SHARDS: an optical spectro-photometric survey of distant galaxies
  (Abridged) We present the Survey for High-z Absorption Red and Dead Sources
(SHARDS), an ESO/GTC Large Program carried out with GTC/OSIRIS. SHARDS is an
ultra-deep optical spectro-photometric survey of the GOODS-N field (130
arcmin^2) at wavelengths 500 to 950 nm and using 24 contiguous medium-band
filters (spectral resolution R 50). The data reach 26.5 mag (>3-sigma level)
with sub-arcsec seeing in all bands. SHARDS main goal is obtaining accurate
physical properties of interm- and high-z galaxies using well-sampled optical
SEDs with sufficient spectral resolution to measure absorption and emission
features. Among the different populations of high-z galaxies, SHARDS principal
targets are massive quiescent galaxies at z>1. In this paper, we outline the
observational strategy and include a detailed discussion of the special
reduction and calibration procedures applied to the GTC/OSIRIS data. We present
science demonstration results about the detection and study of emission-line
galaxies (star-forming and AGN) at z=0-5. We also analyze the SEDs for a sample
of 27 quiescent massive galaxies at 1.0<z<1.4. We discuss on the improvements
introduced by the SHARDS dataset in the analysis of their SFH and stellar
properties. We discuss the systematics arising from the use of different
stellar population libraries. We find that the UV-to-MIR SEDs of the massive
quiescent galaxies at z=1.0-1.5 are well described by an exponential decaying
SFH with scale tau=100-200 Myr, age 1.5-2.0 Gyr, solar or slightly sub-solar
metallicity, and moderate extinction, A(V)~0.5 mag. We also find that galaxies
with masses above M* are typically older than lighter galaxies, as expected in
a downsizing scenario of galaxy formation. This trend is, however, model
dependent, i.e., it is significantly more evident in the results obtained with
some stellar population synthesis libraries and almost absent in others.
