Gas rich sub-galactic halos with mass M _ { t } \leq 10 ^ { 7.5 } { M _ { \odot } } , 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 M _ { b } \sim 10 ^ { 4 } -10 ^ { 7 } { M _ { \odot } } in low spin ( initial dimensionless spin parameter \lambda \leq 0.01 ) halos with a velocity dispersion \sigma _ { v } \leq 11 ~ { } km/s would be compressed by a factor of \sim 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 \sim 2 - 40 ~ { } pc , velocity dispersion \sim 1 - 10 km/s and a total stellar mass of M _ { * } \sim 10 ^ { 3 } -10 ^ { 6 } { M _ { \odot } } .

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 \sim - 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 .