OBSERVATIONS OF MILLIMETER-WAVELENGTH HYDROGEN RECOMBINATION LINES INTHE GALAXY NGC-253

The millimeter-wavelength hydrogen recombination lines H40 alpha, H53 alpha, and H66 alpha have been observed in the nucleus of the nearby s tarburst galaxy NGC 253. The line fluxes have been compared with exist ing radio recombination line measurements and are found to be consiste nt with spontaneous emission from optically thin H II regions. The inf erred hydrogen ionization rate is (3.7 +/- 0.8) x 10(53) S-1. This val ue is up to an order of magnitude larger than those inferred from the recombination lines Br gamma and Br alpha, implying substantial extinc tion due to dust even at near-infrared wavelengths, Comparison with ex isting 3.3 mm continuum measurements implies an electron temperature f or the gas of (5000 +/- 900) K if the continuum at this wavelength is entirely due to free-free emission.