Recurrent Seyfert activity in spiral galaxy nuclei

Abstract

Hills1 has pointed out that in some conditions black holes in galactic nuclei would rapidly grow by disruption and accretion of stars thus producing Seyfert luminosities (>1043 erg s−1). In this hypothesis only those galactic nuclei with a high central stellar density are susceptible to Seyfert pathology, and they evolve through only one Seyfert phase. Here, I suggest an alternative model for recurring activity in normal galactic nuclei based on the assumed presence of a massive nuclear black hole (∼107 M⊙) and the observation of a very clumpy distribution of interstellar gas in the inner 200 pc of our Galaxy. The system of massive molecular clouds with low net angular momentum could provide an ‘accretion event’ every 107 yr with a duration of ∼105 yr. In such a picture, most spiral-galaxy nuclei evolve through recurring Seyfert episodes of rather short duration; the 1% accretion duty cycle is roughly consistent with the fact that a few per cent of all spiral galaxies are Seyferts2.