line of sight velocity dispersion, $\Sigma_{\rm disk}$ is the average surface density of the disk, $\kappa$=$a$$v_{\rm max}$/$R$ where $v_{\rm max}$ is the rotational velocity of the disk, $R$ is the disk radius and $a$=$\sqrt2$ for a flat rotation curve. Galaxy whose disks have $Q< $1 are unstable to local gravitational collapse and will fragment into clumps, whereas those with $Q\gsim $1 have sufficient rotational support for the gas to withstand and collapse. As @Hopkins12b [e.g. see also @Cacciato12] point out, gas-rich galaxies are usually driven to $Q\sim $1 since regions with $Q< $1 begin forming stars, leading to super-linear feedback which eventually arrests further collapse due to energy/momentum injection (recovering $Q\sim$ 1). For galaxies with $Q\gg $1, there is no collapse, no dense regions form and hence no star formation (and so such galaxies would not be selected as star-forming systems).

Following @Rafikov01, and focusing on the largest unstable fluctuations, the appropriate combination of gas and stellar surface density ($\Sigma_{\rm gas}$ and $\Sigma_{\star}$ respectively) is $$\Sigma_{\rm disk}\,=\,\Sigma_{\rm gas}\,+\,\left(\frac{2}{1+f_{\sigma}^2}\right)\Sigma_{\star}
   \label{eqn:KSlaw}$$ where $f_{\sigma}$=$\sigma_{\star}$/$\sigma_{g}$ is the ratio of the velocity dispersion of the stellar component to that of the gas (see also the discussion in @Romeo11)..

Next, @KS98 show that the gas and 