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

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  • 15 January 2001
Abstract
Gas rich sub-galactic halos with mass Mt <= 10^8Msun, 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 external radiation field causes a synchronous inward propagation of an ionization front towards each halo resulting in an inward, convergent shock. We show that the resident gas of mass Mb~10^4-10^7.5Msun 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 ~1-20pc, velocity dispersion ~1-20km/s and a total stellar mass of M*~10^3-10^6.5Msun. Their characteristics seem to match the observed properties of halo globular clusters. The expected abundance of halo globular clusters is consistent with what is observed. 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. They should display similar properties, regardless of the host galaxy type, age, size or luminosity. Metallicity is, on average, expected to be low and should not correlate with any other quantities, 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 old halo globular clusters today.

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