Infrared and optical observations of the hydrogen lines in quasars

Abstract

Infrared and optical spectrophotometric observations of the hydrogen lines in a sample of 16 quasars are reported. Combining these observations with previously published observations of hydrogen lines in quasars brings to 12 the number of quasars for which the Pɑ/Hɑ/Hβ line ratios have been measured and brings to nine the number of quasars for which the Lɑ/Hɑ line ratio has been measured. The Pɑ/Hɑ ratios in low-redshift quasars are distributed around the case B value with a tendency toward values lower than that predicted by case B. The Hɑ/Hβ values in these same quasars are generally greater than the case B value. The trend in the Pɑ/Hɑ/Hβ ratios is for the Pɑ/Hɑ ratio to decrease as Hɑ/Hβ increases. The decrease of the Pɑ/Hɑ ratio with increasing Hɑ/Hβ ratio is the most significant correlation found from the present data that any valid model of the line-emitting regions must explain. The low values of the Lɑ/Hɑ compared to the case B value are confirmed for a large sample of high-redshift quasars. Lɑ is apparently destroyed rather than Hɑ being enhanced. We conclude that reddening external to the einission-line regions cannot satisfactorily explain all the observed hydrogen line ratios. The current observations do not support the model of internal dust destroying resonantly scattered Lɑ, but such a model cannot be ruled out on the basis of the present data. Models of the emission-line regions that apply radiative transfer and collisional excitation effects in the lines appear to be necessary to explain the current observations.