Star Formation and Metal Production as a Function of Redshift: The Role of the Multiphase 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 the transition to a multiphase interstellar medium earlier than less massive and/or more flattened systems. Therefore, more flattened and/or lower mass systems start to form stars actively at smaller redshifts. A natural explanation for this result 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 by axial outflow along the symmetry axis of a nonspherical protogalaxy (blow-out). We suggest that the observed predominance of spheroidal systems observed at high redshift, e.g., in the Hubble Space Telescope deep field, comes about from this effect: at z > 2, roundish (proto)galaxies with total (dark + baryonic) masses of ≈ 1011 M☉ and/or the inner spheroidal cores of similarly massive flattened systems sustain a multiphase interstellar medium, and therefore a high star-formation rate, the magnitude of which 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 spheroidal systems, as well as of massive protogalactic disks.