Herbert J. Rood

A Compilation of Redshifts and Velocity Dispersions for ACO Clusters

We present a compilation of redshifts for 1572 Abell, Corwin, & Olowin (ACO) clusters, referenced to both the heliocentric and cosmic background radiation reference frames, and 395 velocity dispersions corrected to the reference frame of the cluster, available from the literature as of 1998 December. We present an additional list of 81 ACO clusters with published redshifts which are probably those of galaxies or groups superimposed on, or near, the ACO cluster position.

A compilation of redshifts and velocity dispersions for Abell clusters (epoch 1991.2)

A list of redshifts for 758 Abell clusters, and velocity dispersions for 121, published as of 1991 March, and another list of 33 Abell clusters with published redshifts, most of which are probably redshifts of foreground or background galaxies superposed on, or near, the Abell clusters, are presented. Over the past 4 years, the published number of redshifts of member galaxies in clusters increased by a factor of 2.0, but the number of cluster redshifts increased by only a factor of 1.3, while the number of cluster velocity dispersions derived from 50 or more redshifts of member galaxies increased by a factor of 3.8 (from 6 to 23)

Neutral hydrogen in compact groups of galaxies

Integrated H I profiles were detected for 34 of 51 Hickson compact groups (HCGs) of galaxies, and sensitive upper limits to the H I flux density were measured for the other 17. About 60 percent of the galaxies within compact groups are spirals, and a significant tendency exists for the fraction of elliptical galaxies to increase with group surface brightness. The amount of dark matter within the compact group region is negligibly small. An HCG on average contains half as much neutral hydrogen as a loose group with a similar spectrum of galaxy luminosities and morphological types, implying that compact groups are independent dynamical entities and not transient or projected configurations of loose groups. The observed fraction of galaxies which are luminous enough to be possible merger products of compact groups is small compared with the fraction required by the theory of dynamical friction. A clear discrepancy thus exists between solid empirical evidence and a straightforward prediction of Newtonian dynamical theory in a setting which does not permit a dark matter explanation. 44 references.

Size of the Coma cluster

STUDIES of counts of galaxies in large regions centred on the Coma cluster led Zwicky to conclude that the radius of the Coma cluster is at least 6° (13 Mpc for H = 55 km s−1 Mpc−1)1. Outside this radius, the contribution to the counts from foreground and background galaxies overwhelms the contribution from the cluster, rendering it invisible. The technology of image intensifier tubes has now advanced to such an extent that it is feasible to obtain line of sight (radial) velocities of large numbers of galaxies in fields of a cluster. These velocities can be used to distinguish cluster members from foreground and background galaxies2. Such studies have confirmed that the Coma cluster extends to at least 5° (ref. 3).

SPATIAL COINCIDENCE BETWEEN A NUMBER OF HICKSON COMPACT GROUPS AND LOOSE GROUPS OR CLUSTERS

Spatial coincidences have been detected between Hickson compact groups and 36 loose groups and seven Abell clusters. The coincidences were found from examination of (a) prints of the Palomar Sky Survey, (b) galaxy redshift information in the de Vaucouleurs et al. (1992) Third Reference Catalog of Bright Galaxies, (c) locational information for loose groups in recent catalogs by Fouque et al. (1992), Nolthenius (1993), Garcia (1993) and (d) locational information for Abell clusters. Seven-tenths (0.7) of the Hickson compact groups with redshift z

The nature of compact groups of galaxies

Monte Carlo numerical simulation is used to calculate the probability for the chance occurrence of four galaxies projected on the sky satisfying the Hickson isolation criterion within a loose group of eight members. For the models which match most closely the size and galaxy multiplicity function of observed groups, this chance occurrence is found to be smaller by a factor of about 100 than the value obtained previously by Mamom from numerical simulations of dynamical models of groups. This and other direct independent observational results from the literature constitute strong evidence that nearly all of the Hickson compact groups are real physical systems. It is concluded that the tendency for the spiral fraction of a compact group to be larger than the value inferred from the galaxy morphology-group density relation of rich clusters and loose groups is a real physical effect indicating that galaxy morphology depends strongly on a second parameter which, it is suggested, is the velocity dispersion of a system. 21 references.

The distance to the Coma cluster using the B-band Tully-Fisher relation

The present estimate of the distance to the Coma cluster on the basis of the B-band Tully-Fisher (T-F) relation has conducted photographic surface photometry on all galaxies for which H I line width data in a circle of 4-deg radius centered on the Coma cluster. A detailed estimate is made of the cluster population incompleteness bias for the distance estimation. While the correction for sample incompleteness, in conjunction with the T-F relation, increases the distance modulus, the result cannot be as large as the value indicated by Kraan-Kortweg et al. (1988). 65 refs.

Gamma-Ray Bursts from Rich Concentrated Abell-Corwin-Olowin Clusters: The Luminosity-Duration Relation

A likelihood statistic constructed from positions and positional uncertainties identifies 82 associations of BATSE sources with rich, concentrated Abell clusters. Random simulations indicate that 42 ± 9 of these pairs are chance associations. Among the approximately 40 physically associated cluster-gamma-ray burst (GRB) pairs (the nearest containing the Coma Cluster and GRB 2193), we find (1) a plateau between 15.8 and 16.2 mag in the integral (observed-minus-random) number distribution of mR(10), the magnitude of 10th brightest cluster member, which may indicate the presence of two burst populations with different characterstic intrinsic luminosities; (2) the dispersion in GRB fluences between 100 and 300 keV (represented as apparent magnitude, m[100-300 keV]) is smaller for bursts paired with mR(10) ≤ 16.2 clusters than for those with mR(10) > 16.2; (3) the average 100-300 keV isotropic intrinsic energy of GRBs associated with mR(10) ≤ 16.2 clusters with known redshift z 99.9% confidence level). The slope of this relation indicates t90∝L−11024. These results suggest that the anticorrelation of apparent burst magnitude with duration is an intrinsic property of the bursts and is not caused by cosmological time dilation.

RECONSTRUCTED BINARY SUPERGIANT GALAXIES IN CLUSTERS

Among the 112 Abell clusters (statistical sample) with a central supergiant cD galaxy (BM I,I-II), 84 are single (cD1_) and 28 are binary (cD2_); the ratio of the observed number of cD1_ to cD2_ galaxies is ηid_ ~3.0. The merger of the primary and secondary components of a cD2_ galaxy by means of dynamical friction transforms the cD2_ source galaxy into a cD1_ product galaxy. Application of the best available data to results of the best available theoretical and N-body studies leads to a predicted product-to-source ratio ηdyn_ ~ 17. A ratio ηdyn_/ηid_~ 5 signifies to Tremaine (1981) that ~5 cD2_ formation events occur in a BM I,I-II cluster over its age. This explanation requires that the cluster must contain the building-block components of a cD2_ galaxy. We report and analyze new data which demonstrate that not a single non-cD2_ galaxy is sufficiently luminous to serve as a building block of another cD2_ galaxy in 85% (22 of 26) observed cD2_ clusters. It follows that a missing ingredient exists in our understanding of dynamical friction in the present extragalactic context.

Supplemental topics on voids

1. In the spring of 1975 on Kitt Peak, the cosmological significance of superclusters and voids was clearly recognized by the small group of astronomers who were completing redshift surveys that first demonstrated the existence of the Coma supercluster and void. 2. A redshift survey to a faint magnitude limit over a large region of the sky can include, e. g., (a) all galaxies, (b) the galaxies in representative probes, or (c) randomly-sampled galaxies. Each of these map-making strategies has its own special virtues. 3. Redshifts for the Abell clusters and very distant objects are being measured from spectra of (a) individual galaxies recorded electronically and (b) several galaxies recorded simultaneously by means of (1) multiobject spectroscopy via multiaperture or fiber-optic coupling devices, (2) analysis of features on objective-prism spectral plates of Schmidt telescopes, and (3) computer synthesization of spectra from observed multicolor CCD images of a field. 4. A beautiful consistency now exists between the observed kinematics of the solar system and the predictions of Newtonian/general relativistic dynamics. However, a century ago serious discrepancies existed, and explanations were sought in terms of hypothesized missing mass and non-Newtonian dynamics. Today, the same approach is being applied toward resolving discrepancies apparent in extragalactic dynamics. 5. Future observational research on voids will include redshift surveys of galaxies and other objects to very faint magnitude limits. These objects will be selected from catalogs of data measured off direct-survey photographic plates by means of an automated plate scanner.