Optical Proper Motion Measurements of the M87 Jet: New Results from the
  Hubble Space Telescope
  We report new results from an HST archival program to study proper motions in
the optical jet of the nearby radio galaxy M87. Using over 13 years of archival
imaging, we reach accuracies below 0.1c in measuring the apparent velocities of
individual knots in the jet. We confirm previous findings of speeds up to 4.5c
in the inner 6" of the jet, and report new speeds for optical components in the
outer part of the jet. We find evidence of significant motion transverse to the
jet axis on the order of 0.6c in the inner jet features, and superluminal
velocities parallel and transverse to the jet in the outer knot components,
with an apparent ordering of velocity vectors possibly consistent with a
helical jet pattern. Previous results suggested a global deceleration over the
length of the jet in the form of decreasing maximum speeds of knot components
from HST-1 outward, but our results suggest that superluminal speeds persist
out to knot C, with large differentials in very nearby features all along the
jet. We find significant apparent accelerations in directions parallel and
transverse to the jet axis, along with evidence for stationary features in
knots D, E, and I. These results are expected to place important constraints on
detailed models of kpc-scale relativistic jets.
