Skip to main content

Non-Equilibrium Processes in Galaxy Clusters

Final Report Summary - NEPAL (Non-Equilibrium Processes in Galaxy Clusters)

The project targets the dynamical and nonthermal processes in the intracluster medium (ICM) of galaxy clusters. The main objectives are to (i) resolve the nonthermal components of the ICM, and (ii) facilitate a self-consistent model of cool cores. The program is based on the combination of a phenomenological analysis of observational data, a theoretical study, and dedicated numerical simulations.

Plans for both grant periods were largely achieved. In some aspects, progress has exceeded expectations, due to surprising discoveries. The work has branched into several new lines of research, as anticipated in light of scientific and personnel developments. For example, considerable research in the second grant period was devoted to the Fermi bubbles, as local examples of galactic outflows enriching the ICM.

Significant results that were already advertised include:
1) Analytic models for galaxy cluster spiral flows suggest that the core structure is intimately related to spiral flows.
2) Indications that a dynamical mechanism shapes the universal, linear entropy profile of galaxy clusters and groups.
3) Generalizing the spiral flow model for extended CFs naturally reproduces the linear entropy profile observed.
4) Measurement of strong, ~10 percent level magnetization beneath non-merger CFs in galaxy clusters.
5) Substructure below the cold front in Virgo, discovered in a very deep Chandra observation, may arise from entrained AGN bubbles.
6) A tentative identification of the first virial shock, using the VERITAS gamma-ray significance map around Coma.
7) The first analytic approximation for the flow in front of a blunt object spanning the subsonic, transonic, and supersonic regime.
8) An analytic solution to the magnetization by clumps and bubbles, using path or stream functions.
9) Modelling the evolution of electromagnetic fields due to the cumulative motion of clumps and bubbles.
10) A natural model for the radio to gamma-ray Fermi bubble spectrum, yielding a magnetic field estimate and a CR proton upper limit.
11) New analytic and numeric methods to compute the magnetic diffusion function and particle acceleration in collisionless shocks.

These results highlight the importance of dynamical processes in governing the structure of a galaxy cluster. In particular, an extended spiral flow appears essential in shaping the thermal structure, and in stemming catastrophic cooling. The flow likely extends out to the virial radius. We showed that it magnetizes the plasma, at least near the CFs, and is likely responsible for the universal, linear entropy profile discovered. Preliminary detections of the nonthermal components associated with the CFs and with the virial shock have been obtained.

The research also facilitates new analytical and numerical tools for the study of galaxy clusters. This includes hydrodynamic and MHD solutions for motions in the ICM, a consistent galactic outflow model, a simplified description of particle diffusion in collisionless shocks, and the adaptation of a new AMR (DAFNA) code for galaxy cluster simulations.

Due to the multiple new lines of research, some work is still ongoing, with five to eight additional papers expected during 2016. Part of the research, in particular pertaining to numerical work, may culminate only during 2017, due to a shortage of computational resources and manpower.

One Ph.D student (I. Reiss) and two M.Sc. students (Y. Naor, now a Ph.D student, and E. Malka) have been fully committed to the project. A post-doc (I. Gurwich), an additional Ph.D student (Y. Nagar), three additional M.Sc. students (N. Sherf, I. Raveh, I. Wallerstein), and a few undergraduate students, have worked part-time on the project.