Cosmic ray feedback in galaxies and active galactic nuclei

Abstract

Recent cosmological simulations of galaxy formation have demonstrated that feedback by star formation, supernovae and active galactic nuclei appears to be critical in obtaining realistic disk galaxies, to slow down star formation to the small observed rates, to move gas and metals out of galaxies, and to balance radiative cooling of low-entropy gas at the centers of galaxy clusters. However the particular physical processes underlying these feedback processes still remain elusive. In particular, these simulations neglected cosmic rays (CRs) and magnetic fields, which provide a comparable pressure support in comparison to turbulence in our Galaxy, and are known to couple dynamically and thermally to the gas. We will present our recent efforts to model CR physics in the cosmological simulation code arepo and demonstrate that CRs matter on all scales relevant for galaxy formation. Presenting global simulations of galaxy formation that couple CRs to the magneto-hydrodynamics, we show how CRs can launch powerful galactic winds, which reduces the available amount of gas for star formation. In particular, we highlight the importance of the γ-ray window in understanding the properties of galactic winds and discuss the non-thermal radio and γ-ray emission of Milky-Way like galaxies with bubble-shaped outflows. On scales of galaxy clusters, we show that cosmic-ray heating can balance radiative cooling of the low-entropy gas at the centers of galaxy clusters and helps in mitigating the star formation of the brightest cluster galaxies. Combining low-frequency radio and γ-ray emission of M87, the closest active galaxy interacting with the cooling cluster plasma, enable us to put forward a comprehensive, physics-based model of feedback by active galactic nuclei.

Galactic Winds Driven by Isotropic and Anisotropic Cosmic-Ray Diffusion in Disk Galaxies

“…CRs are modeled as a relativistic fluid with a constant adiabatic index of 4/3 in the two fluid approximation (Pfrommer et al 2016). …”

“…Indeed, recent hydrodynamical simulations of the formation and evolution of disk galaxies have shown that CR pressure can drive strong bipolar outflows in disk galaxies provided they are allowed to stream (Uhlig et al 2012;Ruszkowski et al 2016) or diffuse (Booth et al 2013; relative to the rest frame of the gas. However, they also showed that injecting CRs at supernova remnants and only accounting for their advective transport is not sufficient for launching winds (Jubelgas et al 2008;Pfrommer et al 2016).…”

Cosmic rays and non-thermal emission in simulated galaxies: III. probing cosmic ray calorimetry with radio spectra and the FIR-radio correlation

“…The ideal MHD prescription (Pakmor & Springel 2013) governs the evolution of the magnetic field. Starting from an initial seed magnetic field 𝐵 0 permeating the gas cloud before collapse, the magnetic field is exponentially amplified by a small-scale dynamo, before it grows further via an inverse cascade until saturation (Pfrommer et al 2021). We chose two different values for the initial magnetic field of 𝐵 0 = 10 −10 and 10 −12 G, which represent the pre-amplified magnetic field in a proto-galactic environment.…”

“…For the FRC, we chose to calculate the radio luminosity for a face-on configuration of our galaxies. The corresponding edge-on luminosities are typically a factor of about two smaller, which introduces a natural scatter in the FRC (Pfrommer et al 2021). We correlate the face-on radio synchrotron luminosities against the SFR ( 𝑀 ★ ) of our simulated galaxies with an initial magnetic field of 𝐵 0 = 10 −10 G and 𝜁 SN = 0.05 in Fig.…”

“…Here, we apply the same formalism as described in Paper I and study the resulting radio synchrotron emission from primary and secondary CR electrons. In a companion paper, Pfrommer et al (2021) investigate the origin of the global FRC. In particular, the role of the turbulent small-scale dynamo is identified and the processes leading to the observed scatter in the FRC are analyzed.…”