Christopher J. Miller

We present a combined optical and X-ray analysis of three optically selected X-ray bright groups with giant elliptical galaxies in the center. These massive ellipticals were targeted for XMM-Newton X-ray observations based on their large velocity dispersions and their proximity to a nearby ROSAT X-ray source. Additionally, these targets are significantly brighter in the optical than their nearest neighbors. We show that one of these systems meets the standard criteria for a fossil group. While the other two systems have a prominent magnitude gap in the E/S0 ridgeline, they do not appear to have reached the fossil-like final stage of group evolution.

We investigate the star formation properties of dynamically relaxed galaxy clusters as a function of cluster mass for 308 low-redshift clusters drawn from the Sloan Digital Sky Survey (SDSS) C4 cluster catalog. It is important to establish if cluster star formation properties have a mass dependence before comparing clusters at different epochs, and here we use cluster velocity dispersion, σ, as a measure of cluster mass. We select clusters with no significant substructure, a subset of the full C4 sample, so that velocity dispersion is an accurate tracer of cluster mass. We find that the total stellar mass, the number of star-forming galaxies, and total star formation rate scale linearly with the number of member galaxies, with no residual dependence on cluster velocity dispersion. With the mass-dependence of cluster star formation rates established, we compare the SDSS clusters with a sample of z 0.75 clusters from the literature and find that on average (correcting for the mass growth of clusters between the two redshifts) the total Hα luminosity of the high-redshift clusters is 10 times greater than that of the low-redshift clusters. This can be explained by a decline in the Hα luminosities of individual cluster galaxies by a factor of up to ~10 since z 0.75. The magnitude of this evolution is comparable to that of field galaxies over a similar redshift interval, and thus the effect of the cluster environment on the evolution of star-forming galaxies is at most modest. Our results suggest that the physical mechanism driving the evolution of cluster star formation rates is independent of cluster mass, at least for clusters with velocity dispersion greater than 450 km s⁻¹, and operates over a fairly long timescale such that the star formation rates of individual galaxies decline by an order of magnitude over ~7 billion years.

We investigate the star-formation properties of dynamically relaxed galaxy clusters as a function of cluster mass for 308 low-redshift clusters drawn from the Sloan Digital Sky Survey (SDSS) C4 cluster catalog. It is important to establish if cluster star-formation properties have a mass dependence before comparing clusters at different epochs, and here we use cluster velocity dispersion as a measure of cluster mass. We find that the total stellar mass, the number of star-forming galaxies, and total star-formation rate scale linearly with the number of member galaxies, with no residual dependence on cluster velocity dispersion. With the mass-dependence of cluster star-formation rates established, we compare the SDSS clusters with a sample of z = 0.75 clusters from the literature and find that on average the total H-alpha luminosity of the high-redshift clusters is 10 times greater than that of the low-redshift clusters. This can be explained by a decline in the H-alpha luminosities of individual cluster galaxies by a factor of up to 10 since z = 0.75. The magnitude of this evolution is comparable to that of field galaxies over a similar redshift interval, and thus the effect of the cluster environment on the evolution of star-forming galaxies is at most modest. Our results suggest that the physical mechanism driving the evolution of cluster star-formation rates is independent of cluster mass, at least for clusters with velocity dispersion greater than 450 km/s, and operates over a fairly long timescale such that the star-formation rates of individual galaxies decline by an order of magnitude over ~7 billion years. (Abridged)

We present a new technique to compute simultaneously valid confidence intervals for a set of model parameters. We apply our method to the Wilkinson Microwave Anisotropy Probe's (WMAP) Cosmic Microwave Background (CMB) data, exploring a seven dimensional space (tau, Omega_DE, Omega_M, omega_DM, omega_B, f_nu, n_s). We find two distinct regions-of-interest: the standard Concordance Model, and a region with large values of omega_DM, omega_B and H_0. This second peak in parameter space can be rejected by applying a constraint (or a prior) on the allowable values of the Hubble constant. Our new technique uses a non-parametric fit to the data, along with a frequentist approach and a smart search algorithm to map out a statistical confidence surface. The result is a confidence ``ball'': a set of parameter values that contains the true value with probability at least 1-alpha. Our algorithm performs a role similar to the often used Markov Chain Monte Carlo (MCMC), which samples from the posterior probability function in order to provide Bayesian credible intervals on the parameters. While the MCMC approach samples densely around a peak in the posterior, our new technique allows cosmologists to perform efficient analyses around any regions of interest: e.g., the peak itself, or, possibly more importantly, the 1-alpha confidence surface.

We report HST/STIS spectroscopy and Gemini/GMOS-N imaging of the Damped Lyman Alpha (DLA) system toward HS 1543+5921 caused by the host star-forming galaxy (SFG) SBS 1543+593. The Gemini image shows new morphological details of this well resolved DLA galaxy. In combination with previous optical spectra, the new UV spectra enable us to compare for the first time, ionized and neutral gas-phase alpha-element abundances derived from emission- and absorption-line spectroscopy, in a bona fide DLA galaxy. The abundances we determine using emission-line diagnostics agree with those from absorption-line diagnostics. We present our results on a metallicity versus redshift diagram that combines local HII regions and SFGs with high-redshift DLAs, and discuss implications for the chemical evolution of galaxies.

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 which plagued previous optical clusters selection. The present C4 catalog covers ~2600 square degrees of sky with groups containing 10 members to massive clusters having over 200 cluster members with redshifts. We provide cluster properties like sky location, mean redshift, galaxy membership, summed r--band optical luminosity (L_r), velocity dispersion, and measures of substructure. We use new mock galaxy catalogs to investigate the sensitivity to the various algorithm parameters, as well as to quantify purity and completeness. These mock catalogs indicate that the C4 catalog is ~90% complete and 95% pure above M_200 = 1x10^14 solar masses and within 0.03 <=z <= 0.12. The C4 algorithm finds 98% of X-ray identified clusters and 90% of Abell clusters within 0.03 <= z <= 0.12. We show that the L_r of a cluster is a more robust estimator of the halo mass (M_200) than the line-of-sight velocity dispersion or the richness of the cluster. L_r. The final SDSS data will provide ~2500 C4 clusters and will represent one of the largest and most homogeneous samples of local clusters.

During its first ≃100,000 years, the universe was a fully ionized plasma with a tight coupling by Thompson scattering between the photons and matter. The trade-off between gravitational collapse and photon pressure causes acoustic oscillations in this primordial fluid. These oscillations will leave predictable imprints in the spectra of the cosmic microwave background and the present-day matter-density distribution. Recently, the BOOMERANG and MAXIMA teams announced the detection of these acoustic oscillations in the cosmic microwave background (observed at redshift ≃ 1000). Here, we compare these CMB detections with the corresponding acoustic oscillations in the matter-density power spectrum (observed at redshift ≃ 0.1). These consistent results, from two different cosmological epochs, provide further support for our standard Hot Big Bang model of the universe.