Gas perturbations in cool cores of galaxy clusters: effective equation of state, velocity power spectra and turbulent heating

TL;DR

This study analyzes X-ray fluctuations in galaxy cluster cool cores, revealing gentle AGN feedback, low gas velocities, and turbulence levels sufficient to offset cooling, supporting models of non-explosive energy input.

Contribution

It provides detailed statistical analysis of gas fluctuations and velocities in cool cores, supporting gentle AGN feedback models over explosive scenarios.

Findings

Density fluctuations are ~10% on 10-15 kpc scales.

Gas velocities are below 150 km/s on <50 kpc scales.

Non-thermal energy is less than 12% of thermal energy.

Abstract

We present the statistical analysis of X-ray surface brightness and gas density fluctuations in cool cores of ten, nearby and bright galaxy clusters that have deep Chandra observations and show observational indications of radio-mechanical AGN feedback. Within the central parts of cool cores the total variance of fluctuations is dominated by isobaric and/or isothermal fluctuations on spatial scales ~ 10-60 kpc, which are likely associated with slow gas motions and bubbles of relativistic plasma. Adiabatic fluctuations associated with weak shocks constitute less than 10 per cent of the total variance in all clusters. The typical amplitude of density fluctuations is small, ~ 10 per cent or less on scales of ~ 10-15 kpc. Subdominant contribution of adiabatic fluctuations and small amplitude of density fluctuations support a model of gentle AGN feedback as opposed to periodically explosive scenarios which are implemented in some numerical simulations. Measured one-component velocities of gas motions are typically below 100-150 km/s on scales < 50 kpc, and can be up to ~ 300 km/s on ~ 100 kpc scales. The non-thermal energy is < 12 per cent of the thermal energy. Regardless of the source that drives these motions the dissipation of the energy in such motions provides heat that is sufficient to balance radiative cooling on average, albeit the uncertainties are large. Presented results here support previous conclusions based on the analysis of the Virgo and Perseus Clusters, and agree with the Hitomi measurements. With next generation observatories like Athena and Lynx, these techniques will be yet more powerful.