Lensed galaxies in Abell 370. I. Modeling the number counts and redshift distribution of background sources

We present new observations of the cluster-lens Abell 370: a deep HST/WFPC2 F675W image and ESO 3.6m spectroscopy of faint galaxies. These observations shed new light on the statistical properties of faint lensed galaxies. In particular, we spectroscopically confirm the multiple image nature of the B2-B3 gravitational pair (\cite{kneib93}), and determine a redshift of z=0.806+/- 0.002 which is in very good agreement with earlier predictions. A refined mass model of the cluster core (that includes cluster galaxy halos) is presented, based on a number of newly identified multiple images. Following Bézecourt et al. (1998a), we combine the new cluster mass model with a spectrophotometric prescription for galaxy evolution to predict the arclets number counts and redshift distribution in the HST image. In particular, the ellipticity distribution of background sources is taken into account, in order to properly estimate the statistical number and redshift distribution of arclets. We show that the redshift distribution of arclets, and particularly its high redshift tail can be used as a strong constraint to disentangle different galaxy evolution scenarios. A hierarchical model which includes number density evolution is favored by our analysis. Finally, we compute the depletion curves for the faint galaxy number counts and discuss their wavelength dependence. Based on observations with the NASA/ESA Hubble Space Telescope obtained from the data archive at the Space Telescope European Coordinating Facility and with the 3.6m Telescope of the European Southern Observatory, La Silla, Chile and the Canada-France-Hawaii Telescope at Mauna Kea, Hawaii, USA.