DEEP VLA IMAGING OF 12 EXTENDED 3CR QUASARS

A representative sample of 12 extended quasars from the 3CR catalog ha s been imaged at 4.9 GHz using the VLA. These full synthesis observati ons typically achieve an rms noise of 20 mu Jy per beam, at a resoluti on (FWHM) of 0.''34 to 0.''38. Jets are detected on at least one side of every source. The jets are well collimated compared with those in l ess powerful sources, but spreading is detected in most of them. The o pening angles of several jets are not constant, but show recollimation after an initial regime of rapid spreading. Many of the jets contain quasiperiodic strings of knots, of which the knot closest to the centr al feature is usually the brightest (until the jet nears its hot spot) . The degrees of linear polarization at the jet knots range from <5% t o similar to 50%, but show no common trend with distance along the jet s. In knots that are elongated in directions close to that of the jet, the E vectors tend to be orthogonal to the jet axis. The exceptions-m isaligned knots with misaligned polarizations-tend to be bright featur es near large bends in the jets. Many of the jets are initially straig ht to within a few degrees, but bend more in the outer part of the sou rce. The prominence of the inner, straighter jet segments relative to the extended lobes correlates significantly with the prominence of the milliarcsecond-scale central features, but the prominence of the more bent jet segments does not. Candidates for counterjet emission are de tected in seven sources, but there is no unambiguous, continuous count erjet in any of them. Estimates of the flux density ratios between the straighter jet segments and the counterjets based on these tentative detections range from 1.2:1 to >175:1. There is no evidence in this sa mple that counterjet detectability correlates with such putative incli nation indicators as central feature prominence or projected linear si ze. There is also no evidence that the prominence of the counterjets a nticorrelates with that of the jets as predicted by simple relativisti c-beaming models for the jet/counterjet asymmetry. There is, however, strong evidence that large bends in the main jet favor counterjet dete ction, and there are no counterjet candidates opposite long, uninterru pted straight segments of the main jets.The detectability of the count erjets in these quasars may therefore be strongly influenced by intera ctions between the underlying beams and inhomogeneities in the surroun ding material. We offer a new empirical definition of the term ''hot s pot'' that is intended to improve the distinction between such feature s and ''jet knots.'' Both the compactness of hot spots and their posit ion in the lobe are affected by whether they are fed by a detectable j et. When the hot spots differ significantly in compactness, the more c ompact one is always on the jetted side. Jetted hot spots are also mor e likely to be recessed deeply from the outer edge of their lobes than are their counterjetted counterparts. The jetted hot spot is less pro minent relative to other extended emission if the jet bends through a large angle, particularly if a large bend occurs abruptly. The counter jetted hot spot is also less well defined if the jet is more bent. The lobes of several sources show considerable inhomogeneity, including f ilamentation.There is little difference in the inhomogeneity of the je tted and counterjetted lobes if the hot spots are excluded. The lobes have a common linear polarization pattern, with low polarization at th e center and high polarization (often reaching 40% to 60%) near the ed ges. This pattern matches the expectations of models in which the magn etic field in the lobes is provided by passively expanding the field i n the jets. The lobes in sources with promising counterjet candidates are often S-symmetric relative to the jet axis and their hot spots are more misaligned than in sources without such candidates. Counterjets may therefore be easier to detect if the jets change orientation durin g the lifetime of the source. We outline the implications of our resul ts for various models of the prominence and asymmetries of central fea tures, jets, counterjets, and hot spots. The correlations between the prominence and sidedness of the large-scale straight jet segments and of the small-scale central features favor models in which the kilopars ec-scale jets initially have bulk relativistic velocities. The slope o f the prominence correlation is less than expected if the larger-scale jets have characteristic Lorentz factors as high as those in the mill iarcsecond-scale features, however. This result is fragile within our small sample, but other aspects of our data also suggest that another phenomenon, closely coupled to jet bending, helps to determine the pro minence of features far from the central region. Overall, our data fav or ''tired jet'' models in which the average jet velocity decreases wi th increasing distance from the central object. This makes it harder f or the simplest relativistic-jet models to account for the systematic differences in compactness and placement of jetted and counterjetted h ot spots. The models may need refinement to include a range of Lorentz factors in the jets at any given distance from the quasar.