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