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Abstract Observations of distant galaxies suggest that the physics of galaxy formation at high redshifts differs significantly from later times. In contrast to large, steady disk galaxies like the Milky Way, high-redshift galaxies are often characterized by clumpy, disturbed morphologies and bursty star formation histories. These differences between low-mass, bursty galaxies and higher-mass, steady star-forming galaxies have recently been studied in galaxy formation simulations with resolved multiphase interstellar medium (ISM). These simulation studies indicate that while steady disk galaxies can be well-modeled as “equilibrium disks” embedded in a distinct, hot circumgalactic medium (CGM), bursty galaxies are much more dynamic and their star formation occurs in a dispersion-dominated medium that extends to halo scales, with no clear boundary between the ISM and the CGM. We develop an analytic framework to model star formation in bursty galaxies that are not adequately modeled as equilibrium disks. The framework approximates the gas in low-mass halos as a continuous, supersonically turbulent medium with large density fluctuations. Star formation occurs locally in the high-density tail of a roughly log-normal density distribution. This is analogous to turbulent models of star formation in molecular clouds, but here applied on inner CGM scales. By comparing with galaxy formation simulations from the Feedback in Realistic Environments project, we show that this framework can be used to understand star formation efficiencies (SFEs) and radial profiles in halos. The turbulent framework shows explicitly how the instantaneous galaxy-averaged SFE can be relatively low even if the local efficiency in dense gas approaches unity.
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What Causes The Formation of Disks and End of Bursty Star Formation?
Abstract As they grow, galaxies can transition from irregular/spheroidal with “bursty” star formation histories (SFHs), to disky with smooth SFHs. But even in simulations, the direct physical cause of such transitions remains unclear. We therefore explore this in a large suite of numerical experiments re-running portions of cosmological simulations with widely varied physics, further validated with existing FIRE simulations. We show that gas supply, cooling/thermodynamics, star formation model, Toomre scale, galaxy dynamical times, and feedback properties do not have a direct causal effect on these transitions. Rather, both the formation of disks and cessation of bursty star formation are driven by the gravitational potential, but in different ways. Disk formation is promoted when the mass profile becomes sufficiently centrally-concentrated in shape (relative to circularization radii): we show that this provides a well-defined dynamical center, ceases to support the global “breathing modes” which can persist indefinitely in less-concentrated profiles and efficiently destroy disks, promotes orbit mixing to form a coherent angular momentum, and stabilizes the disk. Smooth SF is promoted by the potential or escape velocity Vesc (not circular velocity Vc) becoming sufficiently large at the radii of star formation that cool, mass-loaded (momentum-conserving) outflows are trapped/confined near the galaxy, as opposed to escaping after bursts. We discuss the detailed physics, how these conditions arise in cosmological contexts, their relation to other correlated phenomena (e.g. inner halo virialization, vertical disk “settling”), and observations.
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- Award ID(s):
- 2108318 1713353 1911233 2045928 2108314 2107872 1652522 2108230 2307327 2108962 1910346 1752913
- PAR ID:
- 10443247
- Date Published:
- Journal Name:
- Monthly Notices of the Royal Astronomical Society
- ISSN:
- 0035-8711
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript
- Journal Article:
- https://doi.org/10.1093/mnras/stad1902
