H-II REGIONS IN M82 - HIGH-RESOLUTION MILLIMETER CONTINUUM OBSERVATIONS

We present an aperture synthesis image of 92 GHz (lambda = 3.3 mm) continuum emission in the central region of the starburst galaxy M82, obtained with the Hat Creek millimeter array. The emission extends approximately 35" along a warped ridge. The image is remarkably similar to those made at longer wavelengths at comparable resolution. The distribution is also similar to far-infrared profiles. The total lambda = 3.3 mm flux density is 0.59 +\- 0.09 Jy, and the emission is spatially confined to a region between the lobes of bright molecular emission. Continuum spectra spanning a range of more than 60 in frequency with effective 6" apertures (the resolution at 92 GHz) were obtained by combining the 92 GHz data with new VLA 22 and 15 GHz data and with previously published VLA 5 and 1.5 GHZ data. A simple model fit to the data is used to estimate the relative contributions of the thermal and nonthermal emission to the continuum spectra. The modeling results suggest that the spectral index of the nonthermal emission in the core is alpha-approximately -0.5 (S(v) oc v-alpha) at 1.5-5 GHz and steepens above approximately 15 GHz at 92 GHz thermal free-free emission dominates. The results agree well with estimates of the thermal emission from recombination line studies. The inferred Lyman continuum flux is 9 +/- 2 x 10(53) s-1. Comparison of this flux with that predicted by the infrared luminosity suggests that the H II regions in M82 have swept most of the surrounding neutral gas and dust into dense shells.