The Fate of Young Radio Galaxies: Decelerations Inside Host Galaxies?

Abstract

We examine the evolution of variously sized radio galaxies (i.e., compact symmetric objects [CSOs], medium-size symmetric objects [MSOs], Fanaroff-Riley type II (FR II) radio galaxies by comparing the relation between the hot spot size and the projected linear size with a coevolution model of hot spots and a cocoon. We take account of the deceleration effect by the cocoon head growth. We find that the advance speed of hot spots and lobes inevitably show the deceleration phase (CSO-MSO phase) and the acceleration phase (MSOYFR II phase). This is ascribed to the change of the power-law index of ambient density profile in the MSO phase ($1 kpc). It is also found that the cocoon shape becomes nearly spherical or disrupted for MSOs, while an elongated morphology is predicted for CSOs and FR II galaxies. This seems to be consistent with the higher fraction of distorted morphology of MSOs than that of CSOs and FR II galaxies. Finally, we predict that only CSOs whose initial advance speed is higher than about 0.1 c can evolve into FR II galaxies, comparing the hot spot speed with the sound speed of the ambient medium.