  We numerically investigate chemodynamical evolution of interstellar medium
(ISM) in gas-rich disk-disk galaxy mergers in order to explore the origin of
fundamental chemical properties of halo ISM observed in elliptical galaxies.
Main results obtained in this chemodynamical study are the following three. (1)
Elliptical galaxies formed by gas-rich mergers show steep negative metallicity
gradients of ISM especially in the outer part of galaxies. The essential reason
for this is that chemical evolution of ISM in mergers proceeds in such an
inhomogeneous way that in the central part of mergers, metal-enrichment of ISM
is more efficient owing to radial inflow of metal-enriched ISM during
dissipative galaxy merging, whereas in the outer part, metal-enrichment is less
efficient owing to a larger amount of metal-enriched ISM tidally stripped away
from mergers. This result provides a clue to the origin of gaseous metallicity
gradients of elliptical halo recently revealed by ASCA. (2) Because of
inhomogeneous chemical evolution of ISM in mergers, some merger remnants show
mean gaseous metallicity discernibly smaller than mean stellar one. The degree
of difference in mean stellar and gaseous metallicity in a merger remnant
depends on chemical mixing length, galactic mass, and the effectiveness of
supernova feedback. (3) Elliptical galaxies formed by multiple mergers are more
likely to have metal-poor gaseous halo components and steep gaseous metallicity
gradients than those formed by pair mergers. This is principally because a
larger amount of less-metal enriched ISM can be tidally stripped away more
efficiently from galaxies in multiple mergers.
