MERLIN observations of superluminal motion in the jet of 3C120

Abstract

We present evidence for changes in the radio structure of the jet in 3C120 between December 1980 and December 1988 derived from MERLIN observations at 1.67 GHz. The changes are consistent with outward motion of 19±2 mas (2.4 ± 0.3 mas yr⁻¹, i.e. 3.5 ± 0.4h⁻¹ c, for H₀ = 100h km s⁻¹ Mpc⁻¹) in the inner jet, about 0.15 arcsec (∼ 75h⁻¹ pc) from the core. This confirms the suggestion by Benson etal. (1988) that the jet does not slow down quickly just outside the core region usually imaged by VLBI. However, we find no evidence for motion at the position of the knot 4 arcsec (∼ 2h⁻¹ kpc) from the nucleus as suggested by Walker et al. (1988). Our limit is 1.26 mas yr⁻¹, i.e. <1.8h⁻¹ c, compared with Walker et al. ’s measurement of 2.4 ± 0.8 mas yr⁻¹ i.e. 3.7 ± 1.2h⁻¹ c.