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5.4 Isotropic Properties of BL Lac Objects and FR I Galaxies

5.4.1 Extended Radio Emission

More than ten years ago it was noted that the extended radio emission of BL Lacs was comparable to that of low-luminosity (i.e., FR I) radio galaxies (Browne 1983; Wardle et al. 1984; Antonucci and Ulvestad 1985), although these early studies were hampered by the lack of redshift information and small and incomplete samples. Now that sizeable complete BL Lac samples are available, indeed the mean extended radio powers at 5 GHz for the 1 Jy radio-selected BL Lac sample (Stickel et al. 1991) and a subsample of the 3CR FR Is are essentially the same, < Pext > ~ 1025 W Hz-1 (Padovani 1992a). Figure 10 shows the overlapping histograms of Pext observed for the 1 Jy BL Lacs and the 2 Jy FR Is (Wall and Peacock 1985; di Serego Alighieri et al. 1994b), which have similar flux limits and selection frequencies. It is not clear whether the BL Lac morphologies are consistent with FR I morphologies seen at small angles; if the angle separating the two classes is large enough, one might expect to see more double-jet radio structures in BL Lacs, although the FR I morphologies are quite varied (Parma et al. 1992).

Figure 10
Figure 10. A comparison of the extended radio powers at 5 GHz for FR I radio galaxies (dashed line) from the 2 Jy sample (Wall and Peacock 1985; di Serego Alighieri et al. 1994b), radio-selected BL Lac objects (solid line) from the 1 Jy sample (Stickel et al. 1991) and X-ray-selected BL Lacs (dot-dashed line) from the EMSS sample (Stocke et al. 1991). Data for the FR Is come from Morganti et al. (1993); those for RBL have been derived from the R values (Appendix C) given by Padovani (1992a), updated with data from Kollgaard et al. (1992) and Murphy et al. (1993) plus lower limits from Ulvestad et al. (1981); data for the XBL are taken from Perlman and Stocke (1993). Upper limits on extended power (<) and lower limits on redshift (>) for BL Lac objects are indicated.

There is similar agreement between B2 FR Is and X-ray selected BL Lac objects (XBL) from the Einstein Medium Sensitivity Survey (EMSS; Perlman and Stocke 1993) or 3CR and B2 FR Is and XBL from the HEAO-1 Large Area Sky Survey (LASS; Laurent-Muehleisen et al. 1993). Figure 10 includes the distribution of Pext for the EMSS XBL; both the 3CR and B2 samples have slightly lower fluxes at 5 GHz than the 2 Jy sample, which results in their extended powers reaching smaller values.

It is notable that the mean extended radio power of XBL is more than an order of magnitude lower than for radio-selected BL Lacs (RBL; see also Laurent-Muehleisen et al. 1993). Moreover, the weak radio cores of XBL appear relatively unbeamed, as if they are ``off-axis'' in the radio band (Padovani 1992a; Perlman and Stocke 1993) or intrinsically less luminous (Giommi and Padovani 1994; Padovani and Giommi 1995a). The large difference in extended power supports the latter interpretation (see Sec 6.2.5, 6.2.6).

Some of the 1 Jy RBL, which extend to higher redshifts and thus to higher luminosities, appear to have radio morphologies more consistent with FR IIs than with FR Is (or rather, with what FR IIs would look like at a small angle with the line of sight; Kollgaard et al. 1992; Murphy et al. 1993). This is not a serious challenge to the idea of FR Is being the parent population of BL Lac objects (see also Maraschi and Rovetti 1994, who consider the continuity explicitly), as it affects only a relatively few objects at the high end of the FR I luminosity function, but it reinforces the blurriness of the distinction between FR Is and FR IIs in the overlapping luminosity range. Just as the luminosities overlap at high frequencies, perhaps the clear morphological separation breaks down in higher frequency maps (which are needed to get high spatial resolution in the more luminous, higher redshift objects). Also, none of the BL Lacs observed by Kollgaard et al. (1992) displays FR II characteristics as clearly as some core-dominated quasars observed by the same group (albeit with somewhat better resolution; Kollgaard et al. 1990). Finally, a substantial number of 3CR FR II sources have low-excitation optical spectra, i.e., with lines such as [O III] that are very weak (or undetectable) in comparison to the hydrogen lines (Laing et al. 1994). Since the narrow-line spectra of these objects are clearly different from those of classical FR II radio galaxies and radio quasars, instead resembling those of FR Is, it is possible that the low-excitation sources, despite their FR II morphology, could well belong to the low-power unification scheme. It would then follow that some BL Lacs are expected to display FR II morphology and also that that does not necessarily associate them with all the high-power sources.

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