Are the black hole masses in narrow-line Seyfert 1 galaxies actually small?

Abstract

Narrow-line Seyfert 1 galaxies (NLS1s) are generally considered peculiar objects among the broad class of type 1 active galactic nuclei, due to the relatively small width of the broad lines, strong X-ray variability, soft X-ray continua, weak [O III], and strong Fe II line intensities. The mass MBH of the central massive black hole (MBH) is claimed to be lighter than expected from known MBH-host galaxy scaling relations, while the accretion rate on to the MBH larger than the average value appropriate to Seyfert 1 galaxies. In this Letter, we show that NLS1 peculiar MBH and L/LEdd turn out to be fairly standard, provided that the broad-line region is allowed to have a disc-like, rather than isotropic, geometry. Assuming that NLS1s are rather 'normal' Seyfert 1 objects seen along the disc axis, we could estimate the typical inclination angles from the fraction of Seyfert 1 classified as NLS1s, and compute the geometrical factor relating the observed full width at half-maximum of broad lines to the virial mass of the MBH. We show that the geometrical factor can fully account for the 'black hole mass deficit' observed in NLS1s, and that L/LEdd is (on average) comparable to the value of the more common broad-line Seyfert 1 galaxies. © 2008 The Authors. Journal compilation © 2008 RAS.