  A class of autonomous quantum heat baths satisfying the eigenstate
thermalization hypothesis (ETH) criteria is proposed. We show that such systems
are expected to cause thermal relaxation of much smaller quantum systems
coupled to one of the baths local observables. The process of thermalization is
examined through residual fluctuations of local observables of the bath around
their thermal values predicted by ETH. It is shown that such fluctuations
perturb the small quantum system causing its decoherence to the thermal state.
As an example, we investigate theoretically and numerically thermalization of a
qubit coupled to a realistic ETH quantum heat bath.
