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We present high-resolution ALMA observations of the C$^{18}$O, $^{13}$CO and$^{12}$CO $J$=1-0 isotopologues in 3 nearby ultra-luminous infrared galaxies(ULIRGS; Arp 220, IRAS 13120-5453, IRAS 17208-0014) and 1 nearby post-mergergalaxy (NGC 2623). In all 4 systems, we measure high $^{12}$CO/C$^{18}$O and$^{12}$CO/$^{13}$CO integrated line ratios while the $^{13}$CO/C$^{18}$O ratiois observed to be extremely low in comparison to typical star-forming disks,supporting previous work. We investigate whether these unusual line ratios aredue to dynamical effects, astrochemistry within the gas, or nucleosynthesis instars. Assuming both lines are optically thin, low $^{13}$CO/C$^{18}$O valuessuggest that C$^{18}$O is more abundant than $^{13}$CO in the interstellarmedium of these systems. A plausible explanation is that local ULIRGs and theirprogeny have an excess in massive star formation; in other words, they areproducing a top-heavy stellar initial mass function.

Extreme CO Isotopologue Line Ratios in ULIRGS: Evidence for a top-heavy IMF