Molecular and atomic hydrogen line emission from star-forming galaxies
Citations Over Time
Abstract
Models of the generation of molecular and atomic hydrogen line emission from star-forming regions are discussed. These take into account the variations in the efficiency with which fluorescent H_2_ emission is produced as the ratio of the incident soft UV to the gas density changes (the lower this ratio, the higher the efficiency). The models deal with hot stars embedded within molecular clouds, large compact clusters of young stars generating intense UV fields, and emission from clouds bathed in a relatively diffuse UV field. The results from the models are compared with the ratio of the observed intensities of the H_2_ v = 1-0 S(1) and Brγ lines from the nuclear regions for a sample of 29 galaxies whose optical spectra are dominated by H II regions. Unlike previous investigators, we find that the scatter in the ratio of the two lines is quite small, suggesting a common mechanism for the H_2_ excitation. The line ratios are found to be consistent with H_2_ emission from photodissociation zones and in most of the sample can be understood in terms of ensembles of hot stars intimately mixed with molecular clouds, and perhaps forming blisters at their surface, although a few galaxies require much higher ratios of the UV intensity to the gas density, conditions which imply the presence of large numbers of massive stars in compact clusters.