2004ApJ...609...80C

The masses of central massive black holes in BL Lac objects are estimated from their host galaxy absolute magnitude M_Rat R-band by using the empirical relation between M_Rand black hole mass M_bh. 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 those expected by the empirical correlation R_BLR-λL_λ(3000 Å) defined by a sample of Seyfert galaxies and quasars. We discuss a variety of possibilities and suggest that this difference can probably be attributed to anisotropic motion of the BLR clouds in these BL Lac objects. If the BLR geometry of these sources is disklike, the viewing angles between the axis and the line of sight are in the range ∼2°-12°, which is consistent with the unification schemes.