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

TL;DR

This study identifies and confirms two high-redshift galaxies at z=4.05 in the cluster Abell 2390 using lens modeling, photometric redshifts, and spectroscopy, revealing their properties and supporting hierarchical galaxy formation.

Contribution

First spectroscopic confirmation of two multiple-imaged galaxies at z=4.05 in a cluster lens, combining lens modeling, photometry, and spectroscopy to study their properties.

Findings

Confirmed redshift z=4.05 for both galaxies via Ly-alpha emission.

Galaxies are bright, clumpy, with near-solar metallicities.

Results support hierarchical merging and early star formation in the universe.

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 A2390. The 2 objects presented in this paper, namely H3 and H5, were identified through lens modelling as multiple images of high-redshift sources at z>3.5. 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 3 different images at z~4: the brightest image of H3, which 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 sources located ~100 kpc part on the source plane, with mean metallicities compatible with a fraction of solar or even solar values. All these results seem to favour a hierarchical merging scenario, where we are actually seeing a relatively advanced step for these 2 z~4 objects, with stars forming locally and efficiently from a preenriched gas.