Estimating supermassive black hole masses in active galactic nuclei using polarization of broad Mg ii, H α, and H β 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 position angle ϕ. So far this method has been used for 30 objects but only for Hα 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 λ2798 Åspectral line. We use 3D Monte Carlo radiative transfer code stokes for simultaneous modeling of equatorial scattering of Hα, Hβ and Mg II lines. We included vertical inflows and outflows in the Mg II broad line region (BLR). We find that polarization states of Hα and Hβ lines are almost identical and SMBH mass estimates differ by 7%. For Mg II line, we find that ϕ exhibits an additional “plateau” with a constant ϕ, 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 Hα and Hβ 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.

Past, Present, and Future of the Scaling Relations of Galaxies and Active Galactic Nuclei

“…The dependence on orientation can be overcome by spectropolarimetric measurements: if the emission line light is scattered by an equatorial scatterer, then the width of the polarized line flux should be related to the velocity field as measured by an observer in the equatorial plane of the accretion disk, i.e., as if the viewing angle were θ 90 from the disk axis, de facto removing the orientation effect. Spectropolarimetric measurements allowed for the estimate of the black hole mass in a few tens of type-1 AGN (Savić et al, 2018;Afanasiev et al, 2019;Savić et al, 2020;Capetti et al, 2021). The technique requires large-aperture telescopes even for nearby, bright AGN, whose polarization is notoriously low ( ≲1%; Sniegowska et al 2021, in preparation).…”

Polarization in broad emission lines of active galactic nuclei

“…In the case that the BLR is dominant by outflow/inflow component, the velocity across the line profile will be systematically shifted to the blue or red (see Figure 2), the velocity of a part of the BLR should have radial and tangential components, where radial component is mostly due to the Keplerian motion and in a combination with the equatorial scattering will result in a swing in the PA. However, in this case the rate of polarization can be affected by the outflow/inflow (see Savić et al 2018, 2020).…”

“…It seems that the (PA), in the case of the Keplerian motion in combination with equatorial scattering could be used for the SMBH estimates. First of all, one can expect in the case of rotation motion caused by gravitation, the PA should have a swing in the profile across the broad lines (see models in Piotrovich et al 2019; Savić et al 2018, 2020). This is caused by the equatorial scattering of radial component velocity component.…”

Past, present and Future of the Scaling Relations of Galaxies and Active Galactic Nuclei

“…The dependence on orientation can be overcome by spectropolarimetric measurements: if the emission line light is scattered by an equatorial scatterer, then the width of the polarized line flux should be related to the velocity field as measured by an observer in the equatorial plane of the accretion disk, i.e., as if the viewing angle were θ = 90 from the disk axis, de facto removing the orientation effect. Spectropolarimetric measurements allowed for the estimate of the black hole mass in a few tens of type-1 AGN [689,690,691,692]. The technique requires large-aperture telescopes even for nearby, bright AGN, whose polarization is notoriously low ( 1%; Sniegowska et al 2021, in preparation).…”