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Astron. Astrophys. 356, 815-826 (2000)

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3. Velocity distribution along the line of sight

We first discuss the overall properties derived from the velocity distribution along the line of sight.

3.1. Overall characteristics of the structures detected along the line of sight

A wavelet reconstruction of the velocity distribution along the line of sight is displayed in Fig. 1 (466 galaxies). We remind the reader that this type of reconstruction takes into account structures at a significance level of at least 3[FORMULA], and detects structures at various scales. The sample was analyzed with 256 points, and the smallest scale was excluded because it is very noisy. A more detailed description of this technique can be found in Fadda et al. (1998).

[FIGURE] Fig. 1. Wavelet reconstruction of the velocity distribution in the direction of Abell 496. The numbers above the peaks correspond to those of the structures described in the text and in Table 1. The density units correspond to a total integrated galaxy density of 1.

Nine "groups" or velocity substructures are found with this method, and their velocity characteristics are given in Table 1. The group number is given in Column 1, the number of galaxies in Column 2, the BWT mean velocity (Beers et al. 1990) and corresponding BWT velocity dispersion in Columns 3 and 4, and the velocity interval in Column 5. Foreground groups 1 and 2 and background groups 7 and 8 are most probably not real groups, since they are widely spread both on the sky and in velocity distribution; because of the small number of galaxies involved in the first three of these groups, we did not calculate mean velocities or velocity dispersions for these structures. For group 8 these values are only indicative, but characteristic of a low mass structure. Group 3 appears to be the cluster Abell 496 itself. Except for 7 objects, all the galaxies in group 4 appear to be located north of Abell 496. Group 5 has the same kind of shape and size as group 4 and is roughly coaligned with Abell 496 along the line of sight. Group 6 appears strongly concentrated both spatially and in velocity space, all but two galaxies being located west of Abell 496. Moreover, its velocity dispersion is also low.


[TABLE]

Table 1. Structures detected along the line of sight


These results are confirmed when we apply the same method as for the ENACS clusters (Katgert et al. 1996, Mazure et al. 1996) to detect velocity structures along the line of sight. This method consists in sorting the galaxies in order of increasing velocity, and plotting their rank as a function of velocity (hereafter the rank-velocity classification). If the distribution of galaxies in redshift space is strictly gaussian, we expect to see a regular S-shape in the sequence/gap space. When there are more than 5 galaxies between two successive gaps, we consider that the galaxies belong to a structure.

3.2. A finer analysis of structures 4, 5, 6 and 9

In order to understand whether groups 4, 5, 6 and 9 can be physically coherent structures, we performed a Serna & Gerbal (1996) analysis for each of these groups separately. Since this type of analysis takes into account galaxy magnitudes, we had to eliminate one galaxy in group 4 and one in group 5, for which we have no magnitude in our redshift catalogue. We also tried keeping these galaxies and assigning them an "average" magnitude R=17. The results in both cases were similar. Note that the redshift catalogue completeness is about 50% in regions 4 and 5, and 55% in region 9, within the magnitude limit R=18.8. It could therefore be argued that the Serna & Gerbal method is meaningless for these samples. However, in this type of analysis it is the brightest galaxies which mainly contribute to the dynamics of the system (since the mass to luminosity ratio is taken to be constant for all galaxies). If the samples are limited to magnitudes R[FORMULA]17.0, the redshift catalogue completeness then becomes 72%, 72% and 79% for regions 4, 5 and 9 respectively, and the Serna & Gerbal method is therefore fully applicable.

The characteristics of the substructures found with the Serna & Gerbal method are given in Table 2. Structure 4 has subgroups 4c and 4d well defined in space; they extend over 5 and 3.6 Mpc respectively and could therefore be members of two different clusters. Due to their large spatial extent, subgroups 4a and 4b respectively seem to be just forward and background galaxies, with the exception of the five galaxies of group 4a at the north west extremity (see Fig. 2).

[FIGURE] Fig. 2a-d. Isodensity contours for galaxies in structures 4, 5, 6 and 9 (from a to d ); galaxy positions are superimposed with the following symbols: structure 4: empty rectangles=4a, filled rectangles=4b, empty triangles=4c, filled triangles=4d, crosses=other galaxies in structure 4; structure 5: empty rectangles=5a, filled rectangles=5b, empty triangles=5c, crosses=other galaxies in structure 5; structure 6: all galaxies; structure 9: filled rectangles=9a, filled triangles=9b, empty triangles=9c, crosses=other galaxies in structure 9. Positions are relative to the cluster center defined in the text.


[TABLE]

Table 2. Substructures detected along the line of sight


Subgroups 5a and 5b form structures with a small velocity dispersion, but extending over about 7 Mpc, a size which appears rather large for these groups to be members of two background clusters; the extent and the velocity dispersion of 5c are even larger (Fig. 2).

While the Serna & Gerbal (1996) method finds dynamical sub-structures for the other groups along the line of sight, the same method reveals no substructures in group 6, except for two pairs of galaxies; group 6 therefore appears well defined both in velocity distribution and in space. Eighteen galaxies are included in an ellipse with a major and minor axes of about 8 and 3 Mpc, suggesting that this is a poor, diffuse and low mass cluster (Fig. 2).

Finally, three subgroups are apparently found in structure 9, 9a and 9b having very small velocity dispersions but spanning a rather large spatial region. The overall velocity field in group 9 shows an interesting pattern looking like a velocity gradient (Fig. 3). This could be a filament more or less aligned along the line of sight.

[FIGURE] Fig. 3. Isocontours of the velocity field in group 9, from 51000 (right) to 54000 km s-1 (left) by steps of 500 km s-1. Positions are relative to the cluster center defined in the text.

A rank-velocity classification applied to each group confirms that group 4 and possibly group 9 appear to have three substructures (two clear breaks in the curves), group 5 has three or four substructures and group 6 has no clear substructure except perhaps for the two or three first galaxies which are probably infalling objects.

To summarize, groups 4 and 9 clearly appear as filaments or at least elongated structures along the line of sight, but not really massive clusters. This analysis is confirmed by the iso-velocity contours of group 9. The continuous velocity gradient could be interpreted as the result of a merger, with the infalling groups not perfectly aligned along the line of sight. Group 6 has a low velocity dispersion and is probably a poor cluster. Group 5 exhibits two low velocity dispersion sub-structures and a moderately high velocity dispersion group (5c), but the number of galaxies in 5c is too low to provide a robust estimation and we assume that this structure is not a cluster.

We will now discuss the dynamical state of the main structure on the line of sight: the cluster Abell 496 itself (group 3).

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© European Southern Observatory (ESO) 2000

Online publication: April 17, 2000
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