RADIO RECOMBINATION LINES FROM THE NUCLEAR REGIONS OF STARBURST GALAXIES

摘要

Using the Very Large Array (VLA) with an angular resolution of 3″, we have detected the hydrogen recombination line H92α from the galaxies Arp 220, M83, and NGC 2146. The line emission arises from the nuclear regions with a line-to-continuum ratio of 1% or less. In order to fit both the observed H92α line and continuum data in the nuclear regions, we have considered two types of models. First, we utilize a model with a collection of H II regions. A large number of compact H II regions are required in this model. With electron temperatures in the range 5 x 10~3-1 x 10~4 K and a range of electron densities, this model can account for both the line intensity and the continuum spectrum. In most cases, the H92α line is dominated by internal stimulated emission due to free-free continuum arising within the H II regions. In a low-density case (n_e = 50 cm~(-3)) for Arp 220, about half the line emission comes from external stimulated emission due to the background nonthermal source. Typical rates of ionizing photons predicted from these models are ～5 x 10~(52) s~(-1) for M83, ～4 x 10~(53) s~(-1) for NGC 2146, and 5 x 10~(54) s~(-1) for Arp 220.