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Astron. Astrophys. 317, 670-675 (1997)

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4. Segregation in velocity

Radial velocities and magnitudes had to be normalized before a kinematical analysis on the galaxy sample as a whole could be started. We chose the most straightforward methods, using absolute magnitudes M and velocities normalized by cluster mean velocity and velocity dispersion, i.e.:


This implies that [FORMULA] has a mean value of 0 and a standard deviation of 1 in each cluster. Normalized velocity dispersions


will be used in the following as kinematical indicators for different samples. Only galaxies brighter than M  =  [FORMULA] have been included in the following analysis, which roughly corresponds to the completeness limit of the present dataset. Choosing only objects brighter than M  = -19 also excludes dwarf galaxies, which could contaminate fainter samples (Binggeli et al. 1988).

4.1. Dependence on galaxy types

First, we looked at differences in the kinematical behaviour of galaxies depending on their morphology. It can be seen in Fig. 1 that there is a continuous trend of the velocity dispersion to increase from early to late galaxy types. Velocity dispersions have been computed with a biweight estimator of scale (Beers et al. 1990), which has shown to be superior when only few objects are involved. Error bars come from a bootstrapping calculation with 1000 iterations. Between E and S galaxy types a rise in the velocity dispersion of 30% can be observed. The hypothesis that E-galaxies have the same velocity dispersion as S0-galaxies cannot be excluded by an F-test (17% likelihood). The same is true for the difference between S0 and S-galaxies (23% likelihood of same underlying distribution), while the difference between E and S-galaxies is significant at a level of 3%.

[FIGURE] Fig. 1. Velocity dispersion for galaxies of different morphological classes.

4.2. Dependence on galaxy luminosity

In the present sample of galaxy clusters there are clear signs of velocity dispersion dependence on absolute magnitude for galaxies brighter than [FORMULA], as can be seen in Fig. 2. Again, velocity dispersions have been computed with a biweight estimator of scale and error bars come from bootstrapping with 1000 repetitions. Bin limits were set every 0.5 mag between [FORMULA] and [FORMULA], taking the biweight mean value [FORMULA] in each bin as the x-position instead of the bin center. Further dividing the sample into galaxies of different types reveals that mainly the most luminous early type galaxies are responsible for the lower velocity dispersion. However, there is a general tendency of ellipticals to have lower dispersions than spirals also at the faint end. Former effect cannot be due exclusively to the existence of D/cD galaxies residing in the bottom of the potential well, because only 4 of the clusters are given a Bautz-Morgan type I (Abell et al. 1989). Another hint is that there are no E-galaxies with normalized velocities larger than 1.2 down to [FORMULA], while 6 or 7 would be expected from a normal distribution with [FORMULA] = 1. No clear indication about the kinematical status of S0 galaxies was found. It is widely known that there is considerable danger of confusion while classifying S0 galaxies as an intermediate class between E and S (Bender 1992). This uncertainty in the morphological classification, together with the low number of S0 galaxies brighter than [FORMULA] in our sample, makes it difficult to state about the presence of luminosity segregation in this class. It seems that the brightest S0 galaxies lie very close to the kinematical center of the cluster, indicating that these galaxies have been residing in the centers of clusters for long periods of time. On the other side, no signs of velocity dispersion changes with luminosity can be seen for E or S0 galaxies fainter than [FORMULA]. This can be explained by the fact that the time-scale for dynamical friction exceeds the Hubble time when galaxies of luminosity lower than L [FORMULA] are involved (Sarazin 1988).
As can be seen in Fig. 2, there is a deviating galaxy with [FORMULA] and [FORMULA], which was classified as S by Nilson (1973). Inspection of the corresponding photographic plate reveals that the object is of peculiar nature, possibly interacting with its neighbours. Its morphological type is given as uncertain in several other catalogues, ranging between S0 and Scd.

[FIGURE] Fig. 2. Velocity dispersions for galaxies of different absolute magnitudes.
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© European Southern Observatory (ESO) 1997

Online publication: July 8, 1998