Impact of Cosmic-Ray Transport on Galactic Winds

Abstract

The role of cosmic rays generated by supernovae and young stars has very recently begun to receive significant attention in studies of galaxy formation and evolution due to the realization that cosmic rays can efficiently accelerate galactic winds. Microscopic cosmic ray transport processes are fundamental for determining the efficiency of cosmic ray wind driving. Previous studies focused on modeling of cosmic ray transport either via constant diffusion coefficient or via streaming proportional to the Alfvén speed. However, in predominantly cold, neutral gas, cosmic rays can propagate faster than in the ionized medium and the effective transport can be substantially larger; i.e., cosmic rays can decouple from the gas. We perform three-dimensional magnetohydrodynamical simulations of patches of galactic disks including the effects of cosmic rays. Our simulations include the decoupling of cosmic rays in the cold, neutral interstellar medium. We find that, compared to the ordinary diffusive cosmic ray transport case, accounting for the decoupling leads to significantly different wind properties such as the gas density and temperature, significantly broader spatial distribution of cosmic rays, and larger wind speed. These results have implications for X-ray, γ-ray and radio emission, and for the magnetization and pollution of the circumgalactic medium by cosmic rays.

Hadronic Interactions of Energetic Charged Particles in Protogalactic Outflow Environments and Implications for the Early Evolution of Galaxies

“…This would be the case in regions of near the base of the outflow cone, where the low outflow velocity would not be able to compete against CR diffusion -indeed, such an effect is seen in numerical simulations (e.g. Farber et al 2018). The opposite would be true at high altitudes, where the flow velocity is greater and could advect CRs faster than they would typically be able to diffuse.…”

“…This fraction is consistent with the level found in the numerical models of Girichidis et al (2018) (between 5 and 25%), which consider a combined transport scenario where outflow velocities close to the mid-plane are small, much like the diffusion limit and combined transport picture considered in the present work. Farber et al (2018) also considers a pure advection scenario. They find substantial CR energy is harboured within a few kpc of the galactic plane because outflow velocities are low near h = 0.…”

“…& Breitschwerdt, D. 2012;Uhlig et al 2012;Heesen et al 2016;Taylor & Giacinti 2017;Farber et al 2018), the structures and strengths of the local magnetic field (e.g. Parker 1964;Jokipii 1966; Berezin-events, are therefore expected be abundant in energetic CRs (Karlsson 2008;Lacki et al 2011;Lacki & Thompson 2012;Wang & Fields 2014;Farber et al 2018). Likewise protogalaxies, which have vibrant star forming activity and hence high SN event rates, should also be abundant in CRs.…”

Cosmological simulations of galaxy formation

“…They are accelerated through diffusive shock acceleration mostly in supernova remnants and jets of active galactic nuclei (first-order Fermi acceleration) and turbulence (second-order Fermi acceleration). Cosmic rays contribute to the pressure in the interstellar medium 209,210 , provide an important heating channel 211,212 , and potentially play a role in driving galactic gas outflows [213][214][215][216][217][218][219][220][221][222] due to their shallow equation of state (P cr ∝ ρ 4/3 cr ), their long cooling time, and their ability to transfer energy to outflows outside of star-forming discs 223 . The propagation of cosmic rays is dictated by the strength and topology of the underlying magnetic fields.…”

The Role of Pressure Anisotropy in Cosmic-Ray Hydrodynamics

“…Both theories have been implemented in models of galactic winds and star formation feedback Breitschwerdt et al (1991); Everett et al (2008); Uhlig et al (2012); Agertz et al (2013); Booth et al (2013); Salem & Bryan (2014); Girichidis et al (2016); Ruszkowski et al (2017); Farber et al (2018); Mao & Ostriker (2018); Chan et al (2019). These works show that the mass flux, momentum flux, thermal structure, and even the existence of galactic winds are sensitive to the model of cosmic ray transport, as is the degree to which cosmic ray feedback suppresses star formation.…”