Astron. Astrophys. 335, 41-48 (1998)

1. Introduction

One of the major changes in our understanding of our universe has been the realization of the rich and complex structures which are apparent in the large scale distribution of galaxies. From both redshift surveys and projected galaxy distributions, the appearance of galaxy voids and supercluster filaments has become clear. The large scale structure also is an important constraint for different cosmological scenarios. On smaller scales, investigations of the relationships between nearby galaxy clusters suggest that clusters retain information about the large scale structures from which they form (e.g., van Haarlem & van de Weygaert 1993; West et al. 1995, West 1997; Colberg et al. 1997). The frequency of substructure may also provide constraints on (Richstone et al. 1992, Mohr et al. 1995, Buote & Xu 1997; see also Kauffmann & White 1993).

As part of a survey for substructure, we have studied the large scale mass distribution around the richness class 1 cluster ABCG 85 (Abell et al. 1989), using X-ray and optical observations. As a bright, luminous, relatively nearby cluster, ABCG 85 has been studied extensively. At a redshift of , the angular scale for ABCG 85 corresponds to  kpc arcmin^-1 (we assume H₀ = 50 km sec^-1 Mpc^-1 throughout). ABCG 85 contains a cD galaxy close to its center. The cluster X-ray characteristics include a peaked emission profile, harboring a cooling flow, emission from individual cluster member galaxies, an X-ray emitting subcluster south of the cluster center (hereafter the south blob), a superposed foreground group of galaxies (to the west-northwest), as well as additional foreground and background structures detected from the optical spectroscopic observations (see Jones et al. 1979, Pislar et al. 1997, Lima-Neto et al. 1997, Slezak et al. 1998, Durret et al. 1998, and in preparation for detailed discussions of the X-ray and optical observations).

In this contribution, we report on the X-ray and optical properties of a larger region around ABCG 85 than has been considered in previous studies, and in particular include the nearby clusters ABCG 87 and ABCG 89. We find that ABCG 85 itself exhibits preferential alignments on scales from 100 kpc to Mpc (in projection on the sky). In particular, the cluster cD galaxy is elongated along the same position angle as a large filamentary structure in X-rays roughly coinciding with the optical position of ABCG 87. Located to the east of ABCG 85 is ABCG 89, not detected as an extended X-ray source. We also address why ABCG 87 appears different in the optical and X-ray and why ABCG 89 appears as a cluster in the optical observations and not in X-rays.

Based on these results, we discuss the ABCG 85/87/89 complex and describe the alignments and/or structures in the context of a large scale structure formation scenario such as that proposed by West et al. (1995; see also van Haarlem & van de Weygaert 1993 and Colberg et al. 1997). In Sect. 2 we present the X-ray imaging analysis and galaxy distributions. Sect. 3 discusses in detail the galaxy velocity distribution. A model is proposed in Sect. 4 and discussed in Sect. 5.

© European Southern Observatory (ESO) 1998

Online publication: June 12, 1998

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