The Supernova Rate–Velocity Dispersion Relation in the Interstellar Medium

We investigate the relationship between the velocity dispersion of the gasand the SN rate and feedback efficiency in the ISM. We explore the constancy ofthe velocity dispersion profiles in the outer parts of galactic disks at~6-8 kms^-1, and the transition to the starburst regime. Our results show that a) SNdriving leads to constant velocity dispersions of sig~6 km s^-1 for the totalgas and sigHI~3 km s^-1 for the HI gas, independent of the SN rate, for valuesof the rate between 0.01-0.5 the Galactic rate R_{G},b) the position of thetransition to the starburst regime at SFR/Area~5*10^-3-10^-2 M_sol yr^-1 kpc^-2observed in the simulations, is in good agreement with the transition to thestarburst regime in the observations, c) for the high SN rates, no HI gas ispresent in the simulations box, however, for the total gas velocity dispersion,there is good agreement between the models and the observations,d) at theintermediate SN rates R/R_{G}~0.5-1, taking into account the thermal broadeningof the HI line helps reach a good agreement in that regime between the modelsand the observations,e) for R/R_{G}<0.5, sig and sigHI fall below the observedvalues by a factor of~2. However, a set of simulation with different values ofepsilon indicates that for larger values of the supernova feedbackefficiencies, velocity dispersions of the HI gas of the order of 5-6 km s^{-1}can be obtained, in closer agreement with the observations. The fact that forR/R_{G}<0.5, the HI gas velocity dispersions are a factor ~2 smaller than theobserved values could result from the fact that we might have underestimatedthe SN feedback efficiency. It might also be an indication that other physicalprocesses couple to the stellar feedback in order to produce the observed levelof turbulence in galactic disks.Comment: 44 pages, 22 figures. Accepted to Ap