CEERS: Increasing Scatter along the Star-forming Main Sequence Indicates Early Galaxies Form in Bursts

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 σ ( log S F R 100 ) = 0.4 dex, compared to the SFR₁₀₀-M_* relation, with σ ( log S F 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 log M * / M ⊙ ≥ 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.