Abstract
The regulation of the baryonic content in dwarf galaxies is a long-standing problem. Supernovae (SNe) are supposed to play a key role in forming large-scale galactic winds by removing important amounts of gas from galaxies. SNe are efficient accelerators of non-thermal particles, so-called cosmic rays (CRs), which can substantially modify the dynamics of the gas and conditions to form large-scale galactic winds. We investigate how CR injection by SNe impacts the star formation and the formation of large-scale winds in dwarf galaxies, and whether it can produce galaxy star-formation rates (SFR) and wind properties closer to observations. We ran CR magneto-hydrodynamical simulations of dwarf galaxies at high resolution (9 pc) with the adaptive mesh refinement codeRAMSES. Those disc galaxies are embedded in isolated halos of mass of 1010and 1011 M⊙, and CRs are injected by SNe. We included CR isotropic and anisotropic diffusion with various diffusion coefficients, CR radiative losses, and CR streaming. The injection of CR energy into the interstellar medium smooths out the highest gas densities, which reduces the SFR by a factor of 2–3. Mass outflow rates are significantly greater with CR diffusion, by 2 orders of magnitudes for the higher diffusion coefficients. Without diffusion and streaming, CRs are inefficient at generating winds. CR streaming alone allows for the formation of winds but which are too weak to match observations. The formation of galactic winds strongly depends on the diffusion coefficient: for low coefficients, CR energy stays confined in high density regions where CR energy losses are highest, and higher coefficients, which allow for a more efficient leaking of CRs out of dense gas, produce stronger winds. CR diffusion leads to colder and denser winds than without CRs, and brings outflow rates and mass loading factors much closer to observations.
Highlights
It has been well established that feedback – the processes by which star formation and supermassive black hole growth regulate themselves by deposition of energy and momentum in the ambient medium – is a key mechanism to regulate the amount of baryons in galaxies
Booth et al (2013), performing simulations of Small Magellanic Cloud and Milky Way sized disc galaxies, showed that the star-formation rate (SFR) was suppressed for both masses when accounting for isotropic cosmic rays (CRs) diffusion
In accordance with recent work from Chan et al (2019) and Hopkins et al (2020), we find that overall, CR injection has a modest effect on star formation
Summary
It has been well established that feedback – the processes by which star formation and supermassive black hole growth regulate themselves by deposition of energy and momentum in the ambient medium – is a key mechanism to regulate the amount of baryons in galaxies (see Somerville & Davé 2015; Naab & Ostriker 2017, for recent reviews). Booth et al (2013), performing simulations of Small Magellanic Cloud and Milky Way sized disc galaxies, showed that the star-formation rate (SFR) was suppressed for both masses when accounting for isotropic CR diffusion (i.e. diffusion of CRs that is independent of the magnetic field lines). Chan et al (2019) presented isolated simulations of dwarf and Milky Way-type galaxies including CR feedback They found that only constant isotropic diffusion coefficients of 3 × 1028−29 cm s−1 reproduce observations of γ-ray emission. We study the impact of CR feedback released by SNe on galaxies and wind formation using high resolution MHD simulations of dwarf galaxies of halo mass of 1010 M and 1011 M , and varying the physics of CRs (isotropic or anisotropic diffusion, with different values of the diffusion coefficient, and testing for the effect of the streaming instability).
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