Kinetic energy stored in interstellar medium (ISM) bulk/turbulent motions is a crucial ingredient in properly describing most properties of observed galaxies. By using Monte Carlo simulations, we investigate how this energy is injected by supernovae and dissipated via cloud collisions, and derive the corresponding ISM velocity probability distribution function (PDF). The functional form of the PDF for the modulus of the velocity dispersion is<formula format="display" id="m1">.The power-law index of the PDF depends only on the value of the average cloud collision elasticity <∊> as β= 2 exp(<∊>- 1). If β and the gas velocity dispersion σ are known, the specific kinetic energy dissipated by collisions is found to be ~σ2 ln(2/β)/(β- 0.947); in steady state, this is equal to the energy input from supernovae (SNe). We predict that in a multiphase, low-metallicity (Z~ 5 × 10-3Zsolar) ISM the PDF should be close to a Maxwellian (β= 2) with velocity dispersion σ>~ 11 km s-1 in more metal rich systems (Z>~ 5 × 10-2Zsolar), on the other hand, we expect to observe almost exponential PDFs. This is in good agreement with a number of observations that we review and might explain the different star formation modes seen in dwarfs and spiral galaxies.
Monthly Notices of the Royal Astronomical Society
- Pub Date:
- August 2002
- shock waves;
- ISM: kinematics and dynamics;
- Accepted for publication of MNRAS. 10 pages including 7 figures