  We present a many-body description for two-component ultracold bosonic gases
when one of the species is in the weakly interacting regime and the other is
either weakly or strongly interacting. In the one-dimensional limit the latter
case system is a hybrid in which a Tonks-Girardeau gas is immersed in a
Bose-Einstein condensate, which is an example of a new class of quantum system
involving a tunable, superfluid environment. We describe the process of phase
separation microscopically and semiclassically in both situations and show that
the quantum correlations are maintained in the separated phase.
