Stochastic star formation activity of galaxies within the first billion years probed by JWST

Abstract: JWST highlighted an excess of UV-bright galaxies at z>10, with a derived UVLF that exhibits a softer evolution than expected. In this work, we aim at characterizing the burstiness level of high-redshift galaxy SFHs and its evolution. We implement a stochastic SFH module in CIGALE using power spectrum densities, to estimate the burstiness level of star formation in galaxies at 6<z\<12. We find that SFHs with a high level of stochasticity better reproduce the SEDs of z\>6 galaxies, while smoother assumptions introduce biases when applied to galaxies with bursty star-formation activity. The assumed stochasticity level of the SFH also affects the constraints on galaxies' physical properties, including the main sequence. Successively assuming different levels of burstiness, we determined the best-suited SFH for each 6<z\<12 galaxy in the JADES sample from a Bayes Factor analysis. Galaxies are classified according to their level of burstiness, and the corresponding physical properties are associated to them. For massive galaxies (log Mstar/Msun> 8.8), the fraction of bursty galaxies increases from 0.42+/-0.08 to 0.76+/-0.20 from z=6 to 12, respectively. At all redshifts, only <20% of low-mass galaxies are classified as bursty, due to their faintness resulting in low S/N. For bursty galaxies, the log10(SFR10/SFR100) ratio, another indicator of bursty star formation, does not evolve with redshift, but the fraction of galaxies with high log10(SFR10/SFR100) slightly increases from 0.25+/-0.06 to 0.37+/-0.11 between z=6 and z=9. We include additional constraints from observations on sigmaUV and SFE, finding a maximum of 0.72+/-0.02 mag and 0.06+/-0.01, for sigmaUV and SFE, respectively. This confirms that neither alone is responsible for the weak evolution of the UVLF at z>10. Our results add further evidence that a combination with other mechanisms is likely responsible for the high-z UVLF.