Two fossil groups of galaxies at z~0.4 in the COSMOS: accelerated
  stellar-mass build-up, different progenitors
  We report on 2 fossil groups of galaxies at z=0.425 and 0.372 discovered in
the Cosmic Evolution Survey (COSMOS) area. Selected as X-ray extended sources,
they have total masses (M_200) of 1.9(+/-0.41)E13 and 9.5(+/-0.42)E13 M_sun,
respectively, as obtained from a recent X-ray luminosity-mass scaling relation.
The lower mass system appears isolated, whereas the other sits in a well-known
large-scale structure (LSS) populated by 27 other X-ray emitting groups. The
identification as fossil is based on the i-band photometry of all the galaxies
with a photo-z consistent with that of the group at the 2-sigma confidence
level and within a projected group-centric distance equal to 0.5R_200, and
i_AB<=22.5-mag limited spectroscopy. Both fossil groups exhibit high
stellar-to-total mass ratios compared to all the X-ray selected groups of
similar mass at 0.3<=z<=0.5 in the COSMOS. At variance with the composite
galaxy stellar mass functions (GSMFs) of similarly massive systems, both fossil
group GSMFs are dominated by passively evolving galaxies down to M^stars~1E10
M_sun (according to the galaxy broad-band spectral energy distributions). The
relative lack of star-forming galaxies with 1E10<=M^stars<=1E11 M_sun is
confirmed by the galaxy distribution in the b-r vs i color-magnitude diagram.
Hence, the 2 fossil groups appear as more mature than the coeval, similarly
massive groups. Their overall star formation activity ended rapidly after an
accelerated build up of the total stellar mass; no significant infall of
galaxies with M^stars>=1E10 M_sun took place in the last 3 to 6 Gyr. This
similarity holds although the 2 fossil groups are embedded in two very
different density environments of the LSS, which suggests that their galaxy
populations were shaped by processes that do not depend on the LSS. However,
their progenitors may do so. ...
