Optical dropout galaxies lensed by the cluster A2667
  We investigate the nature and the physical properties of z, Y and J-dropout
galaxies selected behind the lensing cluster A2667. This field is part of our
project aimed at identifying z~7-10 candidates accessible to spectroscopic
studies, based on deep photometry with ESO/VLT HAWK-I and FORS2 (zYJH and
Ks-band images, AB(3 sigma)~26-27) on a sample of lensing clusters extracted
from our multi-wavelength combined surveys with SPITZER, HST, and Herschel. In
this paper we focus on the complete Y and J-dropout sample, as well as the
bright z-dropouts fulfilling the selection criteria by Capak et al. (2011). 10
candidates are selected within the common field of ~33 arcmin2 (effective area
once corrected for contamination and lensing dilution). All of them are
detected in H and Ks bands in addition to J and/or IRAC 3.6/4.5, with
H(AB)~23.4 to 25.2, and have modest magnification factors. Although best-fit
photometric redshifts place all these candidates at high-z, the contamination
by low-z interlopers is estimated at 50-75% level based on previous studies,
and the comparison with the blank-field WIRCAM Ultra-Deep Survey (WUDS). The
same result is obtained when photometric redshifts include a luminosity prior,
allowing us to remove half of the original sample as likely z~1.7-3 interlopers
with young stellar pulations and strong extinction. Two additional sources
among the remaining sample could be identified at low-z based on a detection at
24 microns and on the HST z_850 band. These low-z interlopers are not well
described by current templates given the large break, and cannot be easily
identified based solely on optical and near-IR photometry. Given the estimated
dust extinction and high SFRs, some of them could be also detected in the IR or
sub-mm bands. After correction for likely contaminants, the observed counts at
z>7.5 seem to be in agreement with an evolving LF. (abridged)
