Design and Goals of Study

Overview

Our primary objective is to determine the evolutionary status of clusters - both in the dynamic and galaxy population sense - within the range . The observational foundations upon which this is based are comprehensive multi-band (optical and X-ray) imaging and intermediate resolution spectroscopy going all the way out to the infall radius (Mpc) for a statistically significant () sample of clusters in this redshift regime. Of central importance is the use of the new ``Two Degree Field'' (2dF) spectrograph facility on the 3.9m Anglo-Australian Telescope (AAT) which, with its superlative (x400) multiplex gain, will allow spectra to be gathered per cluster - almost an order of magnitude greater than that achieved in other previous or ongoing surveys (eg., Dressler & Schechtman 1988; the low-z component of the CNOC survey, Schade et al. 1997; Zabludoff 1998, in preparation). Hence, our study will provide a significant step forward by tracking population changes not just as a function of look-back time but also as a function of the broad range of environment probed within each cluster.

Dynamical characterisation of the clusters will proceed by following and extending the technique developed by ZZ95, which combines multi-colour imaging, high resolution X-ray imaging, and optical spectroscopy. The colour and spectral information also provides a powerful tool for quantifying the present and past star formation activity within cluster galaxies (Couch & Sharples 1987) and, in this context, the aim is to specifically look for the remnants of the blue population responsible for the Butcher-Oemler effect at higher redshifts. Our large spatial coverage will allow us to trace the variation in population characteristics from the central core - the focus of most previous studies - right out to the surrounding low-density field. The importance local galaxy density has on such variations, analogous to the morphology-local density relation of Dressler (1980), will also be investigated. In this context, the 250,000 galaxy spectra obtained as part of the 2dF Galaxy Redshift Survey (Colless et al. 1998), will also provide an invaluable reference sample, particularly of the low-density field regions.

Cluster Selection

Our sample is drawn from the X-ray Brightest, Abell-type Cluster Survey (XBACs) catalogue of 242 clusters published by Ebeling et al. (1996; hereafter E96). XBACs is an X-Ray follow-up of optically-selected clusters from the Abell, Corwin, Olowin (1989; ACO) catalogue. Clusters were selected from this catalogue according to the following criteria: (i)erg s, (ii), (iii)Declination < +10 degrees, and (iv)mag (based on values from Burstein & Heiles 1984). A total of 45 clusters satisfied these criteria from which a randomly-selected sub-sample of 20 targets was selected; these are listed in Table 1. Column 1 lists the cluster name from the ACO catalogue, columns 2 and 3 give the Right Ascension and Declination (B1950) of the X-ray centre of the cluster respectively, column 4 lists the redshift from E96, column 5 lists the cluster's X-ray luminosity in erg s (E96), column 6 gives the cluster's Bautz-Morgan type (ACO), column 7 lists the cluster's richness class (ACO), and column 8 denotes the filters through which the images were obtained, with those in bold-face indicating cases where the data were taken in photometric conditions. It can be seen from Table 1 that our selected clusters incorporate a range of global properties such as richness class and Bautz-Morgan type. We therefore have some scope to investigate whether these properties have any bearing on the dynamical and evolutionary state of clusters.

Table 1: Our cluster sample and its properties.

Cluster	RA (1950)	Dec (1950)	z		B-M type	Richness	CCD
				erg s		class	Imagery
A0022	00 18 06.7	-26 00 00	0.1310	5.31	I	3	B,R
A2811	00 39 42.0	-28 49 00	0.1082	5.43	I-II	1
A3112	03 16 11.8	-44 25 11	0.0703	7.70	I	2	B,R
A0550	05 50 43.6	-21 03 36	0.1251	7.06		2	B,R
A3378	06 04 06.2	-35 17 42	0.1410	6.87	I	1	B,R
A0644	08 14 58.8	-07 21 22	0.0711	7.92	III	0	B,R
A1084	10 42 01.5	-06 48 19	0.1346	7.42	III	0	B,R
A1285	11 27 49.0	-14 17 55	0.1050	5.47	II-III	1	B,R
A1437	11 57 51.4	+03 37 47	0.1339	7.72	I-II	3	B,R
A1650	12 56 07.0	-01 29 28	0.0845	7.81	I-II	2	B,R
A1651	12 56 47.2	-03 55 33	0.0846	8.25	I-II	1	B,R
A1664	13 01 00.4	-23 58 34	0.1276	5.36		2	B,R
A2055	15 16 17.3	+06 24 51	0.1021	4.78	III	0	B,R
A2104	15 37 28.5	-03 08 41	0.1554	7.89	III	0	B,R
A2204	16 30 17.6	+05 10 44	0.1524	20.58	II	3	B,R
A3814	21 46 12.0	-30 56 00	0.1177	3.85	II	3
A3888	22 31 38.7	-37 59 31	0.1510	14.52	I-II	2	B,R,U
A3921	22 46 41.6	-64 41 47	0.0953	5.40	II	2	B,R,U
A2496	22 48 18.0	-16 40 00	0.1220	3.71	I-II	2
A2597	23 22 42.0	-12 23 00	0.0852	7.97	III	0	B,R,U
Field 0	23 10 00.7	-12 23 48	-	-	-	-	B,R
Field 1	01 45 00.0	-30 00 00	-	-	-	-	B,R
Field 2	03 00 00.0	-36 00 00	-	-	-	-	R

© Copyright Astronomical Society of Australia 1997