  We present a new set of galaxy scaling relations for the relative mass
content of atomic gas, molecular gas and stars. Such relations are driven by
disc gravitational instability, and originate from the low galaxy-to-galaxy
variance of Toomre's $Q$ stability parameter. We test such relations using more
than 100 galaxies, from massive spirals to dwarf irregulars, thus spanning
several orders of magnitude in stellar mass
($M_{\star}\approx10^{6\mbox{-}11}\,\mbox{M}_{\odot}$) and atomic gas mass
($M_{\mathrm{HI}}\approx10^{7\mbox{-}10.5}\,\mbox{M}_{\odot}$). Such tests
demonstrate (i) that our scaling relations are physically motivated and tightly
constrained, (ii) that the mass-averaged gravitational instability properties
of galaxy discs are remarkably uniform across the sequence Sa-dIrr, and (iii)
that specific angular momentum plays an important role in such a scenario.
Besides providing new insights into a very important topic in galaxy evolution,
this work provides a simple formula (Eq. 5) that one can use for generating
other galaxy relations driven by disc instability. We explain how to do that,
mention a few possible applications, and stress the importance of testing our
approach further.
