  We provide a joint theoretical and experimental investigation of the
temperature dependence of the collective oscillations of first sound nature
exhibited by a highly elongated harmonically trapped Fermi gas at unitarity,
including the region below the critical temperature for superfluidity.
Differently from the lowest axial breathing mode, the hydrodynamic frequencies
of the higher nodal excitations show a temperature dependence, which is
calculated starting from Landau two-fluid theory and using the available
experimental knowledge of the equation of state. The experimental results agree
with high accuracy with the predictions of theory and provide the first
evidence for the temperature dependence of the collective frequencies near the
superfluid phase transition.
