  Several authors have found a statistically significant excess of galaxies
with higher redshifts relative to the group centre, so-called discordant
redshifts, in particular in groups where the brightest galaxy, identified in
apparent magnitudes, is a spiral. Our aim is to explain the observed redshift
excess. We use a semi-analytical galaxy catalogue constructed from the
Millennium Simulation to study redshift asymmetries in spiral-dominated groups
in the Lambda cold dark matter (LambdaCDM) cosmology. We show that discordant
redshifts in small galaxy groups arise when these groups are gravitationally
unbound and the dominant galaxy of the group is misidentified. The redshift
excess is especially significant when the apparently brightest galaxy can be
identified as a spiral, in full agreement with observations. On the other hand,
the groups that are gravitationally bound do not show a significant redshift
asymmetry. When the dominant members of groups in mock catalogues are
identified by using the absolute B-band magnitudes, our results show a small
blueshift excess. This result is due to the magnitude limited observations that
miss the faint background galaxies in groups. When the group centre is not
correctly identified it may cause the major part of the observed redshift
excess. If the group is also gravitationally unbound, the level of the redshift
excess becomes as high as in observations. There is no need to introduce any
"anomalous" redshift mechanism to explain the observed redshift excess.
Further, as the Friends-of-Friends percolation algorithm picks out the
expanding parts of groups, in addition to the gravitationally bound group
cores, group catalogues constructed in this way cannot be used as if the groups
are purely bound systems.
