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

Recently Taniguchi and Ohyama found that the higher $^{12}$CO to $^{13}$COintegrated intensity ratios at a transition $J$=1--0, $R =I(^{12}$CO)$/I(^{13}$CO) $\gtrsim 20$, in a sample of starburst merginggalaxies such as Arp 220 are mainly attributed to the depression of $^{13}$COemission with respect to $^{12}$CO. Investigating the same sample of galaxiesanalyzed by Taniguchi & Ohyama, we find that there is a tight, almost linearcorrelation between the dust mass and $^{13}$CO luminosity. This implies thatdust grains are also depressed in the high-$R$ starburst mergers, leading tothe higher dust temperature ($T_{\rm d}$) in them because of the relativeincrease in the radiation density. Nevertheless, the average dust mass ($M_{\rmd}$) of the high-$R$ starburst mergers is higher significantly than that ofnon-high $R$ galaxies. This is naturally understood because the galaxy mergerscould accumulate a lot of dust grains from their progenitor galaxies togetherwith supply of dust grains formed newly in the star forming regions. Since$L$(FIR) $\propto M_{\rm d} T_{\rm d}^5$ given the dust emissivity law, $S_\nu\propto \lambda^{-1}$, the increases in both $M_{\rm d}$ and $T_{\rm d}$explain well why the starburst mergers are so bright in the FIR. We discussthat the superwind activity plays an important role in destroying dust grainsas well as dense gas clouds in the central region of mergers.Comment: 10 pages (aaspp4.sty), 3 postscript figures (embedded). Accepted for publication in Astrophysical Journal Letter