摘要
We present the star-formation-rate -- stellar-mass (SFR-M $_\ast$ ) relation for galaxies in the CEERS survey at$4.5\leq z\leq 12$ . We model the ȷwst\ and \ rest-UV and rest-optical photometry of galaxies with flexible star-formation histories (SFHs) using We consider SFRs averaged from the SFHs over 10~Myr () and 100~Myr (), where the photometry probes SFRs on these timescales, effectively tracing nebular emission lines in the rest-optical (on$\sim10$ ~Myr timescales) and the UV/optical continuum (on$\sim100$Myr timescales). We measure the slope, normalization and intrinsic scatter of the SFR-M $_\ast$relation, taking into account the uncertainty and the covariance of galaxy SFRs and$M_\ast$ . From$z\sim 5-9$there is larger scatter in the$\sfrten-M_\ast$relation, with$\sigma(\log \sfrcen)=0.4$ ~dex, compared to the$\sfrcen-M_\ast$relation, with$\sigma(\log \sfrten)=0.1$ ~dex. This scatter increases with redshift and increasing stellar mass, at least out to$z\sim 7$ . 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$R_\mathrm{SFR}=\log(\sfrten/\sfrcen)$in which, on average,$R_\mathrm{SFR}$decreases with increasing stellar mass and increasing redshift. This yields a star-formation ``duty cycle'' of$\sim40\%$for galaxies with$\log M_\ast/M_\odot\geq 9.3$ , at$z\sim5$ , declining to$\sim20\%$at$z\sim9$ . 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$z\gtrsim~6$to smoother SFR evolution at$z\lesssim~4.5$ .