Fossil group origins X. Velocity segregation in fossil systems
  We want to study how the velocity segregation and the radial profile of the
velocity dispersion depend on the prominence of the brightest cluster galaxies
(BCGs). We divide a sample of 102 clusters and groups of galaxies into four
bins of magnitude gap between the two brightest cluster members. We then
compute the velocity segregation in bins of absolute and relative magnitudes.
Moreover, for each bin of magnitude gap we compute the radial profile of the
velocity dispersion. When using absolute magnitudes, the segregation in
velocity is limited to the two brightest bins and no significant difference is
found for different magnitude gaps. However, when we use relative magnitudes, a
trend appears in the brightest bin: the larger the magnitude gap, the larger
the velocity segregation. We also show that this trend is mainly due to the
presence, in the brightest bin, of satellite galaxies in systems with small
magnitude gaps: in fact, if we study separately central galaxies and
satellites, this trend is mitigated and central galaxies are more segregated
than satellites for any magnitude gap. A similar result is found in the radial
velocity dispersion profiles: a trend is visible in central regions (where the
BCGs dominate) but, if we analyse the profile using satellites alone, the trend
disappears. In the latter case, the shape of the velocity dispersion profile in
the centre of systems with different magnitude gaps show three types of
behaviours: systems with the smallest magnitude gaps have an almost flat
profile from the centre to the external regions; systems with the largest
magnitude gaps show a monothonical growth from the low values of the central
part to the flat ones in the external regions; finally, systems with $1.0 <
\Delta m_{12} \le 1.5$ show a profile that peaks in the centres and then
decreases towards the external regions. We suggest that two mechanisms could be
respons....
