AAS Meeting #194 - Chicago, Illinois, May/June 1999
Session 73. AGN: Radio Galaxies, QSO's and Blazars
Display, Wednesday, June 2, 1999, 10:00am-6:30pm, Southeast Exhibit Hall

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[73.08] Unusual Morphologies from the HST Snapshot Survey of BL Lac Objects: Three Optical Jets and A Possible Einstein Ring

C. M. Urry, R. Scarpa, M. O'Dowd (Space Telescope Science Institute), T. Broadhurst (University of California at Berkeley), R. Falomo (Padua Observatory), A. Treves (University of Insubria), R. Webster (University of Melbourne), A. Sillanp\"aa, L. Takalo (Turku Observatory), H. Heidt (Landessternwarte Heidelberg)

The HST snapshot survey of 108 BL Lac objects has revealed several instances of unusual morphologies, three jets and a possible Einstein ring. Of the three detected optical jets, two were already known (PKS 0521-365 and 3C371) and one is a new discovery (PKS 2201+044). At HST resolution the jets are well resolved and individual knots clearly seen, closely corresponding to the radio knots. The derived radio-to-optical spectral index is remarkably constant along each jet, implying similar conditions in each knot, and indeed similarity among the three sources. Considerations of the energy budgets of these and other known optical jets constrain the presence of bulk relativistic motion on kiloparsec scales, the intrinsic magnetic field strengths, and the need for reacceleration along the jet.

The most unusual morphology in the survey is a ring-like structure around an unresolved BL Lac object at z>0.7. The most plausible of several possible explanations is that the BL Lac object has lensed one or two background galaxies into arcs approximating an Einstein ring. We present composite images from a joint deconvolution of the HST image and ground-based images from the NOT and CFHT telescopes, as well as a tentative model of the system.

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