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 ${10}^{8.5}{\mathrm{M}}_{\odot }$ where the sample is largely representative, estimating star-formation rates (SFRs) using the H$\alpha$ 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 (${\sigma }_{\mathrm{i}\mathrm{n}\mathrm{t}}$) increases with decreasing stellar mass for the H$\alpha$-based SFMS, and we find no mass dependence of ${\sigma }_{\mathrm{i}\mathrm{n}\mathrm{t}}$ in the UV-based SFMS. Additionally, we find that ${\sigma }_{\mathrm{i}\mathrm{n}\mathrm{t}}$ decreases with increasing redshift, from $0.36_{-0.02}^{+0.02}$ dex to $0.22_{-0.07}^{+0.08}$ dex (H$\alpha$ SFMS), and from $0.28_{-0.02}^{+0.02}$ dex to $0.20_{-0.07}^{+0.08}$ dex (UV SFMS) between $z\sim 2$ and $z\sim 6.5$. We also measure the redshift evolution of the specific SFR and find that, assuming $\mathrm{s}\mathrm{S}\mathrm{F}\mathrm{R}\propto (1+\mathrm{z}{)}^{\gamma }$, $\gamma =1.89_{-0.15}^{+0.16}$ for the H$\alpha$-based SFMS, and $\gamma =1.36_{-0.13}^{+0.13}$ for the UV-based SFMS. Analyzing the observed H$\alpha$/UV luminosity ratios and star-formation histories from the {\sc 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 ${\mathrm{L}}_{\mathrm{H}\alpha }\mathrm{/}\nu {\mathrm{L}}_{\nu ,1600}$ vs. $\mathrm{\Delta }\log ({\mathrm{L}}_{\mathrm{H}\alpha })$. Taken together, these results are consistent with theoretical predictions of bursty star formation in the early Universe and provide valuable constraints for theoretical models of galaxy evolution.