Spectropolarimetry of low redshift Quasars: origin of the polarization and implications for black hole mass estimates

TL;DR

This study uses spectropolarimetry of low-redshift quasars to understand the polarization origin, revealing a single scattering medium and proposing a method to improve black hole mass estimates by accounting for inclination effects.

Contribution

It demonstrates that polarization measurements can trace the scattering medium and correct inclination bias in black hole mass estimates in AGN.

Findings

The polarization is produced by a single equatorial scattering medium.

The polarization angle swing indicates the inclination of the system.

Polarized flux line width provides a less biased black hole mass estimate.

Abstract

We present the results of high signal-to-noise ratio VLT spectropolarimetry of a representative sample of 25 bright type 1 AGN at z<0.37, of which nine are radio-loud. The sample covers uniformly the 5100 A optical luminosity at $L_{5100}\sim 10^{44}-10^{46}$ erg s$^{-1}$, and H$\alpha$ width at FWHM$\sim 1000-10,000$~ km/s. We derive the continuum and the H$\alpha$ polarization amplitude, polarization angle, and angle swing across the line, together with the radio properties. We find the following: 1. The broad line region (BLR) and continuum polarization are both produced by a single scattering medium. 2. The scattering medium is equatorial, and at right angle to the system axis. 3. The scattering medium is located at or just outside the BLR. The continuum polarization and the H$\alpha$ polarization angle swing, can both serve as an inclination indicator. The observed line width is found to be affected by inclination, which can lead to an underestimate of the black hole mass by a factor of $\sim 5$ for a close-to face-on view. The line width measured in the polarized flux overcomes the inclination bias, and provides a close-to equatorial view of the BLR in all AGN, which allows to reduce the inclination bias in the BLR based black hole mass estimates.