Millimeter Recombination Line Emission in the Starburst Galaxy M82

Abstract

view Abstract Citations (33) References (27) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Millimeter Recombination Line Emission in the Starburst Galaxy M82 Seaquist, E. R. ; Carlstrom, J. E. ; Bryant, P. M. ; Bell, M. B. Abstract We present and compare observations of M82 in the H41α (92.0344 GHz), H29α (256.3022 GHz), H27α (316.4156 GHz), and H26α (354.5055 GHz) radio recombination lines (RRLs) of hydrogen. The H41α 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 H41α and the 92 GHz continuum and the HCO^+^ (1-0) emission at 89.1885 GHz, both of which were observed simultaneously with H41α. The distribution of H41α 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 H41α distribution and that of other ionized gas indicators, which agree more with the continuum. The H41α 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 H41α 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 H41α 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 (<= 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 H41α 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 H27α. This is especially the case at the tangent point of the molecular ring on the west side of the nucleus, where H27α appears anomalously strong compared to H41α in the range -30 to +100 km s^-1^. This is also the region in which large velocity deviations are seen at H41α. 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. Publication: The Astrophysical Journal Pub Date: July 1996 DOI: 10.1086/177454 Bibcode: 1996ApJ...465..691S Keywords: GALAXIES: INDIVIDUAL MESSIER NUMBER: M82; GALAXIES: ISM; GALAXIES: STARBURST; RADIO LINES: GALAXIES full text sources ADS | data products SIMBAD (3) NED (1)