Radio recombination lines from the starburst galaxy NGC 3256

Abstract

We have detected the radio recombination lines H91α and H92α with rest frequencies of 8.6 GHz and 8.3 GHz from the starburst nucleus NGC 3256 at an angular resolution of 16.4'' × 9.6'' using the Australia Telescope Compact Array and at an angular resolution of 12.0'' × 2.9'' using the VLA. The line was detected at ~1 mJy beam-1 peak with a width of 160 km s-1 with the ATCA and at ~0.5 mJy beam-1 peak with a width of 114 km s-1 with the VLA. Modelling the line emitting region as a collection of H II regions, we derive constraints on the required number of H II regions, their temperature, density, and distribution. We find that a collection of 10 to 300 H II regions with temperatures of 5000 K, densities of 1000 cm-3 to 5000 cm-3 and diameters of 15 pc produced good matches to the line and continuum emmission. The Lyman continuum production rate required to maintain the ionization is 2 × 1052~s-1 to 6 × 1053~s-1, which requires 600 to 17 000 O5 stars to be produced in the starburst.