Globular cluster formation with multiple stellar populations from
  hierarchical star cluster complexes
  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]x10^5 Msun can accrete
more than 10^5 Msun 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 of GCs.
