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

We investigate the effects of the cosmic microwave background (CMB) radiation field on the collapse of pre-stellar 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 tens of M. _⊙) are formed when the metallicity is 10^-4 Z_⊙ ≤ Z ≤ 10^-2.5 Z_⊙; at higher metallicities, the CMB inhibits fragmentation and only very large masses (of approximately hundreds of M. _⊙) are formed. These effects become even more dramatic at z > 10 and the fragmentation mass scales are always ≥hundreds of M. _⊙, independent of the initial metallicity.When dust grains are present, sub-solar mass fragments are formed at any redshift for metallicities Z ≥ 10^-6 Z_⊙ because dust cooling remains relatively insensitive to the presence of the CMB. When Z > 10^-3 Z_⊙, heating of dust grains by the CMB at z ≥ 5 favours the formation of larger masses, which become super-solar when Z ≥ 10^-2 Z_⊙ 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.