  Heat transport is a fundamental property of all physical systems and can
serve as a fingerprint identifying different states of matter. In a normal
liquid a hot spot diffuses while in a superfluid heat propagates as a wave
called second sound. Despite its importance for understanding quantum
materials, direct imaging of heat transport is challenging, and one usually
resorts to detecting secondary effects, such as changes in density or pressure.
Here we establish thermography of a strongly interacting atomic Fermi gas, a
paradigmatic system whose properties relate to strongly correlated electrons,
nuclear matter and neutron stars. Just as the color of a glowing metal reveals
its temperature, the radiofrequency spectrum of the interacting Fermi gas
provides spatially resolved thermometry with sub-nanokelvin resolution. The
superfluid phase transition is directly observed as the sudden change from
thermal diffusion to second sound propagation, and is accompanied by a peak in
the second sound diffusivity. The method yields the full heat and density
response of the strongly interacting Fermi gas, and therefore all defining
properties of Landau's two-fluid hydrodynamics. Our measurements serve as a
benchmark for theories of transport in strongly interacting fermionic matter.
