Revisiting the Black Hole Masses of Soft X-ray-Selected Active Galactic Nuclei

Abstract

In our previous work, using luminosity and the H-beta FWHM as surrogates for black hole mass (M_BH), we compared the black hole masses of narrow-line Seyfert 1 galaxies (NLS1s) and broad-line Seyfert 1 galaxies (BLS1s) in a sample of soft X-ray-selected active galactic nuclei. We found that the distributions of black hole masses in the two populations are statistically different. Recent work shows that the second moment of the H-beta emission line (the line dispersion) is a better estimator of black hole mass than FWHM. To test whether changing the width measure affects our results, we calculate line dispersion-based black hole masses for our sample. We find that using the line dispersion rather than the FWHM as a measure of the gas velocity shifts NLS1 and BLS1 virial product distributions closer together, but they remain distinct. On the M_BH-sigma plane, we find that using the line dispersion leaves NLS1s below the M_BH-sigma relation, but to a less significant degree than when FWHM is used to calculate black hole masses (the [OIII] 5007A FWHM is used as a surrogate for the bulge stellar velocity dispersion). The level of significance of our findings is such that we cannot draw firm conclusions on the location of the two samples on the M_BH-sigma plane. We are still left with two alternative scenarios: either (1) NLS1s lie below the M_BH-sigma relation indicating that their black hole masses are growing, or (2) NLS1s lie on the M_BH-sigma relation, so they preferentially reside in smaller mass, less luminous galaxies; the present data do not allow us to choose one over the other. More trustworthy stellar velocity dispersions and accurate black hole mass measurements are required for a firmer statement about the locus of NLS1s on the M_BH-sigma plane.