Astrophys. J., 710, 764-782 (2010/February-2)
Extended radio emission in MOJAVE blazars: challenges to unification.
KHARB P., LISTER M.L. and COOPER N.J.
Abstract (from CDS):
We present the results of a study on the kiloparsec-scale radio emission in the complete flux density limited MOJAVE sample, comprising 135 radio-loud active galactic nuclei. New 1.4 GHz Very Large Array (VLA) radio images of six quasars and previously unpublished images of 21 blazars are presented, along with an analysis of the high-resolution (VLA A-array) 1.4 GHz emission for the entire sample. While extended emission is detected in the majority of the sources, about 7% of the sources exhibit only radio core emission. We expect more sensitive radio observations, however, to detect faint emission in these sources, as we have detected in the erstwhile "core-only" source, 1548+056. The kiloparsec-scale radio morphology varies widely across the sample. Many BL Lac objects exhibit extended radio power and kiloparsec-scale morphology typical of powerful FRII jets, while a substantial number of quasars possess radio powers intermediate between FRIs and FRIIs. This poses challenges to the simple radio-loud unified scheme, which links BL Lac objects to FRIs and quasars to FRIIs. We find a significant correlation between extended radio emission and parsec-scale jet speeds: the more radio powerful sources possess faster jets. This indicates that the 1.4 GHz (or low-frequency) radio emission is indeed related to jet kinetic power. Various properties such as extended radio power and apparent parsec-scale jet speeds vary smoothly between different blazar subclasses, suggesting that, at least in terms of radio jet properties, the distinction between quasars and BL Lac objects, at an emission-line equivalent width of 5 Å, is essentially an arbitrary one. While the two blazar subclasses display a smooth continuation in properties, they often reveal differences in the correlation test results when considered separately. This can be understood if, unlike quasars, BL Lac objects do not constitute a homogeneous population, but rather include both FRI and FRII radio galaxies for their parent population. It could also just be due to small number statistics. We find that the ratio of the radio core luminosity to the k-corrected optical luminosity (Rv) appears to be a better indicator of orientation for this blazar sample than the traditionally used radio core prominence parameter (Rc). Based on the assumption that the extended radio luminosity is affected by the kiloparsec-scale environment, we define the ratio of extended radio power to absolute optical magnitude (Lext/Mabs) as a proxy for environmental effects. Trends with this parameter suggest that the parsec-scale jet speeds and the parsec-to-kiloparsec jet misalignments are not affected by the large-scale environment, but are more likely to depend upon factors intrinsic to the active galactic nucleus, or its local parsec-scale environment. The jet speeds could, for instance, be related to the black hole spins, while jet misalignments could arise due to the presence of binary black holes, or kicks imparted to black holes via black hole mergers, consistent with both radio morphologies resembling precessing jet models observed in some MOJAVE blazars and the signature of a 90° bump in the jet misalignment distribution, attributed to low-pitch helical parsec-scale jets in the literature. We suggest that some of the extremely misaligned MOJAVE blazar jets could be "hybrid" morphology sources, with an FRI jet on one side and an FRII jet on the other. It is tempting to speculate that environmental radio boosting (as proposed for Cygnus A) could be responsible for blurring the Fanaroff-Riley dividing line in the MOJAVE blazars, provided a substantial fraction of them reside in dense (cluster) environments.
BL Lacertae objects: general - galaxies: active - quasars: general - radio continuum: galaxies
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<Available at CDS (J/ApJ/710/764): table1.dat>
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