Star formation in the first galaxies - II. Clustered star formation and the influence of metal line cooling

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 on to cluster stars to be followed for ∼4 Myr corresponding to multiple local free-fall times. At metallicities at least 10^-3 Z_⊙, gas is able to reach the cosmic microwave background temperature floor and fragment pervasively resulting in a stellar cluster of size ∼1 pc and total mass ∼1000 M_⊙. The masses of individual sink particles vary, but are typically ∼100 M_⊙, consistent with the Jeans mass at T_CMB, though some solar mass fragments are also produced. Below 10^-4 Z_⊙, fragmentation is strongly suppressed on scales greater than 0.01 pc and total stellar mass is lower by a factor of ∼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 ∼10^-3 Z_⊙.