Injection spectroscopy of impurities in a thermal Bose gas

We spectroscopically study mobile impurities immersed in a homogeneous thermal Bose gas, varying the bath temperature and the strength of impurity-bath interactions. For weak interactions, we observe a mean-field shift of the impurity energy proportional to the scattering length, with the spectral width dominated by thermal fluctuations. For strong interactions, the energy shift is suppressed; at high temperature, the width is set by unitary-limited two-body collisions, but many-body effects become important in the quantum-degenerate regime. We contrast the spectra with those of an impurity immersed in a Bose-Einstein condensate and show good agreement with theoretical expectations.