4. Three dimensional structure of the ABCG 85 complex
Fig. 13 shows the distribution in the plane of the sky of galaxies with measured velocities, colour coded for various velocity ranges. This figure illustrates the difficulty in untangling structures projected on the plane of the sky, even if they have very different velocities. We can note however that two structures are visible: a condensation of "yellow galaxies" in the region of ABCG 89 (but galaxies in this velocity range also can be found elsewhere), and "dark blue galaxies" corresponding to the "Foreground Group". Besides, there seems to be an enhancement of galaxies aligned along PA .
We show two cone diagrams for the velocity distribution, which are displayed in Fig. 14. Both panels show evidence for various substructures along the line of sight. The bulk of the galaxies (the purple and red points in the figures) correspond to ABCG 85.
In the left panel of Fig. 14, the "Foreground Group" is clearly visible (dark blue points), and it is apparent from Fig. 13 that this group is quite compact and concentrated (in projection) towards the center-west of the cluster.
The ABCG 87 cluster itself, or rather the region south-east of ABCG 85, appears to be composed of several subgroups with velocities somewhat lower than the median cluster velocity (green points). These subgroups, defined in velocity space, have X-ray counterparts (Fig. 2). In Fig. 15 we place each subgroup at its X-ray position and represent it in the right ascension-velocity plane (the velocities are in fact the various peaks observed in Fig. 10), we observe a filament pointing towards ABCG 85. This suggests that a stream of material is falling onto the main core of ABCG 85. The south blob (the extended X-ray feature south of the cD) would then be the meeting point (see Fig. 2) and corresponds to a hot region in the ASCA temperature map (Markevitch et al. 1998).
The two structures with the largest velocities (yellow and cyan points) seem to trace two sheets of galaxies (referred to as the near and remote sheets in Fig. 15). Both structures are inclined along the line of sight, and appear to intersect. These two sheets may lie on the surface of a large bubble which intersects our field of view.
Note also the coalignment, along the line of sight, of the Foreground Group, ABCG 85, A89b, and A89c. This could indicate the presence of a second filament directly along the line of sight. Furthermore, the two filaments could be concentrations in a sheet. Notice the existence of a dozen galaxies with velocities slightly higher than that for most of the ABCG 85 galaxies which indicate a lower density region of the line of sight filament.
© European Southern Observatory (ESO) 1998
Online publication: June 12, 1998