  We report first principles characterization of the structural and electronic
properties of (SrTiO$_{3}$)$_{5}$/(SrRuO$_{3}$)$_{1}$ superlattices. We show
that the system exhibits a spin-polarized two-dimensional electron gas,
extremely confined to the 4$d$ orbitals of Ru in the SrRuO$_{3}$ layer. Every
interface in the superlattice behaves as a minority-spin half-metal
ferromagnet, with a magnetic moment of $\mu$ = 2.0 $\mu_{\rm B}$/SrRuO$_{3}$
unit. The shape of the electronic density of states, half metallicity and
magnetism are explained in terms of a simplified tight-binding model,
considering only the $t_{2g}$ orbitals plus (i) the bi-dimensionality of the
system, and (ii) strong electron correlations.
