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

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### Abstract

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.

### Author and article information

###### Journal
20 July 2020
###### Article
2007.10474