of nearly 9000 early-type galaxies, in the redshift range 0.01 < z < 0.3, was selected from the Sloan Digital Sky Survey using morphological and spectral criteria. This paper describes how the sample was selected, presents examples of images and seeing corrected fits to the observed surface brightness profiles, describes our method for estimating K-corrections, and shows that the SDSS spectra are of sufficiently high quality to measure velocity dispersions accurately. It also provides catalogs of the measured photometric and spectroscopic parameters. In related papers, these data are used to study how early-type galaxy observables, including luminosity, effective radius, surface brightness, color, and velocity dispersion, are correlated with one another.

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arXiv2003

Early-type galaxies in the SDSS. III. The Fundamental Plane

A magnitude limited sample of nearly 9000 early-type galaxies, in the redshift range 0.01 < z < 0.3, was selected from the Sloan Digital Sky Survey using morphological and spectral criteria. The Fundamental Plane relation in this sample is R_o ~ sigma^{1.49\pm 0.05} I_o^{-0.75\pm 0.01} in the r* band. It is approximately the same in the g*, i* and z* bands. Relative to the population at the median redshift in the sample, galaxies at lower and higher redshifts have evolved only little. If the Fundamental Plane is used to quantify this evolution then the apparent magnitude limit can masquerade as evolution; once this selection effect has been accounted for, the evolution is consistent with that of a passively evolving population which formed the bulk of its stars about 9 Gyrs ago. One of the principal advangtages of the SDSS sample over previous samples is that the galaxies in it lie in environments ranging from isolation in the field to the dense cores of clusters. The Fundamental Plane shows that galaxies in dense regions are slightly different from galaxies in less dense regions.

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arXiv2003

Early-type galaxies in the SDSS. IV. Colors and chemical evolution

The colors and chemical abundances of early-type galaxies at redshifts z<0.3 are studied using a sample of nearly 9000 galaxies, selected from the Sloan Digital Sky Survey using morphological and spectral criteria. In this sample, redder galaxies have larger velocity dispersions. Color also correlates with magnitude and size, but these correlations are entirely due to the L-sigma and R_o-sigma relations: the primary correlation is color-sigma. The red light in early-type galaxies is, on average, slightly more centrally concentrated than the blue. Because of these color gradients, the strength of the color--magnitude relation depends on whether or not the colors are defined using a fixed metric aperture. Chemical evolution and star formation histories of early-type galaxies are investigated using co-added spectra of similar objects. At fixed sigma, the population at z ~ 0.2 had weaker Mg_2 and stronger H_beta absorption compared to the population at z ~ 0. It was also bluer. Comparison of these colors and line-strengths, and their evolution, with single-burst stellar population models suggests a formation time of 9 Gyrs ago, consistent with a Fundamental Plane analysis of this sample.

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arXiv2003

Early-type galaxies in the SDSS. II. Correlations between observables

A magnitude limited sample of nearly 9000 early-type galaxies, in the redshift range 0.01 < z < 0.3, was selected from the Sloan Digital Sky Survey using morphological and spectral criteria. The sample was used to study how early-type galaxy observables, including luminosity L, effective radius R_o, surface brightness I_o, color, and velocity dispersion sigma, are correlated with one another. Measurement biases are understood with mock catalogs which reproduce all of the observed scaling relations and their dependences on fitting technique. At any given redshift, the intrinsic distribution of luminosities, sizes and velocity dispersions in our sample are all approximately Gaussian. A maximum likelihood analysis shows that sigma ~ L^{0.25\pm 0.012}, R_o ~ L^{0.63\pm 0.025}, and R_o ~ I^{-0.75\pm 0.02} in the r* band. In addition, the mass-to-light ratio within the effective radius scales as M_o/L ~ L^{0.14\pm 0.02} or M_o/L ~ M_o^{0.22\pm 0.05}, and galaxies with larger effective masses have smaller effective densities: Delta_o ~ M_o^{-0.52\pm 0.03}. These relations are approximately the same in the g*, i* and z* bands. Relative to the population at the median redshift in the sample, galaxies at lower and higher redshifts have evolved only little, with more evolution in the bluer bands. The luminosity function is consistent with weak passive luminosity evolution and a formation time of about 9 Gyrs ago.

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arXiv2002

The Broad-band Optical Properties of Galaxies with Redshifts 0.0 < z < 0.2

Using photometry and spectroscopy of 144,609 galaxies from the Sloan Digital Sky Survey, we present bivariate distributions of pairs of seven galaxy properties: four optical colors, surface brightness, radial profile shape as measured by the Sersic index, and absolute magnitude. In addition, we present the dependence of local galaxy density (smoothed on 8 h^{-1} Mpc scales) on all of these properties. Several classic, well-known relations among galaxy properties are evident at extremely high signal-to-noise ratio: the color-color relations of galaxies, the color-magnitude relations, the magnitude-surface brightness relation, and the dependence of density on color and absolute magnitude. We show that most of the i-band luminosity density in the universe is in the absolute magnitude and surface brightness ranges used. Some of the relationships between parameters, in particular the color--magnitude relations, show stronger correlations for exponential galaxies and concentrated galaxies taken separately than for all galaxies taken together. We provide a simple set of fits of the dependence of galaxy properties on luminosity for these two sets of galaxies.

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arXiv2002

The Type IIn SN 1995G: Interaction with the CSM

We present the photometric and spectroscopic evolution of the type IIn SN 1995G in NGC 1643, on the basis of 4 years of optical and infrared observations. This supernova shows very flat optical light curves similar to SN 1988Z, with a slow decline rate at all times. The spectra are characterized by strong Balmer lines with multiple components in emission and with a P-Cygni absorption component blueshifted by only 700 km/s. This feature indicates the presence of a slowly expanding shell above the SN ejecta as in the case of SNe 1994aj and 1996L. As in other SNe IIn the slow luminosity decline cannot be explained only with a radioactive energy input and an additional source of energy is required, most likely that produced by the interaction between supernova ejecta and a pre--existent circumstellar medium. It was estimated that the shell material has a density n(H) >> 10^8 cm^-3, consistent with the absence of forbidden lines in the spectra. About 2 years after the burst the low velocity shell is largely overtaken by the SN ejecta and the luminosity drops at a faster rate.

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arXiv2001

Early-type galaxies in the SDSS

A sample of nearly 9000 early-type galaxies, in the redshift range 0.01<z<0.3, was selected from the Sloan Digital Sky Survey using morphological and spectral criteria. The sample was used to study how early-type galaxy observables, including luminosity L, effective radius R, surface brightness I, color, and velocity dispersion V, are correlated with one another. Measurement biases are understood with mock catalogs which reproduce all of the observed scaling relations. At any given redshift, the intrinsic distribution of luminosities, sizes and velocity dispersions in our sample are all approximately Gaussian. In the r* band L ~ V^3.91, L ~ R^1.58, R ~ I^(-0.75), and the Fundamental Plane relation is R ~ V^(1.49) I^(-0.75). These relations are approximately the same in the g*, i* and z* bands. At fixed luminosity, the mass-to-light ratio scales as M/L ~ L^0.14. The g*-r* color scales as V^0.25. Color also correlates with magnitude and size, but these correlations are entirely due to the L-V and R-V relations. Chemical evolution and star formation histories are investigated using co-added spectra of similar objects in our sample. Chemical abundances correlate primarily with velocity dispersion. At fixed V, the higher redshift population is bluer, is weaker in Mg2, and is stronger in Hbeta than the population nearby. In addition, the population at higher redshifts is slightly more luminous. These differences are consistent with that of a passively evolving population which formed the bulk of its stars about 9 Gyrs ago. The Fundamental Plane suggests that galaxies in dense regions are slightly different from those in less dense regions, but the co-added spectra and color--magnitude relations show no statistically significant dependence on environment.

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arXiv2001

Spectroscopic Target Selection for the Sloan Digital Sky Survey: The Luminous Red Galaxy Sample

We describe the target selection and resulting properties of a spectroscopic sample of luminous, red galaxies (LRG) from the imaging data of the Sloan Digital Sky Survey (SDSS). These galaxies are selected on the basis of color and magnitude to yield a sample of luminous, intrinsically red galaxies that extends fainter and further than the main flux-limited portion of the SDSS galaxy spectroscopic sample. The sample is designed to impose a passively-evolving luminosity and rest-frame color cut to a redshift of 0.38. Additional, yet more luminous, red galaxies are included to a redshift of 0.5. Approximately 12 of these galaxies per square degree are targeted for spectroscopy, so the sample will number over 100,000 with the full survey. SDSS commissioning data indicate that the algorithm efficiently selects luminous (M_g=-21.4), red galaxies, that the spectroscopic success rate is very high, and that the resulting set of galaxies is approximately volume-limited out to z=0.38. When the SDSS is complete, the LRG spectroscopic sample will fill over 1h^-3 Gpc^3 with an approximately homogeneous population of galaxies and will therefore be well suited to studies of large-scale structure and clusters out to z=0.5.

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arXiv2001

A New Very Cool White Dwarf Discovered by the Sloan Digital Sky Survey

Early data taken during commissioning of the SDSS have resulted in the discovery of a very cool white dwarf. It appears to have stronger collision induced absorption from molecular hydrogen than any other known white dwarf, suggesting it has a cooler temperature than any other. While its distance is presently unknown, it has a surprisingly small proper motion, making it unlikely to be a halo star. An analysis of white dwarf cooling times suggests that this object may be a low-mass star with a helium core. The SDSS imaging and spectroscopy also recovered LHS 3250, the coolest previously known white dwarf, indicating that the SDSS will be an effective tool for identifying these extreme objects.

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arXiv1999

Interstellar Dust Emission as a CMBR Foreground

This chapter discusses Galactic dust and how its thermal emission confuses CMBR measurements. Interstellar dust grains are composed of many differing species, and observational evidence has only begun to disentagle their composition and sizes. Fortunately, their far-IR/millimeter emission is less complex. We describe how a very simple two-species model can describe the emission at 200 < nu < 3000 GHz to high precision. At lower frequencies, other non-thermal processes may dominate the emission from dust.

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