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The impact of cosmic rays on dynamical balance and disc–halo interaction in L⋆ disc galaxies

Chan, T K and Kereš, Dušan and Gurvich, Alexander B and Hopkins, Philip F and Trapp, Cameron and Ji, Suoqing and Faucher-Giguère, Claude-André (2022) 'The impact of cosmic rays on dynamical balance and disc–halo interaction in L⋆ disc galaxies.', Monthly Notices of the Royal Astronomical Society, 517 (1). pp. 597-615.

Abstract

Cosmic rays (CRs) are an important component in the interstellar medium, but their effect on the dynamics of the disc–halo interface (<10 kpc from the disc) is still unclear. We study the influence of CRs on the gas above the disc with high-resolution FIRE-2 cosmological simulations of late-type L galaxies at redshift z ∼ 0. We compare runs with and without CR feedback (with constant anisotropic diffusion κ ∼ 3 × 1029 cm2 s−1 and streaming). Our simulations capture the relevant disc–halo interactions, including outflows, inflows, and galactic fountains. Extra-planar gas in all of the runs satisfies dynamical balance, where total pressure balances the weight of the overlying gas. While the kinetic pressure from non-uniform motion (1 kpc scale) dominates in the mid-plane, thermal and bulk pressures (or CR pressure if included) take over at large heights. We find that with CR feedback, (1) the warm (∼104 K) gas is slowly accelerated by CRs; (2) the hot (>5 × 105 K) gas scale height is suppressed; (3) the warm-hot (2 × 104–5 × 105 K) medium becomes the most volume-filling phase in the disc–halo interface. We develop a novel conceptual model of the near-disc gas dynamics in low-redshift L galaxies: with CRs, the disc–halo interface is filled with CR-driven warm winds and hot superbubbles that are propagating into the circumgalactic medium with a small fraction falling back to the disc. Without CRs, most outflows from hot superbubbles are trapped by the existing hot halo and gravity, so typically they form galactic fountains.

Item Type:Article
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Available under License - Creative Commons Attribution 4.0.
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Status:Peer-reviewed
Publisher Web site:https://doi.org/10.1093/mnras/stac2236
Publisher statement:This is an Open Access article distributed under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse, distribution, and reproduction in any medium, provided the original work is properly cited.
Date accepted:02 August 2022
Date deposited:21 November 2022
Date of first online publication:19 August 2022
Date first made open access:21 November 2022

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