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NASA NTRS1987

Studies of the Virgo cluster. VI - Morphological and kinematical structure of the Virgo cluster

The structure of the Virgo cluster is analyzed on the basis of the positions, Hubble types, and radial velocities of 1277 Virgo cluster galaxies. The surface distribution of galaxies is considered according to type, and is discussed using maps, isopleths, strip counts, and radial-density distributions. It is found that the Virgo cluster shows pronounced double structure. The main concentration has a large velocity dispersion and is made up predominantly of early-type galaxies, while the secondary concentration has a much smaller velocity dispersion and contains late types. There is a strong spatial segregation of the Hubble types, the early-type galaxies being more concentrated toward the cluster center. There is significant substructure in the cluster core. The irregularity of the Virgo cluster in both configuration and velocity space shows that the core and the envelope are still forming, and hence that the cluster is young.

Binggeli, Bruno↗
NASA NTRS1992

Spectroscopy of galaxies in distant clusters. IV - A catalog of photometry and spectroscopy for galaxies in seven clusters with z in the range of 0.35 to 0.55

Photometry and spectroscopy for seven deep fields containing distant clusters of galaxies with z in the range of 0.35 to 0.55 are presented. Positions and photometric parameters, including r-magnitudes g - r and r - i colors, surface brightnesses, and photometric profile types are given for about 2000 galaxies. Low-resolution spectroscopy is obtained from which redshifts are determined for 289 objects, of which 190 are cluster members. These are classified according to dominant spectral features, and examples are plotted in each cluster. Color-magnitude and color-color diagrams are formed which show trends in the cluster populations, and maps are made of the cluster field using the color-color relations to increase the contrast of cluster over field. Galaxies with spectra typical of old stellar populations cluster most strongly, with active galaxies, those with recent or ongoing star formation, or an active nucleus, distributed more diffusely. The g - r color is well correlated with active star formation as judged from spectral features.

Dressler, Alan↗
NASA NTRS1993

When clusters collide - A numerical Hydro/N-body simulation of merging galaxy clusters

A 3D numerical simulation of two merging clusters of galaxies, using a hybrid Hydro/N-body code, is presented. The hydrodynamics of the code is solved by an Eulerian finite difference method. Initial results disclose that the X-ray emission of the dominant cluster becomes elongated and broadened; heating occurs at the core of the dominant cluster as a result of multiple shocks, and high velocity gas motions within the intracluster medium. It is predicted that clusters which have undergone recent mergers and do not have cooling flows will have high peculiar gas velocities and that the shocks and turbulence generated during the merger may power cluster-wide radio halos. Prolonged high-velocity gas motions through the dominant cluster core possibly play a major role in the formation and shaping of wide-angle tailed radio sources associated with central dominant galaxies. The N-body component of the simulation reveals the subcluster to be dispersed as it passes through the dominant cluster.

Roettiger, Kurt↗
NASA NTRS1995

Radial velocities of stars in the globular cluster M4 and the cluster distance

The internal stellar velocity distribution of the globular cluster M4 is evaluated from nearly 200 new radial velocity measurements good to 1 km/s and a rederivation of existing proper motions. The mean radial velocity of the cluster is 70.9 +/- 0.6 km/s. The velocity dispersion is 3.5 +/- 0.3 km/s at the core, dropping marginally towards the outskirts. Such a low internal dispersion is somewhat at odds with the cluster's orbit, for which the perigalacticon is sufficiently close to the galactic center that the probability of cluster disruption is high; a tidal radius two-thirds the currently accepted value would eliminate the discrepancy. The cluster mass-to-light ratio is also small, M/L(sub V) = 1.0 +/- 0.4 in solar units. M4 thus joins M22 as a cluster of moderate and concentration with a mass-to-light ratio among the lowest known. The astrometric distance to the cluster is also smaller than expected, 1.72 +/- 0.14 kpc. This is only consistent with conventional estimates of the luminosity of horizontal branch stars provided an extinction law R = A(sub V)/E(B-V) approximately 4 is adopted, as has been suggested recently by several authors.

Peterson, R. C.↗
NASA NTRS1978

OSO-8 X-ray spectra of clusters of galaxies. 1. Observations of twenty clusters: Physical correlations

OSO-8 X-ray spectra from 2 to 20 keV were analyzed for 26 clusters of galaxies. Temperature, emission integrals, iron abundances, and low energy absorption measurements are given. Eight clusters have positive iron emission line detections at the 90% confidence level, and all twenty cluster spectra are consistent with Fe/H=0.000014 by number with the possible exception of Virgo. Physical correlations between X-ray spectral parameters and other cluster properties are examined. It is found that: (1) the X-ray temperature is approximately proportional to the square of the velocity dispersion of the galaxies; (2) the emission integral and therefore the bolometric X-ray luminosity is a strong function of the X-ray temperature; (3) the X-ray temperature and emission integral are better correlated with cluster central galaxy density than with richness; (4) temperature and emission integral are separately correlated with Rood-Sastry type; and (5) the fraction of galaxies which are spirals is correlated with the observed ram pressure in the cluster core.

Mushotzky, R. F.↗
NASA NTRS1979

A search for X-ray emission from rich clusters, extended halos around clusters, and superclusters

HEAO A-2 X-ray observations of rich clusters, extended halos around clusters, and superclusters are discussed. Concerning the optically richest clusters, results in the 0.2-60 keV energy range show that they appear to be relatively underluminous in X-rays. It is found that X-ray and microwave measurements for some rich clusters and for Abell 576 can be used to meaningfully constrain intracluster gas parameters and H(0); in addition, observations of Abell 576 show its luminosity to be less than earlier estimates. Extended halos around clusters were not detected in the data, and spectral observations of Abell 401 and 2147 reveal that they have typical cluster spectra with iron-line emission, while intensity measurements indicate that supercluster X-ray emission is not required.

Pravdo, S. H.↗
NASA NTRS1983

The dynamics of rich clusters of galaxies. II - The Perseus cluster

The dynamics of the Perseus cluster are analyzed using self-consistent equilibrium analytical models. Using existing data in the literature plus new radial velocities reported here, composite surface density and velocity dispersion profiles are derived. These profiles have been compared with dynamical models described by Kent and Gunn (1982). The best fit suggest the presence of a significant degree of anisotropy in the velocity distribution: galaxy orbits are constrained to pass within a radius of seven cluster radii, or 1.3 deg of the cluster center. For Hubble constant = 50, a core radius of 340 kpc 11 arcmin and a mass to visual light ratio M/L = 300 are found. Using these results, X-ray observations of the intracluster medium in Perseus are reanalyzed. A previously noted discrepancy between the observed temperature of the hot gas and the cluster velocity dispersion is reduced but not eliminated. A cooling accretion flow previously deduced to exist in this cluster is shown to extend to only about one-third of the cluster radius.

Kent, S. M.↗
NASA NTRS1989

The nearby Abell clusters. III - Luminosity functions for eight rich clusters

Red photographic data on eight rich Abell clusters are combined with previous results on four other Abell clusters to study the luminosity functions of the clusters. The results produce a mean value of the characteristic galaxy magnitude (M asterisk) that is consistent with previous results. No relation is found between the magnitude of the first-ranked cluster galaxy and M asterisk, suggesting that the value of M asterisk is not changed by dynamical evolution. The faint ends of the luminosity functions for many of the clusters are quite flat, validating the nonuniversality in the parametrization of Schechter (1976) functions for rich clusters of galaxies.

Oegerle, William R.↗
NASA NTRS1994

The X-ray luminosity functions of Abell clusters from the Einstein Cluster Survey

We have derived the present epoch X-ray luminosity function of northern Abell clusters using luminosities from the Einstein Cluster Survey. The sample is sufficiently large that we can determine the luminosity function for each richness class separately with sufficient precision to study and compare the different luminosity functions. We find that, within each richness class, the range of X-ray luminosity is quite large and spans nearly a factor of 25. Characterizing the luminosity function for each richness class with a Schechter function, we find that the characteristic X-ray luminosity, L(sub *), scales with richness class as (L(sub *) varies as N(sub*)(exp gamma), where N(sub *) is the corrected, mean number of galaxies in a richness class, and the best-fitting exponent is gamma = 1.3 +/- 0.4. Finally, our analysis suggests that there is a lower limit to the X-ray luminosity of clusters which is determined by the integrated emission of the cluster member galaxies, and this also scales with richness class. The present sample forms a baseline for testing cosmological evolution of Abell-like clusters when an appropriate high-redshift cluster sample becomes available.

Burg, R.↗
NASA NTRS2002

Toward An Understanding of Cluster Evolution: A Deep X-Ray Selected Cluster Catalog from ROSAT

In the past year, we have focussed on studying individual clusters found in this sample with Chandra, as well as using Chandra to measure the luminosity-temperature relation for a sample of distant clusters identified through the ROSAT study, and finally we are continuing our study of fossil groups. For the luminosity-temperature study, we compared a sample of nearby clusters with a sample of distant clusters and, for the first time, measured a significant change in the relation as a function of redshift (Vikhlinin et al. in final preparation for submission to Cape). We also used our ROSAT analysis to select and propose for Chandra observations of individual clusters. We are now analyzing the Chandra observations of the distant cluster A520, which appears to have undergone a recent merger. Finally, we have completed the analysis of the fossil groups identified in ROM observations. In the past few months, we have derived X-ray fluxes and luminosities as well as X-ray extents for an initial sample of 89 objects. Based on the X-ray extents and the lack of bright galaxies, we have identified 16 fossil groups. We are comparing their X-ray and optical properties with those of optically rich groups. A paper is being readied for submission (Jones, Forman, and Vikhlinin in preparation).

Jones, Christine↗
NASA NTRS1981

The Ophiuchus cluster - A bright X-ray cluster of galaxies at low galactic latitude

The discovery of an extended X-ray source identified with a cluster of galaxies at low galactic latitude is reported. The source, designated the Ophiuchus cluster, was detected near 4U 1708-23 with the HEAO 1 Scanning Modulation Collimator, and identified with the cluster on the basis of extended X-ray size and positional coincidence on the ESO/SRC (J) plate of the region. An X-ray flux density in the region 2-10 keV of approximately 25 microJ was measured, along with an X-ray luminosity of 1.6 x 10 to the 45th ergs/sec and an X-ray core radius of approximately 4 arcmin (0.2 Mpc) for an assumed isothermal sphere surface brightness distribution. The X-ray spectrum in the range 2-10 keV obtained with the HEAO 1 A-2 instrument is well fit by a thermal bremsstrahlung model with kT = 8 keV and a 6.7-keV iron line of equivalent width 450 eV. The steep-spectrum radio source MSH 17-203 also appears to be associated with the cluster, which is the closest and brightest representative of the class of X-ray clusters with a dominant central galaxy.

Johnston, M. D.↗
NASA NTRS1981

The X-ray structure of a galaxy cluster at z = 0.54 - Implications for cluster evolution and cosmology

High-resolution X-ray observations of the rich cluster 0016+16 at a redshift of 0.541 are presented. The emitting gas in this cluster is hot and extremely luminous, and its structure resembles that seen in the brightest nearby cluster sources. In most of its properties, 0016+16 resembles a richer version of the Coma cluster, and it offers little support to the hypothesis that clusters at z greater than 0.5 differ fundamentally from nearer objects.

White, S. D. M.↗
NASA NTRS1983

A nonparametric clustering technique which estimates the number of clusters

In applications of cluster analysis, one usually needs to determine the number of clusters, K, and the assignment of observations to each cluster. A clustering technique based on recursive application of a multivariate test of bimodality which automatically estimates both K and the cluster assignments is presented.

Ramey, D. B.↗
NASA NTRS1983

CCD photometry of Abell clusters. I - Magnitudes and redshifts for 84 brightest cluster galaxies

The way in which the luminosity of brightest cluster galaxy varies with cluster parameters is investigated in a sample of 84 Abell clusters of moderate redshift and of richness classes in the zero-to-5 range. A photometric system is used which is well matched with the low noise CCD detector employed, as well as to the arrangement of galaxies with different redshifts on a standard system. The data, together with measurements from earlier work which forms a low redshift sample of the 199 brightest cluster galaxies, confirm that the luminosity dispersion in first-rank cluster galaxies is about 0.34 mag. The colors of giant ellipticals have not changed since redshifts of 0.25, and the richness correction is well established at 0.10 mag/Abell richness class. Application of these corrections reduces the luminosity dispersion to 0.29 mag.

Schneider, D. P.↗
NASA NTRS1985

CCD observations of Abell clusters. IV - Surface photometry of 175 brightest cluster galaxies

The present data from a consistent body of CCD aperture and surface photometry for the brightest galaxies in 175 clusters from the Abell catalog confirm the dispersion in the aperture luminosities for first-ranked cluster galaxies of about 0.32 mag, as well as the documented trends of luminosity with Bautz-Morgan class and cluster richness. Half of these objects are multiple-nucleus systems; multiplicity is not strongly correlated with cluster morphology or richness, and the fraction of multiples is 2-10 times higher than would be produced by random projection in the cores of Hubble-law clusters. The average distance of the secondary nuclei from the luminosity center of the galaxy is significantly less than the value expected from random projections.

Hoessel, J. G.↗
NASA NTRS1988

Population studies in groups and clusters of galaxies. I - The luminosity function of galaxies in the Fornax Cluster

The luminosity functions of galaxies of various Hubble types in the central 2.4-deg-radius core of the Fornax Cluster and the 6-deg-radius core of the Virgo Cluster have been compared. Although properties such as density, velocity dispersion, and X-ray luminosity differ in the two clusters, no significant differences are noted in their luminosity functions or in their morphological mix of galaxies. The ratio of the distances to the Fornax and Virgo clusters has been obtained. Evidence is provided for a radial variation in the luminosity function of dwarf ellipticals in the two clusters.

Ferguson, Henry C.↗
NASA NTRS1988

Color gradients in cooling flow cluster central galaxies and the ionization of cluster emission line systems

Preliminary results are given for a program to measure color gradients in the central galaxies in clusters with a variety of cooling flow rates. The objectives are to search for extended blue continuum regions indicative of star formation, to study the spatial distribution of star formation, and to make a quantitative measure of the amount of light from young stars, which can lead to a measure of the star formation rate (for an assumed initial mass function). Four clusters with large masses and large cluster H-alpha emission fluxes are found to have an excess of blue light concentrated to the centers of the cluster central galaxy. Assumption of a disk IMF leads to the conclusion that the starlight might play a major role in ionizing the emission line gas in these clusters.

Romanishin, W.↗
NASA NTRS1989

Population studies in groups and clusters of galaxies. II - A catalog of galaxies in the central 3.5 deg of the Fornax Cluster

A catalog of 2678 galaxies within an area of about 40 sq deg centered on the Fornax Cluster has been compiled based on 26 deep large-scale plates obtained with the 2.5-m Las Campanas Observatory reflector. The catalog includes 340 likely cluster members and 2338 likely background galaxies. Radial velocities are given for 89 of the galaxies. The spatial distributions of various types of galaxies have been modeled as the sum of a King (1962) model cluster component superimposed on a uniform background. Using maximum-likelihood fits to these distributions, a core radius of 0.7 deg is found for a King model fit to the cluster, suggesting that there are few cluster members contained in the sample of background galaxies.

Ferguson, Henry C.↗