  The radio source Sagittarius A* (Sgr A*), is thought to be a supermassive
black hole located at the centre of our Galaxy, that is accreting gas from the
surrounding region. Using the high inferred accretion rates, however, standard
accretion models are unable to explain the low luminosity and observed spectrum
from Sgr A*. A new accretion model has been proposed - an advection dominated
accretion flow (ADAF), where most of the accretion energy is stored in the gas
and lost into the black hole. The gas therefore has a two-temperature structure
with the protons being much hotter than the electrons. The model explains the
low luminosity from Sgr A* and most of the millimeter to hard X-ray spectrum,
but has had serious difficulty in agreeing with the low energy radio
observations. Here we report an emission process associated with the protons
that naturally resolves the observed discrepancy. This provides, for the first
time, observational evidence for a two-temperature plasma in hot accretion
flows, and gives strong support to the idea that an ADAF does accrete onto a
2.5*10^6 solar mass black hole at the Galactic Centre.
