Probing the star formation main sequence down to 10⁷ M_⊙ at 1  <  z  <  9

¹ Institute for Computational Astrophysics and Department of Astronomy and Physics, Saint Mary’s University 923 Robie Street Halifax NS B3H 3C3, Canada
² Columbia Astrophysics Laboratory, Columbia University 550 West 120th Street New York NY 10027, USA
³ Space Telescope Science Institute 3700 San Martin Drive Baltimore Maryland 21218, USA
⁴ National Research Council of Canada, Herzberg Astronomy & Astrophysics Research Centre 5071 West Saanich Road Victoria BC V9E 2E7, Canada
⁵ Faculty of Mathematics and Physics Jadranska ulica 19 SI-1000 Ljubljana, Slovenia
⁶ Department of Physics and Astronomy, University of California Davis 1 Shields Avenue Davis CA 95616, USA
⁷ Kapteyn Astronomical Institute, University of Groningen P.O. Box 800 9700AV Groningen, The Netherlands
⁸ Department of Physics and Astronomy, York University 4700 Keele St. Toronto Ontario M3J 1P3, Canada
⁹ Observatorio Astronómico de Córdoba, Universidad Nacional de Córdoba Laprida 854 X5000 Córdoba, Argentina

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 26 September 2025
Accepted: 12 January 2026

Abstract

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 10^7.6 M_⊙ at z ∼ 1 (10^8.8 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 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.