Star Formation and Metal Production as a Function of Redshift: The Role of the Multi-Phase ISM
Abstract
We present models of the cosmological star formation and metal production history of (proto-)galaxies with varying axis ratios. More massive and/or roughly spherical systems reach the threshold-metallicity for a transition to a multi-phase interstellar medium earlier than less massive, more flattened systems. Therefore, more flattened, lower-mass systems start to form stars actively at smaller redshifts. A natural explanation is found in the overall robustness of the interstellar medium against complete expulsion (blow-away) at high total masses, and in the prevention of metal enrichment in the outer regions due to axial outflow along the symmetry axis of a non-spherical proto-galaxy (blow-out). We suggest that the observed predominance of spheroidal systems observed at high redshift, e.g. in the Hubble Deep Field, is due to this effect: At z>2, roundish (proto-)galaxies with total (dark+baryonic) masses of ~1011 Mo and/or the inner spheroidal cores of similarly massive flattened systems sustain a multi-phase interstellar medium, and therefore a high star-formation rate, whose magnitude depends on the fraction of baryonic matter in the systems. Conversely, the peak at z~1-2 in the observed cosmological metal production rate coincides with the epochs of star formation of lower mass spheroidals, as well as of massive proto-galactic disks.
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