The slope and scatter of the star-forming main sequence at z ∼ 5: Reconciling observations with simulations

Galaxies exhibit a tight correlation between their star formation rate (SFR) and stellar mass over a wide redshift range known as the star-forming main sequence (SFMS). With JWST, the SFMS can now be investigated at high redshifts down to masses of ∼10 6 M ⊙ , using sensitive star formation rate tracers such as the H α emission, which allow us to probe the variability in the star formation histories. We present inferences of the SFMS based on 316 H α -selected galaxies at z ∼ 4 − 5 with log( M ★ / M ⊙ ) = 6.4 − 10.6. These galaxies were identified behind the Abell 2744 lensing cluster with NIRCam grism spectroscopy from the survey All the Little Things (ALT). At face value, our data suggest a shallow slope in the SFMS (SFR ∝ M ★ α , with α = 0.45). After we corrected this for the H α -flux limited nature of our survey using a Bayesian framework, the slope steepened to α = 0.59 +0.10 −0.09 , whereas current data on their own are inconclusive on the mass dependence of the scatter. These slopes differ significantly from the slope of ∼1 that is expected from the observed evolution of the galaxy stellar mass function and from simulations. When we fixed the slope to α = 1, we found evidence for a decreasing intrinsic scatter with stellar mass (from ∼0.5 dex at M ★ = 10 8 M ⊙ to 0.4 dex at M ★ = 10 10 M ⊙ ). This difference might be explained by a (combination of) luminosity-dependent SFR(H α ) calibration, a population of (mini)-quenched low-mass galaxies, or underestimated dust attenuation in high-mass galaxies. Future deep observations with different facilities can quantify these processes, which will enable us to achieve better insights into the variability of the star formation histories.