CONSEQUENCES OF COSMIC MICROWAVE BACKGROUND-REGULATED STAR FORMATION

It has been hypothesized that the cosmic microwave background (CMB) provides a temperature floor for collapsing protostars that can regulate the process of star formation and result in a top-heavy initial mass function (IMF) at high metallicity and high redshift. We examine whether this hypothesis has any testable observational consequences. First we determine, using a set of hydrodynamic galaxy formation simulations, that the CMB temperature floor would have influenced the majority of stars formed at redshifts between z = 3 and 6, and probably even to higher redshift. Five signatures of CMB-regulated star formation are (1) a higher supernova rate than currently predicted at high redshift; (2) a systematic discrepancy between direct and indirect measurements of the high redshift star formation rate; (3) a lack of surviving globular clusters that formed at high metallicity and high redshift; (4) a more rapid rise in the metallicity of cosmic gas than is predicted by current simulations; and (5) an enhancement in the abundances of α elements such as O and Mg at metallicities −2 ≲ [Fe/H] ≲ −0.5. Observations are not presently able to either confirm or rule out the presence of these signatures. However, if correct, the top-heavy IMF of high-redshift high-metallicity globular clusters could provide an explanation for the observed bimodality of their metallicity distribution.