5.4.2 Narrow Emission Lines
It is not yet clear whether the narrow-line luminosities of BL Lac objects, in the few cases when such lines can be observed, are consistent with those of FR I galaxies. [O III] luminosities are available for eight of the eleven 1 Jy BL Lac objects with certain redshift z 0.5 (Stickel et al. 1993). The mean emission line luminosity is LO III = 1041.30 ± 0.11 erg s-1, with a spread of less than an order of magnitude. Figure 11 shows these values compared to the available [O III] luminosities for the FR I galaxies in the 2 Jy sample (Tadhunter et al. 1993), plotted versus redshift to compensate simultaneously for any trends with observed wavelength range or luminosity (which correlates well with redshift in flux-limited samples).
|Figure 11. The [O III] emission line luminosity for the 1 Jy BL Lacs (filled circles), 2 Jy FR Is (open circles), and 2 Jy flat-spectrum radio quasars (open squares), including some upper limits for the FR Is, plotted versus redshift to allow comparisons at similar radio luminosity (which is well correlated with redshift in radio-flux-limited samples) and/or observed wavelength. At a given redshift, the line luminosities of FR Is are generally smaller than those of BL Lacs, although the highest FR I [O III] luminosities are comparable. The line luminosities of quasars are larger than those of BL Lacs by more than a factor of ten. (The one quasar located in the BL Lac region, PKS 0521-365, is discussed in the text; Sec. 7.1.) This argues against the idea that the high-redshift, high-luminosity BL Lac objects are more closely related to flat-spectrum radio quasars than to low-redshift, low-luminosity BL Lacs, although the BL Lac data extend only to z ~ 0.4.|
For z 0.2, the maximum redshift of the FR Is, and excluding non-detections, the mean [O III] luminosities are LO III = 1041.10 ± 0.07 and 1040.49 ± 0.24 erg s-1 for five BL Lacs and five FR Is, respectively. The difference is a factor of ~ 4, significant at the ~ 94% level, and is more than an order of magnitude if one includes the upper limits. However, the FR I line fluxes may be underestimated due to small slits and/or contamination from a strong stellar continuum (Tadhunter et al. 1993). Also, a comparable set of uniform upper limits for BL Lac objects is not available for comparison; possibly the [O III] luminosity in most BL Lac objects is well below the few detections available. High signal-to-noise-ratio spectra through large apertures of a reasonably large sample of FR I radio galaxies (and BL Lac objects) are needed to determine whether LO III in FR Is is really smaller than in BL Lacs.
The apparent difference in [O III] luminosities for BL Lacs and FR Is could also arise if [O III] were emitted anisotropically, as for radio-quasars and FR II radio galaxies (Sec 5.3). The fact that broad Mg II emission lines are seen in some BL Lac objects (Stickel et al. 1993) means that for unification to be correct, some FR Is (albeit high-redshift ones) must have obscured broad line regions. By analogy to quasars (Sec. 5.3.2), the obscuring matter could affect the [O III] emission more than the [O II], thus explaining the apparent discrepancy. While there is no direct evidence for an obscuring torus surrounding the [O III]-emitting clouds in BL Lac objects or FR Is, neither is there evidence against it.
Recent HST spectra of the nucleus of M87, a nearby FR I galaxy of modest luminosity (Harms et al. 1994), do show relatively broad emission lines, ranging from FWHM ~ 1400 km s-1 for H up to ~ 1900 km s-1 for [O I]. These lines are broader than the typical widths for Seyfert 2 galaxies (FWHM ~ 250-1200 km s-1; Osterbrock 1989) and the [O III] / H ratio is much larger than 1, unlike the typical case for Seyfert 1 galaxies. If M87 has a hidden nuclear broad-line region, further spectroscopic investigation of this and other FR Is is obviously of great interest.
Unlike the quasar - FR II unification scheme, where SSRQ appear at intermediate angles, the BL Lac - FR I scheme has essentially no transition objects (the low-luminosity BLRG 3C 120 might be an exception). These would be FR I-like radio sources, with steep radio spectra and FR I-like morphologies because they are oriented outside the relativistic beaming cone, but would show BL Lac-like emission lines (possibly broad lines) because they are oriented inside any obscuration cone. It may be that BL Lacs just do not have any appreciable emission, lines or continuum, beyond that relativistically beamed from the jet. Alternatively, broad lines could be hidden in FR Is, a phenomenon implied by unification with the high-redshift BL Lac objects, in which case there should be (as for quasars) broad-line FR Is. The lack of such objects could be explained if the opening angle of the obscuring torus is narrower at lower powers (Lawrence 1991; Falcke et al. 1995; see also Baum et al. 1995). Far infrared observations of BL Lacs and FR Is with ISO should indicate whether or not there is obscuring matter in low-luminosity radio sources.