Optical Proper Motion Measurements of the M87 Jet: New Results from the Hubble Space Telescope

TL;DR

This study uses over 13 years of HST data to measure optical proper motions in the M87 jet, revealing complex velocity structures, superluminal motions, and accelerations that challenge previous models of jet deceleration.

Contribution

It provides the first detailed optical proper motion measurements of the outer jet of M87, showing persistent superluminal speeds and complex velocity patterns over a large spatial extent.

Findings

Superluminal speeds up to 4.5c confirmed in the inner jet.

Evidence of transverse motion around 0.6c in inner features.

Superluminal velocities persist out to the outer knots, contradicting previous deceleration models.

Abstract

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.