Redshift and velocity dispersion of the cluster of galaxies around NGC 326

Redshifts of several galaxies thought to be associated with NGC 326 are determined. The results confirm the presence of a cluster and find a mean redshift of z = 0.0477 +/- 0.0007 and a line-of-sight velocity dispersion sigma_{z} = 599 (+230, -110) km/s. The velocity dispersion and previously measured X-ray gas temperature of kT ~ 1.9 keV are consistent with the cluster sigma_{z}/kT relation, and NGC 326 is seen to be a slowly-moving member of the cluster.Comment: 3 pages, to appear in MNRA

Another X-ray-Discovered Poor Cluster of Galaxies Associated with CL 0016+16

We report optical spectroscopic observations of RX J0018.8+1602, a ROSAT X-ray source proposed to be an intermediate redshift cluster of galaxies. Our observations confirm the identification of RX J0018.8+1602 and provide measurements of its mean radial velocity (z=0.5406 +/- 0.0006) and velocity dispersion (sigma_p = 200 [+110, -80] km/s). This is the second poor cluster that has been found to be a companion to CL 0016+16 (z=0.5455), the other one being RX J0018.3+1618 (z=0.5506). The 0.2-2 keV band source-frame X-ray luminosity summed over both companion clusters is 5E44 ergs/s, which is a significant fraction, ~23%, of the X-ray luminosity of the main cluster. The companions are located at angular distances of 10 arcmin to 25 arcmin (minimum physical scales of 5 Mpc to 12 Mpc) from CL 0016+16 and we propose that they represent a new large-scale component of the X-ray emission from clusters of galaxies. Similar low X-ray luminosity poor clusters surrounding nearby Abell clusters can explain the excess power observed in the angular cross-correlation function between Abell clusters and the X-ray background on inferred physical scales of 14--20 Mpc (Soltan et al. 1996)

Determining Tangential Peculiar Velocities of Clusters of Galaxies using Gravitational Lensing

We propose two new methods for measuring tangential peculiar velocities of rich clusters of galaxies. Our first method is based on weak gravitational lensing and takes advantage of the differing images of background galaxies caused by moving and stationary gravitational potentials. Our second method is based on measuring relative frequency shifts between multiple images of a single strongly lensed background galaxy. We illustrate this method using the example of galaxy cluster CL 0024+1654.Comment: LateX, 4 pages, 2 figures, accepted for publication in Part 1 of The Astrophysical Journa

A Merger Scenario for the Dynamics of Abell 665

We present new redshift measurements for 55 galaxies in the vicinity of the rich galaxy cluster Abell 665. When combined with results from the literature, we have good velocity measurements for a sample of 77 confirmed cluster members from which we derive the cluster's redshift z=0.1829 +/- 0.0005 and line-of-sight velocity dispersion of 1390 +/- 120 km/s. Our analysis of the kinematical and spatial data for the subset of galaxies located within the central 750 kpc reveals only subtle evidence for substructure and non-Gaussianity in the velocity distribution. We find that the brightest cluster member is not moving significantly relative to the other galaxies near the center of the cluster. On the other hand, our deep ROSAT high resolution image of A665 shows strong evidence for isophotal twisting and centroid variation, thereby confirming previous suggestions of significant substructure in the hot X-ray--emitting intracluster gas. In light of this evident substructure, we have compared the optical velocity data with N-body simulations of head-on cluster mergers. We find that a merger of two similar mass subclusters (mass ratios of 1:1 or 1:2) seen close to the time of core-crossing produces velocity distributions that are consistent with that observed.Comment: 30 pages and 7 figures. Accepted by the Astrophysical Journal Full resoultion figures 1 and 3 available in postscript at http://www.physics.rutgers.edu/~percy/A665paper.htm

A spectroscopic study of NGC 6251 and its companion galaxies

Measurements of the velocities of galaxies thought to be associated with the giant radio galaxy NGC 6251 confirm the presence of a poor cluster with a systemic redshift of z= 0.0244 +/- 0.0004 and a line-of-sight velocity dispersion of sigma_{z}= 283 (+109, -52) km/s. This suggests a cluster atmosphere temperature of T = 0.7 (+0.6, -0.2) keV, which is not enough to confine the radio jet by gas pressure. The core of NGC 6251 shows strong emission lines of [O III] and H alpha + [N II], but there is no evidence for line emission from the jet (detected in optical continuum by Keel (1988)).Comment: 7 pages, 2 figures, to be published in MNRA

Preliminary Sunyaev Zel'dovich Observations of Galaxy Clusters with OCRA-p

We present 30 GHz Sunyaev Zel'dovich (SZ) observations of a sample of four galaxy clusters with a prototype of the One Centimetre Receiver Array (OCRA-p) which is mounted on the Torun 32-m telescope. The clusters (Cl0016+16, MS0451.6-0305, MS1054.4-0321 and Abell 2218) are popular SZ targets and serve as commissioning observations. All four are detected with clear significance (4-6 sigma) and values for the central temperature decrements are in good agreement with measurements reported in the literature. We believe that systematic effects are successfully suppressed by our observing strategy. The relatively short integration times required to obtain these results demonstrate the power of OCRA-p and its successors for future SZ studies.Comment: 9 pages, 2 figures. Accepted by MNRAS, online earl