Gemini Preprint #18

Observations of Millimetre-Wavelength Hydrogen Recombination Lines in the Galaxy NGC253

P. J. Puxley and C. M. Mountain
Gemini 8m Telescopes Project, 950 N. Cherry Ave., Tucson AZ 85719

P. W. J. L. Brand
Institute for Astronomy, University of Edinburgh, Blackford Hill, Edinburgh, UK, EH9 3HJ

T. J. T. Moore
Astrophysics Group, Liverpool John Moores University, Byrom St., Liverpool UK L3 3AF

N. Nakai
Nobeyama Radio Observatgory, Minamimaki, Minamisaku, Nagano 384-13, Japan


The millimetre-wavelength hydrogen recombination lines H40alpha, H53alpha and H66alpha have been observed in the nucleus of the nearby starburst galaxy NGC253. The line fluxes have been compared with existing radio recombination line measurements and are found to be consistent with spontaneous emission from optically thin HII regions. The inferred hydrogen ionization rate is (3.7±0.8) × 10^-53 s^-1. This value is up to an order of magnitude larger than those inferred from the recombination lines Brgamma and Bralpha implying substantial extinction due to dust even at near-infrared wavelengths. Comparison with existing 3.3mm continuum measurements implies an electron temperature for the gas of (5000±900)K if the continuum at this wavelength is entirely due to free-free emission.

Appears in the Astrophysical Journal, volume 485.

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Ruth A. Kneale / / January 28, 1997