R. A. White

A Catalog of Nearby Poor Clusters of Galaxies

A catalog of 732 optically selected, nearby poor clusters of galaxies covering the entire sky north of -3° declination is presented. The poor clusters, called WBL clusters, were identified as concentrations of three or more galaxies with photographic magnitudes brighter than 15.7, possessing a galaxy surface overdensity of 104/3. These criteria are consistent with those used in the identification of the original Yerkes poor clusters, and this new catalog substantially increases the sample size of such objects. These poor clusters cover the entire range of galaxy associations up to and including Abell clusters, systematically including poor and rich galaxy systems spanning over 3 orders of magnitude in the cluster mass function. As a result, this new catalog contains a greater diversity of richness and structures than other group catalogs, such as the Hickson and Yerkes catalogs. The information on individual galaxies includes redshifts and cross-references to other galaxy catalogs. The entries for the clusters include redshift (where available) and cross-references to other group and cluster catalogs.

The X-Ray Luminosity Function and Gas Mass Function for Optically Selected Poor and Rich Clusters of Galaxies

We present the first X-ray luminosity function (XLF) for an optically selected sample of 49 nearby poor clusters of galaxies and a sample of 67 Abell clusters with z ≤ 0.15. We have extended the measured cluster XLF by more than a factor of 10 in X-ray luminosity. Our poor cluster sample was drawn from an optical catalog of groups with 0.01 ≤ z ≤ 0.03 composed of Zwicky galaxies. The X-ray emission was measured from the ROSAT all-sky survey. About 45% of the poor clusters were detected, with 0.5-2.0 keV luminosities from 1.7 to 65 × 1041 h-2 ergs s-1. These are among the X-ray brightest, optically selected poor clusters in the northern hemisphere. For this sample, the poor cluster XLF was found to be a smooth extrapolation of the rich cluster XLF. A new Hydro/N-body simulation of a hot + cold dark matter model with Ωtotal = 1, Ων = 0.2, and a baryon fraction of 7.5% was used to model and understand our observational selection effects. We found that the observed cluster gas mass function was consistent with our model.

Mass and Metallicity of Five X-Ray-bright Galaxy Groups

We present ASCA X-ray observations of a sample of five groups selected from a cross-correlation of the ROSAT All-Sky Survey with the White et al. optical catalog of groups. These X-ray-bright groups significantly increase the number of known systems with temperatures between 2 and 3 keV. They have element abundances of roughly 0.3☉, which are typical of clusters, but their favored ratio of Si-to-Fe abundance is lower than the cluster value. Combining the ASCA results with ROSAT imaging data, we calculate total masses of a few to several times 1013 M☉, gas mass fractions of ~10%, and baryonic mass fractions of at least 15%-20% within a radius of 0.5 Mpc. Upper limits for the ratios of gas to galaxy mass and of the iron mass to galaxy luminosity overlap with the range observed in rich clusters and extend to lower values, but not to such low values as seen in much poorer groups. These results support the idea that groups, unlike clusters, are subject to the loss of their primordial and processed gas and show that this transition occurs at the mass scale of the 2-3 keV groups. A discussion of ASCA calibration issues and a comparison of ROSAT and ASCA temperatures are included in an Appendix.

A 20 centimeter VLA survey of Abell clusters of galaxies. III - Images and optical identifications

Radio contour maps, models, and optical identifications for 250 radio galaxies in Abell clusters of galaxies are presented. Consideration is given to the effect of environment on radio galaxies, the evolution of radio galaxies in rich clusters, and the importance of beaming in a complete sample of radio galaxies.

Redshift and Optical Properties for S Statistically Complete Sample of Poor Galaxy Clusters

From the poor cluster catalog of White et al. (1996), we define a sample of 71 optically-selected poor galaxy clusters. The surface-density enhance- ment we require for our clusters falls between that of the loose associations of Turner and Gott (1976) and the Hickson compact groups (Hickson, 1982). We review the selection biases and determine the statistical comleteness of the sample. For this sample, we report new velocity measurements made with the ARC 3.5-m Dual-Imaging spectrograph and the 2.3-m Steward Observatory MX fiber spectrograph. Combining our own measurements with those from the literature, we examine the velocity distributions, velocity dispersions, and 1-d velocity substructure for our poor cluster sample, and compare our results to other poor cluster samples. We find that approximately half of the sample may have significant 1-d velocity substructure. The optical morphology, large-scale environment, and velocity field of many of these clusters is indicative of young, dynamically evolving systems. In future papers, we will use this sample to derive the poor cluster X-ray luminosity function and gas mass function (see astro-ph/9606120), and will examine the optical/X-ray properties of the clusters in more detail.From the poor cluster catalog of White et al. (1996), we define a sample of 71 optically-selected poor galaxy clusters. The surface-density enhancement we require for our clusters falls between that of the loose associations of Turner and Gott (1976) and the Hickson compact groups (Hickson, 1982). We review the selection biases and determine the statistical completeness of the sample. For this sample, we report new velocity measurements made with the ARC 3.5-m Dual-Imaging spectrograph and the 2.3-m Steward Observatory MX fiber spectrograph. Combining our own measurements with those from the literature, we examine the velocity distributions, velocity dispersions, and 1-d velocity substructure for our poor cluster sample, and compare our results to other poor cluster samples. We find that approximately half of the sample may have significant 1-d velocity substructure. The optical morphology, large-scale environment, and velocity field of many of these clusters are indicative of young, dynamically evolving systems. In future papers, we will use this sample to derive the poor cluster X-ray luminosity function and gas mass function, and will examine the optical/X-ray properties of the clusters in more detail. – 2 –

The GLOBE Visualization Project: Using WWW in the Classroom.

Though commercial interest in the World Wide Web is growing, the potential uses of the medium as a learning tool are numerous. The GLOBE Visualization Project is one such educational application. We have designed and implemented a WWW-based, user-friendly, language-independent, graphical user interface providing access to visualizations created for GLOBE, a multinational program of education and science. The target users of the system are K–12 students and their teachers from over 1100 schools in 39 countries; other clients include the GLOBE scientific investigators and members of the public internationally. Navigation is intuitive, and employs the metaphors of a “Control Panel” which changes the image appearing in a “Viewscreen.” The interface can be learned empirically by persons of all ages regardless of technical expertise or native language; context-sensitive help is provided for users who prefer documentation. The GLOBE Visualization server is on the World Wide Web at URL http://globe.gsfc.nasa.gov/globe/.

Radio emission in the directions of cD and related galaxies in poor clusters. III - VLA observations at 20 cm

VLA radio maps and optical identifications of a sample of sources in the directions of 21 Yerkes poor cluster fields are presented. The majority of the cluster radio sources are associated with the dominant D or cD galaxies (approx.70%). Our analysis of dominant galaxies in rich and poor clusters indicates that these giant galaxies are much more often radio emitters (approx.25% of cDs are radio active in the poor clusters), have steeper radio spectra, and have simpler radio morphologies (i.e., double or other linear structure) than other less bright ellipticals. A strong continuum of radio properties in cD galaxies is seen from rich to poor clusters. We speculate that the location of these dominant galaxies at the cluster centers (i.e., at the bottom of a deep, isolated gravitational potential well) is the crucial factor in explaining their multifrequency activity. We briefly discuss galaxy cannibalism and gas infall models as fueling mechanisms for the observed radio and x-ray emission.

A VLA 20 CM survey of poor groups of galaxies

The paper reports on VLA 20 cm observations of an extensive sample of galaxies in 139 poor groups. These groups, composed of galaxies down to the limit of the Zwicky et al. (CGCG) catalog, were chosen using a percolation algorithm set at a high surface-density threshold. Approximately 50 percent of the groups have measured redshifts. These groups were surveyed using a snapshot mode of the VLA with a resolution of about 13 arcsec. Analysis of the resulting radio and optical properties reveals that the presence of a nearby companion galaxy has an important role in generating radio emission in a galaxy. CCD observations of two radio-loud, disturbed galaxies with companions are presented and are used to discuss models of radio-source production. Nine tailed radio galaxies are found in the poor groups, which is much more than had been expected from previous work on rich clusters and from theoretical models. The paper discusses previous statistical biases and proposes a method for bending head-tail sources in poor groups. From the confinement of extended radio features associated with tailed sources, the presence of a substantial intracluster medium that should radiate significantly at soft-X-ray energies is predicted. 56 references.

A 1400-MHz survey of 1478 Abell clusters of galaxies

Observations of 1478 Abell clusters of galaxies with the NRAO 91-m telescope at 1400 MHz are reported. The measured beam shape was deconvolved from the measured source Gaussian fits in order to estimate the source size and position angle. All detected sources within 0.5 corrected Abell cluster radii are listed, including the cluster number, richness class, distance class, magnitude of the tenth brightest galaxy, redshift estimate, corrected cluster radius in arcmin, right ascension and error, declination and error, total flux density and error, and angular structure for each source.

Databases from Cosmic Background Explorer (COBE)

The Cosmic Background Explorer (COBE)1 was launched in November 1989. Its scientific objectives are to search for spatial anisotropics and spectral distortions in the 2.7 K cosmic microwave background (CMB) radiation, to detect the diffuse infrared background radiation from the first objects to form after the Big Bang, and to study all other sources of diffuse radiation from 1 micron to 1 centimeter. These other sources include interplanetary and interstellar dust, hot electrons in the Galaxy, faint stars in the Galaxy, and possibly IR galaxies and hot gas in galaxy clusters. To map these primeval and local sources, the three scientific instruments on COBE scan the sky repeatedly, building up signal-to-noise statistics until the data are limited only by the astrophysical environment. The three instruments are the DIRBE (Diffuse Infrared Background Experiment) covering 1 to 300 micron with a 10 band filter photometer and a 0.7° beamwidth; the FIRAS (Far Infrared Absolute Spectrophotometer) covering 100 microns to 1 cm with an absolutely calibrated polarizing Michelson interferometer with a 5 percent spectral resolution and a 7° beamwidth; and the DMR (Differential Microwave Radiometers) covering 31.5,53, and 90 GHz with 7° beamwidth. COBE is described further elsewhere (see Gulkis et al., 1990).