A note on black hole masses estimated by the second moment in Narrow Line Seyfert 1 Galaxies

TL;DR

This study calculates black hole masses in NLS1 galaxies using the second moment of the Hβ emission line, revealing dependencies on line profile components and aligning NLS1s with the standard - relation when using refined velocity dispersion estimates.

Contribution

It introduces a method to estimate SMBH masses in NLS1s via the Hβ second moment, accounting for line profile complexities and improving consistency with the - relation.

Findings

Second moment depends on the broad component of Hβ line.

Mass estimates from the second moment are about 0.50 dex larger than FWHM-based estimates.

NLS1s with lower masses are close to the - relation when using stellar velocity dispersion.

Abstract

The second moment of the H$\beta$ emission line is calculated for 329 narrow line Seyfert 1 galaxies (NLS1s) selected from the Sloan Digital Sky Survey (SDSS), which is used to calculate the central supermassive black hole (SMBHs) mass of each. We find that the second moment depends strongly on the broader component of the H$\beta$ line profile. We find that for the NLS1s requiring two Gaussians to fit the H$\beta$ line the mean value of the SMBH mass from the H$\beta$ second moment is larger by about 0.50 dex than that from the full width at half maximum (FWHM). Using the gas velocity dispersion of the core/narrow component of \oiii $\lambda$ 5007 to estimate the stellar velocity dispersion, $\sigma_{*}$, the new mass makes NLS1s fall very close to the $\mbh - \sigma_{*}$ relation for normal AGNs. By using $\sigma_{*}$ measured directly from SDSS spectra with a simple stellar population synthesis method, we find that for NLS1s with mass lower than $10^7 \msun$, they fall only marginally below the $\mbh - \sigma_{*}$ relation considering the large scatter in the mass calculation.