Is Thermal Instability Significant in Turbulent Galactic Gas?

We investigate numerically the role of thermal instability (TI) as agenerator of density structures in the interstellar medium (ISM), both byitself and in the context of a globally turbulent medium. Simulations of theinstability alone show that the condenstion process which forms a dense phase(``clouds'') is highly dynamical, and that the boundaries of the clouds areaccretion shocks, rather than static density discontinuities. The densityhistograms (PDFs) of these runs exhibit either bimodal shapes or a single peakat low densities plus a slope change at high densities. Final static situationsmay be established, but the equilibrium is very fragile: small densityfluctuations in the warm phase require large variations in the density of thecold phase, probably inducing shocks into the clouds. This result suggests thatsuch configurations are highly unlikely. Simulations including turbulentforcing show that large- scale forcing is incapable of erasing the signature ofthe TI in the density PDFs, but small-scale, stellar-like forcing causeserasure of the signature of the instability. However, these simulations do notreach stationary regimes, TI driving an ever-increasing star formation rate.Simulations including magnetic fields, self-gravity and the Coriolis force showno significant difference between the PDFs of stable and unstable cases, andreach stationary regimes, suggesting that the combination of the stellarforcing and the extra effective pressure provided by the magnetic field and theCoriolis force overwhelm TI as a density-structure generator in the ISM. Weemphasize that a multi-modal temperature PDF is not necessarily an indicationof a multi-phase medium, which must contain clearly distinct thermalequilibrium phases.Comment: 18 pages, 11 figures. Submitted to Ap