The Inner Jet of the Radio Galaxy M87

TL;DR

This paper presents high-resolution VLBA images of M87's inner jet, revealing limb brightening, a faint counter-jet, and slow-moving features, supporting a spine-sheath velocity structure and raising questions about jet symmetry and flow speed.

Contribution

It provides detailed imaging evidence of the jet's limb brightening and counter-jet, and discusses implications for jet velocity structure and symmetry in M87.

Findings

Detection of limb-brightened jet structure.

Observation of a faint, limb-brightened counter-jet.

Jet features exhibit slow apparent speeds despite relativistic expectations.

Abstract

We report new 2 cm VLBA images of the inner radio jet of M87 showing a limb brightened structure and unambiguous evidence for a faint 3 mas long counter-feature which also appears limb brightened. Multi-epoch observations of seven separate jet features show typical speeds of less than a few percent of the speed of light, despite the highly asymmetric jet structure and the implications of the canonical relativistic beaming scenario. The observed morphology is consistent with a two stream spine-sheath velocity gradient across the jet, as might be expected from the recently discovered strong and variable TeV emission as well as from numerical modeling of relativistic jets. Considering the large jet to counter-jet flux density ratio and lack of observed fast motion in the jet, we conclude that either the inner part of the M87 jet is intrinsically asymmetric or that the bulk plasma flow speed is much greater than any propagation of shocks or other pattern motions.