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

Abstract

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 Å optical luminosity at L5100 ∼ 1044−1046 erg s−1, and H α width at FWHM ∼ 1000−10 000 $\rm {\, km \, s}^{-1}$. We derive the continuum and the H α 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 α 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 ∼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.