Matter content in AGN jets: constraint from cocoon dynamics?

The matter content of jets in active galactic nuclei is examined in a new way. We model the dynamical expansion of its cocoon embedded in the intra-cluster medium (ICM). By comparing the observed shape of the cocoon with that expected from the theoretical model, we estimate the total pressure ($P_{\rm c}$) and electron temperature ($T_{e}$) of the cocoon. The number density of the total electrons ($n_{e^{-}}$) is constrained by using the non-thermal spectrum of the hot spot and the analysis of the momentum balance between the jet thrust and the rum pressure of ICM. Together with the obtained $P_{\rm c}$, $T_{e}$ and $n_{e^{-}}$, we constrain the matter content in the jets. We find that, in the case of Cygnus A, the ratio of number density of protons to that of electrons is of order of $10^{-3}$ . This implies the existence of a large number of positron in the jet.Comment: 4 pages, to appear in the proceedings of "Astrophysical Sources of High Energy Particles and Radiation", Torun, 20-24 June 200

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

We examine the fate of a dead radio source in which jet injection from the central 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 the emission from both the lobe and the shell, which are composed of shocked jet matter and a shocked ambient medium, respectively. Based on a simple dynamical model of expanding lobe and shell, we clarify how the broadband spectrum of each component evolves before and after the cessation of the jet activity. It is shown that the spectrum is strongly dominated by the lobe emission while the jet is active ($t \leq t_j$). On the other hand, once the jet activity has ceased ($t > t_j$), the lobe emission fades out rapidly, since fresh electrons are no longer supplied from the jet. Meanwhile, shell emission only shows a gradual decrease, since accelerated electrons are continuously supplied from the bow shock that is propagating into the ambient medium. As a result, overall emission from the shell overwhelms that from the lobe at wide range of frequencies 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 by shell emission. We suggest that the emission from the shell can be probed in particular at a radio wavelengths with the Square Kilometer Array (SKA) phase 1.Comment: 9 pages, 3 figures, accepted for publication in Ap