Two multiple-imaged z=4.05 galaxies in the cluster-lens Abell 2390

We present the first results on the identification and study of very distan t field galaxies in the core of cluster-lenses, using a selection criterium based on both lens modelling and photometric redshifts. We concentrate on two multiple-imaged sources at z = 4.05 in the cluster Abell 2390. The two objects presented in this paper, namely H3 (cusp are) and H5 (fold are), we re identified through lens modelling as multiple images of high-redshift so urces at z greater than or similar to 3.5 (Kneib et al. 1999). We confirm t he excellent agreement between this identification and both their photometr ic redshifts and morphologies. Our CFHT/WHT program for a systematic redshi ft survey of arcs in clusters has allowed to obtain a set of spectra on thr ee different images at z similar to 4: the brightest image of H3, whose red shift was already confirmed by Frye & Broadhurst (1998), and the two bright est images of H5. The later is then confirmed spectroscopically as a multip le image, giving a strong support to the lens model. The main feature in ea ch of these spectra is a strong emission line, identified as Ly a, leading to z. = 4.05 for both H3 and H5. The spectrophotometric properties of these galaxies are studied, in particular the degeneracy in the parameter-space defined by the SFR type, age, metallicity and reddening. H3 and H5 are intr insically bright and clumpy galaxies (M-B* to M-B* -2 magnitudes), located similar to 100h(50)(-1) kpc apart on the source plane, with mean metallicit ies compatible with a fraction of solar or even solar values. These results seem to favour a hierarchical merging scenario, when we are seeing a relat ively evolved phase in these two z similar to 4 objects, with stars forming locally and efficiently.