The orientation dependence of quasar single-epoch black hole mass scaling relationships

Abstract

Black hole masses are estimated for radio-loud quasars using several self-consistent scaling relationships based on emission-line widths and continuum luminosities. The emission lines used, Hβ, Mg ii λ2798 and C iv λ1549, have different dependencies on orientation as estimated by radio core dominance. We compare differences in the log of black hole masses estimated from different emission lines and show that they depend on radio core dominance in the sense that core-dominated, jet-on objects have systematically smaller Hβ and Mg ii determined masses compared to those from C iv, while lobe-dominated edge-on objects have systematically larger Hβ and Mg ii determined masses compared to those from C iv. The effect is consistent with the Hβ line width, and to a lesser extent that of Mg ii, being dependent upon orientation in the sense of an axisymmetric velocity field plus a projection effect. The size of the effect is nearly an order of magnitude in black hole mass going from one extreme orientation to the other. We find that radio spectral index is a good proxy for radio core dominance and repeating this analysis with radio spectral index yields similar results. Accounting for orientation could in principle significantly reduce the scatter in black hole mass scaling relationships, and we quantify and offer a correction for this effect cast in terms of radio core dominance and radio spectral index.