Investigations of clusters of galaxies. IV. Properties of the galaxy cluster Abell 1795

Abstract

The structure of the centre of the galaxy cluster A 1795 and its surroundings, representing different cluster components, has been investigated in several wavelength ranges. A preferred orientation deviating about 10 -20" from the north-south direction has been found for all cluster components. The orientation of the central cD galaxy is in good agreement with this direction, too. There seem to exist hints for antisegregation in luminosity as well as for colour segregation.Key words: clusters of galaxies -Abell 1795: photometry -structure A A A subject classification: 160 IntroductionThe galaxy cluster Abell 1795 is one of the most interesting clusters of galaxies. It is a cD cluster with an extremely dominant central galaxy classified as BM-type I and Abell richness class 2. A total cluster population N = 115, a redshift z = 0.062 and a radial velocity dispersion or = 821 km s K 1 are given by STRUBLE and ROOD (1982) on the basis of 20 redshift measurements. HILL et al. (1988) published redshifts of 41 cluster members with 6, = 838 km s K 1 and after a correction for relativistic effects according to DANEsEet al. (1980) with or = 786 km s-'. STRUBLE and ROOD (1982) obtained a value 20 arcmin for the total cluster radius and determined the morphological type of the 2nd and 3rd brightest galaxy as SO. Abell 1795 is an amazingly strong X-ray source (MUSHOTSKY 1984) with a smoothed X-ray distribution centered at the cD galaxy (JONES et al. 1979). From this observation and from its strong Ha emission (HECKMAN 1981, VAN BREUGEL et al. 1984) the cD galaxy was recognized to be an extremely strong accretor of cooling gas. The accretion rate amounts to 400 -480 solar masses per year (MUSHOTSKY and SZYMKOWIAK 1988).There appears to be a correlation between the X-ray luminosity and the velocity dispersion of clusters of galaxies, L, z o : (SOLINGER and TUCKER 1972). With the enlargement of the sample of X-ray clusters an increasing scatter about the assumed correlation becomes obvious, however, the basic relationship remains the same (QUINTANA and MELNICK 1982). Hence, the extreme excess of the central X-ray emission of A 1795 above the basic relationship (similar to SC 0316, A 1904, Coma, A 2142, A 262, and A 119) is of special interest.Another point of interest is the same orientation of the cD galaxy and the X-ray emission as discussed by BAIER et al. The formation of cD galaxies at the centres of galaxy clusters is yet an unsolved problem. Several scenarios were suggested, the most promising ones are galaxy cannibalism (HAUSMANN and OSTRIKER 1978) or growth by cooling flow (MUSHOTSKY et al. 1981).In this connection it is remarkable that the presence or absence of a cD galaxy does not depend on cluster properties (MERRIT 1985). cD galaxies appear with nearly equal frequency in clusters of all richness classes and velocity dispersions and even in very poor clusters. They can be observed at a local density peak within a cluster, too. Besides, approximately 25 -50% of all cD galaxies have multiple nuclei , seeming to be ...