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Register a new userTurbulent Heating in Galaxy Clusters Brightest in X-rays
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The hot, X-ray-emitting intracluster medium (ICM) is the dominant baryonic constituent of clusters of galaxies. In the cores of many clusters, radiative energy losses from the ICM occur on timescales significantly shorter than the age of the system. Unchecked, this cooling would lead to massive accumulations of cold gas and vigorous star formation, in contradiction to observations. Various sources of energy capable of compensating these cooling losses have been proposed, the most promising being heating by the supermassive black holes in the central galaxies through inflation of bubbles of relativistic plasma. Regardless of the original source of energy, the question of how this energy is transferred to the ICM has remained open. Here we present a plausible solution to this question based on deep Chandra X-ray observatory data and a new data-analysis method that enables us to evaluate directly the ICM heating rate due to the dissipation of turbulence. We find that turbulent heating is sufficient to offset radiative cooling and indeed appears to balance it locally at each radius - it might therefore be the key element in resolving the gas cooling problem in cluster cores and, more universally, in atmospheres of X-ray gas-rich systems.
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The discrepancy between expected and observed cooling rates of X-ray emitting gas has led to the {it cooling flow problem} at the cores of clusters of galaxies. A variety of models have been proposed to model the observed X-ray spectra and resolve the cooling flow problem, which involves heating the cold gas through different mechanisms. As a result, realistic models of X-ray spectra of galaxy clusters need to involve both heating {it and} cooling mechanisms. In this paper, we argue that the heating time-scale is set by the magnetohydrodynamic (MHD) turbulent viscous heating for the Intracluster plasma, parametrised by the Shakura-Sunyaev viscosity parameter, $alpha$. Using a cooling+heating flow model, we show that a value of $alphasimeq 0.05$ (with 10% scatter) provides improved fits to the X-ray spectra of cooling flow, while at the same time, predicting reasonable cooling efficiency, $epsilon_{cool} = 0.33^{+0.63}_{-0.15}$. Our inferred values for $alpha$ based on X-ray spectra are also in line with direct measurements of turbulent pressure in simulations and observations of galaxy clusters. This simple picture unifies astrophysical accretion, as a balance of MHD turbulent heating and cooling, across more than 16 orders of magnitudes in scale, from neutron stars to galaxy clusters.
Due to their late formation in cosmic history, clusters of galaxies are not fully in hydrostatic equilibrium and the gravitational pull of their mass at a given radius is expected not to be entirely balanced by the thermal gas pressure. Turbulence may supply additional pressure, and recent (X-ray and SZ) hydrostatic mass reconstructions claim a pressure support of $sim 5-15%$ of the total pressure at $R_{rm 200}$. In this work we show that, after carefully disentangling bulk from small-scale turbulent motions in high-resolution simulations of galaxy clusters, we can constrain which fraction of the gas kinetic energy effectively provides pressure support in the clusters gravitational potential. While the ubiquitous presence of radial inflows in the cluster can lead to significant bias in the estimate of the non-thermal pressure support, we report that only a part of this energy effectively acts as a source of pressure, providing a support of the order of $sim 10%$ of the total pressure at $R_{rm 200}$.
Buoyant bubbles of relativistic plasma in cluster cores plausibly play a key role in conveying the energy from a supermassive black hole to the intracluster medium (ICM) - the process known as radio-mode AGN feedback. Energy conservation guarantees that a bubble loses most of its energy to the ICM after crossing several pressure scale heights. However, actual processes responsible for transferring the energy to the ICM are still being debated. One attractive possibility is the excitation of internal waves, which are trapped in the clusters core and eventually dissipate. Here we show that a sufficient condition for efficient excitation of these waves in stratified cluster atmospheres is flattening of the bubbles in the radial direction. In our numerical simulations, we model the bubbles phenomenologically as rigid bodies buoyantly rising in the stratified cluster atmosphere. We find that the terminal velocities of the flattened bubbles are small enough so that the Froude number ${rm Fr}lesssim 1$. The effects of stratification make the dominant contribution to the total drag force balancing the buoyancy force. In particular, clear signs of internal waves are seen in the simulations. These waves propagate horizontally and downwards from the rising bubble, spreading their energy over large volumes of the ICM. If our findings are scaled to the conditions of the Perseus cluster, the expected terminal velocity is $sim100-200{,rm km,s^{-1}}$ near the cluster cores, which is in broad agreement with direct measurements by the Hitomi satellite.
Emission of AGNs and neutral pion decay - are the two most natural mechanisms, that could make a galaxy cluster be a source of gamma-rays in the GeV regime. We revisited this problem by using 52.5-month FERMI-LAT data above 10 GeV and stacking 55 clusters from the HIFLUGS sample of the X-ray brightest clusters. The choice of >10 GeV photons is optimal from the point of view of angular resolution, while the sample selection optimizes the chances of detecting signatures of the neutral pion decay, arising from hadronic interactions of relativistic protons with an intra-cluster medium, which scale with the X-ray flux. In the stacked data we detected a signal for the central 0.25 deg circle at the level of 4.3 sigma. An evidence for a spatial extent of the signal is marginal. A subsample of cool-core clusters has higher count rate 1.9+/-0.3 per cluster compared to the subsample of non-cool core clusters 1.3+/-0.2. Several independent arguments suggest that the contribution of AGNs to the observed signal is substantial if not dominant. No strong support for the large contribution of pion decay was found. In terms of a limit on the relativistic protons energy density, we got an upper limit of ~1.5% relative to the gas thermal energy density, provided that the spectrum of relativistic protons is hard (s=4.1 in dN/dp=p^-s). This estimate assumes that relativistic and thermal components are mixed. For softer spectra the limits are weaker.
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.
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