[Paper Review] Evolution of Buoyant Bubbles in M87

E. Churazov, M. Brüggen+3 more|arXiv (Cornell University)|Aug 15, 2000

Methane Hydrates and Related Phenomena3 citations

TL;DR

This paper proposes that buoyant cosmic-ray bubbles, inflated by past AGN activity in M87, rise through the cooling X-ray gas at ~half the sound speed, transforming from spheres into tori due to hydrodynamic instabilities. These rising bubbles uplift cold, dense X-ray-emitting gas, creating elongated X-ray features trailing radio lobes—explaining the observed anticorrelation between radio and X-ray morphologies in the central 50 kpc region of M87.

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.

Motivation & Objective

• Explain the complex X-ray and radio morphology in the central 50 kpc of M87, particularly the anticorrelation between radio lobes and X-ray surface brightness.
• Investigate how buoyant bubbles of cosmic rays, generated by past AGN activity, evolve in the hot, cooling intracluster medium.
• Understand the origin of elongated X-ray features trailing prominent radio structures, which are colder and denser than ambient gas.
• Explore the role of convection and energy redistribution from rising bubbles in regulating cooling flows and flattening entropy profiles.
• Assess the potential for bubble-driven heating to suppress mass deposition in cooling flows, reconciling observations with theoretical models.

Proposed method

• Performs 2D hydrodynamic simulations to model the rise of buoyant bubbles through a stratified, cooling gas atmosphere.
• Assumes bubbles are initially spherical and evolve into tori due to Rayleigh-Taylor and Kelvin-Helmholtz instabilities during ascent.
• Models the uplift of colder, denser X-ray-emitting gas by rising bubbles, increasing its volume emissivity and enhancing X-ray surface brightness.
• Uses a simplified energy budget to estimate that 10% of jet power (~5×10⁴³ erg s⁻¹) could heat the cooling flow region and prevent mass deposition.
• Compares simulated morphologies with ROSAT HRI X-ray and high-dynamic-range radio maps (327 MHz) of M87 to validate qualitative agreement.
• Analyzes spectral indices from multi-frequency radio data to infer particle age and energy losses, supporting the idea of prolonged AGN activity.

Experimental results

Research questions

• RQ1 How do buoyant cosmic-ray bubbles evolve morphologically in the hot, stratified medium of the M87 cluster core?
• RQ2 What causes the observed elongated X-ray features trailing prominent radio lobes in M87’s halo?
• RQ3 Can the uplift of cold, dense X-ray gas by rising bubbles explain the anticorrelation between radio and X-ray emission in M87?
• RQ4 To what extent can energy from rising bubbles heat the cooling flow and suppress mass deposition?
• RQ5 How do hydrodynamic instabilities (e.g., Rayleigh-Taylor, Kelvin-Helmholtz) influence the transformation of spherical bubbles into toroidal structures?

Key findings

• Buoyant bubbles rising at ~half the sound speed in M87’s cooling flow region evolve from spherical shapes into toroidal (mushroom-like) structures due to hydrodynamic instabilities.
• The uplifted cold X-ray gas trailing the bubbles has higher volume emissivity than ambient gas, producing the observed elongated X-ray features.
• The morphology of the radio halo and X-ray surface brightness distribution show qualitative agreement with simulations of rising bubbles and gas uplift.
• Energy dissipation from subsonic bubble motions, if efficient (≥10% of jet power), can heat the central region and prevent mass deposition over ~10⁸ years.
• Convection driven by rising bubbles may flatten X-ray surface brightness and entropy profiles, promoting distributed mass deposition and filament formation.
• The model provides a plausible explanation for the observed anticorrelation between radio lobes and X-ray surface brightness in M87, consistent with XMM-Newton results.