Detection of an H92 alpha recombination line in the starburst galaxy NGC 660

Abstract

We have used the Very Large Array (VLA) to search for the H92 alpha radio recombination line (RRL) in four starburst galaxies. In NGC 660, the line was detected over a 17 x 8 sq arcsec region near its starburst nucleus. The line and continuum emission indicate that the RRL-emitting gas is most likely in the form of a cluster of H II regions with a small filling factor. Using a simple model, we find that the total ionized mass in the nuclear region is in the range 2-8 x 10 exp 4 solar masses and the rate of production of UV photons NLyc = 1-3 x 10 exp 53/s. The ratio of H92 alpha and Br gamma line intensities in NGC 660 indicates that extinction is significant even at gamma = 2 microns. The velocity field of the ionized gas is consistent with a rotating disk with an average velocity gradient of 15 km/s arcsec. The dynamical mass within the central 500 pc is 4 x 10 exp 88 solar masses and may be about 6 x 10 exp 7 solar masses within the central 120 pc. No line was detected in the other galaxies (NGC 520, NGC 1614, and NGC 6946) to a 3 sigma limit of 300 microJy. In the starburst galaxies in which RRLs have been detected, we find that there is a rough correlation between the integrated H92 alpha line flux density and both the total far-infrared flux density and the radio continuum emission from the central region.