The three videos below show the evolution of a gas-rich galaxy simulated with different assumptions about cosmic ray physics: (1) without cosmic rays, (2) with cosmic rays that diffuse with a constant diffusion coefficient, and (3) with diffusion suppression near the sites of active star formation. The gas mass fraction (fgas) of the galactic disk increases from 20% to 40% as indicated in the upper left corner. Different panels show:
1. Gas density; brighter color corresponds to higher density
2. Star formation rate; brighter color corresponds to a higher rate
3. Pressure due to turbulence on unresolved scales; brighter color corresponds to a larger pressure
4. Cosmic ray pressure (it is zero in the 1st video and therefore not shown); the color range is the same as in the 3rd panel
The case without cosmic ray feedback (fg0.4-noCR in Semenov et al. 2021). The cosmic ray pressure stays zero and therefore it is not shown; the entire right side of the video shows turbulent pressure instead. As the gas mass fraction increases, gaseous disc fragments into massive star-forming clumps. Turbulent pressure alone cannot prevent this fragmentation.
The case with cosmic ray feedback, assuming constant diffusion coefficient (fg0.4-constκ in Semenov et al. 2021). Quickly diffusing cosmic rays establish a smooth pressure component (see the rightmost panel). This extra pressure contributes to the global support of the interstellar medium but it does not generate sufficiently strong local gradients and cannot prevent dense clump formation. As a result, the gaseous disk still fragments into clumps when the gas fraction becomes high.
The case with cosmic ray feedback and diffusion suppression near the sites of active star formation (fg0.4-suppκ in Semenov et al. 2021). Cosmic ray diffusion suppression results in accumulation of CRs near the sites of star formation which produces strong local pressure gradients that can inhibit formation of gaseous clumps. As a result, the gaseous disk maintains grand-design spiral structure even in the regime when it is highly unstable due to high gas mass fraction.