The statistical properties of stars at redshift, z = 5, compared with the present epoch

Abstract

We report the statistical properties of stars and brown dwarfs obtained from three radiation hydrodynamical simulations of star cluster formation with metallicities of 1, 1/10, and 1/100 of the solar value. The star-forming clouds are subjected to cosmic microwave background radiation that is appropriate for star formation at a redshift z = 5. The results from the three calculations are compared to each other, and to similar previously published calculations that had levels of background radiation appropriate for present-day (z = 0) star formation. Each of the calculations treats dust and gas temperatures separately and includes a thermochemical model of the diffuse interstellar medium. We find that whereas the stellar mass distribution is insensitive to the metallicity for present-day star formation, at z = 5 the characteristic stellar mass increases with increasing metallicity and the mass distribution has a deficit of brown dwarfs and low-mass stars at solar metallicity compared to the Galactic initial mass function. We also find that the multiplicity of M-dwarfs decreases with increasing metallicity at z = 5. These effects are a result of metal-rich gas being unable to cool to as low temperatures at z = 5 compared to at z = 0 due to the hotter cosmic microwave background radiation, which inhibits fragmentation at high densities.