Angular momentum and local gravitational instability in galaxy discs: does $Q$ correlate with $j$ or $M\,$?

TL;DR

This study introduces a new, simple diagnostic for galaxy disc stability that surprisingly shows little correlation with angular momentum or stellar mass, suggesting a universal self-regulation mechanism in star-forming spirals.

Contribution

The paper presents a novel disc instability diagnostic that is consistent with common angular momentum measurements and reveals a universal stability level across diverse spiral galaxies.

Findings

The diagnostic is nearly constant across galaxy types and masses.

Disc stability does not strongly correlate with stellar angular momentum or mass.

Nearby star-forming spirals self-regulate to a universal disc stability level.

Abstract

We introduce a new diagnostic for exploring the link between angular momentum and local gravitational instability in galaxy discs. Our diagnostic incorporates the latest developments in disc instability research, is fully consistent with approximations that are widely used for measuring the stellar specific angular momentum, $j_{\star}=J_{\star}/M_{\star}$, and is also very simple. We show that such a disc instability diagnostic hardly correlates with $j_{\star}$ or $M_{\star}$, and is remarkably constant across spiral galaxies of any given type (Sa$\!-\!$Sd), stellar mass ($M_{\star}=10^{9.5}\!-\!10^{11.5}\,\mathrm{M}_{\odot}$) and velocity dispersion anisotropy ($\sigma_{z\star}/\sigma_{R\star}=0\!-\!1$). The fact that $M_{\star}$ is tightly correlated with star formation rate ($\mathrm{SFR}$), molecular gas mass ($M_{\mathrm{mol}}$), metallicity ($12+\log\mathrm{O/H}$) and other fundamental galaxy properties thus implies that nearby star-forming spirals self-regulate to a quasi-universal disc stability level. This proves the existence of the self-regulation process postulated by several star formation models, but also raises important caveats.