Hubble Space Telescope observations of superluminal motion in the M87 jet

We present observations of the M87 jet made with the Faint Object Camera on board the Hubble Space Telescope at five epochs between 1994 and 1998. The se observations reveal 10 superluminal features within the first 6 " of the jet, with eight of these having apparent speeds in the range 4c-6c. Two ad ditional features within the first arcsecond of the jet have subluminal spe eds of 0.63c and 0.84c. The latter of these, named HST-1 East, appears to e mit new superluminal features moving at 6c, which subsequently fade with a half-intensity timescale of similar to 2 yr. The fastest speeds we observe require a Lorentz factor gamma greater than or equal to 6 for the bulk flow and a jet orientation within 19 degrees of the line of sight, in the conte xt of the relativistic jet model. Finding such large gamma in an FR I radio source like M87 strongly supports BL Lac/FR I unification models. These la rge speeds help to mitigate the particle lifetime problem posed by the opti cal emission, as well as the jet confinement problem.