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

Abstract

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. © 2009 The Authors. Journal compilation © 2009 RAS.