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C.M. Urry (STScI)
The rapid variability and high luminosity of blazars are signatures of beamed emission from powerful relativistic jets closely aligned with the line of sight. The jet emission is magnified a thousand-fold or more by relativistic beaming, providing a unique opportunity to probe how jets are formed and collimated near supermassive black holes. That such jets are common to all radio-loud active galaxies --- the so-called ``unification'' hypothesis --- is supported by statisical studies and by direct observations of properties unaffected by relativistic beaming, such as host galaxies and local environments. Our recent HST studies of blazar host galaxies confirm they are luminous but otherwise normal ellipticals, well matched to the hosts of their ``parent'' radio galaxies. There is no sign that the active nucleus strongly affects the host galaxy or vice versa, and no correlation between jet/nuclear power and host galaxy luminosity/mass, as might have been expected given an underlying relation between black hole mass and bulge mass. These results are consistent with the idea that all massive ellipticals harbor supermassive black holes, with (low) duty cycles of activity, and perhaps point to scenarios where the cosmic formation of black holes is closely linked to that of the most massive galaxies.