Characterising uniform star formation efficiencies with marginally-stable galactic disks

TL;DR

This paper investigates the near-constant star formation efficiency in galaxies by modeling galactic disks as marginally stable, linking observed uniform SFE to disk stability and molecular gas formation processes.

Contribution

It introduces a novel model connecting uniform HI-based star formation efficiency to constant marginal stability disks in galaxies, tested with two molecular gas fraction prescriptions.

Findings

SFE_HI remains fairly constant across five orders of magnitude in stellar mass.

The hydrostatic pressure prescription better predicts SFE_HI in low-mass galaxies.

The model links uniform SFE_HI to star-forming disks with constant marginal stability.

Abstract

We examine the HI-based star formation efficiency (SFE_HI), the ratio of star formation rate to the atomic Hydrogen (HI) mass, in the context of a constant stability star-forming disk model. Our observations of HI-selected galaxies show SFE to be fairly constant (log SFE_HI = -9.65 yr-1 with a dispersion of 0.3 dex) across ~5 orders of magnitude in stellar masses. We present a model to account for this result, whose main principle is that the gas within galaxies forms a uniform stability disk and that stars form within the molecular gas in this disk. We test two versions of the model differing in the prescription that determines the molecular gas fraction, based on either the hydrostatic pressure, or the stellar surface density of the disk. For high-mass galaxies such as the Milky Way, we find that either prescription predicts SFE_HI similar to the observations. However, the hydrostatic pressure prescription is a more accurate SFE_HI predictor for low-mass galaxies. Our model is the first model that links the uniform SFE_HI observed in galaxies at low redshifts to star-forming disks with constant marginal stability. While the rotational amplitude Vmax is the primary driver of disk structure in our model, we find the specific angular momentum of the galaxy may play a role in explaining a weak correlation between SFE_HI and effective surface brightness of the disk.