  We use single or few Cs atoms as thermometer for an ultracold, thermal Rb
cloud. Observing the thermometer atoms' thermalization with the cold gas using
spatially resolved fluorescence detection, we find an interesting situation,
where a fraction of thermometer atoms thermalizes with the cloud while the
other fraction remains unaffected. We compare release-recapture measurements of
the thermometer atoms to Monte-Carlo simulations while correcting for the
non-thermalized fraction, and recover the cold cloud's temperature. The
temperatures obtained are verified by independent time-of-flight measurements
of the cold cloud's temperature. We also check the reliability of our
simulations by first numerically modelling the unperturbed in-trap motion of
single atoms in absence of the cold cloud, and second by performing
release-recapture thermometry on the cold cloud itself. Our findings pave the
way for local temperature probing of quantum systems in non-equilibrium
situations.
