We have determined detailed radio spectra for 26 compact sources in the
starburst nucleus of M82, between 74 and 1.3 cm. Seventeen show low-frequency
turnovers. One other has a thermal emission spectrum, and we identify it as an
HII region. The low frequency turnovers are due to absorption by the
interstellar gas in M82. New information on the AGN candidate 44.01+595, shows
it to have a non-thermal falling powerlaw spectrum at the highest frequencies,
and that it is strongly absorbed below 2 GHz. We derive large magnetic fields
in the supernova remnants, of order 1-2 milliGauss, hence large pressures in
the sources suggest that the brightest ones are either expanding or are
strongly confined by a dense interstellar medium. From the largest source in
our sample, we derive a supernova rate of 0.016 SN/yr.Comment: 19 pages, 7 tables, 29 figures, LaTeX, requires AAS macros v. 4.0. To
appear in ApJ July 20, 199