  Thermalization of an isolated quantum system has been a nontrivial problem
since the early days of quantum mechanics. In generic isolated quantum systems,
nonequilibrium dynamics is expected to result in thermalization, indicating the
emergence of statistical mechanics from quantum dynamics. However, what feature
of a many-body quantum system facilitates quantum thermalization is still not
well understood. Recent experimental advancements have shown that entanglement
may act as a thermalizing agent, not universally but particularly. Here, we
theoretically show that the thermal averages of an observable in an isolated
many-body quantum system with a large number of degrees of freedom emerge from
the entangled energy eigenstates of the system. In particular, we show that the
expectation values of an observable in entangled energy eigenstates and its
marginals are equivalent to the microcanonical and canonical averages of the
observable.
