Are the black hole masses in Narrow Line Seyfert 1 galaxies actually small?

TL;DR

This study suggests that the small black hole masses in Narrow Line Seyfert 1 galaxies are due to geometric effects, and when accounting for a disc-like broad line region, their masses and accretion rates are similar to typical Seyfert 1 galaxies.

Contribution

The paper introduces a geometrical model with a disc-like broad line region to explain the apparent small black hole masses in NLS1s, aligning their properties with standard Seyfert 1 galaxies.

Findings

Black hole mass estimates in NLS1s are affected by the geometry of the broad line region.

A disc-like geometry can explain the observed narrow line widths in NLS1s.

When accounting for geometry, NLS1s have similar Eddington ratios to typical Seyfert 1 galaxies.

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 [OIII], and strong FeII line intensities. The mass M_BH 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 onto the MBH larger than the average value appropriate to Seyfert 1 galaxies. In this Letter, we show that NLS1 peculiar M_BH and L/L_Edd 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 FWHM 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/L_Edd is (on average) comparable to the value of the more common broad line Seyfert 1 galaxies.