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

Abstract

We have used the Very Large Array (VLA) to search for the H92α radio recombination line (RRL) in four starburst galaxies. In NGC 660, the line was detected over a 17Å-8 arcsec² 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 2-8Å-104 M⊙ and the rate of production of UV photons N_Lyc~1-3Å-10⁵³ s^-1. The ratio of H92α and Brγ line intensities in NGC 660 indicates that extinction is significant even at λ=2 µm. The velocity field of the ionized gas is consistent with a rotating disc with an average velocity gradient of ~15 km s^-1 arcsec^-1. The dynamical mass within the central 500 pc is ~4Å-10⁸ M_⊙ and may be about ~6Å-107 M_⊙ within the central 120 pc. No line was detected in the other galaxies (NGC 520, NGC 1614 and NGC 6946) to a 3s limit of 300 µJy. In the starburst galaxies in which RRLs have been detected, we find that there is a rough correlation between the integrated H92α line flux density and both the total far-infrared flux density and the radio continuum emission from the central region.