On the degree of dust extinction in major galaxy mergers with dusty starburst

We numerically investigate how fundamental properties of the spectral energ y distribution (SED) of a major gas-rich galaxy merger with dusty starburst are determined by the initial orbital configuration of the merger. We foun d that an infrared luminous galaxy with dusty starburst formed by a nearly retrograde-retrograde merger suffers the most remarkable dust extinction of stellar light and consequently shows very red colours. Considering that a retrograde-retrograde merger does not produce strong and long tidal tails, this result suggests that a luminous infrared galaxy without clear signs of interaction and merging shows Very large internal dust extinction and redd er colours. These numerical results furthermore imply that the morphology o f a luminous infrared galaxy can correlate with the degree of internal dust extinction and thus with the shape of the SED, principally because the mor phology of a merger also depends strongly on the initial orbital configurat ion of the merger. Based on these results, we discuss the origin of ultra-l uminous infrared galaxies.