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

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5. Discussion

We presented a three dimensional model of the complex ABCG 85/87/89 region. We find a general extension observed toward the south-east along PA [FORMULA] both in X-rays and in optical photometric observations:

- In X-rays, the elongated structure extends to the edge of the ROSAT PSPC field of view; the X-ray appearance includes several individual group-like systems lying up to about 4 Mpc (in projection) from the center of the cluster (see Fig. 1) as well as very elongated diffuse emission;

- from the optical photometric catalogue, ABCG 85 is elongated towards the ABCG 87 cluster (see Fig. 4). The PA [FORMULA] is characteristic for ABCG 85: the direction of the major axis, the brightest galaxies in ABCG 85, and even the major axis of the central cD galaxy itself are elongated along the same direction.

East of ABCG 85 lies ABCG 89 which is not bright in X-rays. Our velocity data reveal that ABCG 89 is not a cluster, but the superposition on the sky of two groups well separated in velocity space. These two groups (to which we refer as A89b & A89c in Fig. 15) are located in intersecting sheets on opposite sides of a large bubble.

We have shown that ABCG 87 is not a rich cluster, but is resolved into individual groups possibly falling onto ABCG 85. These groups are organized as a filament almost perpendicular to the plane of the sky. The superposition of these groups gives the appearance of a single optical cluster, while in X-rays, several groups are still visible: This is probably due to the fact that the emissivity in X-rays is proportional to the density squared, therefore enhancing the contrast, compared to optical imaging.

ABCG 85 itself is probably not fully relaxed, even if it appears smooth and symmetric; the distribution of velocities is obviously not gaussian and probably multi-modal.

In his scenario for the Coma cluster, West (1997) links the orientations of Coma to the filament in which Coma is embedded. He suggests that matter, including galaxies, groups and gas, falls onto the cluster along this filament. The case of ABCG 85 reinforces this cosmological scenario because we actually observe the infall of material (a filament of groups of galaxies and gas) onto ABCG 85. This result is consistent with the X-ray temperature map derived from ASCA data by Markevitch et al. (1998), which shows a temperature enhancement in a region south of ABCG 85 and roughly perpendicular to the general direction of [FORMULA] along which the various structures are aligned. Such a temperature increase could be interpreted as shock heating due to the compression of the X-ray gas by infalling matter. The fact that there also appears to be a radio relic, also roughly perpendicular to the [FORMULA] direction, in this zone (Bagchi et al. 1998) also supports recent merger activity, since relativisitic electrons can be produced during a merger.

Remarkably, the ABCG 85/87 filament is coaligned with a much larger structure including from northwest to southeast: ABCG 70, ABCG 85 and 89, ABCG 87, ABCG 91, the NGC 255 group and ABCG 106 (Fig. 1 in Slezak et al. 1998). Such a structure extends over more than [FORMULA] on the sky, corresponding to a linear distance of 28 Mpc at the redshift of ABCG 85 (z=0.0555). This is obviously a lower limit, since the filament may be inclined to the line of sight. Although this projected value is smaller than that found for example for the Perseus Pisces structure (50 [FORMULA] Mpc, Haynes & Giovanelli 1986), it is nevertheless much larger than typical cluster sizes. Unfortunately, there are only very few redshifts available for this large structure outside the ABCG 85/87/89 complex. Further optical observations are required to confirm the cohesiveness of this remarkable filamentary structure.

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© European Southern Observatory (ESO) 1998

Online publication: June 12, 1998