Laird A. Thompson

The COMA/A1367 Supercluster and Its Environs

The three-dimensional galaxy distribution in the region of space surrounding the two rich clusters Coma and A1367 is analyzed by using a nearly complete redshift sample of 238 galaxies with m, 20 h-l Mpc where there appear to be no galaxies whatever. Since tidal disruption is probably responsible for the isolated component of supercluster galaxies, the observations suggest that all galaxies are (or once were) members of groups or clusters. A number of related topics with more general significance are also discussed. (1) The size-to-separation ratio for foreground groups indicates that the redshift of group formation is z 8 9. (2) There is a general correlation between the volume mass density of a galaxy system and the morphologies of the component galaxies. (3) Finally, we speculate that all clusters of richness class z 2 2 are located in superclusters. Subject headings: galaxies : clusters of galaxies: redshifts

Ground Turbulence At Mauna Kea Observatory: Location And Ground Height For Future Telescopes

The turbulent surface boundary layer has been monitored within the summit area of Mauna Kea on the island of Hawaii. Measurements were made using three meteorological towers instrumented with microthermal sensors at three levels, ambient temperature sensors at two levels, and anemometers at two levels. Spectra of the temperature fluctuations were also measured. The spatial and temporal variations in microthermal activity associated with ground turbulence are quantified and explained. From these findings, the best locations and ground heights for telescope facilities are determined. The relative importance of turbulence near the ground in the degradation of image quality is deduced.

An Active Mirror Image Stabilizing Instrument System (ISIS) For Use On Mauna Kea

On nights when the seeing is at its best on Mauna Kea, the cores of images produced by the large optical telescopes (UH 2.24-m and CFHT 3.6-m) shrink to sizes < 0.5 arcsec. During these periods, the image cores show translational motion over a patch < 1 arcsec at frequencies < 30 Hz. To remove this translation motion, an Image Stabilizing Instrument System (ISIS) has been built and used on Mauna Kea. The instrument contains a microprocessor-controlled active plane mirror capable of being repositioned every 2 ms, a photon-counting guide probe, and an active shutter to remove moments of excessive image blur. We describe the instruments design detail, discuss the optimal way to use such an instrument, and show samples of observational data taken with it.

Globular Clusters Detected in the Coma Cluster’s Central Giant Galaxy NGC 4874

We have used the Canada-France-Hawaii Telescope on Mauna Kea to obtain two deep CCD exposures in the central region of the Coma cluster, and we have used the new images to detect globular clusters in the central giant galaxy NGC 4874. One exposure with a total integration time of 2 hours lies in the halo of the giant galaxy. A second (comparison) exposure of the same integration time was taken within the cluster core but well away from any other bright galaxy. Our analysis, which includes both photometry and image classification with the automated routines called FOCAS, extends to a limiting B magnitude of 26.25. Image classification is a necessary part of this study because at the faintest limiting magnitudes globular clusters are easily confused with the rich population of faint background galaxies. We have been particularly successful in sorting stellar objects from galaxies because the image quality on our images is quite good: FWHM ~ 0.6 arcsec. The data were taken with the fast autoguiding instrument called ISIS (Thompson and Ryerson 1984)

Galaxy Morphology in Distant Clusters

Direct information on the evolution of galaxies and of rich clusters may be available in the images of clusters at high red-shift. While this type of investigation will flourish once Space Telescope is operating, ground-based observations can provide a valuable starting point. A program is underway at Mauna Kea Observatory to obtain subarcsecond images of distant clusters. The program is facilitated by the recent completion of a precise, high-frequency autoguiding system capable of removing image motion from the telescope focal plane. CCD images of clusters out to z = 0.4 have been obtained under good seeing conditions through a filter that mimics standard photographic bandpasses used in studies of nearby clusters (IIIa-J + GG 385).

A2197 + A2199 Supercluster Region

A2197 and A2199 are among the nearest rich clusters of galaxies. They are worthy of special attention because (i) they form a close binary system, the analysis of which provides an independent estimate of the clusters’ mass to light ratio, and (ii) these two clusters fall along a bridge of interconnected clusters that stretches at least 50 Mpc. We report 78 new redshifts in the A2197+A2199 region, thus tripling the number of known redshifts.

Mauna Kea Site Survey

The summit of Mauna Kea on the island of Hawaii is widely recognized as one of the best ground-based sites in the world for optical and infrared astronomy and, as such, it is a potential location for at least two of the advanced technology telescopes now in the planning stages. Because of the importance of further understanding the site characteristics for undeveloped areas near the summit of Mauna Kea, the University of Hawaiis Institute for Astronomy, which is responsible within the state for the site, has undertaken a comprehensive site survey program to (1) study the orographic properties of the mountaintop and (2) to relate these properties to the image quality as observed through a large telescope. This site survey program will be closely coordinated with the U.S. National New Technology Telescope site survey. In our survey, we will be particularly interested in simultaneous measurements throughout a two-year period of wind speed, wind direction, and microthermal turbulence over a grid of test towers placed within the summit area. With such data--and simultaneous monitoring of image quality at the summit of Mauna Kea--we hope to identify the best potential locations for large telescopes on Mauna Kea and, in turn, to effect the most responsible use of this international resource.