GRAPE-SPH chemodynamical simulation of elliptical galaxies - I. Evolution of metallicity gradients

We simulate the formation and chemodynamical evolution of 124 elliptical galaxies using a GRAPE-SPH code that includes various physical processes that are 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 cold dark matter (CDM)-based scenario, galaxies form through the successive merging of subgalaxies 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. We examine the physical conditions during 151 merging events that occur in our simulation. 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; and (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 metallicity gradient Deltalog Z/Deltalog rsimilar or equal to- 0.3 with dispersion of +/- 0.2 and no correlation between gradient and galaxy mass are consistent with observations of Mg-2 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 either by monolithic collapse or by major merger alone. Rather it requires a model in which both formation processes arise, such as the present CDM scheme.