Detection of H53-alpha emission from M82 - A reliable measure of the ionization rate and its implications
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Abstract
Emission in the hydrogen recombination line H53α has been detected, for the first time, in another galaxy: the nearby "starburst" system M82. This line is produced primarily by spontaneous emission and provides the most direct, extinction-independent estimate of the ionization rate in the star-forming complex. The line strength implies an ionization rate of 1.1 x 10^54^ s^-1^, approximately 5 times larger than that inferred from Brα observations, and indicates a dust extinction at 4 microns of more than 1 mag (A_v_ ~ 27 mag). Comparison of the 3.3 mm free-free continuum and H53α fluxes implies an average electron temperature of T_e_ ~ 5 x 10^3^ K. Analysis of the line excitation conditions, using the H53α emission in conjunction with that in the far-IR O^2+^ and N21 forbidden lines, together with the IR luminosity, suggests that only a very restricted range of stellar masses are formed. Less plausible interpretations are (1) that approximately two-thirds of the direct stellar emission escapes from the central 40" without encountering dust and (2) that much of the line emission and IR luminosity arises via fast (~ 100 km s^-1^) shocks.