Probing the star formation main sequence down to 10 7 M ⊙ at 1 < z < 9

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 (> 10 9 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 10 7.6 M ⊙ at z ∼ 1 (10 8.8 M ⊙ at z ∼ 9). The overall intrinsic MS slope (based on the SFR 100 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 10 9.5 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.