  We report a new scanning nanomechanical noise microscopy platform enabling to
both heat and acquire the fluctuations of mechanical nanostructures with
nanometric resolution. We use this platform to image the thermally activated
nanomechanical dynamics of a model system consisting of a $40\,\mathrm{nm}$
diameter single-defect nanowire, while scanning a localized heat source across
its surface. We develop a thermal backaction model, which we use to demonstrate
a close connection between the structure of the nanowire, its thermal response,
its dissipation and its fluctuations. We notably show that the defect behaves
as a single fluctuation hub, whose e-beam excitation yields a far
off-equilibrium vibrational state, largely dominated by the quantum
fluctuations of the heating source. Our platform is of interest for future
quantitative investigation of fundamental nanoscale dynamical phenomena, and
appears as a new playground for investigating quantum thermodynamics in the
strongly dissipative regime and at room temperature.
