CO spectral energy distributions of high-redshift main-sequence galaxies

Characterize the CO spectral line energy distributions of main-sequence galaxies at redshifts greater than 4 in order to determine the appropriate excitation ratios for inferring their molecular gas masses from mid-J CO transitions.

Background

The molecular gas masses inferred from mid-J CO transitions depend on the CO spectral line energy distribution (SLED), which is needed to convert the luminosity of an observed transition into the luminosity of the fundamental CO(10) line. The relevant excitation ratios also depend on the physical conditions of the interstellar medium, including metallicity, temperature, density, and dynamical state.

For main-sequence galaxies at redshifts above 4, the paper states that the CO SLED is not yet constrained. The analysis therefore adopts excitation ratios measured for main-sequence galaxies at redshifts 22.7 as the closest available analogues, while acknowledging the substantial uncertainty associated with extrapolating those values to z approximately 4.5.