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The ISM is a turbulent, multi-phase, and multi-scale medium following scalingrelations. Analytical models of galactic gaseous disks need to take intoaccount the multi-scale and multi-phase nature of the interstellar medium. Theycan be described as clumpy star-forming accretion disks in vertical hydrostaticequilibrium, with the mid-plane pressure balancing the gravity of the gaseousand stellar disk. ISM turbulence is taken into account by applying Galacticscaling relations to the cold atomic and molecular gas phases. Turbulence ismaintained through energy injection by supernovae. With the determination ofthe gas mass fraction at a given spatial scale, the equilibrium gas temperaturebetween turbulent heating and line cooling, the molecular abundances, and themolecular line emission can be calculated. The resulting model radial profilesof IR, H{\sc i}, CO, HCN, and HCO$^+$ emission are compared to THINGS,HERACLES, EMPIRE, SINGS, and GALEX observations of 17 local spiral galaxies.The Toomre parameter, which measures the stability against star formation(cloud collapse), exceeds unity in the inner disk of a significant number ofgalaxies. In two galaxies it also exceeds unity in the outer disk. Therefore,in spiral galaxies $Q_{\rm tot}=1$ is not ubiquitous. The model gas velocitydispersion is consistent with the observed H{\sc i} velocity dispersion whereavailable. Within our model HCN and HCO$^+$ is already detectable in relativelylow-density gas ($\sim 1000$~cm$^{-3}$). CO and HCN conversion factors andmolecular gas depletion time were derived. Both conversion factors areconsistent with values found in the literature. Whereas in the massive galaxiesthe viscous timescale greatly exceeds the star formation timescale, the viscoustimescale is smaller than the star formation timescale within$\rm{R}~\sim~2~\rm{R}_{\rm d}$, the disk scale length, in the low-massgalaxies.

astronomy and astrophysics

Predicting HCN, HCO + , multi-transition CO, and dust emission of  star-forming galaxies: Constraining the properties of resolved gas and dust  disks of local spiral galaxies