Synchronized formation of subgalactic systems at cosmological reionization: Origin of halo globular clusters

Gas-rich subgalactic halos with mass M(t)less than or equal to 10(7.5) M-circle dot, while incapable of forming stars because of a lack of adequate coolants, contain a large fraction of baryonic mass at cosmological reionization. We show that the reionization of the universe at z=6-20 has an interesting physical effect on these halos. The external radiation field causes a synchronous inward propagation of an ionization front toward each halo, resulting in an inward, convergent shock. The resident gas of mass M(b)similar to 10(4)-10(7) M-circle dot in low-spin (initial dimensionless spin parameter lambda less than or equal to0.01) halos with a velocity dispersion of sigma (v) less than or equal to1 1km s(-1) would be compressed by a factor of about 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 of about 2-40 pc, a velocity dispersion of about 1-10 km s(-1), and a total stellar mass of M(*)similar to 10(3)-10(6) M-circle dot. 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 of about -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; however, 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.