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Session 21 - Luminous Infrared Galaxies.
Display session, Wednesday, January 07
Exhibit Hall,

[21.09] Far-IR Hydrogen Recombination Lines in M82

V. I. Harvey (U. Nevada, Las Vegas and Maria Mitchell Obs.), S. Satyapal (NASA/GSFC and Maria Mitchell Obs.), H. A. Smith (SAO), V. Strelnitski (Maria Mitchell Obs.)

The possibility to use far-IR hydrogen recombination \alpha-lines to determine the level of the optically thin spontaneous emission of hydrogen recombination lines in the starburst galaxy M82 is investigated. Observations of M82 in the wavelength region 43-196\mum were obtained with the Infrared Space Observatory's (ISO) Long Wavelength Spectrometer (LWS). This region encompasses the hydrogen \alpha-lines H10\alpha to H15\alpha. The data were reduced using the ISO Spectral Analysis Package and IRAF package SPLOT. Only upper limits for these lines were measured, as the strong thermal dust emission in M82 causes a very low line-to-continuum (L/C) ratio at these wavelengths. These upper limits, when compared with the available fluxes in other \alpha-lines, from centimeter radio lines to H4\alpha at 4.05\mum, confirm the hypothesis that the centimeter lines are masing. We discuss the problem of L/C ratio and argue that the far-IR hydrogen recombination \alpha-lines may be unobtainable for starburst galaxies, in principal, with the existing spectroscopic infrared facilities and those facilities planned for the near future. We show that the L/C problem is much less severe in the mid-IR than in the far-IR and discuss possible ground based observations of mid-IR and short centimeter \alpha-lines. These measurements may be decisive in determining the proper level of spontaneous optically thin emission for all hydrogen \alpha-lines in M82.

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