# $^{13}$CO/C$^{18}$O Gradients Across the Disks of Nearby Spiral Galaxies

## TL;DR

This study measures 13CO/C18O ratios across nine nearby spiral galaxies, revealing gradients that inform on chemical abundance variations and stellar nucleosynthesis effects within galaxy disks.

## Contribution

It provides high-resolution measurements of isotopologue ratios across galaxy disks, which are rare and enhance understanding of chemical processes in galaxies.

## Key findings

- Mean 13CO/C18O ratio of 6.0 in galaxy centers
- Ratios increase with galactocentric radius
- Ratios decrease with star formation rate surface density

## Abstract

We use the IRAM Large Program EMPIRE and new high-resolution ALMA data to measure 13CO(1-0)/C18O(1-0) intensity ratios across nine nearby spiral galaxies. These isotopologues of CO are typically optically thin across most of the area in galaxy disks, and this ratio allows us to gauge their relative abundance due to chemistry or stellar nucleosynthesis effects. Resolved 13CO/C18O gradients across normal galaxies have been rare due to the faintness of these lines. We find a mean 13CO/C18O ratio of 6.0$\pm$0.9 for the central regions of our galaxies. This agrees well with results in the Milky Way, but differs from results for starburst galaxies (3.4$\pm$0.9) and ultraluminous infrared galaxies (1.1$\pm$0.4). In our sample, the 13CO/C18O ratio consistently increases with increasing galactocentric radius and decreases with increasing star formation rate surface density. These trends qualitatively agree with expectations for carbon and oxygen isotopic abundance variations due to stellar nucleosynthesis, with a possible effect of fractionation.