  If and how an isolated quantum system thermalizes despite its unitary time
evolution is a long-standing, open problem of many-body physics. The eigenstate
thermalization hypothesis (ETH) postulates that thermalization happens at the
level of individual eigenstates of a system's Hamiltonian. However, the ETH
requires stringent conditions to be validated, and it does not address how the
thermal state is reached dynamically from an inital non-equilibrium state. We
consider a Bose-Einstein condensate (BEC) trapped in a double-well potential
with an initial population imbalance. We find that the system thermalizes
although the initial conditions violate the ETH requirements. We identify three
dynamical regimes. After an initial regime of undamped Josephson oscillations,
the subsystem of incoherent excitations or quasiparticles (QP) becomes strongly
coupled to the BEC subsystem by means of a dynamically generated, parametric
resonance. When the energy stored in the QP system reaches its maximum, the
number of QPs becomes effectively constant, and the system enters a
quasi-hydrodynamic regime where the two subsystems are weakly coupled. In this
final regime the BEC acts as a grand-canonical heat reservoir for the QP system
(and vice versa), resulting in thermalization. We term this mechanism dynamical
bath generation (DBG).
