Evidence for anisotropic motion of the clouds in broad-line regions of BL Lacertae objects

Abstract

The masses of central massive black holes in BL Lac objects are estimated from their host galaxy absolute magnitude at R-band by using the empirical relation between absolute magnitude of host galaxy and black hole mass. Only a small fraction of BL Lac objects exhibit weak broad-line emission, and we derive the sizes of the broad-line regions (BLRs) in these BL Lac objects from the widths of their broad emission lines on the assumption of the clouds being virilized in BLRs. It is found that the sizes of the BLRs in these sources are usually 2-3 orders of magnitude larger than that expected by the empirical correlation between BLR size and optical luminosity defined by a sample of Seyfert galaxies and quasars. We discuss a variety of possibilities and suggest it may probably be attributed to anisotropic motion of the BLR clouds in these BL Lac objects. If the BLR geometry of these sources is disk-like, the viewing angles between the axis and the line of sight are in the range of 2-12 degrees, which is consistent with the unification schemes.