Relatively fast and reasonably furious: evidence for increased burstiness in smaller haloes at cosmic dawn

ABSTRACT We introduce an effective framework to model star-formation burstiness and use it to jointly fit galaxy ultraviolet (UV) luminosity functions (UVLFs), clustering, and H$\alpha$/UV ratios, providing the first robust empirical evidence that early galaxies hosted in lower mass haloes are burstier. Using $z\sim 4\!-\!6$ observations, we find that galaxies show approximately 0.6 dex of star formation rate variability if hosted in haloes of $M_h = 10^{11}\, M_\odot$ (typical of $M_{\rm UV}\approx -19$ galaxies at $z = 6$). This translates into a scatter of $\sigma _{M_{\rm UV}}\approx 0.75$ mag in the UVLF, in line with past findings. Strikingly, we find that burstiness grows for galaxies hosted in smaller haloes, reaching $\gtrsim 1$ dex for $M_h \le 10^{9}\, M_\odot$ (corresponding to $\sigma _{M_{\rm UV}} \approx 1.5$ mag for faint $M_{\rm UV} \gtrsim -15$ galaxies). Extrapolating to higher redshifts, when small haloes were more prevalent, the inferred mass-dependent burstiness can reproduce observed UVLFs up to $z\sim 17$ within 1$\sigma$, potentially alleviating the tension between pre- and post-James Webb Space Telescope galaxy-formation models. Current observations allow us to constrain the main burst time-scale to approximately 20 Myr, consistent with expectations from supernova feedback, and suggest broad distributions of ionizing efficiencies at fixed $M_{\rm UV}$. Our results demonstrate that mass-dependent burstiness, as predicted by hydrodynamical simulations, is critical for understanding the mass assembly of early galaxies.