AGN Jet Power, Formation of X-ray Cavities, and FR I/II Dichotomy in Galaxy Clusters

TL;DR

This study examines how AGN jets interact with galaxy cluster gas, revealing that less powerful jets tend to form FR I sources and explaining the observed FR I/II distribution based on jet power and evolution.

Contribution

It demonstrates that jets with observed powers below 10^{44} erg s^{-1} evolve as FR I sources, providing a physical explanation for the FR I/II dichotomy in galaxy clusters.

Findings

Jets with <~ 10^{44} erg s^{-1} cannot become FR II sources.

FR I sources are more common in clusters due to jet deceleration.

Predicted X-ray cavity sizes match observations within a factor of two.

Abstract

We investigate the ability of jets in active galactic nuclei (AGNs) to break out of the ambient gas with sufficiently large advance velocities. Using observationally estimated jet power, we analyze 28 bright elliptical galaxies in nearby galaxy clusters. Because the gas density profiles in the innermost regions of galaxies have not been resolved so far, we consider two extreme cases for temperature and density profiles. We also follow two types of evolution for the jet cocoons: being driven by the pressure inside the cocoon (Fanaroff-Riley [FR] I type), and being driven by the jet momentum (FR II type). Our main result is that regardless of the assumed form of density profiles, jets with observed powers of <~ 10^{44} erg s^{-1} are not powerful enough to evolve as FR II sources. Instead, they evolve as FR I sources and appear to be decelerated below the buoyant velocities of the cocoons when jets were propagating through the central dense regions of the host galaxies. This explains the reason why FR I sources are more frequent than FR II sources in clusters. Furthermore, we predict the sizes of X-ray cavities from the observed jet powers and compare them with the observed ones --- they are consistent within a factor of two if the FR I type evolution is realized. Finally, we find that the jets with a power >~ 10^{44} erg s^{-1} are less affected by the ambient medium, and some of them, but not all, could serve as precursors of the FR II sources.