  Notable attention has been devoted to the problem of thermalization of
observables in isolated quantum setups by individual eigenstates. Here, we
approach this issue from an open quantum system perspective, by considering
isolated setups which consists of a small system coupled to a macroscopic bath.
We argue that in such a case, even in fully integrable models, the system
observables exhibit thermalization when the system-bath setup is in a typical
eigenstate of its Hamiltonian (which is called weak eigenstate thermalization).
This occurs unless thermalization is suppressed by localization due to strong
coupling. We illustrate our claim for a single fermionic level coupled to a
noninteracting fermionic bath. We further show that upon quenching the system
Hamiltonian, the system occupancy typically relaxes to the thermal value
corresponding to the new Hamiltonian. Finally, we demonstrate that system
thermalization also arises for a system coupled to a bath initialized in a
typical eigenstate of its Hamiltonian. Our findings suggest that chaos and
nonintegrability are not the sole drivers of thermalization and complementary
approaches are needed to offer a more comprehensive understanding of how
statistical mechanics emerges.
