Observing Strategy

CCD Imagery

The first major step of our program has been to secure high quality, broad-band B and R CCD images of each cluster in order to conduct a detailed photometric study of their galaxy populations and to identify targets for subsequent spectroscopic follow-up. Coarse morphological information on the galaxies in each cluster field is also being derived from these images. The imaging was performed at Las Campanas Observatory in Chile (LCO) using the 40-inch Swope telescope equipped with a thinned , m pixel Tektronix CCD camera. The field of view of the CCD camera is with each pixel corresponding to 0.696´´ on the sky. Each cluster was imaged over a field, thereby well encompassing its infall radius and providing coverage of the surrounding field. To image a field of this size with this instrumental set-up, a mosaic of 21 pointings (a square without the corners; see Fig 1) was required.

For each pointing two exposures were taken, each offset by 15 arcsec. Total combined exposure times were 600s in the B-band and 500s in the R-band. There was a deliberate 44 arcsec overlap between neighbouring fields in order to check and tie together the photometry and astrometry across the whole field. Photometric calibration was obtained via observations of Landolt (1992) standards taken at frequent intervals between the science frames and at a variety of airmasses throughout the night. These provided both photometric zero-points for our data and measurements of the extinction coefficients on each night.

2dF Spectroscopy

The 2dF facility is ideally matched to the requirements of our study and indeed its utility in spectroscopically surveying rich clusters in unprecedented detail was a major driver in conducting this program. Its large field allows, for the first time, clusters to be easily explored right out to their infall radius. The high throughput and sensitivity of the system (with it being on a 4m-class telescope) makes it possible to probe well down the cluster luminosity function, to a depth sufficient to detect and study any faint, remnant star-forming population. With just two 2dF configurations involving a total exposure of hrs, spectra for galaxies within the field of each cluster can be obtained, realising a sample of member galaxies, a number sufficient to distinguish statistically between the properties of independent subsamples, eg., the dynamics of star-forming versus quiescent galaxies and the higher-order kinematics of galaxies in the core and infall regions of the cluster.

The goal, therefore, is to obtain intermediate (Å) resolution spectroscopy of magnitude-limited (mag) samples of galaxies within each cluster field, with candidates sourced from the photometric catalogues generated from our optical images (see §4). This spectral resolution allows both robust measurement of key absorption and emission line indices (eg. [OII]3727, CaII H & K, H, H) and the determination of the internal velocity dispersions of the bright, early-type cluster members. The latter provides an important additional constraint on the cluster physics through measurement of the bright end mass function.

X-ray Imagery

Pointed observations with ROSAT using its HRI camera are being conducted to determine the distribution and morphology of the hot X-ray gas in each cluster. This will provide imagery over a 40arcmin field-of-view, thereby well covering the central Mpc from which most of the X-ray emission in rich clusters is seen (Fabricant et al. 1986; Evrard 1990). The arcsec spatial resolution of the ROSAT HRI will enable us to look at the intra-cluster medium on scales of kpc, corresponding to that of individual galaxies. Its primary use, however, is to assess the merger history of each cluster via its morphology and its positional relation to the optical (cluster galaxy) distribution (§1, ZZ95).

© Copyright Astronomical Society of Australia 1997