Neta A. Bahcall

Early-type galaxies in the sloan digital sky survey. 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: g*-r* ∝ σ0.26±0.02. Color also correlates with magnitude, g* - r* ∝ (-0.025 ± 0.003)M, and size, but these correlations are entirely due to the L-σ and Ro-σ relations: the primary correlation is color-σ. 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; the color-σ relation is less sensitive to this choice. Chemical evolution and star formation histories of early-type galaxies are investigated using co-added spectra of similar objects. The resulting library of co-added spectra contains spectra that represent a wide range of early-type galaxies. Chemical abundances correlate primarily with velocity dispersion: Hβ ∝ σ-0.24±0.03, Mg2 ∝ σ0.20±0.02, Mg b ∝ σ0.32±0.03, and Fe ∝ σ0.11±0.03. At fixed σ, the population at z ~ 0.2 had weaker Mg2 and stronger Hβ 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 Gyr ago, consistent with a fundamental plane analysis of this sample. Although the fundamental plane shows that galaxies in dense regions are slightly different from galaxies in less dense regions, the co-added spectra and color-magnitude relations show no statistically significant dependence on environment.

Cosmology and the Halo Occupation Distribution from Small-Scale Galaxy Clustering in the Sloan Digital Sky Survey

We use the projected correlation function wp(rp) of a volume-limited subsample of the Sloan Digital Sky Survey (SDSS) main galaxy-redshift catalog to measure the halo occupation distribution (HOD) of the galaxies of the sample. Simultaneously, we allow the cosmology to vary within cosmological constraints imposed by cosmic microwave background experiments in a ΛCDM model. We find that combining wp(rp) for this sample alone with observations by the Wilkinson Microwave Anisotropy Probe (WMAP), Arcminute Cosmology Bolometer Array Receiver (ACBAR), Cosmic Background Imager (CBI), and Very Small Array (VSA) can provide one of the most precise techniques available to measure cosmological parameters. For a minimal, flat, six-parameter ΛCDM model with an HOD with three free parameters, we find Ωm = 0.278, σ8 = 0.812, and H0 = 69.8 km s-1 Mpc-1; these errors are significantly smaller than from cosmic microwave background (CMB) alone and similar to those obtained by combining CMB with the large-scale galaxy power spectrum assuming scale-independent bias. The corresponding HOD parameters describing the minimum halo mass and the normalization and cutoff of the satellite mean occupation are Mmin = (3.03) × 1012 h-1 M☉, M1 = (4.58) × 1013 h-1 M☉, and κ = 4.44. These HOD parameters thus have small fractional uncertainty when cosmological parameters are allowed to vary within the range permitted by the data. When more parameters are added to the HOD model, the error bars on the HOD parameters increase because of degeneracies, but the error bars on the cosmological parameters do not increase greatly. Similar modeling for other galaxy samples could reduce the statistical errors on these results, while more thorough investigations of the cosmology dependence of nonlinear halo bias and halo mass functions are needed to eliminate remaining systematic uncertainties, which may be comparable to statistical uncertainties.

The Hubble Space Telescope Snapshot Survey. IV - A summary of the search for gravitationally lensed quasars

We report the concluding results of the HST Snapshot Survey for gravitationally lensed quasars. New observations of 153 high-luminosity z above 1 quasars are presented, bringing to 498 the total number of quasars observed in the survey. The new observations do not reveal new candidates for gravitational lensing. We present tables summarizing all of the snapshot observations, with measured V-magnitudes, accurate to 0.1 mag, for each of the quasars successfully observed. The observed frequency of lensing of quasars into multiple images is 3-6 out of 502, depending on whether one counts candidates that are not yet securely confirmed and cases in which clusters play a role. This frequency is in the range predicted by calculations with a vanishing cosmological constant, assuming galaxies can be modeled by unevolving isothermal spheres dominated in their centers by dark matter. The observed frequency is an order of magnitude lower than expected in such models when the universe is strongly dominated by a cosmological constant. This conclusion is, however, sensitive to the model assumptions and to the precise number of actual lensed quasars.

Detection of massive tidal tails around the globular cluster palomar 5 with Sloan digital sky survey commissioning data

Abstract : Globular clusters are self-gravitating stellar systems that experience a time-varying tidal potential as they orbit through their parent galaxy. Their dynamical evolution is driven by internal effects such as stellar evolution, two-body relaxation and binary heating, and by external effects induced by the galactic force field, i.e., heating by tidal shocks during disk and bulge passages and tidal stripping. Both internal and external effects should lead to a permanent loss of cluster members and to the eventual dissolution of the cluster. The Galactic globular clusters observed today are believed to be survivors from an initially much more numerous population. They appear to be in various stages of evolution and dissolution, depending on their initial conditions and their galactic orbits (e.g., Cherno & Weinberg 1990, Djorgovsky & Meylan 1994). Numerical simulations predict that possibly as many as half of the present-day Galactic globulars will not survive for another Hubble time (Gnedin & Ostriker 1997). Observational conformation of the gradual dissolution of globular clusters and determination of their mass loss rates is important in itself, but can also shed light on the formation history and structure of the Galactic halo and provide constraints on the Galactic potential.

Detection of massive tidal tails around the globular cluster Palomar 5 with sloan digital sky survey commissioning data

Abstract : Globular clusters are self-gravitating stellar systems that experience a time-varying tidal potential as they orbit through their parent galaxy. Their dynamical evolution is driven by internal effects such as stellar evolution, two-body relaxation and binary heating, and by external effects induced by the galactic force field, i.e., heating by tidal shocks during disk and bulge passages and tidal stripping. Both internal and external effects should lead to a permanent loss of cluster members and to the eventual dissolution of the cluster. The Galactic globular clusters observed today are believed to be survivors from an initially much more numerous population. They appear to be in various stages of evolution and dissolution, depending on their initial conditions and their galactic orbits (e.g., Cherno & Weinberg 1990, Djorgovsky & Meylan 1994). Numerical simulations predict that possibly as many as half of the present-day Galactic globulars will not survive for another Hubble time (Gnedin & Ostriker 1997). Observational conformation of the gradual dissolution of globular clusters and determination of their mass loss rates is important in itself, but can also shed light on the formation history and structure of the Galactic halo and provide constraints on the Galactic potential.