The ALMaQUEST Survey: The Molecular Gas Main Sequence and the Origin of the Star-forming Main Sequence

The origin of the star-forming main sequence (SFMS; i.e., the relation between star formation rate and stellar mass, globally or on kpc scales) remains a hotly debated topic in galaxy evolution. Using the ALMA-MaNGA QUEnching and STar formation (ALMaQUEST) survey, we show that for star-forming spaxels in the main-sequence galaxies, the three local quantities, star formation rate surface density (ΣSFR), stellar mass surface density (Σ*), and the H2 mass surface density (${{\rm{\Sigma }}}_{{{\rm{H}}}_{2}}$) are strongly correlated with one another and form a 3D linear (in log) relation with dispersion. In addition to the two well-known scaling relations, the resolved SFMS (ΣSFR versus Σ*) and the Schmidt–Kennicutt (SK) relation (ΣSFR versus ${{\rm{\Sigma }}}_{{{\rm{H}}}_{2}}$), there is a third scaling relation between ${{\rm{\Sigma }}}_{{{\rm{H}}}_{2}}$ and Σ*, which we refer to as the molecular gas main sequence (MGMS). The latter indicates that either the local gas mass traces the gravitational potential set by the local stellar mass or both quantities follow the underlying total mass distributions. The scatter of the resolved SFMS (σ ∼ 0.25 dex) is the largest compared to those of the SK and MGMS relations (σ ∼ 0.2 dex). A Pearson correlation test also indicates that the SK and MGMS relations are more strongly correlated than the resolved SFMS. Our result suggests a scenario in which the resolved SFMS is the least physically fundamental and is the consequence of the combination of the SK and the MGMS relations.