On the Non-Thermal Energy Content of Cosmic Structures
Abstract
:1. Introduction
2. Results
2.1. The Distribution of NT Energy Inside Galaxy Clusters
- The turbulent kinetic energy ranges from the case in which all velocity components larger than are filtered out (lower line), to the upper limit in which all the kinetic energy resolved in the cells is assumed to be turbulent. While our previous studies suggest that the typical outer scale of turbulent motions is of the order of 100–300 kpc, in the presence of large-scale accretion this scale can increase. Hence the hatched region here is meant to bracket the plausible range of ICM turbulence. The turbulent energy support increases with radius, because the driving from infall motions in the outskirts is increasingly supersonic, due to the radial drop in the ICM temperature.
- The magnetic energy ranges from the primordial seed field model (trend increasing with radius) to the trend of the AGN seeding (trend decreasing with radius). In the AGN scenario the magnetic energy drops quickly with radius, as the overall activity by AGN is unable to significantly magnetise large volumes outside of clusters (It shall be noted that also the underlying distribution of the thermal is slightly modified in the astrophysical seeding scenarios, due to the balance of cooling and feedback, which results into a denser cluster core). Both these scenarios are probably underestimating the magnetic energy in the cluster centre, because the finite Reynolds number achieved in our run (, based on , where N is the 1-dimensional size of the high-resolution domain of the simulations, in number of cells, e.g., [48]) likely causes a delayed start of the small-scale dynamo amplification, compared to reality (e.g., [37]). Limited to the single case of the Coma cluster, our primordial seeding run seems to be in better agreement with the observational results of Faraday Rotation (e.g., [49,50]), which suggests a distribution of magnetic energy that scales with the gas thermal energy, and a significant magnetisation in the outskirts (even if limited to a single narrow sector of Coma, e.g., [50]);
- The cosmic-ray energy goes from the upper limit obtained with out post-processing modelling of tracers [39] to zero in case no CR-protons are accelerated by shocks within the cluster. The increasing trend with radius of the CR-energy follows from the sharp increase of the acceleration efficiency as a function of Mach number, which rapidly increase towards cluster outskirts (e.g., [38]).
2.2. The Large-Scale Distribution of Non-Thermal Energy
- The turbulent kinetic energy is estimated as in the previous case via small-scale filtering (lower limit), or assuming that the entire post-shock kinetic energy is channeled into turbulence. This second option is particularly significant in the rarefied warm-hot intergalactic medium (WHIM) outside cluster, where our coarse resolution might underestimate vorticity injected by strong accretion shocks [32] . Again, the turbulent kinetic energy budget increases towards lower densities, becoming nearly supersonic at the scale of the linear structures of the Universe (e.g., filaments). At the over density probed through CIV absorption lines in the WHIM, the limits on shear motions at high-z are ∼10% percent of the thermal energy [52], which is at the level of our highest estimate of turbulence. For the cluster cores, the turbulent support is instead in line with present upper limits from X-ray analysis by XMM-Newton observations [53,54], consistent with our estimate at higher resolution in the previous Section.
- The magnetic energy is here measured by combining runs including only primordial seeding at high redshift, or with the additional seeding by AGN at run-time. Outside of clusters the uncertainties are very big because the fields are in the range depending on the seeding model. Even more significantly than in the previous Section, our simulated magnetic fields are found to be smaller than what has been observed through Faraday Rotation (e.g., [49,50,55]), due to the lack of resolution that prevents the formation of a small-scale dynamo. However, in the high density range of our distribution the magnetic energy can be significant, owing to the assumed fixed thermal/magnetic energy per event, which can have a higher impact on low mass systems compared to the previous analysis of the Coma-like cluster.
- The cosmic-ray energy is limited by the requirement of staying within the γ-ray limits by FERMI [27]. While the uncertainties in the acceleration function produce a large uncertainty in the CR energy budget within clusters (owing to the fact that the acceleration efficiency of weak, shocks is very uncertain), the numerical uncertainties are smaller outside clusters, where shocks are predicted to be strong () and the acceleration efficiency is expected to be ∼10%–30% based on the the modelling of strong supernova shocks (e.g., [56]).
3. Discussion and Conclusions
Acknowledgments
Author Contributions
Conflicts of Interest
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Vazza, F.; Wittor, D.; Brüggen, M.; Gheller, C. On the Non-Thermal Energy Content of Cosmic Structures. Galaxies 2016, 4, 60. https://doi.org/10.3390/galaxies4040060
Vazza F, Wittor D, Brüggen M, Gheller C. On the Non-Thermal Energy Content of Cosmic Structures. Galaxies. 2016; 4(4):60. https://doi.org/10.3390/galaxies4040060
Chicago/Turabian StyleVazza, Franco, Denis Wittor, Marcus Brüggen, and Claudio Gheller. 2016. "On the Non-Thermal Energy Content of Cosmic Structures" Galaxies 4, no. 4: 60. https://doi.org/10.3390/galaxies4040060