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

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  • 12 January 2001
Abstract
Gas rich sub-galactic halos with mass Mt <= 10^8 Msun, while incapable of forming stars due to lack of adequate coolants, do exist in abundance at cosmological reionization. The reionization of the universe has an interesting physical effect on these halos. The sudden rise of the external radiation field causes a synchronous inward propagation of an ionization radiation front towards each halo and a large thermal pressure jump is immediately established at a halo-centric radius (outside the virial radius) of approximately unity optical depth. An inward, spherical, convergent shock then commences. We show that the resident gas of mass Mb~10^4-10^7Msun in low spin (initial dimensionless spin parameter lambda <= 0.01) halos with a velocity dispersion sigma_v <= 10km/s would be compressed by a factor of ~100 in radius and form self-gravitating baryonic systems. Such gaseous systems fragment to form stars, blow away remaining gas and become stellar systems of size ~1-10pc, velocity dispersion ~1-10km/s and a total stellar mass of M*~10^3-10^6Msun. Their characteristics seem to match the observed properties of halo globular clusters. We suggest that these stellar systems are seen as old halo globular clusters today.

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