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 \leq 10 moderate characteristic masses ( of 10s of M _ { \odot } ) are formed when the metallicity is 10 ^ { -4 } Z _ { \odot } \leq Z \leq 10 ^ { -2.5 } Z _ { \odot } ; at higher metallicities , the CMB inhibits fragmentation and only very large masses ( of \sim 100s of M _ { \odot } ) are formed .
These effects become even more dramatic at z > 10 and the fragmentation mass scales are always \geq 100s of M _ { \odot } , independent of the initial metallicity .

When dust grains are present , sub-solar mass fragments are formed at any redshift for metallicities Z \geq 10 ^ { -6 } Z _ { \odot } because dust-cooling remains relatively insensitive to the presence of the CMB .
When Z > 10 ^ { -3 } Z _ { \odot } , heating of dust grains by the CMB at z \geq 5 favors the formation of larger masses , which become super-solar when Z \geq 10 ^ { -2 } Z _ { \odot } and z \geq 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 .