3 Optical analysis

For two of the clusters - A901 and A3570 - optical spectra could be taken. In A901 redshifts of three galaxies could be well determined. These three galaxies are at similar redshifts (see Table 2). We derive a cluster redshift of z_A901=0.17. Unfortunately, the number of galaxies is not sufficient to estimate the velocity distribution.

In A3570 we found 17 cluster member galaxies (see Table 2) in addition to the three galaxies known previously (Postman & Lauer 1995). From these 20 galaxies we derive a cluster redshift z_A3570=0.0375(very similar to the redshift 0.0372 by Abell et al. 1989). The velocity distribution is shown in Fig. 1. The velocity dispersion based on these 20 galaxies $\sigma=460$ km s^-1 is relatively small. Such a small velocity dispersion is a first hint for a virialised state of the cluster. If merging of subclusters would take place and not all of the mergers would happen perpendicular to the line-of-sight a broad velocity distribution would be expected. Assuming virial equilibrium we estimate the cluster mass from the velocity dispersion and the estimate of the virial radius by Girardi et al. (1998) to $M_{{\rm opt}}(A3570)=2.7~10^{14}{M}_{\odot}$.

 

 

Table 2: Redshifts and positions of galaxies in the clusters A901 and A3570

cluster	$\alpha$(J2000)	$\delta$(J2000)	redshift
A901	09 54 57.5	-10 11 50	0.171
	09 55 38.2	-10 10 19	0.169
	09 56 53.2	-10 10 14	0.164
A3570	13 45 17.2	-38 10 21	0.0372
	13 45 46.0	-37 56 45	0.0406
	13 45 49.7	-37 37 18	0.0365
	13 46 09.9	-38 00 01	0.0364
	13 46 18.5	-38 20 18	0.0405
	13 46 54.0	-37 51 48	0.0376
	13 47 04.1	-38 03 57	0.0348
	13 47 08.9	-37 55 23	0.0366
	13 47 11.7	-38 07 33	0.0379
	13 47 23.9	-37 51 00	0.0390
	13 47 27.3	-37 57 54	0.0390
	13 47 45.8	-37 50 41	0.0376
	13 47 51.0	-38 00 52	0.0383
	13 47 57.5	-37 56 29	0.0373
	13 48 07.2	-38 16 47	0.0346
	13 48 38.2	-37 43 41	0.0355
	13 49 12.0	-37 55 54	0.0370

Figure 1: Velocity distribution of 20 galaxies in A3570