Millimeter recombination line emission in the starburst galaxy M82

We present and compare observations of M82 in the H41 alpha (92.0344 GHz), H29 alpha (256.3022 GHz), H27 alpha (316.4156 GHz), and H26 alpha (354.5055 GHz) radio recombination lines (RRLs) of hydrogen. The H41 alpha observations were made with the Millimeter Array of the Owens Valley Radio Observatory and the others with the James Clerk Maxwell Telescope. A comparison is made between the distributions of H41 alpha and the 92 GHz continuum and the HCO+ (1-0) emission at 89.1885 GHz, both of which were observed simultaneously with H41 alpha. The distribution of H41 alpha at 4 '' resolution is remarkably different from that of the underlying continuum, considering that both are expected to reflect the distribution of ionized gas. Differences are also noted between the H41 alpha distribution and that of other ionized gas indicators, which agree more with the continuum. The H41 alpha emission appears more clumpy and reveals bright regions with an anomalous ratio of line to continuum. In addition, there is a systematic variation of line-to-continuum ratio perpendicular to the inner disk of M82. The H41 alpha brightness distribution, however, shows a similarity to the HCO+ emission and hence to the distribution of dense molecular gas. There exist also peculiar kinematic effects with velocity deviations of up to 150 km s(-1) from normal rotation. We interpret the intensities and the kinematics of H41 alpha emission in terms of two ionized components: (1) a layer exhibiting spontaneous emission with a comparatively high filling factor and a temperature near 10,000 K, and (2) a layer of compact (less than or equal to 1 pc) dense (n(e) > 10(4.5) cm(-3)) regions with a small filling factor exhibiting primarily stimulated emission. The latter regions may either be compact H II regions or clouds of molecular gas ionized by shocks resulting from interaction between molecular clouds and the outflowing wind from the starburst region. Comparisons between spectra at H41 alpha and the higher frequency transitions show that while most of the millimeter RRL emission in M82 is consistent with spontaneous emission, some regions may emit predominantly by maser amplification at H27 alpha. This is especially the case at the tangent point of the molecular ring on the west side of the nucleus, where H27 alpha appears anomalously strong compared to H41 alpha in the range -30 to +100 km s(-1). This is also the region in which large velocity deviations are seen at H41 alpha. Thus, stimulated emission may also be present at submillimeter wavelengths, which would require even higher densities. Analysis of the frequency dependence of the global line luminosities indicates no large departures from spontaneous emission in the slope of the relation. However, the foregoing results indicate that a significant fraction of the line flux may be stimulated, and that the stimulated fraction may remain roughly constant with frequency in the millimeter region.