Exploring the CO/CN line ratio in nearby galaxies with the ALMA archive

TL;DR

This study uses ALMA archival data to analyze the CO/CN line ratio in 17 nearby galaxies, revealing significant spatial variations and differences between active galactic nuclei and starburst regions, with implications for understanding dense gas conditions.

Contribution

First systematic survey of CO/CN line ratios in nearby galaxies using ALMA archival data, highlighting spatial variations and differences in active galactic nuclei.

Findings

Line ratios vary from 7 to 140 across galaxies.

Active galactic nuclei show higher CO/CN ratios than starburst regions.

Spatial variations in line ratios suggest changes in chemical abundance or excitation.

Abstract

We describe an archival project using Cycle 0 data from the Atacama Large Millimeter/submilleter Array to survey the CO/CN line ratio in 17 nearby galaxies. CN is an interesting molecule that traces dense gas exposed to ultraviolet radiation and its N=1-0 lines can be observed simultaneously with the CO J=1-0 line. We identify 8 galaxies with distances < 200 Mpc for which both lines are detected. Signal-to-noise matched CO/CN ratios range from as low as 7 to as high as 65, while ratios using the total detected flux range from 20 to 140. Spatial variations greater than a factor of 3 are seen in several galaxies. These line ratio changes are likely due to changes in the [CN]/[H2] abundance ratio and/or the CN excitation. Additional measurements of the warm gas pressure and the CN excitation should help to distinguish between these two possibilities. 3 of the 4 active galactic nuclei in our sample show CO/CN line ratios that are roughly a factor of 2-3 larger than those seen in starburst-dominated regions, which may be in conflict with models of molecular abundances in X-ray dominated regions.