Spectral detection of multiple stellar populations in z~1 early-type
  galaxies
  We present a spectroscopic analysis based on measurements of two mainly
age-dependent spectrophotometric indices in the 4000A rest frame region, i.e.
H+K(CaII) and Delta4000, for a sample of 15 early-type galaxies (ETGs) at 0.7 <
z_{spec} < 1.1, morphologically selected in the GOODS-South field. Ages derived
from the two different indices by means of the comparison with stellar
population synthesis models, are not consistent with each other for at least
nine galaxies (60 per cent of the sample), while for the remaining six
galaxies, the ages derived from their global spectral energy distribution (SED)
fitting are not consistent with those derived from the two indices. We then
hypothesized that the stellar content of many galaxies is made of two stellar
components with different ages. The double-component analysis, performed by
taking into account both the index values and the observed SED, fully explains
the observational data and improves the results of the standard one-component
SED fitting in 9 out of the 15 objects, i.e. those for which the two indices
point towards two different ages. In all of them, the bulk of the mass belongs
to rather evolved stars, while a small mass fraction is many Gyr younger. In
some cases, thanks to the sensitivity of the H+K(CaII) index, we find that the
minor younger component reveals signs of recent star formation. The
distribution of the ages of the younger stellar components appears uniformly in
time and this suggests that small amounts of star formation could be common
during the evolution of high-z ETGs. We argue the possibility that these new
star formation episodes could be frequently triggered by internal causes due to
the presence of small gas reservoir.
