  Rydberg atoms immersed in a Bose-Einstein condensate interact with the
quantum gas via electron-atom and ion-atom interaction. To suppress the
typically dominant electron-neutral interaction, Rydberg states with principal
quantum number up to $n = 190$ are excited from a dense and tightly trapped
micron-sized condensate. This allows us to explore a regime where the Rydberg
orbit exceeds the size of the atomic sample by far. In this case, a detailed
lineshape analysis of the Rydberg excitation spectrum provides clear evidence
for ion-atom interaction at temperatures well below a microkelvin. Our results
may open up ways to enter the quantum regime of ion-atom scattering for the
exploration of charged quantum impurities and associated polaron physics.
