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

## TL;DR

This study uses high-resolution ALMA observations to analyze isotopologue line ratios in ULIRGs, providing evidence that these galaxies may have a top-heavy initial mass function due to unusual chemical abundance ratios.

## Contribution

It presents new measurements of CO isotopologue line ratios in ULIRGs, supporting the hypothesis of a top-heavy IMF in these galaxies based on chemical abundance evidence.

## Key findings

- High $^{12}$CO/C$^{18}$O and $^{12}$CO/$^{13}$CO ratios observed
- Low $^{13}$CO/C$^{18}$O ratios suggest C$^{18}$O is more abundant
- Evidence supports a top-heavy stellar initial mass function in ULIRGs

## Abstract

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-merger galaxy (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 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 $^{13}$CO/C$^{18}$O values suggest that C$^{18}$O is more abundant than $^{13}$CO 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.