Evolution of Buoyant Bubbles in M87

TL;DR

This paper investigates how buoyant bubbles formed by past active galactic nucleus activity in M87 rise through the galaxy's cooling gas, transforming into tori and uplifting cooler gas, supported by hydrodynamic simulations.

Contribution

It provides a qualitative hydrodynamic model explaining the evolution and observable features of buoyant bubbles in M87's environment.

Findings

Bubbles transform into tori as they rise.

Rising bubbles uplift cooler X-ray emitting gas.

Simulations support observed radio and X-ray features.

Abstract

The morphology of the X-ray and radio emitting features in the central $\sim$ 50 kpc region around the galaxy M87 strongly suggests that buoyant bubbles of cosmic rays (inflated by an earlier nuclear active phase of the galaxy) rise through the cooling gas at roughly half the sound speed. In the absence of strong surface tension, initially spherical bubbles will transform into tori as they rise through an external medium. Such structures can be identified in the radio images of the halo of M87. During their rise, bubbles will uplift relatively cool X-ray emitting gas from the central regions of the cooling flow to larger distances. This gas is colder than the ambient gas and has a higher volume emissivity. As a result, rising ``radio'' bubbles may be trailed by elongated X-ray features as indeed is observed in M87. We performed simple hydrodynamic simulations to qualitatively illustrate the evolution of buoyant bubbles in the M87 environment.