We present results from three cosmological simulations , only differing in gas metallicity , that focus on the impact of metal fine-structure line cooling on stellar cluster formation in a high-redshift atomic cooling halo .
Sink particles allow the process of gas hydrodynamics and accretion onto cluster stars to be followed for \sim 4 Myr corresponding to multiple local free-fall times .
At metallicities at least 10 ^ { -3 } Z _ { \odot } , gas is able to reach the CMB temperature floor and fragment pervasively resulting in a stellar cluster of size \sim 1 \mathrm { pc } and total mass \sim 1000 M _ { \odot } .
The masses of individual sink particles vary , but are typically \sim 100 M _ { \odot } , consistent with the Jeans mass at T _ { \mathrm { CMB } } , though some solar mass fragments are also produced .
Below 10 ^ { -4 } Z _ { \odot } , fragmentation is strongly suppressed on scales greater than 0.01 \mathrm { pc } and total stellar mass is lower by a factor of \sim 3 than in the higher metallicity simulations .
The sink particle accretion rates , and thus their masses , are determined by the mass of the gravitationally unstable gas cloud and prolonged gas accretion over many Myr , exhibiting features of both monolithic collapse and competitive accretion .
Even considering possible dust induced fragmentation that may occur at higher densities , the formation of a bona fide stellar cluster seems to require metal line cooling and metallicities of at least \sim 10 ^ { -3 } Z _ { \odot } .