Detection of an H92α recombination line in the starburst galaxy NGC 660

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 arcsec(2) 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(4) M. and the rate of production of UV photons N-Lyc similar to 1-3 x 10(53) s(-1), The ratio of H92 alpha and Br gamma line intensities in NGC 660 indicates that extinction is significant even at lambda = 2 mu m. The velocity field of the ionized gas is consistent with a rotating disc with an average velocity gradient of similar to 15 km s(-1) arcsec(-1). The dynamical mass within the central 500 pc is similar to 4 x 10(8) M. and may be about similar to 6 x 10(7) M. 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 mu Jy, 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.