CO Excitation in High-z Main Sequence Analogues: Resolved CO(4-3)/CO(3-2) Line Ratios in DYNAMO Galaxies

TL;DR

This study uses high-resolution ALMA observations to measure CO line ratios in local galaxy analogues of high-redshift main sequence galaxies, revealing higher excitation than typical local galaxies and insights into star formation conditions.

Contribution

It provides the first resolved measurements of CO(4-3)/CO(3-2) line ratios in local analogues of high-z galaxies, linking molecular gas excitation to star formation surface density.

Findings

Median CO(4-3)/CO(3-2) ratio is 0.54, higher than local galaxies.

Line ratios increase with star formation rate surface density.

Models over-predict observed line ratios at low star formation surface densities.

Abstract

The spectral line energy distribution of carbon monoxide contains information about the physical conditions of the star forming molecular hydrogen gas; however, the relation to local radiation field properties is poorly constrained. Using ~ 1-2 kpc scale ALMA observations of CO(3-2) and CO(4-3), we characterize the CO(4-3)/CO(3-2) line ratios of local analogues of main sequence galaxies at z ~ 1-2, drawn from the DYNAMO sample. We measure CO(4-3)/CO(3-2) across the disk of each galaxy and find a median line ratio of $R_{43} = 0.54^{+0.16}_{-0.15}$ for the sample. This is higher than literature estimates of local star-forming galaxies and is consistent with multiple lines of evidence that indicate DYNAMO galaxies, despite residing in the local Universe, resemble main-sequence galaxies at z ~ 1-2. Comparing to existing lower resolution CO(1-0) observations, we find $R_{41}$ and $R_{31}$ values in the range $\sim 0.2-0.3$ and $\sim 0.4-0.8$ respectively. We combine our kpc-scale resolved line ratio measurements with HST observations of H$\alpha$ to investigate the relation to star formation rate surface density and compare this relation to expectations from models. We find increasing CO(4-3)/CO(3-2) with increasing star formation rate surface density; however, models over-predict the line ratios across the range of star formation rate surface densities we probe, particularly at the lower range. Finally, SOFIA observations with HAWC+ and FIFI-LS reveal low dust temperatures and no deficit of [CII] emission with respect to the total infrared luminosity.