Are Narrow-line Seyfert 1 Galaxies Powered by Low-mass Black Holes?

Abstract

<p>Narrow-line Seyfert 1 galaxies (NLS1s) are believed to be powered by the accretion of matter onto low-mass black holes (BHs) in spiral host galaxies with BH masses M_BH ∼ 10⁶–10⁸ M_sun. However, the broadband spectral energy distribution of the γ-ray-emitting NLS1s are found to be similar to flat-spectrum radio quasars. This challenges our current notion of NLS1s having low MBH. To resolve this tension of low MBH values in NLS1s, we fitted the observed optical spectrum of a sample of radio-loud NLS1s (RL-NLS1s), radio-quiet NLS1s (RQ-NLS1s), and radio-quiet broad-line Seyfert 1 galaxies (RQ-BLS1s) of ∼500 each with the standard Shakura–Sunyaev accretion disk (AD) model. For RL-NLS1s we found a mean log(M_BH,AD/M_sun) of 7.98 ± 0.54. For RQ-NLS1s and RQ-BLS1s we found mean log(M_BH,AD/M_sun) of 8.00 ± 0.43 and 7.90 ± 0.57, respectively. While the derived M_BH,AD values of RQ-BLS1s are similar to their virial masses, for NLS1s the derived M_BH,AD values are about an order of magnitude larger than their virial estimates. Our analysis thus indicates that NLS1s have MBH similar to RQ-BLS1s and their available virial MBH values are underestimated, influenced by their observed relatively small emission line widths. Considering the Eddington ratio as an estimation of the accretion rate and using M_BH,AD, we found the mean accretion rate of our RQ-NLS1s, RL-NLS1s, and RQ-BLS1s as 0.06, 0.05 and 0.05, respectively. Our results, therefore, suggest that NLS1s have BH masses and accretion rates that are similar to BLS1s.<br /></p>