The Steep Balmer Decrement of Quasars and Seyfert I Galaxies and its Origin

Observations of QSOs and Seyfert I galaxies show an anomalously steep Balmer decrement, which is in contradiction to the prediction of the traditional recombination theory. This is a long standing puzzle in past three decades in the study of AGNs. In this paper, we provide an alternative approach to solve this puzzle by using a newly recognized line emission mechanism, namely, the “Cerenkov line-like radiation”. For this purpose, we collected about a hundred of QSOs and Seyfert I galaxies, whose Balmer decrements have been measured and published in the past 30 years, and make the theoretical calculations on the observed Balmer decrements using an improved formula for the Cerenkov line intensity. The agreement between the calculations and observations is excellent. Therefore, we argue that the broad hydrogen lines of the QSOs and Seyfert I galaxies mainly originate from the Cerenkov line-like radiation of relativistic electrons. If this suggestion is further confirmed, our knowledge about the physics of AGNs will be greatly changed.