Photometric Evolution of Dusty Starburst Mergers: On the Nature of Ultraluminous Infrared Galaxies

By performing N-body simulations of chemodynamical evolution of galaxies with dusty starbursts, we investigate photometric evolution of gas-rich major mergers to explore the nature of ultraluminous infrared galaxies (ULIRGs) with the total infrared luminosity (L_IR for 8-1000 μm) of ~10¹² L_☉. The main results are the following: (1) Global colors and absolute magnitudes during the dusty starburst of a major merger do not change with time significantly because interstellar dust heavily obscures young starburst populations that could cause rapid evolution of photometric properties of the merger. (2) Dust extinction of stellar populations in a galaxy merger with large infrared luminosity (L_IR > 10¹¹ L_☉) is selective in the sense that younger stellar populations are preferentially obscured by dust more than old ones are. This is because younger populations are located in the central region where a larger amount of dusty interstellar gas can be transferred from the outer gas-rich regions of the merger. (3) Both L_IR and the ratio of L_IR to B-band luminosity (L_B) increases as the star formation rate increases during the starburst of the present merger model, resulting in the positive correlation between L_IR and L_IR/L_B. The dust temperature, T_dust, and the flux ratio of f_{60 μm}/f_{100 μm} also increase with the increase of the star formation rate because a larger number of young stars formed by massive starbursts can heat the dusty interstellar gas as the star formation becomes larger. (4) The star formation efficiency, total gas mass, the degree of dust extinction (A_V), T_dust, L_IR, L_IR/L_B, and f_{60 μm}/f_{100 μm} depend strongly on the separation of the two cores of the merger, which clearly reflects the fact that dynamical processes of galaxy merging play an important role in determining the photometric evolution of dusty starbursts in the merger. (5) The two-dimensional distribution of global colors (e.g., R-K) shows a negative color gradient during starburst, mainly because central young starburst populations are preferentially and very heavily obscured by dust. Furthermore, the peak of the two-dimensional distribution of f_{60 μm} coincides with that of A_V. These results clearly suggest that the spatial distribution of gas and stars is one important determinant of the two-dimensional photometric properties of major mergers. Our numerical results clearly demonstrate that dynamical processes of major galaxy merging, which can control the time evolution of the relative spatial distribution of dusty interstellar gas and young stars, play a vital role in determining photometric properties of ULIRGs.