Donald Q. Lamb

Cosmological constraints from the SDSS luminous red galaxies

We measure the large-scale real-space power spectrum P(k) using luminous red galaxies (LRGs) in the Sloan Digital Sky Survey (SDSS) and use this measurement to sharpen constraints on cosmological parameters from the Wilkinson Microwave Anisotropy Probe (WMAP). We employ a matrix-based power spectrum estimation method using Pseudo-Karhunen-Loeve eigenmodes, producing uncorrelated minimum-variance measurements in 20 k-bands of both the clustering power and its anisotropy due to redshift-space distortions, with narrow and well-behaved window functions in the range 0.01h/Mpc 0.1h/Mpc and associated nonlinear complications, yet agree well with more aggressive published analyses where nonlinear modeling is crucial.

Galaxy Clustering in Early Sloan Digital Sky Survey Redshift Data

We present the first measurements of clustering in the Sloan Digital Sky Survey (SDSS) galaxy redshift survey. Our sample consists of 29,300 galaxies with redshifts 5700 km s-1 ≤ cz ≤ 39,000 km s-1, distributed in several long but narrow (25-5°) segments, covering 690 deg2. For the full, flux-limited sample, the redshift-space correlation length is approximately 8 h-1 Mpc. The two-dimensional correlation function ξ(rp,π) shows clear signatures of both the small-scale, fingers-of-God distortion caused by velocity dispersions in collapsed objects and the large-scale compression caused by coherent flows, though the latter cannot be measured with high precision in the present sample. The inferred real-space correlation function is well described by a power law, ξ(r) = (r/6.1 ± 0.2 h-1 Mpc)-1.75±0.03, for 0.1 h-1 Mpc ≤ r ≤ 16 h-1 Mpc. The galaxy pairwise velocity dispersion is σ12 ≈ 600 ± 100 km s-1 for projected separations 0.15 h-1 Mpc ≤ rp ≤ 5 h-1 Mpc. When we divide the sample by color, the red galaxies exhibit a stronger and steeper real-space correlation function and a higher pairwise velocity dispersion than do the blue galaxies. The relative behavior of subsamples defined by high/low profile concentration or high/low surface brightness is qualitatively similar to that of the red/blue subsamples. Our most striking result is a clear measurement of scale-independent luminosity bias at r 10 h-1 Mpc: subsamples with absolute magnitude ranges centered on M* - 1.5, M*, and M* + 1.5 have real-space correlation functions that are parallel power laws of slope ≈-1.8 with correlation lengths of approximately 7.4, 6.3, and 4.7 h-1 Mpc, respectively.

Gamma-Ray Bursts as a Probe of the Very High Redshift Universe

There is increasingly strong evidence that gamma-ray bursts (GRBs) are associated with star-forming galaxies and occur near or in the star-forming regions of these galaxies. These associations provide indirect evidence that at least the long GRBs detected by BeppoSAX are a result of the collapse of massive stars. The recent evidence that the light curves and the spectra of the afterglows of GRB 970228 and GRB 980326 appear to contain a supernova component, in addition to a relativistic shock-wave component, provides more direct clues that this is the case. We show that, if many GRBs are indeed produced by the collapse of massive stars, GRBs and their afterglows provide a powerful probe of the very high redshift (z 5) universe. We first establish that GRBs and their afterglows are both detectable out to very high redshifts. We then show that one expects GRBs to occur out to at least z ? 10, and possibly to z ? 15-20, redshifts that are far larger than those expected for the most distant quasars. This implies that there are large numbers of GRBs with peak photon number fluxes below the detection thresholds of BATSE and HETE 2, and even below the detection threshold of Swift. The mere detection of very high redshift GRBs would give us our first information about the earliest generations of stars. We show that GRBs and their afterglows can be used as beacons to locate core-collapse supernovae at redshifts z 1 and to study the properties of these supernovae. We describe the expected properties of the absorption-line systems and the Ly? forest in the spectra of GRB afterglows and discuss various strategies for determining the redshifts of very high redshift GRBs. We then show how the absorption-line systems and the Ly? forest visible in the spectra of GRB afterglows can be used to trace the evolution of metallicity in the universe and to probe the large-scale structure of the universe at very high redshifts. Finally, we show how measurement of the Ly? break in the spectra of GRB afterglows can be used to constrain, or possibly measure, the epoch at which reionization of the universe occurred by using the Gunn-Peterson test.

A Possible Cepheid-like Luminosity Estimator for the Long Gamma-Ray Bursts

We present a possible Cepheid-like luminosity estimator for the long gamma-ray bursts based on the variability of their light curves. To construct the luminosity estimator, we use CGRO/BATSE data for 13 bursts, Wind/Konus data for five bursts, Ulysses/GRB data for one burst, and NEAR/XGRS data for one burst. Spectroscopic redshifts, peak fluxes, and high-resolution light curves are available for 11 of these bursts; partial information is available for the remaining nine bursts. We find that the isotropic equivalent peak luminosities L of these bursts positively correlate with a rigorously constructed measure V of the variability of their light curves. We fit to these data a model that accommodates both intrinsic scatter (statistical variance) and extrinsic scatter (sample variance). We find that L ~ V. If one excludes GRB 980425 from the fit, on the grounds that its association with SN 1998bw at a redshift of z = 0.0085 is not secure, the luminosity estimator spans ≈2.5 orders of magnitude in L, and the slope of the correlation between L and V is positive with a probability of 1 - (1.4 × 10-4) (3.8 σ). Although GRB 980425 is excluded from this fit, its L and V values are consistent with the fitted model, which suggests that GRB 980425 may well be associated with SN 1998bw and that GRB 980425 and the cosmological bursts may share a common physical origin. If one includes GRB 980425 in the fit, the luminosity estimator spans ≈6.3 orders of magnitude in L, and the slope of the correlation is positive with a probability of 1 - (9.3 × 10-7) (4.9 σ). In either case, the luminosity estimator yields best-estimate luminosities that are accurate to a factor of ≈4, or best-estimate luminosity distances that are accurate to a factor of ≈2. Regardless of whether GRB 980425 should be included in the fit, its light curve is unique in that it is much less variable than the other ≈17 light curves of bursts in our sample for which the signal-to-noise ratio is reasonably good.

The C4 Clustering Algorithm: Clusters of Galaxies in the Sloan Digital Sky Survey

We present the C4 Cluster Catalog, a new sample of 748 clusters of galaxies identified in the spectroscopic sample of the Second Data Release (DR2) of the Sloan Digital Sky Survey (SDSS). The C4 cluster-finding algorithm identifies clusters as overdensities in a seven-dimensional position and color space, thus minimizing projection effects that have plagued previous optical cluster selection. The present C4 catalog covers ~2600 deg2 of sky and ranges in redshift from z = 0.02 to 0.17. The mean cluster membership is 36 galaxies (with measured redshifts) brighter than r = 17.7, but the catalog includes a range of systems, from groups containing 10 members to massive clusters with over 200 cluster members with measured redshifts. The catalog provides a large number of measured cluster properties including sky location, mean redshift, galaxy membership, summed r-band optical luminosity (Lr), and velocity dispersion, as well as quantitative measures of substructure and the surrounding large-scale environment. We use new, multicolor mock SDSS galaxy catalogs, empirically constructed from the ΛCDM Hubble Volume (HV) Sky Survey output, to investigate the sensitivity of the C4 catalog to the various algorithm parameters (detection threshold, choice of passbands, and search aperture), as well as to quantify the purity and completeness of the C4 cluster catalog. These mock catalogs indicate that the C4 catalog is 90% complete and 95% pure above M200 = 1 × 1014 h-1 M⊙ and within 0.03 ≤ z ≤ 0.12. Using the SDSS DR2 data, we show that the C4 algorithm finds 98% of X-ray–identified clusters and 90% of Abell clusters within 0.03 ≤ z ≤ 0.12. Using the mock galaxy catalogs and the full HV dark matter simulations, we show that the Lr of a cluster is a more robust estimator of the halo mass (M200) than the galaxy line-of-sight velocity dispersion or the richness of the cluster. However, if we exclude clusters embedded in complex large-scale environments, we find that the velocity dispersion of the remaining clusters is as good an estimator of M200 as Lr. The final C4 catalog will contain 2500 clusters using the full SDSS data set and will represent one of the largest and most homogeneous samples of local clusters.

A Study of Compact Object Mergers as Short Gamma-Ray Burst Progenitors

We present a theoretical study of double compact objects as potential short/hard gamma-ray burst (GRB) progenitors. An updated population synthesis code, StarTrack, is used to calculate properties of double neutron stars and black hole-neutron star binaries. We obtain their formation rates, estimate merger times, and finally predict their most likely merger locations and afterglow properties for different types of host galaxies. Our results serve for a direct comparison with the recent HETE-2 and Swift observations of several short bursts, for which afterglows and host galaxies were detected. We also discuss the possible constraints these observations put on the evolutionary models of double compact object formation. We emphasize that our double compact object models can successfully reproduce at the same time short GRBs within both young, star-forming galaxies (e.g., GRB 050709 and GRB 051221A), as well as within old, elliptical hosts (e.g., GRB 050724 and probably GRB 050509B).

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 the 16,713 objects in the SDSS First Data Release that have luminosities larger than Mi = -22 (in a cosmology with H0 = 70 km s-1 Mpc-1, ΩM = 0.3, and ΩΛ = 0.7), have at least one emission line with FWHM larger than 1000 km s-1, and have highly reliable redshifts. The area covered by the catalog is ≈1360 deg2. The quasar redshifts range from 0.08 to 5.41, with a median value of 1.43. For each object, the catalog presents positions accurate to better than 02 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 some radio, near-infrared, and X-ray emission properties of the quasars, when available, from other large-area surveys. Calibrated digital spectra of all objects in the catalog, covering the wavelength region 3800–9200 A at a spectral resolution of 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 also 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 observational period covered by this catalog, care must be taken when assembling samples from the catalog for use in statistical studies. A total of 15,786 objects (94%) in the catalog were discovered by the SDSS; 12,173 of the SDSS discoveries are reported here for the first time. Included in the new discoveries are five quasars brighter than i = 16.0 and 17 quasars with redshifts larger than 4.5.

The Sloan Digital Sky Survey Quasar Catalog. I. Early data release

We present the first edition of the Sloan Digital Sky Survey (SDSS) Quasar Catalog. The catalog consists of the 3814 objects (3000 discovered by the SDSS) in the initial SDSS public data release that have at least one emission line with a full width at half-maximum larger than 1000 km s-1, luminosities brighter than Mi* = -23, and highly reliable redshifts. The area covered by the catalog is 494 deg2; the majority of the objects were found in SDSS commissioning data using a multicolor selection technique. The quasar redshifts range from 0.15 to 5.03. For each object the catalog presents positions accurate to better than 02 rms per coordinate, five-band (ugriz) CCD-based photometry with typical accuracy of 0.05 mag, radio and X-ray emission properties, and information on the morphology and selection method. Calibrated spectra of all objects in the catalog, covering the wavelength region 3800–9200 A at a spectral resolution of 1800–2100, are also available. Since the quasars were selected during the commissioning period, a time when the quasar selection algorithm was undergoing frequent revisions, the sample is not homogeneous and is not intended for statistical analysis.

Universal intermittent properties of particle trajectories in highly turbulent flows

We present a collection of eight data sets from state-of-the-art experiments and numerical simulations on turbulent velocity statistics along particle trajectories obtained in different flows with Reynolds numbers in the range R{lambda}in[120:740]. Lagrangian structure functions from all data sets are found to collapse onto each other on a wide range of time lags, pointing towards the existence of a universal behavior, within present statistical convergence, and calling for a unified theoretical description. Parisi-Frisch multifractal theory, suitably extended to the dissipative scales and to the Lagrangian domain, is found to capture the intermittency of velocity statistics over the whole three decades of temporal scales investigated here.

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 (WDs) and 56 hot subdwarfs are identified spectroscopically where only 44 white dwarfs and five 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 C2 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 12,000 K, and the density of these stars for 15 < g < 20 is found to be ~2.2 deg-2 at Galactic latitudes of 29°–62°. 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.