Most old globular clusters ( GCs ) in the Galaxy are observed to have internal chemical abundance spreads in light elements .
We discuss a new GC formation scenario based on hierarchical star formation within fractal molecular clouds .
In the new scenario , a cluster of bound and unbound star clusters ( ‘ star cluster complex ’ , SCC ) that have a power-law cluster mass function with a slope ( \beta ) of 2 is first formed from a massive gas clump developed in a dwarf galaxy .
Such cluster complexes and \beta = 2 are observed and expected from hierarchical star formation .
The most massive star cluster ( ‘ main cluster ’ ) , which is the progenitor of a GC , can accrete gas ejected from asymptotic giant branch ( AGB ) stars initially in the cluster and other low-mass clusters before the clusters are tidally stripped or destroyed to become field stars in the dwarf .
The SCC is initially embedded in a giant gas hole created by numerous supernovae of the SCC so that cold gas outside the hole can be accreted onto the main cluster later .
New stars formed from the accreted gas have chemical abundances that are different from those of the original SCC .
Using hydrodynamical simulations of GC formation based on this scenario , we show that the main cluster with the initial mass as large as [ 2 - 5 ] \times 10 ^ { 5 } { M } _ { \odot } can accrete more than 10 ^ { 5 } { M } _ { \odot } gas from AGB stars of the SCC .
We suggest that merging of hierarchical star cluster complexes can play key roles in stellar halo formation around GCs and self-enrichment processes in the early phase of GC formation .