From massive spirals to dwarf irregulars: a new set of tight scaling relations for cold gas and stars driven by disc gravitational instability

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.