Synchronized Formation of Subgalactic Systems at Cosmological Reionization: Origin of Halo Globular Clusters

Gas rich sub-galactic halos with mass Mt <= 10^7.5 Msun, while incapable offorming stars due to lack of adequate coolants, contain a large fraction ofbaryonic mass at cosmological reionization. We show that the reionization ofthe universe at z=10-20 has an interesting physical effect on these halos. Theexternal radiation field causes a synchronous inward propagation of anionization 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 dimensionlessspin 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 compressedgaseous systems fragment to form stars, the final stellar systems will have asize 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 theobserved properties of halo globular clusters. The expected number density isconsistent with the observed number density of halo globular clusters. Theobserved mass function of slope ~-2 at the high mass end is predicted by themodel. Strong correlation between velocity dispersion and luminosity (orsurface brightness) and lack of correlation between velocity dispersion andsize, in agreement with observations, are expected. Metallicity is, on average,expected to be low and should not correlate with any other quantities ofglobular clusters, except that a larger dispersion of metallicity amongglobular clusters is expected for larger galaxies. The observed trend ofspecific frequency with galaxy type may be produced in the model. We suggestthat these stellar systems are seen as halo globular clusters today.Comment: accepted to ApJ, 7 ApJ page