Detection of the H92alpha recombination line from the starbursts in the   Circinus galaxy and NGC 1808
  Context. Gas ionized by starburst activity radiates radio recombination lines (RRLs), from which one can derive the plasma conditions and the number of massive stars formed in the burst, free of dust obscuration effects.   Aims. We aimed to find RRL emission from additional starburst systems and to use the line properties to estimate the properties of the ionized gas.   Methods. We conducted a search for RRLs in the nearby extragalactic starburst or Seyfert galaxies NGC 1808, the Circinus galaxy, NGC 4038/9, II Zw 40, NGC 6221, NGC 7552, IRAS 18325-5926, IC 5063, and VV 114. We used the VLA and the ATCA to search for the RRLs H91alpha and H92alpha with rest frequencies of 8.6 GHz and 8.3 GHz.   Results. We detected for the first time RRLs from the starburst nuclei in the Circinus galaxy and NGC 1808. The Circinus galaxy was detected with a line strength integrated over the source of 3.2 mJy, making it the fourth-strongest extragalactic RRL emitter known at this frequency (after NGC 4945, M 82, and NGC 253). The line and continuum emission can be matched by a model consisting of a collection of 50 to 10 000 H II regions with 300 to 9000 O5 stars required to maintain the ionization, inferring a star formation rate of 0.2 Msun/yr to 6 Msun/yr. NGC 1808 was detected in RRL emission at 3.9 sigma with a line strength of 0.47 mJy at the expected velocity.   Conclusion. We have detected RRLs from two galaxies, adding to the small but growing number of known extragalactic RRL emitters. The Circinus galaxy is strong and especially suited to high-quality follow-up spectroscopic study. We derived conditions and star formation rates in the starbursting regions. Uncertainties can be reduced by future multi-transition studies. 
A. L. Roy, W. M. Goss, K. R. Anantharamaiah