  We directly observe the hydrodynamic linear response of a unitary Fermi gas
confined in a box potential and subject to a spatially periodic optical
potential that is translated into the cloud at speeds ranging from subsonic to
supersonic. We show that the time-dependent change of the density profile is
sensitive to the thermal conductivity, which controls the relaxation rate of
the temperature gradients and hence the responses arising from adiabatic and
isothermal compression.
