Discovery of a faint R-band drop-out: a strongly reddened lensed star forming galaxy at z=1.68

We report the discovery of an unusual emission line galaxy at redshift z=1.68 found from near-IR imaging and spectroscopic observations with ISAAC/VLT of a faint gravitationally lensed R-band drop-out behind the lensing cluster Abell 1835. From the emission lines of [O_III], and H_beta this galaxy shows a moderate to high excitation and line equivalent widths typical of relatively metal-poor HII galaxies. Its apparent J magnitude translates to an absolute B-band magnitude M_B ~ -16.4 once corrected for a gravitational magnification of 1.8 magnitudes. This makes it one of the faintest intermediate redshift galaxiesknown so far. From the presence of emission lines and the available VRIJHK photometry we determine constraints on its dominant stellar population. The only viable fit found is for a fairly young (~ 6-9 Myr) burst suffering from a considerable extinction (A_v ~ 1.2-1.8 mag). We find that this object resembles strongly HII galaxies and intermediate redshift compact emission line galaxies, albeit suffering from a larger extinction than usual. We also discuss the possible contamination introduced by such R-band drop-out galaxies in searches for z > 5 galaxies.