Radio constraints on the nature of BL Lacertae objects and their parent population

Abstract

The Very Large Array has been used at 5 GHz to make images of 17 BL Lacertae objects, 13 of which are members of the "1 Jy sample" defined by Stickel et al. (ApJ, 374,431(1991)]. Sixteen sources were observed with the D array and 11 with the B array. All but one of the higher resolution images, and some of the lower resolution ones, show extended radio features. In at least four cases the sensitivity is sufficient to show linear polarization features in the extended emission. Of the ten sources that exhibit extended emission and have measured redshifts, at least three appear to be in the Fanaroff-Riley class II (FR II), from both their morphology and polarization properties, in addition to the luminosity of their extended emission. Three other sources are clearly in the FRI class, and four sources near the luminosity division between FR I and FR II sources are difficult to classify morphologically. At milliarcsecond resolution, however, none of these BL Lacertae objects show the polarization properties expected of a FR II nucleus. The characteristic "BL Lacertae-like" polarization on parsec scales is distinct from that of quasars, and is exhibited by both broad-line and narrow-line BL Lacertae objects. This suggests that the broad-line BL Lacertae objects are not simply an extension of the quasar/OVV population, and it also argues against the proposal that most BL Lacertae objects are gravitationally microlensed images of distant quasars. We present several arguments suggesting that not every FR I radio galaxy would appear as a radio-selected BL Lacertae object if viewed at a small angle to the jet direction. It seems likely that only those FR I radio galaxies with intrinsically-stronger cores would do so, while x-ray selected BL Lacertae objects may be found among the weaker cored and less favorably oriented FIR I radio galaxies. This alleviates some of the problems with the current unification schemes. The data for the radio- selected BL Lacertae objects are then consistent with typical jet Lorentz factors of {GAMMA} ~5, thus accounting straightforwardly with the modest superluminal speeds that have been measured so far in these objects.