Stability and Ly$\alpha$ emission of Cold Stream in the Circumgalactic Medium: impact of magnetic fields and thermal conduction

Abstract

Cold streams of gas with temperatures around $10^4\mathrm{K}$ play a crucial role in the gas accretion on to high-redshift galaxies. The current resolution of cosmological simulations is insufficient to fully capture the stability and Ly$\alpha$ emission characteristics of cold stream accretion, underscoring the imperative need for conducting idealized high-resolution simulations. We investigate the impact of magnetic fields at various angles and anisotropic thermal conduction (TC) on the dynamics of radiatively cooling streams through a comprehensive suite of two-dimensional high-resolution simulations. An initially small magnetic field ($\sim 10^{-3}\mu \mathrm{G}$), oriented non-parallel to the stream, can grow significantly, providing stability against Kelvin-Helmholtz instabilities and reducing the Ly$\alpha$ emission by a factor of $<20$ compared to the hydrodynamics case. With TC, the stream evolution can be categorised into three regimes: (1) the Diffusing Stream regime, where the stream diffuses into the surrounding hot circumgalactic medium; (2) the Intermediate regime, where TC diffuses the mixing layer, resulting in enhanced stabilization and reduced emissions; (3) the Condensing Stream regime, where the impact of magnetic field and TC on the stream's emission and evolution becomes negligible. Extrapolating our findings to the cosmological context suggests that cold streams with a radius of $\le 1\mathrm{kpc}$ may fuel galaxies with cold, metal-enriched, magnetized gas ($B\sim 0.1-1\mu \mathrm{G}$) for a longer time, leading to a broad range of Ly$\alpha$ luminosity signatures of $\sim 10^{37}-10^{41}\mathrm{erg}{\mathrm{s}}^{-1}$.

19 pages, 15 figures, 1 video link (https://youtu.be/zhNCGoiUYgE), 3 Appendices; accepted to MNRAS (December 2023)

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