Radio recombination lines from starburst galaxies

Restricted Access. The central regions of the Galaxy and galaxies, Proceedings of the 184th symposium of the International Astronomical Union, held in Kyoto, Japan, August 18-22, 1997. Ed. Yoshiaki Sofue.Starburst activity is often heavily obscured by dust. To see through the dust and measure the full extent, power and dynamics of the starburst, we are surveying hydrogen radio recombination lines (RRLs) from famous nearby starburst galaxies. Exploiting the improved sensitivity and dynamic range of aperture synthesis arrays such as the VLA and the AT and millimter-wave telescopes such as SEST and the IRAM-30m telescope, we have searched for RRLs in about 25 starburst galaxies. RRLs have been detected in 14 galaxies. The detected galaxies are NGC 253, NGC 660, NGC 1068, NGC 1365, NGC 2146, NGC 3256, NGC 3628, NGC 3690, NGC 4151, NGC 4945, IC 694, Arp 220, M83, and the Circinus Galaxy. The observations were made variously at 1.4, 4.9, 8.4, 86, 135 and 232 GHz. In all the cases the detected line originates in the nuclear starburst region. The line and continuum strengths from the starburst regions are used to constrain the physical conditions such as temperature, density and mass of the ionized gas. These in turn have provided information about the number of high-mass stars formed in the starburst and the flux of Lyman continuum photons. Comparison of predicted Hα and Brγ fluxes of these models with observed values suggest that extinction in many nuclear starbust regions is much more than previously thought. The angular resolution of some of the recombinatin line observations are in the range 1 asec - 3 asec which allows the study of kinematics in the nuclear region and an estimation of the dynamical mass. In one case (NGC 253) the observed two dimensional velocity field can be interpreted in terms of two perpendicular rotating rings and a counter-rotating ring of ionized gas within the central 100 pc. It could also be interpreted in terms of a secondary bar inside the known primary bar