7.2 Possible Connections between BL Lac Objects and FSRQ
Because of their similar continuum properties, BL Lac objects and FSRQ are collectively called blazars, yet we saw in the previous section that there are also considerable differences between them. What is the relationship between the two blazar classes? Three suggestions - that FSRQ evolve into BL Lacs, that they are different manifestations of the same physical process, and that BL Lacs are gravitationally micro-lensed FSRQ - are discussed in this section (some of these points were originally discussed by Padovani 1992b).
7.2.1 The Evolutionary Connection
z = 0; Padovani 1992b), but the FSRQ are distributed to much higher redshift. It is a well-known problem that high-redshift quasars must disappear and/or become considerably less luminous by the present epoch (Schmidt 1968). Here we consider the suggestion that FSRQ evolve into BL Lac objects, becoming weak-lined objects by virtue of increased beaming of the continuum, that is, a Lorentz factor increasing with cosmic time (decreasing with redshift; Vagnetti et al. 1991). The merit of this approach is that it includes evolution more directly than usual and it attempts to link the (at present) separate unified schemes for high- and low-luminosity radio-loud AGN. It also predicts the evolution of the critical angle separating blazars from radio galaxies (Vagnetti and Spera 1994). Specific predictions of distributions of jet/counter-jet ratios and superluminal velocities are also possible (Vagnetti, in preparation).
The results in Sec. 7.1, however, demonstrate the lack of continuity in redshift between FSRQ and BL Lac objects. For example, their extended radio powers and line luminosities are very different - by 1-2 orders of magnitude at comparable redshifts. Figures 11 and 19 show that BL Lacs and FSRQ occupy separate, approximately parallel, regions in the L[O III] - z and LMg II - z planes, which is more suggestive of related but distinct histories than of evolution from high-luminosity to low-luminosity blazar.
The strongest objection to the suggestion that strong-lined objects evolve into weak-lined ones because of increased beaming is the fact that BL Lacs have intrinsically weak lines. That is, the equivalent widths are not small because the optical continuum is stronger but because the lines are weaker. There is no evidence that the Lorentz factor, and to first order the Doppler factor, is larger in BL Lacs than in FSRQ. If anything, the limits on Doppler factors derived from SSC arguments are smaller for BL Lacs than for FSRQ (Sec. 6.3). Furthermore, superluminal motion data might actually support an increase with redshift of the Lorentz factor for low values of q0 (Vermeulen and Cohen 1994).
In the context of the radio-galaxy/blazar unified scheme, an evolutionary connection between FSRQ and BL Lac objects has strong implications for the parent populations. If (some) FSRQ evolve into BL Lacs, then (some) FR IIs should evolve into FR Is. Possible mechanisms for this transition exist, like the deceleration of the high-velocity jets that give rise to an FR II radio source by an increase in the density of the intergalactic medium (De Young 1993) or a decrease in the mass accretion rate onto the central engine (Baum et al. 1995). The connection between FR I and FR II radio galaxies is still not fully understood (Owen and Laing 1989; Owen and Ledlow 1994; Baum et al. 1995), but it is interesting to note that studies of cluster environments of radio sources do suggest both FR IIs and radio quasars could evolve into FR Is (Hill and Lilly 1991; Yee and Ellingson 1993).