2025ApJ...979..193C

We present the star formation rate-stellar mass (SFR-M_*) relation for galaxies in the Cosmic Evolution Early Release Science survey at 4.5 ≤ z ≤ 12. We model the JWST and Hubble Space Telescope rest-UV and rest-optical photometry of galaxies with flexible star formation histories (SFHs) using BAGPIPES. We consider SFRs averaged from the SFHs over 10 Myr (SFR₁₀) and 100 Myr (SFR₁₀₀), where the photometry probes SFRs on these timescales, effectively tracing nebular emission lines in the rest-optical (on ∼10 Myr timescales) and the UV/optical continuum (on ∼100 Myr timescales). We measure the slope, normalization and intrinsic scatter of the SFR-M_* relation, taking into account the uncertainty and the covariance of galaxy SFRs and M_*. From z ∼ 5 to 9 there is larger scatter in the SFR₁₀-M_* relation, with $\sigma (\mathrm{log}{\rm{S}}{\rm{F}}{{\rm{R}}}_{100})=0.4$ dex, compared to the SFR₁₀₀-M_* relation, with $\sigma (\mathrm{log}SF{R}_{10})=0.1$ dex. This scatter increases with redshift and increasing stellar mass, at least out to z ∼ 7. These results can be explained if galaxies at higher redshift experience an increase in star formation variability and form primarily in short, active periods, followed by a lull in star formation (i.e., “napping” phases). We see a significant trend in the ratio R_SFR = SFR₁₀/SFR₁₀₀ in which, on average, R_SFR decreases with increasing stellar mass and increasing redshift. This yields a star formation “duty cycle” of ∼40% for galaxies with $\mathrm{log}{M}_{* }/{M}_{\odot }\geqslant 9.3$ at z ∼ 5, declining to ∼20% at z ∼ 9. Galaxies also experience longer lulls in star formation at higher redshift and at higher stellar mass, such that galaxies transition from periods of higher SFR variability at z ≳ 6 to smoother SFR evolution at z ≲ 4.5.