Evolution of Supergiant Star-Forming Clouds in a Λ -CDM Cosmology

Globular cluster formation represents the earliest star formation period during galaxy assembly; we determine whether globular clusters originate within supergiant molecular clouds (SGMCs). A Λ -Cold Dark Matter (CDM) cosmological model is applied in an examination of the hierarchical formation of SGMCs within dark matter haloes. The initial conditions for our simulations are extracted from a large, dark matter only, cosmological simulation. Isolated dark matter haloes are identified, gas particles are added, and the resolution increased to N_dm= N_gas = 34³. The physical properties of self-gravitating SGMCs and of observed globular clusters are compared. As with our previous τ -CDM models, the Λ simulations produce clouds with a power-law mass spectrum, dN/dM M^{-1.7 +/- 0.1}. This is the same mass spectrum observed for globular clusters around galaxies, for molecular clouds and their internal clumps within galaxies, and predicted for the supergiant clouds in which globular clusters may form. Although the clouds themselves are growing, the mass spectra of the gas clouds within the dark matter haloes show no evidence of evolution between redshifts of z=8 and z=2. The initially widely-spaced clouds merge and grow in mass under the gravitational effects of the dominant density concentration. Both cloud-cloud collisions and gas infall into dark haloes induce the growth of clouds. By z=0, each separate cloud will be a part of the galaxy itself. In contrast to the too-young τ -CDM clusters, the Λ cosmology predicts the observed range of globular cluster ages.