Dynamics of Galaxy Merging and the Origin of the Color‐Magnitude Relation of Elliptical Galaxies

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Abstract
We numerically investigate both chemical and dynamical evolution of gas-rich galaxy mergers in a self-consistent manner in order to explore the origin of the color-magnitude (C-M) relation of elliptical galaxies. We especially investigate the dependence of the mean stellar metallicity of merger remnants on the rapidity of gas consumption by star formation during merging. We found that galaxy mergers with more (less) rapid gas consumption by star formation show a larger (smaller) degree of metal enrichment. The reason for this dependence can be explained as follows. For a galaxy merger with more rapid star formation, a lesser amount of interstellar gas is tidally stripped away from the system during galaxy merging principally because a greater amount of initial gas has been already converted to a stellar component before the system suffers more severely from the violently varying gravitational potential of galaxy merging. As a result of this, a greater amount of the gas is consequently enriched to a larger extent with metals and is converted to a stellar component during merging. Thus, greater amounts of metals are shared by the stellar component in the remnant of the galaxy merger with more rapid star formation. This result implies that if more luminous elliptical galaxies are formed by galaxy merging with more rapid star formation, the C-M relation can be reproduced at least in a qualitative way. This result furthermore demonstrates that the origin of the C-M relation of elliptical galaxies can be closely associated with the details of the dynamical process of galaxy merging, which depends principally on the rapidity of gas consumption by star formation in merging galaxies.

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