On the origin of halo globular clusters and spheroid stars

Abstract

A scenario is presented for the simultaneous formation of halo globular clusters and spheroid stars, in which shocks occuring in the gaseous, primordial (Z= 0) Galaxy play the central role. Denser‐than‐average subcondensations are shown to cool quickly and implode. Small condensations (M < Mt= 5 × 104M⊙) are returned to the hot surrounding phase by the reflected shock, large ones (M > Mu= 2 × 107M⊙) fragment into stars in the dense shell behind the implosion shock, thus generating the first spheroid stars. Intermediate clouds, suddenly cooling to 100K because of collisional H2 excitation, are compressed and left in approximate pressure equilibrium with the hot medium: they give rise to halo GCs. The dependence of the proposed formation mechanism upon overall galactic metallicity and galaxy rotation velocity is weak, so that a universal upper mass limit is deduced. A mass range for halo GCs (2 × 103– 6 × 105M⊙) and a velocity dispersion for spheroid stars in agreement with observations are predicted.