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-circle dot) are formed when the metallicity is 10-4 Z(circle dot) < Z < 10-2.5 Z(circle dot); at higher metallicities, the CMB inhibits fragmentation and only very large masses (of approximately hundreds of M-circle dot) are formed. These effects become even more dramatic at z > 10 and the fragmentation mass scales are always >= hundreds of M-circle dot, 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(circle dot) because dust cooling remains relatively insensitive to the presence of the CMB. When Z > 10-3 Z(circle dot), 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(circle dot) 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.