Detection of H92α 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 H92α (ν_rest = 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 H92α 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 × 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 105 Msun, 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 10⁵⁴ 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.