2004ApJ...609L..25O

Recently, Bell et al. accurately measured in several giant H II regions both the width and the area of a great number of hydrogen radio recombination lines (RRLs)–up to the principal quantum number n=274. For observations made at a frequency of 6 GHz, the measured line widths increased with the growth of n, but upon reaching some critical value n=n_cr∼200, the line widths started to decrease rapidly with the further growth of n, while the line areas simultaneously increased above the predicted values. We show that the allowance for collisions with ions–especially for the transition from the impact limit of the ion Stark broadening at relatively low n to the quasi-static limit at relatively high n–could explain the puzzle of the narrowing of the RRLs observed by Bell et al.