Sound wave generation by a spherically symmetric outburst and AGN feedback in galaxy clusters II: impact of thermal conduction

TL;DR

This paper investigates how thermal conduction affects shock-heated gas shells created by black hole outbursts in galaxy clusters, showing conduction can erase observable shells unless suppressed, and discusses sound wave dissipation.

Contribution

It demonstrates that thermal conduction can eliminate observable shock shells unless electron diffusion is heavily suppressed, and proposes the shock-to-bubble radius ratio as a more reliable energy indicator.

Findings

Thermal conduction can erase shock-heated shells unless suppressed.

The shock-to-bubble radius ratio is a robust proxy for energy release.

Sound wave dissipation length is limited by conduction suppression factors.

Abstract

We analyze the impact of thermal conduction on the appearance of a shock-heated gas shell which is produced when a spherically symmetric outburst of a supermassive black hole inflates bubbles of relativistic plasma at the center of a galaxy cluster. The presence of the hot and low-density shell can be used as an ancillary indicator for a high rate of energy release during the outburst, which is required to drive strong shocks into the gas. Here we show that conduction can effectively erase such shell, unless the diffusion of electrons is heavily suppressed. We conclude that a more robust proxy to the energy release rate is the ratio between the shock radius and bubble radius. We also revisited the issue of sound waves dissipation induced by thermal conduction in a scenario, where characteristic wavelength of the sound wave is set by the total energy of the outburst. For a fiducial short outburst model, the dissipation length does not exceed the cooling radius in a typical cluster, provided that the conduction is suppressed by a factor not larger than $\sim$100. For quasi-continuous energy injection neither the shock-heated shell nor the outgoing sound wave are important and the role of conduction is subdominant.