Young massive clusters ( YMCs ) are usually accompanied by lower-mass clusters and unbound stars with a total mass equal to several tens times the mass of the YMC .
If this was also true when globular clusters ( GCs ) formed , then their cosmic density implies that most star formation before redshift \sim 2 made a GC that lasted until today .
Star-forming regions had to change after this time for the modern universe to be making very few YMCs .
Here we consider the conditions needed for the formation of a \sim 10 ^ { 6 } M _ { \odot } cluster .
These include a star formation rate inside each independent region that exceeds \sim 1 M _ { \odot } yr ^ { -1 } to sample the cluster mass function up to such a high mass , and a star formation rate per unit area of \Sigma _ { SFR } \sim 1 M _ { \odot } kpc ^ { -2 } yr ^ { -1 } to get the required high gas surface density from the Kennicutt-Schmidt relation , and therefore the required high pressure from the weight of the gas .
High pressures are implied by the virial theorem at cluster densities .
The ratio of these two quantities gives the area of a GC-forming region , \sim 1 kpc ^ { 2 } , and the young stellar mass converted to a cloud mass gives the typical gas surface density of 500 - 1000 M _ { \odot } pc ^ { -2 } .
Observations of star-forming clumps in young galaxies are consistent with these numbers , suggesting they formed today ’ s GCs .
Observations of the cluster cut-off mass in local galaxies agree with the maximum mass calculated from \Sigma _ { SFR } .
Metal-poor stellar populations in local dwarf irregular galaxies confirm the dominant role of GC formation in building their young disks .