Formation of Metal‐poor Gaseous Halos in Gas‐rich Galaxy Mergers

We numerically investigate chemodynamical evolution of interstellar medium(ISM) in gas-rich disk-disk galaxy mergers in order to explore the origin offundamental 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 metallicitygradients of ISM especially in the outer part of galaxies. The essential reasonfor this is that chemical evolution of ISM in mergers proceeds in such aninhomogeneous way that in the central part of mergers, metal-enrichment of ISMis more efficient owing to radial inflow of metal-enriched ISM duringdissipative galaxy merging, whereas in the outer part, metal-enrichment is lessefficient owing to a larger amount of metal-enriched ISM tidally stripped awayfrom mergers. This result provides a clue to the origin of gaseous metallicitygradients of elliptical halo recently revealed by ASCA. (2) Because ofinhomogeneous chemical evolution of ISM in mergers, some merger remnants showmean gaseous metallicity discernibly smaller than mean stellar one. The degreeof difference in mean stellar and gaseous metallicity in a merger remnantdepends on chemical mixing length, galactic mass, and the effectiveness ofsupernova feedback. (3) Elliptical galaxies formed by multiple mergers are morelikely to have metal-poor gaseous halo components and steep gaseous metallicitygradients than those formed by pair mergers. This is principally because alarger amount of less-metal enriched ISM can be tidally stripped away moreefficiently from galaxies in multiple mergers.Comment: 24 pages 5 figures, ApJ in pres