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

Abstract

We present the first results on the identification and study of very distant 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 arc) and H5 (fold arc), were identified through lens modelling as multiple images of high-redshift sources at z >~ 3.5 (Kneib et al. 1999). We confirm the excellent agreement between this identification and both their photometric redshifts and morphologies. Our CFHT/WHT program for a systematic redshift survey of arcs in clusters has allowed to obtain a set of spectra on three different images at z ~ 4 : the brightest image of H3, whose redshift was already confirmed by Frye & Broadhurst (1998), and the two brightest images of H5. The later is then confirmed spectroscopically as a multiple image, giving a strong support to the lens model. The main feature in each of these spectra is a strong emission line, identified as Ly alpha , 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 intrinsically bright and clumpy galaxies (M(*) _B to M(*) _B -2 magnitudes), located ~ 100 h(-1}_{50) kpc apart on the source plane, with mean metallicities compatible with a fraction of solar or even solar values. These results seem to favour a hierarchical merging scenario, where we are seeing a relatively evolved phase in these two z ~ 4 objects, with stars forming locally and efficiently. Based on observations collected with the Canada-France-Hawaii Telescope (CFHT), the William Herschel Telescope (WHT) and the Hubble Space Telescope (HST).