Radio-loud Narrow Line Seyfert 1 under a different perspective: a revised black hole mass estimate from optical spectropolarimetry

Abstract

Several studies indicate that radio-loud (RL) Active Galactic Nuclei (AGN) are produced only by the most massive black holes (BH), $M_{\rm BH} \sim 10^8$-$10^{10} M_\odot$. This idea has been challenged by the discovery of RL Narrow Line Seyfert 1 (RL NLSy1), having estimated masses of $M_{\rm BH}$$\sim$$10^6$-$10^7$ M$_\odot$. However, these low $M_{\rm BH}$ estimates might be due to projection effects. Spectropolarimetry allows us to test this possibility by looking at RL NLSy1s under a different perspective, i.e., from the viewing angle of the scattering material. We here report the results of a pilot study of VLT spectropolarimetric observations of the RL NLSy1 PKS 2004-447. Its polarization properties are remarkably well reproduced by models in which the scattering occurs in an equatorial structure surrounding its broad line region, seen close to face-on. In particular, we detect a polarized H$\alpha$ line with a width of $\sim$ 9,000 km s$^{-1}$, $\sim 6$ times broader than the width seen in direct light. This corresponds to a revised estimate of $M_{\rm BH}$$\sim$$6\times10^8$ M$_\odot$, well within the typical range of RL AGN. The double-peaked polarized broad H$\alpha$ profile of the target suggests that the rare combination of the orientation effects and a broad line region dominated by the rotation might account for this class of objects, casting doubts on the virial estimates of BH mass for type-I AGN.