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

TL;DR

This paper models the evolution of young radio galaxies, revealing phases of deceleration and acceleration in hot spot speeds, and predicts conditions for their development into larger radio galaxy types based on initial speeds.

Contribution

It introduces a coevolution model accounting for deceleration effects and predicts morphological and dynamical evolution of radio galaxies.

Findings

Hot spots undergo deceleration and acceleration phases.

Cocoon shape varies from spherical to elongated.

Initial hot spot speed >0.1c predicts evolution into FRIIs.

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 radio galaxies (FRIIs)], 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 (MSO-FRII phase). This is ascribed to the change of the power-law index of ambient density profile in the MSO phase ($\sim $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 FRIIs. This seems to be consistent with the higher fraction of distorted morphology of MSOs than that of CSOs and FRI. Finally, we predict that only CSOs whose initial advance speed is higher than about 0.1c can evolve into FRIIs, comparing the hot spot speed with the sound speed of the ambient medium.