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 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 SFR is key to avoiding biases introduced by low-mass, highly star-forming objects. Our combined data set is 80% complete down to 10^7. 6\, M_ at z1 (10^8. 8\, M_ at z9). The overall intrinsic MS slope (based on the SFR₁₀₀ and M_ derived with Dense Basis and nonparametric SFHs) shows little evolution up to z5, 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 z5, 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.