Anthony H. Gonzalez

A Textbook Example of a Bow Shock in the Merging Galaxy Cluster 1E 0657–56

In the representative embodiments of the new and improved methods and apparatus disclosed herein for controlling multicharge perforating guns or core-sampling guns, the gun-control system of the present invention includes a selectively-operable multi-contact switch assembly operative from the surface to sequentially connect a group of electrically-detonatable charges into a firing circuit for consecutively firing the charges on the gun. An array of serially-connected Zener diodes cooperatively associated with the switch assembly provides indications at the surface showing which of the several charges is then connected into the gun-firing circuit. The new and improved gun-control system also includes a shot-monitoring system which provides additional surface indications from which it can be determined whether the charges on the gun are being successfully fired and, at least approximately, that they are being consecutively fired.

The Las Campanas Distant Cluster Survey: The Catalog

We present an optically selected catalog of 1073 galaxy cluster and group candidates at 0.3 z 1. These candidates are drawn from the Las Campanas Distant Clusters Survey (LCDCS), a drift-scan imaging survey of a 130 square degree strip of the southern sky. To construct this catalog we utilize a novel detection process in which clusters are detected as positive surface brightness fluctuations in the background sky. This approach permits us to find clusters with significantly shallower data than other matched-filter methods that are based upon number counts of resolved galaxies. Selection criteria for the survey are fully automated so that this sample constitutes a well-defined, homogeneous sample that can be used to address issues of cluster evolution and cosmology. Estimated redshifts are derived for the entire sample, and an observed correlation between surface brightness and velocity dispersion, σ, is used to estimate the limiting velocity dispersion of the survey as a function of redshift. We find a net surface density of 15.5 candidates per square degree at zest ≥ 0.3, with a false-detection rate of ~30%. At z ~ 0.3 we probe down to the level of poor groups while by z ~ 0.8 we detect only the most massive systems (σ 1000 km s-1). We also present a supplemental catalog of 112 candidates that fail one or more of the automated selection criteria, but appear from visual inspection to be bona fide clusters.

Measuring the Diffuse Optical Light in Abell 1651

Using drift-scan data, a new approach to determining surface brightness profiles, and techniques for detecting low surface brightness signals, we fit the light profile of the brightest cluster galaxy (BCG) in the rich cluster Abell 1651 out to 670 h-1 kpc. This radius is a significant fraction of the virial radius of the cluster (2 h-1 Mpc), indicating that the sizes of the BCG and the cluster are comparable. We find that the profile is consistent with a de Vaucouleurs profile over the radial range probed. We also find that the integrated light profile of the BCG in Abell 1651 contributes 36% of the total cluster light within 500 h-1 kpc. Including all luminous components, we obtain M/LI ~ 160 h for the cluster, which would be overestimated by ~20% without the BCG halo. Furthermore, the relatively red color of the BCG at large radii suggests that recent disruption and tidal stripping of spirals and dwarf ellipticals do not contribute significantly to the halo luminosity. The color and the form of the profile are consistent with a scenario in which the BCG forms from filamentary collapse during the epoch of cluster formation, with relatively little evolution in the past 5 Gyr. We remove the BCG and other detected galaxies from the image and construct a two-dimensional surface brightness map of the cluster core. Several knots of excess emission are found, but the total diffuse component is constrained to contribute less than 5% of the cluster light.

The Velocity Function of Galaxies

We present a galaxy circular velocity function, Ψ(log v), derived from existing luminosity functions and luminosity-velocity relations. Such a velocity function is desirable for several reasons. First, it enables an objective comparison of luminosity functions obtained in different bands and for different galaxy morphologies, with a statistical correction for dust extinction. In addition, the velocity function simplifies comparison of observations with predictions from high-resolution cosmological N-body simulations. We derive velocity functions from five different data sets and find rough agreement among them, but about a factor of 2 variation in amplitude. These velocity functions are then compared with N-body simulations of a ΛCDM model (corrected for baryonic infall) in order to demonstrate both the utility and the current limitations of this approach. The number density of dark matter halos and the slope of the velocity function near v*, the circular velocity corresponding to an ~L* spiral galaxy, are found to be comparable to those of observed galaxies. The primary sources of uncertainty in construction of Ψ(log v) from observations and N-body simulations are discussed, and explanations to account for discrepancies are suggested.

Distant galaxy clusters identified from optical background fluctuations

We present the first high-redshift (0.3 < z < 1.1) galaxy clusters found by systematically identifying optical low surface brightness fluctuations in the background sky. Using spectra obtained with the Keck I telescope and I-band images from the Palomar 1.5 m telescope, we conclude that at least eight of the 10 candidates examined are high-redshift galaxy clusters. The identification of such clusters from low surface brightness fluctuations provides a complement to classic selection methods based on overdensities of resolved galaxies and enables us to search efficiently for rich high-redshift clusters over large areas of the sky. The detections described here are the first in a survey that covers a total of nearly 140 deg2 of the sky and should yield, if these preliminary results are representative, over 300 such clusters.

The Las Campanas Distant Cluster Survey Correlation Function

We present the first nonlocal (z > 0.2) measurement of the cluster-cluster spatial correlation length, using data from the Las Campanas Distant Cluster Survey (LCDCS). We measure the angular correlation function for velocity dispersion-limited subsamples of the catalog at estimated redshifts of 0.35 ≤ zest < 0.575 and derive spatial correlation lengths for these clusters via the cosmological Limber equation. The correlation lengths that we measure for clusters in the LCDCS are consistent both with local results for the APM cluster catalog and with theoretical expectations based upon the Virgo Consortium Hubble Volume simulations and the analytic predictions. Despite samples containing over 100 clusters, our ability to discriminate between cosmological models is limited because of statistical uncertainty.

Constraints on the Size Evolution of Brightest Cluster Galaxies

We measure the luminosity profiles of 16 brightest cluster galaxies (BCGs) at 0.4 < z < 0.8 using high-resolution F160W NICMOS and F814W WFPC2 Hubble Space Telescope imaging. The heterogeneous sample is drawn from a variety of surveys: seven from clusters in the Einstein Medium Sensitivity Survey, five from the Las Campanas Distant Cluster Survey and its Northern Hemisphere precursor, and the remaining four from traditional optical surveys. We find that the surface brightness profiles of all but three of these BCGs are well described by a standard de Vaucouleurs (r1/4) profile out to at least ~2re and that the biweight-estimated NICMOS effective radius of our high-redshift BCGs (re = 8.3 ± 1.4 kpc for H0 = 80 km s-1 Mpc-1, Ωm = 0.2, ΩΛ = 0.0) is ~2 times smaller than that measured for a local BCG sample. If high-redshift BCGs are in dynamical equilibrium and satisfy the same scaling relations as low-redshift ones, this change in size would correspond to a mass growth of a factor of 2 since z ~ 0.5. However, the biweight-estimated WFPC2 effective radius of our sample is 18 ± 5.1 kpc, which is fully consistent with the local sample. While we can rule out mass accretion rates higher than a factor of 2 in our sample, the discrepancy between our NICMOS and WFPC2 results, which after various tests we describe appears to be physical, does not yet allow us to place strong constraints on accretion rates below that level.

Some Implications of the Anisotropic Distribution of Satellite Galaxies

We examine a possible connection between the anisotropic distribution of satellite galaxies around giant spiral galaxies and the evolution of satellite systems. The observed polar anisotropy either is imprinted by initial conditions or develops from an initially symmetric distribution. We attempt to discriminate between these two possibilities by exploring the implications of the latter one. From the observed distribution of satellite galaxies relative to the primary galaxys disk, we derive constraints on the orbital inclinations of the current satellite population. Using this derived inclination limit and assuming that the initial population had no preferred orbital inclination, we estimate the size of the hypothesized original population. We find that our best-fit models imply a population of destroyed (or inhibited) satellites whose combined luminosity (assuming the same M/L as for the observed satellites) is between 18% and 103% of the current disk luminosity.