Synchronized Formation of Sub-Galactic Systems at Cosmological Reionization: Origin of Halo Globular Clusters

Abstract

Gas rich sub-galactic halos with mass Mt <= 10^7.5 Msun, while incapable of forming stars due to lack of adequate coolants, contain a large fraction of baryonic mass at cosmological reionization. We show that the reionization of the universe at z=10-20 has an interesting physical effect on these halos. The external radiation field causes a synchronous inward propagation of an ionization front towards each halo, resulting in an inward, convergent shock. The resident gas of mass Mb~10^4-10^7 Msun in low spin (initial dimensionless spin parameter lambda <= 0.01) halos with a velocity dispersion sigmav <= 11km/s would be compressed by a factor of ~100 in radius and form self -gravitating baryonic systems. Under the assumption that such compressed gaseous systems fragment to form stars, the final stellar systems will have a size 2-40pc, velocity dispersion 1-10km/s and a total stellar mass of M* 10^3-10^6 Msun. The characteristics of these proposed systems seem to match the observed properties of halo globular clusters. The expected number density is consistent with the observed number density of halo globular clusters. The observed mass function of slope ~-2 at the high mass end is predicted by the model. Strong correlation between velocity dispersion and luminosity (or surface brightness) and lack of correlation between velocity dispersion and size, in agreement with observations, are expected. Metallicity is, on average, expected to be low and should not correlate with any other quantities of globular clusters, except that a larger dispersion of metallicity among globular clusters is expected for larger galaxies. The observed trend of specific frequency with galaxy type may be produced in the model. We suggest that these stellar systems are seen as halo globular clusters today.