Non-thermal Sunyaev-Zeldovich signal from radio galaxy cocoons.

Energetic electrons in the cocoons of radio galaxies make them potential sources for not only radio and X-rays but also Sunyaev-Zeldovich (SZ) distortions in the cosmic microwave background (CMB) radiation. Previous works have discussed the energetics of radio galaxy cocoons, but assuming thermal SZ effect, coming from the non-thermal electron population. We use an improved evolutionary model for radio galaxy cocoons to estimate the observed parameters such as the radio luminosities and intensity of SZ-distortions at the redshifts of observation. We, further, quantify the the effects of various relevant physical parameters of the radio galaxies, such as the jet power, the time scale over which the jet is active, the evolutionary time scale for the cocoon, etc on the observed parameters. For current SZ observations towards galaxy clusters, we find that the non-thermal SZ distortions from radio cocoons embedded in galaxy clusters can be non-negligible compared to the amount of thermal SZ distortion from the intra-cluster medium and, hence, can not be neglected. We show that small and young (and preferably residing in a cluster environment) radio galaxies offer better prospects for the detection of the non-thermal SZ signal from these sources. We further discuss the limits on different physical parameters for some sources for which SZ effect has been either detected or upper limits are available. The evolutionary models enable us to obtain limits, previously unavailable, on the low energy cut-off of electron spectrum ($p_{min} \sim 1\hbox{--}2$) in order to explain the recent non-thermal SZ detection \citep{MDCMSNW2017}. Finally, we discuss how future CMB experiments, which would cover higher frequency bands ($>$400 GHz), may provide clear signatures for non-thermal SZ effect.