1 Introduction

In the present accepted paradigm of structure formation, small structures are the first to collapse, then merging hierarchically to build larger objects. In this framework, clusters of galaxies, the largest (nearly) virialized structures, may be accreting galaxies and/or "dark haloes'' even at z=0 (e.g. Lanzoni et al. 2000). Therefore, the study of galaxy clusters may offer important informations for observational cosmology, because cluster properties depend on cosmological parameters and can be used to constrain cosmological scenarios. For instance, the cluster mass function, which may be described by the Press-Schechter formalism (Press & Schechter 1974), depends on the density parameter $\Omega_0$ and on the power spectrum amplitude and shape parameter (e.g., Lacey & Cole 1994; Bahcall & Fan 1998). Also, the morphological and dynamical state of clusters allow us to infer their history and, again, to constrain cosmological theories of large-scale structure formation (e.g., Kauffmann et al. 1999).

The mass of a cluster may be estimated by several methods: the optical virial mass, from the positions and radial velocities of the cluster galaxies; the X-ray mass, from the X-ray emission of the hot intracluster gas; the gravitational lensing mass, from the distortions produced on background object images by the gravitational field of the cluster. However, a discrepancy between these estimators are often found (e.g., Mushotzky et al. 1995; Girardi et al. 1998; Wu et al. 1998). Virial mass estimates rely on the assumption of dynamical equilibrium. X-ray mass estimates also depend on the dynamical equilibrium hypothesis and on the still not well-constrained intra-cluster gas temperature gradient (Irwing et al. 1999). Finally, mass estimates based on gravitational lensing are considered more reliable than the others (e.g., Mellier 1999) because they are completely independent of the dynamical status of the cluster, and their discrepancies with other methods may be due to non-equilibrium effects in the central region of the clusters (Allen 1998).

An important source of departure from equilibrium (that may affect mass estimates) are the substructures. Their very existence supports the current view that clusters grow hierarchically by accreting nearby groups and galaxies. Note that even the frequency and degree of clumpiness in the central regions of the clusters depends on the cosmology (e.g., Richstone et al. 1992). In many cases, substructures are loosely bound and can survive only a few crossing times in the hostile environment of rich clusters. However, they seem to be very common in present-day clusters. A recent estimate by Kolokotronis et al. (2000) indicates that at least 45% of rich clusters present optical substructures. Substructures are detected in both optical and X-ray images in 23% of the clusters. This last number may then be considered a lower limit on the fraction of real substructures in clusters, and it is large! Indeed, it implies that one in four clusters may be out of equilibrium due to the presence of a substructure. The dynamical status of individual clusters should therefore be examined in detail before being used in other studies.

In this paper we present a study of the dynamical status of the cluster Abell 970, from the analysis of the positions and velocities of cluster galaxies, as well as from the intra-cluster gas X-ray emission. Abell 970 has a richness class R=0 and type B-M III (Abell et al. 1989). Together with a few other clusters (A979, A978 and A993), it is member of the Sextans supercluster (number 88 in the catalogue of Einasto et al. 1997; and number 378 in the catalogue of Kalinkov et al. 1998). It has a moderate cooling flow (White et al. 1997).

A search in the NED database indicates that only 4 radial velocities are known in the field of the cluster (see Postman et al. 1992) which, however, have not been published. Here we examine some properties of the cluster Abell 970, using a set of 69 new radial velocities. The observations of radial velocities reported here are part of a program to study the dynamical structure of clusters of galaxies, started some years ago and with several results already published (see e.g. Proust et al. 1987, 1988, 1992, 1995, 2000; Capelato et al. 1991).

This paper is organized as follows. We present in Sect. 2 the details of the observations and data reduction. In Sects. 3 and 4 we discuss the galaxy and the X-ray distributions, respectively. In Sect. 5 we analyze the velocity distribution of the cluster galaxies. In Sect. 6 we present mass estimates for the central region of the cluster, derived from the optical and X-ray observations. In Sect. 7 we discuss the dynamical status of Abell 970. Finally, in Sect. 8 we summarize our conclusions. We adopt here, whenever necessary, $H_0 = 50 ~ h_{50}~~{\rm km\,s^{-1}}$Mpc^-1, $\Omega_0=0.3$ and $\Omega_{\Lambda} = 0.7$.

   

Table 1: Heliocentric radial velocities for galaxies.

GALAXY	R.A	DEC.	TYPE	$b_{\rm J}^{\rm cosmos}$	VELOCITY	R	N
	(2000)	(2000)			$V {\pm {\Delta}V}$
01	10 17 25.7	-10 41 21	E/D	16.57	17525 52	9.72	e
02	10 17 24.6	-10 41 22	S0	18.00	16209 37	7.14	e
03	10 17 28.3	-10 40 59	S0/S	18.67	18424 74	5.48	e
04	10 17 29.7	-10 40 31	S0	17.62	16270 52	9.67	e
05	10 17 26.3	-10 41 34	S0	17.51	18357 54	6.60	e
06	10 17 23.9	-10 42 16	S0/S	18.41	16435 69	5.61	e
07	10 17 27.3	-10 41 51	E/S0	18.36	18145 53	5.89	e
08	10 17 29.5	-10 42 17	S	18.38	17404 71	7.42	e
09	10 17 21.7	-10 42 56	S	17.96	17631 61	6.91	e
10	10 17 24.3	-10 43 29	S	17.95	16341 81	6.76	e
11	10 17 31.8	-10 42 59	S0/S	18.83	17769 81	5.79	e
12	10 17 30.0	-10 43 09	S0	18.45	17327 35	5.57	e
13	10 17 32.9	-10 43 52	S	18.26	16974 36	5.14	e
14	10 17 29.4	-10 44 34	Sa	17.69	18909 75	5.61	e
15	10 17 28.0	-10 44 18	S0	17.67	18030 29	8.46	e
16	10 17 20.7	-10 44 16	S	18.56	17711 26	5.85	e
17	10 17 15.6	-10 43 28	Sa	17.51	19503 96	2.62	e
18	10 17 12.9	-10 42 47	S0	17.83	19450 51	9.10	e
19	10 17 35.6	-10 39 55	Sb	17.15	18789 24	9.41	e
20	10 17 28.1	-10 39 27	Sa	18.16	16842 76	4.79	e
21	10 17 27.6	-10 39 12	S0	18.63	16404 92	4.48	e
22	10 17 21.0	-10 40 13	S0	17.18	18788 45	9.41	e
23	10 17 23.2	-10 40 18	E	18.26	19381 40	10.12	e
24	10 17 22.0	-10 39 45	E	18.00	16847 47	6.37	e
25	10 17 22.5	-10 39 50	E/S0	17.09	19483 54	9.32	e
26	10 17 25.2	-10 41 07	E	18.87	17081 93	3.15	e
27	10 17 28.5	-10 41 13	E	17.52	33586 98	2.84	e
28	10 17 21.9	-10 43 12	E/S0	18.71	17764 64	5.17	e
29	10 17 14.4	-10 39 16	S	18.46	18415 94	3.44	e
30	10 17 12.6	-10 40 05	E/S0	17.69	16533 80	5.36	e
31	10 17 33.6	-10 38 46	SB	17.88	17779 109	4.68	e
32	10 17 44.6	-10 39 14	S0/S	18.09	17992 33	7.55	e
33	10 17 32.4	-10 34 27	S0	19.01	17606 98	2.75	e
34	10 17 30.7	-10 36 24	S	17.43	16722 134	3.55	e
35	10 17 41.6	-10 35 46	S0/S	17.63	21637 37	8.62	e

 

Table 1: continued.

GALAXY	R.A	DEC.	TYPE	$b_{\rm J}^{\rm cosmos}$	VELOCITY	R	N
	(2000)	(2000)			$V {\pm {\Delta}V}$
36	10 17 51.1	-10 35 00	E	17.29	16661 62	4.42	e
37	10 17 36.9	-10 46 01	S	16.8 7	11957 79	4.97	e
38	10 17 36.8	-10 46 05	S0/S	16.8 7	12221 77	4.70	e1
39	10 16 55.8	-10 38 51	Sa	17.83	17570 74	7.53	e
40	10 16 58.5	-10 38 07	S0/S	16.42	17184 44	6.33	e
41	10 16 59.7	-10 37 39	E	18.62	17873 59	6.44	e
42	10 17 00.6	-10 37 12	E	19.16	17194 66	5.58	e
43	10 17 03.9	-10 37 38	E/S0	18.11	17690 52	7.14	e
44	10 17 11.8	-10 36 07	E/S0	16.43	21998 84	8.33	e2
45	10 16 57.5	-10 40 16	E	17.46	17903 48	9.11	e
46	10 17 02.0	-10 39 59	E	19.10	18843 98	4.53	e3
47	10 17 07.6	-10 45 46	S0	16.89	17202 86	15.72	e4
48	10 17 10.2	-10 46 26	E/S0	17.13	17475 46	12.24	e5
49	10 16 49.9	-10 47 23	Sb	16.91	17487 64	6.73	e
50	10 17 00.5	-10 47 17	Sa	17.38	21166 107	3.77	e
51	10 16 54.1	-10 43 10	S0/S	18.45	17257 85	4.71	e
52	10 16 53.2	-10 43 43	E	18.83	17351 85	6.70	e
53	10 17 47.0	-10 45 19	S	19.14	16771 138	2.96	p
54	10 18 18.0	-10 46 48	E/S0	15.95	11579 63	8.04	p
55	10 18 15.6	-10 45 03	S0	17.69	12056 81	3.26	p
56	10 17 51.6	-10 44 47	E	19.44	16372 89	2.53	p
57	10 17 54.6	-10 43 47	S0	19.44	45962 93	2.61	p
58	10 18 06.9	-10 42 43	S	19.13	51814 76	2.41	p
59	10 18 04.0	-10 41 44	SBc	19.68	48068 73	3.01	p
60	10 17 57.7	-10 33 48	S	18.52	16228 44	4.53	p
61	10 18 13.0	-10 34 42	S	18.47	11675 69	5.34	o6
62	10 16 55.2	-10 30 49	S	18.16	18371 133	3.30	o
63	10 16 54.7	-10 33 22	S	17.86	17737 75	4.47	o
64	10 16 36.9	-10 32 38	S0	17.58	17982 82	5.62	o
65	10 16 51.9	-10 36 15	S	17.53	18255 84	5.39	o
66	10 16 48.8	-10 39 10	S0	18.67	18293 63	6.48	o
67	10 17 42.0	-10 45 49	E	18.60	12101 114	3.22	o
68	10 16 37.3	-10 32 39	?	18.77	17589 136	3.04	o
69	10 16 41.3	-10 52 38	S	17.37	17649 42	8.74	o

Emission lines: 1: [OIII], H${\alpha}$, [SII]  $=~12\,084~{\rm km\,s^{-1}}$; 2: H ${\alpha}=~22\,087~{\rm km\,s^{-1}}$; 3: H ${\alpha}=~18\,858~{\rm km\,s^{-1}}$;
4: H ${\alpha}=~17\,234~{\rm km\,s^{-1}}$; 5: H ${\alpha}=~17\,523~{\rm km\,s^{-1}}$; 6: measured on H$\beta$, [OIII], H${\alpha}$; 7: estimated magnitude (see Sect. 4).