SHARDS: stellar populations and star formation histories of a
  mass-selected sample of 0.65<z<1.1 galaxies
  We report on results from the analysis of a stellar mass-selected (log
M*>9.0) sample of 1644 galaxies at 0.65<z<1.1 with ultra-deep (m<26.5) optical
medium-band (R~50) photometry from the Survey for High-z Absorption Red and
Dead Sources (SHARDS). The spectral resolution of SHARDS allows us to
consistently measure the strength of the 4000 Angstrom spectral break
[Dn(4000), an excellent age indicator for the stellar populations of quiescent
galaxies] for all galaxies at z~0.9 down to log M*9. The Dn(4000) index cannot
be resolved from broad-band photometry, and measurements from optical
spectroscopic surveys are typically limited to galaxies at least x10 more
massive. When combined with the rest-frame U-V colour, Dn(4000) provides a
powerful diagnostic of the extinction affecting the stellar population that is
relatively insensitive to degeneracies with age, metallicity or star formation
history. We use this novel approach to estimate de-reddened colours and
light-weighted stellar ages for individual sources. We explore the
relationships linking stellar mass, (U-V), and Dn(4000) for the sources in the
sample, and compare them to those found in local galaxies. The main results
are: a) both Dn(4000) and (U-V) correlate with M*. The dispersion in Dn(4000)
values at a given M* increases with M*, while the dispersion for (U-V)
decreases due to the higher average extinction prevalent in massive
star-forming galaxies. b) for massive galaxies, we find a smooth transition
between the blue cloud and red sequence in the intrinsic U-V colour, in
contrast with other recent results. c) at a fixed stellar age, we find a
positive correlation between extinction and stellar mass. d) the fraction of
sources with declining or halted star formation increases steeply with the
stellar mass, from ~5% at log M*~9.0-9.5 to ~80% at log M*>11, in agreement
with downsizing scenarios.
