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arXiv · 2509.07124

Unveiling the Impact of Cosmic Rays on the Disc Sizes and Outflows from Dwarf Scales to Galaxy Groups

Rebekka Bieri·Rüdiger Pakmor·Freeke van de Voort·Rosie Y. Talbot·Maria Werhahn·Christoph Pfrommer·Volker Springel
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Abstract

Cosmic rays (CRs) are a non-thermal energy component in the interstellar and circumgalactic medium (CGM) that can act as an additional feedback channel beyond thermal and kinetic feedback from stars and AGN. They influence galaxy evolution by altering gas properties, regulating star formation, and shaping galactic outflows. We investigate these effects using a suite of cosmological zoom-in simulations, which incorporate CR transport and feedback on top of the Auriga galaxy formation model. Our simulations span a wide range of halo masses, from dwarf galaxies to small groups ($M_{200\mathrm{c}}= 10^{10} - 10^{13}~\mathrm{M}_\odot$), allowing us to assess the mass-dependent impact of CRs in a cosmological setting. We find that CRs have the strongest impact in lower-mass galaxies ($M_{200\mathrm{c}} < 10^{12}~\mathrm{M}_\odot$), where they suppress star formation rates by up to 50\%, reduce gas and stellar half-light radii, and drive outflows that reach higher velocities at the virial radius compared to simulations without CRs. These CR-enhanced outflows also transport more metals and magnetic fields into the CGM, leading to higher CGM metallicities, stronger magnetisation, and lower CGM temperatures. In more massive galaxies, CRs do not significantly affect star formation or outflow properties, likely because thermal and kinetic feedback from stars and AGN dominate in this regime. However, CRs still influence galaxy morphology across all halo masses by reducing the gas half-mass radius and stellar half-light radius. Finally, variations in CR transport properties, such as different diffusion coefficients or the exclusion of Alfv\'en cooling, significantly affect star formation, CGM properties, and outflows in lower-mass galaxies. This sensitivity makes these galaxies key environments for testing CR transport models and refining our understanding of their role in galaxy evolution.

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Rebekka Bieri, Rüdiger Pakmor, Freeke van de Voort, Rosie Y. Talbot, Maria Werhahn, Christoph Pfrommer, Volker Springel. 2025-09-08. Unveiling the Impact of Cosmic Rays on the Disc Sizes and Outflows from Dwarf Scales to Galaxy Groups. https://arxiv.org/abs/2509.07124

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