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The velocity dispersion of cold interstellar gas, sigma, is one of thequantities that most radically affect the onset of gravitational instabilitiesin galaxy discs, and the quantity that is most drastically approximated instability analyses. Here we analyse the stability of a large sample of nearbystar-forming spirals treating molecular gas, atomic gas and stars as threedistinct components, and using radial profiles of sigma_CO and sigma_HI derivedfrom HERACLES and THINGS observations. We show that the radial variations ofsigma_CO and sigma_HI have a weak effect on the local stability level of galaxydiscs, which remains remarkably flat and well above unity, but is low enough toensure (marginal) instability against non-axisymmetric perturbations and gasdissipation. More importantly, the radial variation of sigma_CO has a strongimpact on the size of the regions over which gravitational instabilitiesdevelop, and results in a characteristic instability scale that is one order ofmagnitude larger than the Toomre length of molecular gas. Disc instabilitiesare driven, in fact, by the self-gravity of stars at kpc scales. This is trueacross the entire optical disc of every galaxy in the sample, with fewexceptions. In the linear phase of the disc instability process, stars andmolecular gas are strongly coupled, and it is such a coupling that ultimatelytriggers local gravitational collapse/fragmentation in the molecular gas.

What drives gravitational instability in nearby star-forming spirals? The impact of CO and HI velocity dispersions