Detection of H92 alpha recombination line from the galaxies NGC 3628, IC 694, and NGC 1365

Abstract

Using the Very Large Array with an angular resolution of 3", we have detected the hydrogen recombination line H92alpha (Vrest = 8309.38 MHz) from the starburst nuclei in the galaxies NGC 3628 and IC 694 and also from the Seyfert II nucleus in NGC t365. In each case the line-emitting region extends over a few hundred parsecs. The detected lines have peak intensities in the range 0.5-1.5 mJy and widths (FWHM) 200- 400 km s 1 The line was not detected in three other galaxies (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 regions in the nuclear region accounts for the observed H92alpha line. The required number of H ii regions, their temperature, electron density, 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 103 cm -3 are permitted by the available constraints. Several hundred H ii regions of a few parsecs in size, with a total mass of a few times 10 M_{0}, are required to account for the observed line flux density. The rate of production of Lyman continuum photons required to maintain the ionization is a few times 10 s -1 Much of the line emission comes from internal stimulated emission due to the continuum generated within the H ii regions which account for 5%-30% of the observed total continuum at 5 GHz. Predictions are made for the expected recombination line and continuum flux density as a function of frequency from the nuclear region of the galaxies.