Metals, dust and the cosmic microwave background: fragmentation of high-redshift star-forming clouds

TL;DR

This study models how the Cosmic Microwave Background influences star-forming cloud fragmentation at high redshifts, revealing that metallicity and dust content significantly affect the characteristic stellar masses formed.

Contribution

It provides a semi-analytic model showing the combined effects of metallicity, dust, and CMB on cloud fragmentation and stellar mass scales in the early universe.

Findings

CMB inhibits fragmentation at high redshift and metallicity.

Dust presence allows formation of sub-solar mass fragments across redshifts.

Fragmentation mass scales increase with metallicity and redshift due to CMB heating.

Abstract

We investigate the effects of the Cosmic Microwave Background (CMB) radiation field on the collapse of prestellar clouds. Using a semi-analytic model to follow the thermal evolution of clouds with varying initial metallicities and dust contents at different redshifts, we study self-consistently the response of the mean Jeans mass at cloud fragmentation to metal line-cooling, dust-cooling and the CMB. In the absence of dust grains, at redshifts z < 10 moderate characteristic masses (of 10s of Msun) are formed when the metallicity is 10^{-4} Zsun < Z < 10^{-2.5} Zsun; at higher metallicities, the CMB inhibits fragmentation and only very large masses (of ~ 100s of Msun) are formed. These effects become even more dramatic at z > 10 and the fragmentation mass scales are always > 100s of Msun, independent of the initial metallicity. When dust grains are present, sub-solar mass fragments are formed at any redshift for metallicities Z > 10^{-6} Zsun because dust-cooling remains relatively insensitive to the presence of the CMB. When Z > 10^{-3} Zsun, heating of dust grains by the CMB at z > 5 favors the formation of larger masses, which become super-solar when Z > 10^{-2} Zsun and z > 10. Finally, we discuss the implications of our result for the interpretation of the observed abundance patterns of very metal-poor stars in the galactic halo.