Non-equilibrium populations of hydrogen in high-redshift galaxies

Abstract

We investigate the possibility of maser amplification in hydrogen recombination lines from the galaxies of first generation, at z 30. Combining analytical and computational approaches, we show that the transitions between the hydrogen Rydberg energy levels induced by the radiation from the ionizing star and by the (warmer than currently) Cosmic Microwave Background can produce noticeable differences in the population distribution, as compared with previous computations for contemporary H + regions, most of which ignored the processes induced by the ionizing star's radiation. In particular, the low (n 30) α-transitions show an increased tendency toward population inversion, when ionization of the H + region is caused by a very hot star at high redshift. The resulting maser/laser amplification can increase the brightness of the emitted lines and make them detectable. However, the limiting effects of maser saturation will probably not allow maser gains to exceed one or two orders of magnitude.