Localization for branching random walks in random environment

We consider branching random walks in d-dimensional integer lattice with time-space i.i.d. offspring distributions. This model is known to exhibit a phase transition: If d >= 3 and the environment is "not too random", then, the total Population grows as fast as its expectation with strictly positive probability. If, on the other hand, d <= 2, or the environment is "random enough", then the total population grows strictly slower than its expectation almost surely. We show the equivalence between the slow population growth and a natural localization property in terms of "replica overlap". We also prove a certain stronger localization property, whenever the total population grows strictly slower than its expectation almost surely. (C) 2008 Elsevier B.V. All rights reserved.