Radio recombination lines from the starburst galaxy NGC 3256

Abstract

We have detected the radio recombination lines H91 α 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 x 9.6 using the Australia Telescope Compact Array and at an angular resolution of 12.0 x 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-' 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 30(1 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 × 10 52 s -1 to 6 x 10 53 s -1 which requires 600 to 17000 O5 stars to be produced in the starbutst.