The Fate of Dead Radio-loud Active Galactic Nuclei: A New Prediction of Long-lived Shell Emission

摘要

We examine the fate of a dead radio source in which jet injection from thecentral engine has stopped at an early stage of its evolution ($t = t_j\lesssim 10^5$ yr). To this aim, we theoretically evaluate the evolution of theemission from both the lobe and the shell, which are composed of shocked jetmatter and a shocked ambient medium, respectively. Based on a simple dynamicalmodel of expanding lobe and shell, we clarify how the broadband spectrum ofeach component evolves before and after the cessation of the jet activity. Itis shown that the spectrum is strongly dominated by the lobe emission while thejet is active ($t \leq t_j$). On the other hand, once the jet activity hasceased ($t > t_j$), the lobe emission fades out rapidly, since fresh electronsare no longer supplied from the jet. Meanwhile, shell emission only shows agradual decrease, since accelerated electrons are continuously supplied fromthe bow shock that is propagating into the ambient medium. As a result, overallemission from the shell overwhelms that from the lobe at wide range offrequencies from radio up to gamma-ray soon after the jet activity has ceased.Our result predicts a new class of dead radio sources that are dominated byshell emission. We suggest that the emission from the shell can be probed inparticular at a radio wavelengths with the Square Kilometer Array (SKA) phase1.