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Journal Article

Resolving small-scale cold circumgalactic gas in TNG50


Nelson,  Dylan
Galaxy Formation, MPI for Astrophysics, Max Planck Society;


Springel,  Volker
Computational Structure Formation, MPI for Astrophysics, Max Planck Society;


Pakmor,  Rüdiger
Stellar Astrophysics, MPI for Astrophysics, Max Planck Society;

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Nelson, D., Sharma, P., Pillepich, A., Springel, V., Pakmor, R., Weinberger, R., et al. (2020). Resolving small-scale cold circumgalactic gas in TNG50. Monthly Notices of the Royal Astronomical Society, 498(2), 2391-2414. doi:10.1093/mnras/staa2419.

Cite as: https://hdl.handle.net/21.11116/0000-0007-D6EC-E
We use the high-resolution TNG50 cosmological magnetohydrodynamical simulation to explore the properties and origin of cold circumgalactic medium (CGM) gas around massive galaxies (M > 1011 M ) at intermediate redshift (⁠z∼0.5⁠). We discover a significant abundance of small-scale, cold gas structure in the CGM of ‘red and dead’ elliptical systems, as traced by neutral H i and Mg ii. Halos can host tens of thousands of discrete absorbing cloudlets, with sizes of order a kpc or smaller. With a Lagrangian tracer analysis, we show that cold clouds form due to strong δρ/ρ¯≫1 gas density perturbations that stimulate thermal instability. These local overdensities trigger rapid cooling from the hot virialized background medium at ∼107 K to radiatively inefficient ∼104 K clouds, which act as cosmologically long-lived, ‘stimulated cooling’ seeds in a regime where the global halo does not satisfy the classic tcool/tff < 10 criterion. Furthermore, these small clouds are dominated by magnetic rather than thermal pressure, with plasma β ≪ 1, suggesting that magnetic fields may play an important role. The number and total mass of cold clouds both increase with resolution, and the mgas ≃ 8 × 104 M cell mass of TNG50 enables the ∼ few hundred pc, small-scale CGM structure we observe to form. Finally, we make a preliminary comparison against observations from the COS-LRG, LRG-RDR, COS-Halos, and SDSS LRG surveys. We broadly find that our recent, high-resolution cosmological simulations produce sufficiently high covering fractions of extended, cold gas as observed to surround massive galaxies.