[论文解读] Relatively Fast and Reasonably Furious: Evidence for Increased Burstiness in Smaller Halos at Cosmic Dawn

Julian B. Muñoz, John Chisholm+8 more|arXiv (Cornell University)|Jan 12, 2026

Galaxies: Formation, Evolution, Phenomena被引用 0

一句话总结

该论文提出一个分析框架，用于建模早期星系的星形成爆发性，结果显示较小的晕中星形成爆发性更强，在 Mh≈1e11 Msun 时 SFR 波动约0.6 dex，在 Mh≤1e9 Msun 时>1 dex，爆发时间尺度约为20 Myr。

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 $M_h = 10^{11}\, M_\odot$ (typical of $M_{ m UV}\approx -19$ galaxies at $z = 6$). This translates into a scatter of $σ_{M_{ m UV}}\approx 0.75$ 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 $σ_{M_{ m UV}} \approx 1.5$ for faint $M_{ m UV} \gtrsim -15$ galaxies). Extrapolating to higher redshifts, when small halos were more prevalent, the inferred mass-dependent burstiness can reproduce observed UVLFs up to $z\sim 17$ within 1$σ$, potentially alleviating the tension between pre- and post-JWST galaxy-formation models. Current observations allow us to constrain burst timescales to approximately $20$ Myr, consistent with expectations from supernova feedback, and suggest broad distributions of ionizing efficiencies at fixed $M_{ m UV}$. Our results demonstrate that mass-dependent burstiness, as predicted by hydrodynamical simulations, is critical for understanding the mass assembly of early galaxies.

研究动机与目标

• Develop an efficient analytic framework to model galaxy star-formation burstiness across a population.
• Constrain burstiness amplitude, timescale, and halo-mass dependence by fitting multi-wavelength JWST+HST data (UVLFs, clustering, Hα/UV).
• Quantify how burstiness propagates into observable UV and Hα luminosities and their ratio across halo masses and redshifts.

提出的方法

• Assume star-formation histories are lognormal fluctuations around a mean SFR for halos of a given mass.
• Model fluctuations x(t) as a damped random walk with correlation function ξ_x(Δt) = (σ_PS^2/2) e^{-|Δt|/τ_PS} and power spectrum P_x(ω) = (σ_PS^2 τ_PS) / [1+(τ_PS ω)^2].
• Compute UV and Hα luminosities via Green’s functions G_λ(t_age) that map past SFR to current light, using SPS models (BC03 and BPASS) and a dust/metallicity-agnostic attenuation parameter A_{Hα/UV}.
• Derive 1D PDFs P(L_λ | M_h) and 2D PDFs P(L_UV, L_Hα | M_h) by exploiting the (approximate) lognormality of sums of lognormal variables and window functions W_λ that filter SFR fluctuations in frequency space.
• Infer burstiness parameters (σ_PS, τ_PS) and their mass/redshift dependence by fitting UV luminosity functions, galaxy clustering, and Hα/UV ratios across z≈4–6.

实验结果

研究问题

• RQ1 Does star-formation burstiness strengthen in lower-mass halos at high redshift?
• RQ2 What are the characteristic burst amplitudes and timescales, and how do they scale with halo mass and redshift?
• RQ3 Can joint modeling of UVLFs, clustering, and Hα/UV constrain the SFR fluctuation PSD and its impact on observables?
• RQ4 What is the implied distribution of ionizing efficiencies at fixed UV luminosity given mass-dependent burstiness?

主要发现

• Galaxies hosted in Mh ≈ 1e11 Msun at z≈4–6 show ~0.6 dex SFR variability (σ_x leading to ~0.75 dex in the UVLF scatter, σ_MUV ≈ 0.75).
• Burstiness increases for smaller halos, reaching >1 dex in SFR variability for Mh ≤ 1e9 Msun (σ_MUV ≈ 1.5 for faint M_UV ≳ −15).
• Mass-dependent burstiness inferred from the model can reproduce observed UVLFs up to z ≈ 17 within 1σ, potentially easing tensions between pre- and post-JWST galaxy-formation models.
• Typical burst timescales constrained to ~20 Myr, consistent with supernova feedback expectations.
• There are broad, mass-dependent distributions of ionizing efficiencies at fixed M_UV, indicating substantial population-level diversity in reionization-era galaxies.