  New BeppoSAX observations of the nearby prototypical starburst galaxies NGC
253 and M82 are presented. A companion paper (Cappi et al.
1998;astro-ph/9809325) shows that the hard (2-10 keV) spectrum of both
galaxies, extracted from the source central regions, is best described by a
thermal emission model with kT ~ 6-9 keV and abundances ~ 0.1-0.3 solar. The
spatial analysis yields clear evidence that this emission is extended in NGC
253, and possibly also in M82. This quite clearly rules out a LLAGN as the main
responsible for their hard X-ray emission. Significant contribution from
point-sources (i.e. X-ray binaries (XRBs) and Supernovae Remnants (SNRs))
cannot be excluded; neither can we at present reliably estimate the level of
Compton emission. However, we argue that such contributions shouldn't affect
our main conclusion, i.e., that the BeppoSAX results show, altogether,
compelling evidence for the existence of a very hot, metal-poor interstellar
plasma in both galaxies.
