γ-ray emission in radio galaxies under the VLBI scope
Authors: Angioni R. , Ros E. , Kadler M. , Ojha R. , Müller C. , Edwards P. G. , Burd P. R. , Carpenter B. , Dutka M. S. , Gulyaev S. , Hase H. , Horiuchi S. , Krauß F. , Lovell J. E. J. , Natusch T. , Phillips C. , Plötz C. , Quick J. F. H. , Rösch F. , Schulz R. , Stevens J. , Tzioumis A. K. , Weston S. , Wilms J. , Zensus J. A. 
Published: July 2020

Aims.This is the second paper in our series studying the evolution of parsec-scale radio emission in radio galaxies in the Southern Hemisphere. Following our study of the radio and high-energy properties ofγ-ray-emitting sources, here we investigate the kinematic and spectral properties of the parsec-scale jets of radio galaxies that have not yet been detected by theFermiLarge Area Telescope (Fermi-LAT) instrument on board NASA’sFermiGamma-ray Space Telescope. For many sources, these results represent the first milliarcsecond resolution information in the literature. These studies were conducted within the framework of the Tracking Active Nuclei with Austral Milliarcsecond Interferometry (TANAMI) monitoring program and in the context of high-energyγ-ray observations fromFermi-LAT.

Methods.We took advantage of the regular 8.4 GHz and 22.3 GHz Very Long Baseline Interferometry (VLBI) observations provided by the TANAMI monitoring program, and explored the kinematic properties of sixγ-ray-faint radio galaxies. We complemented this with ∼8.5 years ofFermi-LAT data, deriving updated upper limits on theγ-ray emission from this subsample of TANAMI radio galaxies. We included publicly available VLBI kinematics ofγ-ray-quiet radio galaxies monitored by the MOJAVE program and performed a consistentFermi-LAT analysis. We combined these results with those from our previous paper to construct the largest sample of radio galaxies with combined VLBI andγ-ray measurements to date. The connection between parsec-scale jet emission and high-energy properties in the misaligned jets of radio galaxies was explored.

Results.For the first time, we report evidence of superluminal motion up toβapp = 3.6 in the jet of theγ-ray-faint radio galaxy PKS 2153−69. We find a clear trend of higher apparent speed as a function of distance from the jet core, which indicates that the jet is still being accelerated on scales of tens of parsecs, or ∼105 Rs, 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 apparent motion for PKS 1549−79, PKS 1733−565, and PKS 2027−308. For all these sources, TANAMI provides the first multi-epoch kinematic analysis on parsec scales. We then compare the VLBI properties ofγ-ray-detected and undetected radio galaxies, and find that the two populations show a significantly different distribution of median core flux density, and, possibly, of median core brightness temperature. In terms of correlation between VLBI andγ-ray properties, we find a significant correlation between median core flux density andγ-ray flux, but no correlation with typical Doppler boosting indicators such as median core brightness temperature and core dominance.

Conclusions.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. This implies thatγ-ray loudness does not necessarily reflect a higher prevalence of boosting effects.