Gamma-ray emission in radio galaxies under the VLBI scope -- II. The relationship between gamma-ray emission and parsec-scale jets in radio galaxies

R. Angioni, E. Ros, M. Kadler, R. Ojha, C. Müller, P. G. Edwards, P. R. Burd, B. Carpenter, M. S. Dutka, S. Gulyaev, H. Hase, S. Horiuchi, F. Krauß, J. E. J. Lovell, T. Natusch, C. Phillips, C. Plötz, J. F. H. Quick, F. Rösch, R. Schulz, J. Stevens, A. K. Tzioumis, S. Weston, J. Wilms, J. A. Zensus

Published 2020-07-20Version 1

Following our study of the radio and high-energy properties of $\gamma$-ray-emitting radio galaxies, here we investigate the kinematic and spectral properties of the parsec-scale jets of radio galaxies that have not yet been detected by Fermi-LAT. We take advantage of the regular VLBI observations provided by the TANAMI monitoring program, and explore the kinematic properties of six $\gamma$-ray-faint radio galaxies. We include publicly available VLBI kinematics of $\gamma$-ray-quiet radio galaxies monitored by the MOJAVE program and perform a Fermi-LAT analysis, deriving upper limits. We combine these results with those from our previous paper to construct the largest sample of radio galaxies with combined VLBI and $\gamma$-ray measurements to date. We find superluminal motion up to $\beta_\mathrm{app}=3.6$ in the jet of PKS 2153$-$69. We find a clear trend of higher apparent speed as a function of distance from the jet core on scales of $\sim10^5\,R_s$, corresponding to the end of the collimation and acceleration zone in nearby radio galaxies. We find evidence of subluminal apparent motion in the jets of PKS 1258$-$321 and IC 4296, and no measurable motion for PKS 1549$-$79, PKS 1733$-$565 and PKS 2027$-$308. We compare the VLBI properties of $\gamma$-ray-detected and undetected radio galaxies, and find significantly different distributions of median core flux density, and, possibly, of median core brightness temperature. We find a significant correlation between median core flux density and $\gamma$-ray flux, but no correlation with typical Doppler boosting indicators such as median core brightness temperature and core dominance. Our study suggests that high-energy emission from radio galaxies is related to parsec-scale radio emission from the inner jet, but is not driven by Doppler boosting effects, in contrast to the situation in their blazar counterparts.