The Star-forming Main Sequence and Bursty Star-formation Histories at z>1.4 in JADES and AURORA

We analyze JWST spectroscopic and HST+JWST photometric observations of 659 star-forming galaxies at 1.4<z<9 from DR3 of the JADES survey and the AURORA Cycle 1 program. We measure the star-forming main sequence (SFMS) for galaxies above 10 8.5 M where the sample is largely representative, estimating star-formation rates (SFRs) using the H α line flux and rest-frame far UV (1600Å) continuum measurements, each independently corrected for dust attenuation. We find that the intrinsic, measurement-error-subtracted scatter in the SFMS (σ int) increases with decreasing stellar mass for the H α-based SFMS, and we find no mass dependence of σ int in the UV-based SFMS. Additionally, we find that σ int decreases with increasing redshift, from 0.36+0.02-0.02 dex to 0.22+0.08-0.07 dex (H α SFMS), and from 0.28+0.02-0.02 dex to 0.20+0.08-0.07 dex (UV SFMS) between z about2 and z about 6.5. We also measure the redshift evolution of the specific SFR and find that, assuming sSFR proportional to (1+z) γ, γ=1.89+0.16-0.15 for the H α-based SFMS, and γ=1.36+0.13-0.13 for the UV-based SFMS. Analyzing the observed H α /UV luminosity ratios and star-formation histories from the prospector fitting code, we find that 41--60% of the sample is inconsistent with having a constant star-formation history. Finally, we find tentative evidence for shorter SFR burst timescales with increasing redshift based on the distribution of L Hα/νL ν,1600 vs. Δ (L Hα). Taken together, these results are consistent with recent theoretical predictions (e.g., the thesan-zoom simulations) of bursty star formation in the early Universe and provide valuable constraints for theoretical models of galaxy evolution.