Spectrally resolved cosmic rays: II -- Momentum-dependent cosmic ray diffusion drives powerful galactic winds

TL;DR

This paper introduces a spectrally resolved cosmic ray model in galaxy formation simulations, revealing how energy-dependent diffusion influences galaxy morphology, star formation, and galactic outflows.

Contribution

The study presents the first implementation of spectrally resolved cosmic rays coupled with magneto-hydrodynamics in galaxy simulations, enabling more accurate modeling of CR cooling and diffusion.

Findings

Galaxies with spectral CRs differ in morphology and star formation rates.

Outflow fronts are driven by high-momentum CRs (~200-600 GeV/c).

Mass loading of outflows is primarily caused by lower-momentum CRs (~8-15 GeV/c).

Abstract

Recently, cosmic ray (CR) feedback has been identified as a critical process in galaxy formation but most previous simulations have integrated out the energy-dependence of the CR distribution, despite its large extent over more than twelve decades in particle energy. To improve upon this simplification, we present the implementation and first application of spectrally resolved CRs which are coupled to the magneto-hydrodynamics in simulations of galaxy formation. The spectral model for the CRs enables more accurate cooling of CRs and allows for an energy-dependent spatial diffusion, for which we introduce a new stable numerical algorithm that proves essential in highly dynamical systems. We perform galaxy formation simulations with this new model and compare the results to a grey CR approach with a simplified diffusive transport and effective cooling that assumes steady-state spectra. We find that the galaxies with spectrally resolved CRs differ in morphology, star formation rate, and strength and structure of the outflows. Interestingly, the first outflow front is driven by CRs with average momenta of $\sim200-600\,\mathrm{Gev}~c^{-1}$. The subsequent formation of outflows, which reach mass loading factors of order unity, are primarily launched by CRs of progressively smaller average momenta of $\sim8-15\,\mathrm{GeV}~c^{-1}$. The CR spectra in the galactic centre quickly approach a steady state, which does not significantly vary over time. In the outer disc and outflow regions, the spectral shape approaches steady state only after $\sim2\,\mathrm{Gyr}$ of evolution. Furthermore, the shapes of the approximate steady state spectra differ for individual regions of the galaxy, which highlights the importance of actively including the full CR spectrum.