- The paper investigates second order Fermi reacceleration mechanisms responsible for synchrotron radio emission in intra-cluster bridges, structures connecting galaxy clusters.
- Using numerical simulations, the study suggests weak shocks and the reacceleration of fossil electrons from AGN are crucial for sustaining the observed large-scale radio emissions.
- Findings imply radio observations can probe cosmic web dynamics and the model predicts spectral indices aligning with observations, offering insights for future high-frequency surveys.
Second Order Fermi Reacceleration Mechanisms in Intra-Cluster Bridges
The exploration of synchrotron radio emission within intra-cluster bridges has been significantly advanced by this paper, which elaborates on the mechanisms of second order Fermi reacceleration. These bridges, observed as radio arcs interconnecting galaxy clusters, present a complex dynamical environment. The authors propose that these large-scale structures could be sustained by the reacceleration of electrons through interactions with Mpc-scale turbulence, driven by weak shocks and super-Alfvénic turbulence.
The paper employs numerical simulations to examine these intra-cluster bridges. The results suggest that weak shocks with Mach numbers approximately between 2 and 3 are prevalent, affecting a notable fraction (up to 10%) of the volume in the past billion years. However, the rarity of strong shocks favors the notion that processes beyond simple shock acceleration, particularly involving fossil electrons, are essential to account for the observed radio emissions. These electrons, remnants from the activity of active galactic nuclei (AGN) and star-forming galaxies, make up the supra-thermal population available for reacceleration.
A critical aspect of this research centers on the interactions of relativistic electrons with the super-Alfvénic turbulence prevalent in these regions. This turbulence is suggested to energize both the particles, through stochastic processes, and the magnetic fields within these volumes, providing the framework for wide-area synchrotron emissions. The magnetic fields, estimated to average around 0.5–0.6 μG, are shown to be amplified by factors of magnitude compared to primordial magnetic fields.
The implications for observational cosmology are substantial. The findings imply that radio observations can probe energy dissipation on scales larger than individual galaxy clusters, thus serving as a new observational window into the dynamics of the cosmic web. Additionally, this synchrotron emission model derived from second order Fermi processes aligns with the spectra observed between 0.15-1.5 GHz with a spectral index, α, of about 1.3-1.5 or steeper, offering predictions for future high-frequency radio surveys.
Theoretical implications arise from the ability of second order Fermi mechanisms to sustain such extensive regions of the intracluster medium. The paper suggests adjustments to our understanding of cosmic ray energetics and magnetic field dynamics, with potential impacts on fields studying cosmic ray propagation, and mergers and AGN activity in the high redshift universe. Moreover, the research invites further investigation with current and future observational data to test the predominance of volume vs. localized phenomena as contributors to radio emissions.
Future developments in radio astronomy, equipped to explore these frequency ranges and spatial extents, will provide a more detailed understanding of the acceleration processes at play. This will further elucidate the lifecycle of cosmic rays and the means of energy dissipation across massive cosmic structures, shaping our understanding of these grand scales of the universe.