Cosmic-ray diffusion and the multi-phase interstellar medium in a dwarf galaxy. I. Large-scale properties and $γ$-ray luminosities

Abstract

Context. Dynamically, cosmic rays with energies above about one GeV/nucleon may be important agents of galaxy evolution. Their pressure and pressure gradients compare with the thermal and magnetic ones to alter gas accretion onto a galaxy, drive fountains and massive galactic outflows, and alter the mass cycling between the diffuse gas reserves and the dense clouds where stars form. The feedback efficiency depends on the actual properties of cosmic-ray transport in the different media, so we crucially need theoretical clues and observational constraints on these properties in order to assess the impact of cosmic rays on galaxy evolution. Aims. We aim to study the dynamical role of cosmic rays in shaping the interstellar medium of a galaxy when changing their propagation mode. As a first step, we perform high-resolution simulations of the evolution of the same isolated galaxy and compare the impact of the simplest cosmic-ray transport assumption of uniform diffusion. We also compare the total γ-ray luminosity produced in the galaxy by hadronic interactions between cosmic rays and the gas as this observable encapsulates the convolution of the gas and cosmic-ray spatial distributions and it can be compared to Fermi LAT data. Methods. We have simulated the evolution of a gas-rich dwarf galaxy (∼10 11 M in total mass, forming about 1 M yr −1 of stars) with the magnetohydrodynamic adaptive-mesh-refinement code RAMSES, down to 9-pc resolution. Cosmic rays are advected by the gas and they diffuse either isotropically or preferentially along the magnetic field with uniform diffusion coefficients ranging from 3 × 10 27 to 10 29 cm 2 s −1 in order to bracket the average value inferred in the Milky Way. The simulations use gas cooling and heating functions that model the multiphasic structure of the gas down to a much lower resolution than used here. We have also updated the observational relation seen between the γ-ray luminosities and star-formation rates of galaxies using the latest detection and characterisations of Fermi LAT sources. Results. We focus this article on the impact of CR transport on the large-scale properties of the galaxy. We find that the radial and vertical distributions of the gas in the different phases, as well as the mass ranking between the phases, are marginally altered when changing cosmic-ray transport. We observe a positive feedback of cosmic rays on the amplification of the magnetic field in the inner half of the galaxy, except for fast isotropic diffusion. The increase in cosmic-ray pressure for slow or anisotropic diffusion can suppress star formation by up to 50%, but the dual effect of cosmic-ray pressure and magnetic amplification can reduce star formation by a factor 2.5. The global γ-ray luminosities and star-formation rates of the simulated galaxies are fully consistent with the best-fit trend seen in the observations in the case of anisotropic 10 27.5−29 cm 2 s −1 diffusion and for isotropic diffusion slower or equal to 3 × 10 28 cm 2 s −1 . These results therefore do not confirm clai...