The Starburst Nuclear Region in M82 Compared in Several Wave Bands

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Abstract
We utilize new, sensitive radio continuum images of M82 at 1.5, 15 and 22 GHz for a comparison with images at other wave bands that have recently appeared in the literature. Overlays of the radio data with images in the optical V band (Hubble Space Telescope observations), mid-infrared, the [Ne II] line, and the CO (1 → 0) and the HCN line are presented as a reference for future analyses and interband comparisons. [Ne II] emission-line data are used to derive expected thermal fractions of the radio emission and then compared with the radio data to identify thermal and nonthermal radio sources. Two wisps emerging from the disk are traced in both the [Ne II] line and at 1.5 GHz. They appear to have enhanced thermal fractions of 25%-35% at 1.5 GHz and probably represent early stages of Parker loops. The radio-IR correlation applied to the mid-infrared-emitting dust grain population holds down to ~30 pc and is slightly better for the thermal component of the radio emission. Polycyclic aromatic hydrocarbons, which are depleted in the central starburst because of the strong UV radiation, do not appear to represent a very significant component of the mid-infrared-emitting population of very small grains. No associations occur between the ~40 supernova remnants apparent in our 025 resolution radio data and in the optically visible super star clusters. We conclude that M82 probably hosts about 20-40 times more super star clusters than are currently optically visible. A relatively poor correlation of the CO emission with the 1.5 GHz radio emission indicates (1) varying star formation efficiency across M82's disk and (2) a peculiar CO-emitting region of low star-forming efficiency in the northwest. The high-density molecular gas tracer, HCN, correlates better with the radio emission and star formation rate. We propose that the northwestern region is devoid of high-density gas because of a lower cloud collision rate, in turn caused by its different position with respect to the bar potential of M82.