An Origin of the Huge Far-infrared Luminosity of Starburst Mergers

Recently Taniguchi and Ohyama found that the higher CO to CO integrated intensity ratios at a transition J=1–0, R = I(CO)/I(CO) ∼ > 20, in a sample of starburst merging galaxies such as Arp 220 are mainly attributed to the depression of CO emission with respect to CO. Investigating the same sample of galaxies analyzed by Taniguchi & Ohyama, we find that there is a tight, almost linear correlation between the dust mass and CO luminosity. This implies that dust grains are also depressed in the high-R starburst mergers, leading to the higher dust temperature (Td) in them because of the relative increase in the radiation density. Nevertheless, the average dust mass (Md) of the high-R starburst mergers is higher significantly than that of non-high R galaxies. This is naturally understood because the galaxy mergers could accumulate a lot of dust grains from their progenitor galaxies together with supply of dust grains formed newly in the star forming regions. Since L(FIR) ∝ MdT 5 d given the dust emissivity law, Sν ∝ λ , the increases in both Md and Td explain well why the starburst mergers are so bright in the FIR. We discuss that the superwind activity plays an important role in destroying dust grains as well as dense gas clouds in the central region of mergers. Subject headings: galaxies: emission lines galaxies: starburst interstellar: molecules interstellar: dust