Context. The identification and study of the first galaxies remains one of the most exciting topics in observational cosmology. The determination of the best possible observing strategies is a very important choice in order to build up a representative sample of spectroscopically confirmed sources at high-z (z > ∼ 7), beyond the limits of present-day observations. Aims. This paper is intended to precisely adress the relative efficiency of lensing and blank fields in the identification and study of galaxies at 6 < ∼ z < ∼ 12.Methods. The detection efficiency and field-to-field variance are estimated from direct simulations of both blank and lensing fields observations. Present known luminosity functions in the UV are used to determine the expected distribution and properties of distant samples at z > ∼ 6 for a variety of survey configurations. Different models for well known lensing clusters are used to simulate in details the magnification and dilution effects on the backgound distant population of galaxies. Results. The presence of a strong-lensing cluster along the line of sight has a dramatic effect on the number of observed sources, with a positive magnification bias in typical ground-based "shallow" surveys (AB < ∼ 25.5). The positive magnification bias increases with the redshift of sources and decreases with both depth of the survey and the size of the surveyed area. The maximum efficiency is reached for lensing clusters at z ∼ 0.1−0.3. Observing blank fields in shallow surveys is particularly inefficient as compared to lensing fields if the UV LF for LBGs is strongly evolving at z > ∼ 7. Also in this case, the number of z ≥ 8 sources expected at the typical depth of JWST (AB ∼ 28−29) is much higher in lensing than in blank fields (e.g. a factor of ∼10 for AB < ∼ 28). All these results have been obtained assuming that number counts derived in clusters are not dominated by sources below the limiting surface brightness of observations, which in turn depends on the reliability of the usual scalings applied to the size of high-z sources. Conclusions. Blank field surveys with a large field of view are needed to prove the bright end of the LF at z > ∼ 6−7, whereas lensing clusters are particularly useful for exploring the mid to faint end of the LF.

Context. We investigate the nature and the physical properties of ten z, Y, and J-dropout galaxies selected in the field of the lensing cluster A2667. Aims. This cluster is part of our project aimed at obtaining deep photometry at ∼0.8-2.5 microns with ESO/VLT HAWK-I and FORS2 on a representative sample of lensing clusters extracted from our multi-wavelength combined surveys with Spitzer, HST, and Herschel. The goal is to identify a sample of redshift z ∼ 7-10 candidates accessible to detailed spectroscopic studies. Methods. Our selection is performed using the usual dropout technique based on deep I, z, Y, J, H, and Ks-band images (AB ∼ 26-27, 3σ), targeting z 7.5 galaxy candidates. We also include IRAC data between 3.6 and 8 μm, and MIPS 24 μm when available. In this paper, we concentrate on the complete Y and J-dropout sample among the sources detected with a high signal-to-noise ratio in both H and Ks bands, as well as the bright z-dropout sources fulfilling the color and magnitude selection criteria adopted by Capak and collaborators. SED-fitting and photometric redshifts were used to constrain the nature and the properties of these candidates. Results. Ten photometric candidates are selected within the ∼7 × 7 HAWK-I field of view (∼33 arcmin 2 of effective area once corrected for contamination and lensing dilution at z ∼ 7-10). All of these are detected in H and Ks bands in addition to J and/or IRAC 3.6 μm/4.5 μm images, with H AB ranging from 23.4 to 25.2, and have modest magnification factors between 1.1 and 1.4. Although best-fit photometric redshifts are obtained at high-z for all these candidates, the contamination by low-z interlopers is expected to be in the range ∼50-75% based on previous studies, and on comparison with the blank-field WIRCAM Ultra-Deep Survey (WUDS). The same result is obtained when photometric redshifts are computed using a luminosity prior, allowing us to remove half of the original sample. Among the remaining galaxies, two additional sources could be identified as low-z interlopers based on a detection at 24 μm and the HST z 850 band. These low-z interlopers are not accurately described by current spectral templates given the large break, and cannot be easily identified based on broad-band photometry in the optical and near-IR domains alone. A good fit at z ∼ 1.7-3 is obtained when assuming a young stellar population together with a strong extinction. Given the estimated dust extinction and high SFRs, some of them could also be detected in the IR or sub-mm bands. Conclusions. After correction for contaminants, the observed number counts at z 7.5 seem to agree with expectations for an evolving LF, and be inconsistent with a constant LF since z ∼ 4. At least one and up to three candidates in this sample are expected to be genuine high-z sources, although spectroscopy is still needed to confirm this.