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Hot-mode accretion and the physics of thin-disc galaxy formation

Hafen, Zachary and Stern, Jonathan and Bullock, James and Gurvich, Alexander B and Yu, Sijie and Faucher-Giguère, Claude-André and Fielding, Drummond B and Anglés-Alcázar, Daniel and Quataert, Eliot and Wetzel, Andrew and Starkenburg, Tjitske and Boylan-Kolchin, Michael and Moreno, Jorge and Feldmann, Robert and El-Badry, Kareem and Chan, T K and Trapp, Cameron and Kereš, Dušan and Hopkins, Philip F (2022) 'Hot-mode accretion and the physics of thin-disc galaxy formation.', Monthly Notices of the Royal Astronomical Society, 514 (4). pp. 5056-5073.

Abstract

We use FIRE simulations to study disc formation in z ∼ 0, Milky Way-mass galaxies, and conclude that a key ingredient for the formation of thin stellar discs is the ability for accreting gas to develop an aligned angular momentum distribution via internal cancellation prior to joining the galaxy. Among galaxies with a high fraction (⁠>70 per cent⁠) of their young stars in a thin disc (h/R ∼ 0.1), we find that: (i) hot, virial-temperature gas dominates the inflowing gas mass on halo scales (≳20 kpc), with radiative losses offset by compression heating; (ii) this hot accretion proceeds until angular momentum support slows inward motion, at which point the gas cools to ≲104K⁠; (iii) prior to cooling, the accreting gas develops an angular momentum distribution that is aligned with the galaxy disc, and while cooling transitions from a quasi-spherical spatial configuration to a more-flattened, disc-like configuration. We show that the existence of this ‘rotating cooling flow’ accretion mode is strongly correlated with the fraction of stars forming in a thin disc, using a sample of 17 z ∼ 0 galaxies spanning a halo mass range of 1010.5 M⊙ ≲ Mh ≲ 1012 M⊙ and stellar mass range of 108 M⊙ ≲ M⋆ ≲ 1011 M⊙. Notably, galaxies with a thick disc or irregular morphology do not undergo significant angular momentum alignment of gas prior to accretion and show no correspondence between halo gas cooling and flattening. Our results suggest that rotating cooling flows (or, more generally, rotating subsonic flows) that become coherent and angular momentum-supported prior to accretion on to the galaxy are likely a necessary condition for the formation of thin, star-forming disc galaxies in a ΛCDM universe.

Item Type:Article
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Status:Peer-reviewed
Publisher Web site:https://doi.org/10.1093/mnras/stac1603
Publisher statement:This article has been accepted for publication in Monthly notices of the Royal Astronomical Society. ©: 2022 The Author(s). Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.
Date accepted:08 June 2022
Date deposited:27 July 2022
Date of first online publication:14 June 2022
Date first made open access:27 July 2022

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