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

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2. X-ray imaging analysis and Galaxy distribution

2.1. ROSAT PSPC X-ray observations of ABCG 85

We have analyzed the ROSAT PSPC observations of ABCG 85 using an image in the energy band 0.5-2.0 keV binned into 15 arcsecond pixels and flat fielded. Fig. 1 shows the resulting ROSAT PSPC image of the [FORMULA] radius field centered on ABCG 85, smoothed with a [FORMULA] Gaussian to enhance the signal to noise ratio at large off-angle distances. The image clearly shows emission peaked on the cD at the cluster center, a second region of extended emission (the south blob) approximately [FORMULA] south of ABCG 85 (720 kpc, projected on the sky), as well as point sources scattered over the PSPC field of view (FOV). In addition to these features, there is an excess of emission towards the south south-east, extending almost to the edge of the FOV. The emission to the southeast is concentrated in a narrow range of position angles, approximately between [FORMULA] and [FORMULA] (counter clockwise from north); the positions and net counts of individual sources from this region are listed in Table 1. We also generated an image following the Snowden et al. (1994; see also Snowden 1995) prescription which slightly reduces the field of view but shows the same features in the overlapping regions.

[FIGURE] Fig. 1. Optical digitized sky survey of the ABCG 85 region, with the isophotes of the ROSAT PSPC image of a flat-fielded [FORMULA] radius field superimposed. The energy band used is 0.4-2.0 keV and the image has been smoothed with a [FORMULA] Gaussian after flat-fielding. The cD galaxy at the X-ray cluster center (peak of the X-ray emission) is clearly visible on the optical map; a bright star is located in the region of extended X-ray emission, but it does not significantly contribute to the observed X-ray emission.


Table 1. X-ray source properties

As shown in Table 1, the sources are all of comparable intensity. If we assume a typical source contains 45 net counts, we find a flux of [FORMULA] ergs cm-2 s-1 (using a Galactic column density of [FORMULA] cm-2 and a spectrum characterized by a temperature of 1.4 keV) for an exposure time of 9.8 ksec. At the distance of ABCG 85, this corresponds to a luminosity of [FORMULA] ergs s-1. These luminosities are typical of those of small groups of galaxies (e.g., Henry et al. 1995, Pildis et al. 1995, Ponman et al. 1996).

The azimuthal distribution computed by binning the counts in the PSPC image in [FORMULA] sectors over the radial range from 27.5 to 55.0 arcminutes (110-220 [FORMULA] pixels) centered on the peak of the surface brightness distribution, shows a clear excess of approximately 500 cts (2750 compared to 2250 cts) compared to the mean at other azimuths. The smaller scale sources contain only approximately 50% of this total excess, indicating additional flux on larger scales (clearly seen in Figs. 2 and 3, see below). This excess emission (excluding the four concentrations in Table 1) has a flux corresponding to [FORMULA] ergs cm-2 s-1, assuming it has the same spectral properties as we assumed for the individual sources above. While our assumption for the sources may be reasonable, since their luminosities are equivalent to those of groups, the excess flux could be characterized by a significantly cooler temperature if it lies in still weaker potentials than typical of small groups.

We generated an image of the excess X-ray emission to the south-east of ABCG 85. We built a synthetic image for the whole cluster, using the azimuthal profile of the northern half of ABCG 85, which was then subtracted from the original data (and normalized by dividing by the symmetric model). The result is displayed as a contour map in Fig. 2. The surface brightness enhancement which initially extends to the south from the center of A85, shifts in angle towards the southeast.

[FIGURE] Fig. 2. Residual X-ray emission obtained after subtracting the smooth emission derived from the north half of the image, as described in the text. The crosses indicate the centers of the three clusters ABCG 85, 87 and 89. The numbers correspond to the sources defined in Table 1.

A wavelet transform analysis, which allows extraction of emission at different spatial scales, gives comparable results (Fig. 3).

[FIGURE] Fig. 3. X-ray contour map after wavelet analysis, onto which are superimposed the galaxies with magnitudes brighter than [FORMULA], and belonging to the ABCG 85 cluster. The coding is as follows: black circles: galaxies in the [14500-16100 km s-1 ] velocity range, white circles: galaxies in the [16100-18500 km s-1 ] velocity range.

2.2. The distribution of galaxies in the ABCG 85 region

Slezak et al. (1998) compiled two photometric catalogues, one of 11862 galaxies in a [FORMULA] field, and one of 4232 galaxies in a 1 degree radius region centered on the cluster to a limiting magnitude of [FORMULA] ; the latter is 95% complete to [FORMULA].

By using the first and second moments of the distribution of galaxy positions, we calculated the ellipticity and the major axis position angle of the galaxy distribution; the ellipticity is e=0.82 and the major axis is along PA [FORMULA]. This alignment is also illustrated by the brightest cluster galaxies (11 galaxies brighter than R=15 and having redshifts belonging to the cluster) which are distributed in a narrow band with PA [FORMULA]. We also note that the cD galaxy itself is elongated with PA [FORMULA].

We used the optical photometric catalogues of Slezak et al. to investigate the structure corresponding to the X-ray emission. A kernel map (Silverman 1986) showing the smoothed galaxy number density is displayed in Fig. 4. This figure shows the alignment at PA [FORMULA] of the contours extending to the south east from the center of ABCG 85. The prominent azimuthally symmetric peak located [FORMULA] southeast of the cD galaxy at the position of ABCG 87 also defines the same [FORMULA] PA. Another structure is seen at the optical position of ABCG 89. A comparison of the X-ray and optical images (Figs. 2, 3 and 4) is instructive. All the features found in the optical at PA [FORMULA] are seen in X-rays, with emission extending from ABCG 85 southeast toward ABCG 87.

[FIGURE] Fig. 4. Adaptive kernel map of the galaxy distribution, drawn for galaxies brighter than [FORMULA] in a field of [FORMULA] square degrees (2018 objects). The initial kernel chosen was half the Silverman value and 10 bootstrap resamplings were performed.

Thus both the X-ray emission and the optical galaxies show an excess of matter in a large region southeast of ABCG 85. Nearly half of the X-ray emission arises from a few sources, reminiscent of groups of galaxies.

We find that galaxies with velocities smaller than the cluster bi-weight (BWT; Beers et al. 1990) mean ([FORMULA] km s-1) and smaller than the velocity of the central cD ([FORMULA] km s-1) are concentrated in two regions along the southern filamentary extension (see Fig. 3). These galaxies have velocities in the 14500-16100 km s-1 range and also lie at the same position angle as the previously mentioned structures.

A major difference in the X-ray and optical maps is in the appearance of the region directly to the east of ABCG 85. No extended X-ray emission is found corresponding to the prominent galaxy concentration at the location of ABCG 89. Also, the X-ray appearance at the position of ABCG 87 is not consistent with the presence of a regular, relaxed system, as is suggested by the optical. The X-ray structure is elongated and shows more substructure/clumpiness. On the contrary, a relaxed system would show X-ray emission having less ellipticity than found in the optical (see e.g. Fabricant et al. 1984).

We now investigate the three dimensional galaxy distribution (space and velocity) to clarify the differences and similarities seen in the X-ray and optical.

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© European Southern Observatory (ESO) 1998

Online publication: June 12, 1998