FORMATION OF PROTO-GLOBULAR CLUSTER CLOUDS BY THERMAL INSTABILITY

Many models of globular cluster formation assume the presence of cold dense clouds in early universe. Here we re-examine the Fall & Rees (1985) model for formation of proto-globular cluster clouds (PGCCs) via thermal instabilities in a protogalactic halo. We first argue, based on the previous study of two-dimensional numerical simulations of thermally unstable clouds in a stratified halo of galaxy clusters by Real et al. (1991), that under the protogalactic environments only nonlinear () density inhomogeneities can condense into PGCCs without being disrupted by the buoyancy-driven dynamical instabilities. We then carry out numerical simulations of the collapse of overdense douds in one-dimensional spherical geometry, including self-gravity and radiative cooling down to T = K. Since imprinting of Jeans mass at K is essential to this model, here we focus on the cases where external UV background radiation prevents the formation of molecules and so prevent the cloud from cooling below K. The quantitative results from these simulations can be summarized as follows: 1) Perturbations smaller than cool isobarically, where nh is the unperturbed halo density, while perturbations larger than cool isochorically and thermal instabilities do not operate. On the other hand, intermediate size perturbations ($M_{min} K range , while the isobaric compression factor is only . 3) Isobarically collapsed clouds ($M at the maximum compression owing to the increased density compression. 4) The density profile of simulated PGCCs can be approximated by a constant core with a halo of rather than a singular isothermal sphere.