Traversing the Star-forming Main Sequence with Molecular Gas Stacks of z ∼ 1.6 Cluster Galaxies

The cluster environment has been shown to affect the molecular gas content of cluster members, yet a complete understanding of this often subtle effect has been hindered due to a lack of detections over the full parameter space of galaxy star formation rates (SFRs) and stellar masses. Here, we stack CO(2-1) spectra of z ∼ 1.6 cluster galaxies to explore the average molecular gas fractions of galaxies both at lower mass (log(M_*/M_⊙) ∼ 9.6) and further below the star-forming main sequence (SFMS; ΔMS ∼ −0.9) than other literature studies; this translates to a 3σ gas mass limit of ∼7 × 10⁹ M_⊙ for stacked galaxies below the SFMS. We divide our sample of 54 z ∼ 1.6 cluster galaxies, derived from the Spitzer Adaptation of the Red-Sequence Cluster Survey, into nine groupings, for which we recover detections in 8. The average gas content of the full cluster galaxy population is similar to coeval field galaxies matched in stellar mass and SFR. However, when further split by CO-undetected and CO-detected, we find that galaxies below the SFMS have statistically different gas fractions from the field scaling relations, spanning deficiencies to enhancements from 2σ below to 3σ above the expected field gas fractions, respectively. These differences between z = 1.6 cluster and field galaxies below the SFMS are likely due to environmental processes, though further investigation of spatially resolved properties and more robust field scaling relation calibration in this parameter space are required.