Sound Waves Excitation by Jet-Inflated Bubbles in Clusters of Galaxies

TL;DR

This study demonstrates that sound waves in galaxy cluster gas can be generated by jet-inflated bubbles, explaining observed ripples, through detailed hydrodynamical simulations of jet-driven bubble inflation.

Contribution

It shows that jet-inflated bubbles, not artificially injected ones, can produce multiple sound waves matching observations in galaxy clusters.

Findings

Sound waves are excited by bubble boundary motion caused by vortices and backflow.

Simulations reproduce ripples observed in the Perseus cluster.

Jets are slow, massive, and can be wide or precessing, inflating bubbles that generate sound waves.

Abstract

We show that repeated sound waves in the intracluster medium (ICM) can be excited by a single inflation episode of an opposite bubble pair. To reproduce this behavior in numerical simulations the bubbles should be inflated by jets, rather than being injected artificially. The multiple sound waves are excited by the motion of the bubble-ICM boundary that is caused by vortices inside the inflated bubbles and the backflow (`cocoon') of the ICM around the bubble. These sound waves form a structure that can account for the ripples observed in the Perseus cooling flow cluster. We inflate the bubbles using slow massive jets, with either a wide opening angle or that are precessing. The jets are slow in the sense that they are highly sub-relativistic, $v_j \sim 0.01c-0.1c$, and they are massive in the sense that the pair of bubbles carry back to the ICM a large fraction of the cooling mass, i.e., $\sim 1-50 M_\odot \yr^{-1}$. We use a two-dimensional axisymmetric (referred to as 2.5D) hydrodynamical numerical code (VH-1).