Estimating supermassive black hole masses in AGNs using polarization of broad MgII, H$\alpha$ and H$\beta$ lines

Abstract

For type-1 active galactic nuclei (AGNs) for which the equatorial scattering is the dominant broad line polarization mechanism, it is possible to measure the supermassive black hole mass by tracing the Keplerian motion across the polarization plane positionangle. So far this method has been used for 30 objects but only for H$\alpha$ emission line. We explore the possibilities this method for determining SMBH masses using polarization in broad emission lines by applying it for the first time to Mg II$\lambda$2798AA spectral line. We use 3D Monte Carlo radiative transfer code stokes for simultaneous modeling of equatorial scattering of H$\alpha$, H$\beta$ and Mg II lines. We included vertical inflows and outflows in the MgII broad line region (BLR). We find that polarization states of H$\alpha$ and H$\beta$ lines are almost identical and SMBH mass estimates differ by 7%. For Mg II line, we find that polarization angle $\phi$ exhibits an additional "plateau" with a constant $\phi$, which deviates than the profiles expected for pure Keplerian motion. SMBH mass estimates using Mg II line are higher by up to 35% than those obtained from H$\alpha$ and H$\beta$ lines. Our model shows that for vertical inflows and outflows in the BLR that is higher or comparable to Keplerian velocity, this method can be applied as a first approximation for obtaining SMBH mass.

8 pages, 4 figures, 3 tables, accepted for publishing in MNRAS

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