  We investigate the effect of a hybrid electron population, consisting of both
thermal and non-thermal particles, on the synchrotron spectrum, image size, and
image shape of a hot accretion flow onto a supermassive black hole. We find two
universal features in the emitted synchrotron spectrum: (i) a prominent
shoulder at low (< 10^11 Hz) frequencies that is weakly dependent on the shape
of the electron energy distribution, and (ii) an extended tail of emission at
high (> 10^13 Hz) frequencies whose spectral slope depends on the slope of the
power-law energy distribution of the electrons. In the low-frequency shoulder,
the luminosity can be up to two orders of magnitude greater than with a purely
thermal plasma even if only a small fraction (< 1%) of the steady-state
electron energy is in the non-thermal electrons. We apply the hybrid model to
the Galactic center source, Sgr A*. The observed radio and IR spectra imply
that at most 1% of the steady-state electron energy is present in a power-law
tail in this source. This corresponds to no more than 10% of the electron
energy injected into the non-thermal electrons and hence 90% into the thermal
electrons. We show that such a hybrid distribution can be sustained in the flow
because thermalization via Coulomb collisions and synchrotron self-absorption
are both inefficient. The presence of non-thermal electrons enlarges the size
of the radio image at low frequencies and alters the frequency dependence of
the brightness temperature. A purely thermal electron distributions produces a
sharp-edged image while a hybrid distribution causes strong limb brightening.
These effects can be seen up to frequencies ~10^11 Hz and are accessible to
radio interferometers.
