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

We present high-resolution ALMA observations of the C18O, 13CO, and 12CO J = 1–0 isotopologues in three nearby ultraluminous infrared galaxies (ULIRGS; Arp 220, IRAS 13120-5453, and IRAS 17208-0014) and one nearby post-merger galaxy (NGC 2623). In all four systems, we measure high 12CO/C18O and 12CO/13CO integrated line ratios while the 13CO/C18O ratio is observed to be extremely low in comparison to typical star-forming disks, supporting previous work. We investigate whether these unusual line ratios are due to dynamical effects, astrochemistry within the gas, or nucleosynthesis in stars. Assuming both lines are optically thin, low 13CO/C18O values suggest that C18O is more abundant than 13CO in the interstellar medium of these systems. A plausible explanation is that local ULIRGs and their progeny have an excess in massive star formation; in other words, they are producing a top-heavy stellar initial mass function.