Uncovered Structures and the Cluster Abell 3627

The new redshifts along with published redshifts in adjacent regions evidence the following structures:

The Hydra and Antlia clusters are not isolated clusters, but part of a filamentary structure which can be traced from Hydra through Antlia (cf., Fig. 1), across the Galactic Plane to ), thus constituting a major structure in the nearby Universe ( at a median recession velocity of only 2800 km ). It seems more a filamentary structure, consisting of spiral-rich groups and clusters, rather than a supercluster.

The prominent galaxy overdensity in Vela is part of a previously unrecognized shallow, large-scale, overdensity centered on km . The independent predictions of a supercluster at this position and distance by Hoffman (1994) and Saunders et al. (1991) indicate that it could be quite massive.

The redshifts in the cluster A3627 (<v>=4882 km for 131 reduced to date) put this cluster near the center of the Great Attractor in velocity space. The mass estimate from its velocity dispersion ( km ) is that of a rich cluster (). Its other cluster properties - predominance of early-type galaxies at the center, its core radius R and its central density all are consistent with this being a rich massive cluster (Kraan-Korteweg et al. 1996). It even has, like Coma, two dominant central cD-galaxies (cf., Fig. 3).

Rich massive clusters generally are strong X-ray emitters and were identified early on with the X-ray satellites (Einstein, HEAO, Uhuru). Despite dedicated searches and the fact that the hard X-ray band is hardly affected by HI-absorption, A3627 had never been identified in X-ray (Jahoda & Mushotzky 1989, Lahav et al. 1989). We therefore studied the ROSAT PSPC data of this cluster (Böhringer et al. 1996) and found that A3627, with an X-ray luminosity of L, is in fact the -brightest X-ray cluster in the ROSAT All Sky Survey. Moreover, the independent mass estimate from the X-ray data is consistent with the virial mass.

Fig. 3 displays the X-ray contours (Böhringer et al. 1996) superimposed on an image of the central part of the cluster.

 
Figure 3: The central part (56 x 56) of the cluster A3627 as reproduced from field 136 of the IIIaJ copy of the ESO/SRC survey. Superimposed in the left panel are the X-ray contours from the ROSAT PSPC observations; the right panel shows the residual X-ray contours after substraction of a spherical component, and the 843 MHz radio continuum emission of the wide-angle tail radio galaxy PKS1610-60.8 and the head-tail radio-source B1610-60.5. The strong X-ray point-source (top right corner), a cluster galaxy, is a Seyfert 1 (Woudt et al. 1996).

The X-ray provides interesting insight into the cluster morphology. The X-ray center is offset from the center of the cluster, the strong radio source PKS1610-60.8, one of the two cD galaxies. The latter coincides with the second central X-ray peak. The X-ray emission is not symmetric but extended towards the SE (left-bottom) corner. Subtracting a spherical symmetric model leaves a residual component (cf., right panel). This subcluster contains the wide-angle-tail radio galaxy PKS1610-60.8 whose large radio lobes of 8 arcmin ( kpc) have a bending angle of . The contours of the radio emission (Jones & Mc Adam 1992) are drawn onto the X-ray subgroup as well. Note the alignment of the subclump with the radio emission. The redshift data are yet too sparse to allow a kinematical analysis of this cluster, however, the redshifts in the subclump tend to be higher, suggesting that this subgroup is in front of the main cluster and falling towards it.

The emission from the radio galaxy B1610-60.5 (Jones & Mc Adam 1992) has been drawn into the right panel of Fig. 3 as well. It is one of the longest known head-tail galaxies (26 armin, kpc). The radio emission aligns over nearly its full extent with the 3rd contour of the main X-ray component. The lack of distortion of the radiolobes of both radio galaxies and the compactness of the subclump imply that the suspected merger must be in an early stage. Forthcoming ATCA HI-synthesis observations will allow deeper insight into this cluster.

The cluster A3627 most likely is the previously unidentified core of the Great Attractor overdensity. The mass excess of the GA is presumed to arise within an area of radius of about 20-30 Mpc (Lynden-Bell et al. 1988). This actually matches the emerging picture from our observations quite well. A3627 seems the center of an apparent ``great wall''-like structure, similar to Coma in the (northern) Great Wall: a broad filament reaches from km ) over the GA region towards ( km ). Whether it merges into the Vela-supercluster at ( km ) is not yet certain.

© Copyright Astronomical Society of Australia 1997