What Causes The Formation of Disks and End of Bursty Star Formation?

TL;DR

This paper investigates the physical causes behind the formation of galactic disks and the end of bursty star formation, revealing that gravitational potential, rather than other physics, primarily drives these transitions.

Contribution

It identifies the gravitational potential as the key factor influencing disk formation and the cessation of bursty star formation, clarifying the physical mechanisms involved.

Findings

Disk formation correlates with increased central mass concentration.

Smooth star formation occurs when escape velocity confines outflows.

Gravitational potential, not other physics, governs these galactic transitions.

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 (not circular velocity) 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.