Formation of metal-poor gaseous halos in gas-rich galaxy mergers

We numerically investigate the chemodynamical evolution of the 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. There are three main results of this chemodynamical study: (I) Elliptical galaxies formed by gas-rich mergers show steep negative metallicity gradients in the ISM, especially in the outer parts of galaxies. This is because chemical evolution of the ISM in mergers proceeds in such an inhomogeneous way that metal enrichment of the ISM is more efficient in the central part of mergers, as a result of the radial inflow of metal-enriched ISM during dissipative galaxy merging, whereas in the outer part, metal enrichment is less efficient because a larger amount of metal-enriched ISM is tidally stripped away from mergers. This result provides a clue to the origin of gaseous metallicity gradients in elliptical halos recently revealed by ASCA. (2) Because of the inhomogeneous chemical evolution of the ISM in mergers, some merger remnants show a mean gaseous metallicity that is discernibly smaller than the mean stellar one. The degree of difference between the mean stellar and gaseous metallicities 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. These three results demonstrate that dynamical evolution of gas-rich galaxy mergers, in particular tidal stripping of less metal enriched ISM during galaxy merging, greatly determines the chemical evolution of the ISM of galaxy mergers. These results furthermore imply that recent ASCA observational results on the mean and radial chemical properties of halo ISM in elliptical galaxies can be understood in terms of the chemodynamical evolution of gas-rich galaxy mergers.