  We discuss relaxation in bosonic and fermionic many-particle systems. For
integrable systems, the time evolution can cause a dephasing effect, leading
for finite subsystems to certain steady states. We give an explicit derivation
of those steady subsystem states and devise sufficient prerequisites for the
dephasing to take place. We also find simple scenarios, in which dephasing is
ineffective and discuss the dependence on dimensionality and criticality. It
follows further that, after a quench of system parameters, bipartite
entanglement entropy will become extensive. This provides a way of creating
strong entanglement in a controlled fashion.
