Relatively Fast and Reasonably Furious: Evidence for Increased Burstiness in Smaller Halos at Cosmic Dawn

Julian B. Muñoz; John Chisholm; Guochao Sun; Jenna Samuel; Jordan Mirocha; Emily Bregou; Alessandra Venditti; Mahdi Qezlou; Charlotte Simmonds; Ryan Endsley

doi:10.1093/mnras/stag415

paper

Relatively Fast and Reasonably Furious: Evidence for Increased Burstiness in Smaller Halos at Cosmic Dawn

arXiv:2601.07912 · doi:10.1093/mnras/stag415

Abstract

We introduce an effective framework to model star-formation burstiness and use it to jointly fit galaxy UV luminosity functions (UVLFs), clustering, and H $α$ /UV ratios, providing the first robust empirical evidence that early galaxies hosted in lower-mass halos are burstier. Using $z \sim 4 - 6$ observations, we find that galaxies show approximately $0.6$ dex of SFR variability if hosted in halos of $M_{h} = 1 0^{11} M_{⊙}$ (typical of $M_{UV} \approx - 19$ galaxies at $z = 6$ ). This translates into a scatter of $σ_{M_{UV}} \approx 0.75$ mag in the UVLF, in line with past findings. Strikingly, we find that burstiness grows for galaxies hosted in smaller halos, reaching $≳ 1$ dex for $M_{h} \leq 1 0^{9} M_{⊙}$ (corresponding to $σ_{M_{UV}} \approx 1.5$ mag for faint $M_{UV} ≳ - 15$ galaxies). Extrapolating to higher redshifts, when small halos were more prevalent, the inferred mass-dependent burstiness can reproduce observed UVLFs up to $z \sim 17$ within 1 $σ$ , potentially alleviating the tension between pre- and post-JWST galaxy-formation models. Current observations allow us to constrain the main burst timescale to approximately $20$ Myr, consistent with expectations from supernova feedback, and suggest broad distributions of ionizing efficiencies at fixed $M_{UV}$ . Our results demonstrate that mass-dependent burstiness, as predicted by hydrodynamical simulations, is critical for understanding the mass assembly of early galaxies.

arXiv comment18+13 pages, 17+12 figures. Updated to match accepted version

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