The Central Engines of Narrow-Line Seyfert 1 Galaxies

Abstract

(abridged) It has been suggested that Narrow-Line Seyfert 1 (NLS1) galaxies are evolutionarily young objects, powered by the accretion of gas onto central black holes that are lower in mass than those found in typical broad-line Seyferts. We explore this hypothesis through the analysis of high-spatial resolution, near-IR imaging data for a sample of 11 NLS1s. By employing the correlation between black-hole mass and host galaxy bulge luminosity, we determine the mean black-hole mass for our sample to be, in solar units, <(log M_BH)> = 7.9. Using the correlation between the size of the broad-line region and the monochromatic continuum luminosity, we obtain black-hole mass estimates under the assumption that the emission-line gas is in virial equilibrium. The mean black-hole mass derived from this relation is <(log M_BH)> = 6.4. We explore possible causes for this discrepancy and the ramifications for our understanding of the role played by NLS1s in AGN evolution. Because numerical simulations constrain the start of the AGN duty cycle to a time shortly after a significant gravitational interaction, we examine the morphology and near-IR bulge colors of the NLS1 sample for evidence of recent encounters. The mean bulge color is found to be redder than that of both a matched sample of non-active galaxies and published estimates for broad-line Seyferts. The source of the unusual bulge colors may be an excess of flux, peaking at around 2.2 micron, that has been detected near the centers of some NLS1s such as Mrk 1239. No evidence is found for light asymmetries or an extra stellar component that would indicate NLS1s are young objects. Finally, we postulate that there may be some interesting lines of circumstantial evidence suggesting that secular processes may be relevant in NLS1s.