Probing the star formation main sequence down to 10

The main sequence of star-forming galaxies (SFGMS or MS) is a fundamental scaling relation that provides a global framework for studying galaxy formation and evolution, as well as an insight into the complex star formation histories (SFHs) of individual galaxies. In this work, we combine large-area pre-JWST surveys (COSMOS2020, CANDELS), which probe high-M★ sources (> 109 M⊙), with SHARDS/CANDELS FAINT and JWST data from CANUCS, CEERS, JADES, and UNCOVER, to obtain a high-z, star formation rate (SFR) and stellar mass (M★) complete sample spanning both high- and low-M★ regimes. Completeness in both M★ and the SFR is key to avoiding biases introduced by low-mass, highly star-forming objects. Our combined dataset is 80% complete down to 107.6 M⊙ at z ∼ 1 (108.8 M⊙ at z ∼ 9). The overall intrinsic MS slope (based on the SFR100 and M★ derived with Dense Basis and nonparametric SFHs) shows little evolution up to z ∼ 5, with values ∼0.7 − 0.8. The slope in the low-M★ regime becomes steeper than that in the high-M★ end at least up to z ∼ 5, but the strength of this change is highly dependent on the assumptions made on the symmetry of the uncertainties in M★ and SFR. If real, the steepening suggests reduced star formation efficiency or declining gas content with decreasing M★. The transition between the low-M★ regime and the canonical MS occurs around 109.5 M⊙, independent of z. This critical value may coincide with the assembly of galaxies’ disks, which can provide a mechanism for self-regulation that stabilizes them against feedback. The intrinsic scatter is compatible with canonical estimates, also at low-M★, ranging from 0.2 − 0.3 dex. This is indicative of rapid variations in star formation being averaged out over ≲100 Myr.