2010A&A...517A..82R

Hydrogen ionized by young, high-mass stars in starburst galaxies radiates radio recombination lines (RRLs), whose strength can be used as a diagnostic of the ionization rate, conditions and gas dynamics in the starburst region, without problems of dust obscuration. However, the lines are weak and only few extragalactic starburst systems have been detected. We aimed to increase the number of known starburst systems with detectable RRLs for detailed studies, and we used the line properties to study the gas properties and dynamics. We searched for the RRLs H91α and H92α with rest frequencies of 8.6GHz and 8.3GHz in the nearby southern Seyfert galaxy NGC 4945 using the Australia Telescope Compact Array with resolution of 3''. This yielded a detection from which we derived conditions in the starburst regions. We detected RRLs from the nucleus of NGC 4945 with a peak line strength integrated over the source of 17.8mJy, making it the strongest extragalactic RRL emitter known at this frequency. The line and continuum emission from NGC 4945 can be matched by a model consisting of a collection of 10 to 300 H II regions with temperatures of 5000K, densities of 10³cm^–3 to 10⁴cm^–3 and a total effective diameter of 2pc to 100pc. The Lyman continuum production rate required to maintain the ionization is 6x10⁵²s^–1 to 3x10⁵³s^–1, which requires 2000 to 10000 O5 stars to be produced in the starburst, inferring a star formation rate of 2M_☉/yr to 8M_☉/yr. We resolved the rotation curve within the central 70pc region and this is well described by a set of rotating rings that were coplanar and edge on. We found no reason to depart from a simple flat rotation curve. The rotation speed of 120km/s within the central 1'' (19pc) radius infers an enclosed mass of 3x10⁷M_☉, and an average surface density with the central 19pc of 25000pc^–2, which exceeds the threshold gas surface density for star formation. We discovered RRLs from NGC 4945. It is the strongest known extragalactic RRL emitter and is suited to high-quality spectroscopic study. We resolved the dynamics of the ionized gas in the central 70pc and derived conditions and star formation rates in the ionized gas.