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

Relatively Fast and Reasonably Furious: Evidence for Increased Burstiness in Smaller Halos at Cosmic Dawn
Julian B. Mu~noz, John Chisholm, Guochao Sun, Jenna Samuel, Jordan Mirocha, Emily Bregou, Alessandra Venditti, Mahdi Qezlou, Charlotte Simmonds, Ryan Endsley
arXiv:2601.07912v2 Announce Type: replace
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$alpha$/UV ratios, providing the first robust empirical evidence that early galaxies hosted in lower-mass halos are burstier. Using $zsim 4-6$ observations, we find that galaxies show approximately $0.6$ dex of SFR variability if hosted in halos 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 halos, reaching $gtrsim 1$ dex for $M_h leq 10^{9}, M_odot$ (corresponding to $sigma_{M_{rm UV}} approx 1.5$ mag for faint $M_{rm UV} gtrsim -15$ galaxies). Extrapolating to higher redshifts, when small halos were more prevalent, the inferred mass-dependent burstiness can reproduce observed UVLFs up to $zsim 17$ within 1$sigma$, 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 $M_{rm UV}$. Our results demonstrate that mass-dependent burstiness, as predicted by hydrodynamical simulations, is critical for understanding the mass assembly of early galaxies.arXiv:2601.07912v2 Announce Type: replace
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$alpha$/UV ratios, providing the first robust empirical evidence that early galaxies hosted in lower-mass halos are burstier. Using $zsim 4-6$ observations, we find that galaxies show approximately $0.6$ dex of SFR variability if hosted in halos 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 halos, reaching $gtrsim 1$ dex for $M_h leq 10^{9}, M_odot$ (corresponding to $sigma_{M_{rm UV}} approx 1.5$ mag for faint $M_{rm UV} gtrsim -15$ galaxies). Extrapolating to higher redshifts, when small halos were more prevalent, the inferred mass-dependent burstiness can reproduce observed UVLFs up to $zsim 17$ within 1$sigma$, 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 $M_{rm UV}$. Our results demonstrate that mass-dependent burstiness, as predicted by hydrodynamical simulations, is critical for understanding the mass assembly of early galaxies.