  We simulate the formation and chemodynamical evolution of 124 elliptical
galaxies by using a GRAPE-SPH code that includes various physical processes
associated with the formation of stellar systems: radiative cooling, star
formation, feedback from Type II and Ia supernovae and stellar winds, and
chemical enrichment. In our CDM-based scenario, galaxies form through the
successive merging of sub-galaxies with various masses. Their merging histories
vary between a major merger at one extreme, and a monolithic collapse of a
slow-rotating gas cloud at the other extreme. The basic processes driving the
evolution of the metallicity gradients are as follows: i) destruction by
mergers to an extent dependent on the progenitor mass ratio. ii) regeneration
when strong central star formation is induced at a rate dependent on the gas
mass of the secondary. iii) slow evolution as star formation is induced in the
outer regions through late gas accretion. We succeed in reproducing the
observed variety of the radial metallicity gradients. The average gradient dlog
Z/dlog r ~ -0.3 with dispersion of +- 0.2 and no correlation between gradient
and galaxy mass are consistent with observations of Mg2 gradients. The variety
of the gradients stems from the difference in the merging histories. Galaxies
that form monolithically have steeper gradients, while galaxies that undergo
major mergers have shallower gradients. Thus merging histories can, in
principle, be inferred from the observed metallicity gradients of present-day
galaxies. The observed variation in the metallicity gradients cannot be
explained by either monolithic collapse or by major merger alone. Rather it
requires a model in which both formation processes arise, such as the present
CDM scheme.
