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Formation and evolution of galactic disks with a multiphase numerical model

(Received 23 January 2002 / Accepted 3 April 2002)

Abstract
The formation and evolution of galactic disks are complex phenomena, where gas and star dynamics are coupled through star formation and the related feedback. The physical processes are so numerous and intricate that numerical models focus, in general, on one or a few of them only. We propose here a numerical model with particular attention to the multiphase nature of the interstellar medium; we consider a warm gas phase ( 10 ⁴ K), treated as a continuous fluid by an SPH algorithm, and a cold gas phase (down to 10 K), fragmented in clouds, treated by a low-dissipation sticky particle component. The two gas phases do not have the same dynamics, nor the same spatial distribution. In addition to gravity, they are coupled through mass exchanges due to heating/cooling processes, and supernovae feedback. Stars form out of the cold phase, and re-inject mass to the warm phase through SN explosions and stellar winds. The baryons are embedded in a live cold dark matter component. Baryonic disks, initially composed of pure gas, encounter violent instabilities, and a rapid phase of star formation that slows down exponentially. Stars form in big clumps that accumulate in the center to build a bulge. Exponential metallicity gradients are obtained. External infall of gas should be included to maintain a star formation rate in the disk comparable to what is observed in present disk galaxies.