Stellar Populations in Gas‐rich Galaxy Mergers. I. Dependence on Star Formation History

Abstract

We investigate the nature of stellar populations of major galaxy mergers between late-type spirals with considerably abundant interstellar medium by performing numerical simulations designed to solve both the dynamical and chemical evolution of the mergers in a self-consistent manner. We particularly consider that the star formation history of galaxy mergers is a crucial determinant of the nature of stellar populations of merger remnants, and therefore we investigate how the difference in star formation history between galaxy mergers affects the chemical evolution of galaxy mergers. We found that the rapidity of star formation, which is defined as the ratio of the dynamical timescale to the timescale of gas consumption by star formation, is the most important determinant for a number of fundamental characteristics of stellar populations of merger remnants. The main results obtained in this study are the following. 1. A galaxy merger with more rapid star formation becomes elliptical with larger mean metallicity. This is primarily because, in the merger with more rapid star formation, a smaller amount of metal-enriched gas is tidally stripped away during merging, and, consequently, a larger amount of the gas can be converted to stellar components. This demonstrates that the cause of the color-magnitude relation of elliptical galaxies can be closely associated with the details of merging dynamics that depend on the rapidity of star formation in galaxy mergers. 2. A negative metallicity gradient fitted reasonably well by a power law can be reproduced by a dissipative galaxy merger with star formation. The magnitude of the metallicity gradient is larger for an elliptical galaxy formed by a galaxy merger with less rapid star formation. 3. The absolute magnitude of the metallicity gradient correlates with that of the age gradient in galaxy mergers in the sense that a merger remnant with a steeper negative metallicity gradient is more likely to show a steeper age gradient. 4. The outer part of a stellar population is both older and less metal-enriched than the nucleus in an elliptical galaxy formed by a galaxy merger with less rapid star formation. Moreover, the metallicity of the outer part of the gaseous component for some models with less rapid star formation is appreciably smaller than the stellar metallicity. This result implies that the origin of metal-poor hot gaseous X-ray halos in real elliptical galaxies can essentially be ascribed to the dynamics of dissipative galaxy merging. 5. Irrespective of the rapidity of star formation, the epoch of galaxy merging affects both the mean stellar metallicity and the mean stellar age of merger remnants: later galaxy mergers are more likely to become ellipticals with both younger and more metal-enriched stellar populations. This result reflects the fact that in the later mergers, a larger amount of more metal-enriched interstellar gas is preferentially converted into stars during the later period of star formation triggered by galaxy merging. These five results clearly demonstrate that even the chemical evolution of elliptical galaxies can be strongly affected by the details of dynamical evolution of galaxy merging, which are furthermore determined by the rapidity of star formation of galaxy mergers. In particular, tidal stripping of interstellar gas and the total amount of gaseous dissipation during galaxy merging are demonstrated to play vital roles in determining a number of chemical properties of merger remnants. Based on these results, we adopt a specific assumption of luminosity dependence on rapidity of star formation and thereby discuss how successfully the present merger model can reproduce a number of fundamental chemical, photometric, and spectroscopic characteristics of elliptical galaxies.