DETECTION OF H92-ALPHA RECOMBINATION LINE FROM THE GALAXIES NGC-3628,IC-694, AND NGC-1365

Using the Very Large Array with an angular resolution of approximately 3'', we have detected the hydrogen recombination line H92alpha (nu(re st) = 8309.38 MHz) from the starburst nuclei in the galaxies NGC 3628 and IC 694 and also from the Seyfert II nucleus in NGC 1365. In each c ase the line-emitting region extends over a few hundred parsecs. The d etected lines have peak intensities in the range 0.5-1.5 mJy and width s (FWHM) 200-400 km s-1. The line was not detected in three other gala xies (NGC 262, NGC 1068, and NGC 3079) that were observed to a similar sensitivity level. We present a model in which a collection of H ii r egions in the nuclear region accounts for the observed H92alpha line. The required number of H II regions, their temperature, electron densi ty, and linear size are constrained by the observed line flux density, line width, continuum spectrum, and size of the line-emitting region. If the temperature of the H II regions is above 5000 K, then electron densities in the range 5-50 x 10(3) cm-3 are permitted by the availab le constraints. Several hundred H ii regions of a few parsecs in size, with a total mass of a few times 10(5) M., are required to account fo r the observed line flux density. The rate of production of Lyman cont inuum photons required to maintain the ionization is a few times 10(54 ) s-1. Much of the line emission comes from internal stimulated emissi on due to the continuum generated within the H II regions which accoun t for 5%-30% of the observed total continuum at 5 GHz. Predictions are made for the expected recombination line and continuum flux density a s a function of frequency from the nuclear region of the galaxies.