The ($NUV-r$) vs. $M_r$ as a tracer of early-type galaxies evolution in
  USGC U376 and LGG 225 groups
  With the aim of tracing back the evolution of galaxies in nearby groups we
use smooth particle hydrodynamical (SPH) simulations with chemo-photometric
implementation. Here we focus on the evolution of the early-type members (Es
and S0s, ETGs hereafter) in two groups, USGC U376 and LGG 225, both in the Leo
cloud. We use the near-UV (NUV)-optical rest-frame $NUV-r$ versus $M_r$ color
magnitude diagram to follow their evolution, from the blue cloud (BC) to the
red sequence (RS), through the green valley (GV). ETGs brighter than $M_r=-21$
mag are older than 13 Gyr and spend up to 10 Gyr of their overall evolutionary
time in the BC before they reach the RS migrating through the GV. Fainter ETGs
are younger, of about 2 Gyr on average, and evolve for about 7-8 Gyr along the
BC. The turn-off occurs at z ~ 0.3-0.4. Therefore these ETGs spend up to 3-5
Gyr crossing the GV; UGC 06324, the faintest ETG in the sample, still is in the
GV. The mechanism driving their evolution is gravitational, due to merging
and/or interaction. Our SPH simulations suggest that ETG members of these
groups evolved toward the RS before and during the group collapse phase. This
result is consistent with the dynamical analysis of both groups showing that
they are not yet virialized.
