Relations between the mass of the PGCC and the number of SNeII exploding in its central regions for a given SFE = 0.01 (plain curve), three self-enrichment levels (dashed curves), from up to bottom, [Fe/H] = -2, -1.5, -1. The point represents the dynamical constraint described in the text for a hot protogalactic background pressure of 8 × 10-11dyne.cm-2