[论文解读] Seen and unseen: bursty star formation and its implications for observations of high-redshift galaxies with JWST

Guochao Sun, Claude‐André Faucher‐Giguère+2 more|arXiv (Cornell University)|May 4, 2023

Galaxies: Formation, Evolution, Phenomena被引用 10

一句话总结

本论文使用FIRE-2宇宙学缩放 zoom-in 模拟，表明在低质量、高红移的星系中，星形成的爆发性会使 JWST 的可观测性具有极强的时间依赖性，从而在观测样本及相关的星系性质上引入选择偏差。

ABSTRACT

Both observations and simulations have shown strong evidence for highly time-variable star formation in low-mass and/or high-redshift galaxies, which has important observational implications because high-redshift galaxy samples are rest-UV selected and therefore particularly sensitive to the recent star formation. Using a suite of cosmological "zoom-in" simulations at $z>5$ from the Feedback in Realistic Environments (FIRE) project, we examine the implications of bursty star formation histories for observations of high-redshift galaxies with JWST. We characterize how the galaxy observability depends on the star formation history. We also investigate selection effects due to bursty star formation on the physical properties measured, such as the gas fraction, specific star formation rate, and metallicity. We find the observability to be highly time-dependent for galaxies near the survey's limiting flux due to the SFR variability: as the star formation rate fluctuates, the same galaxy oscillates in and out of the observable sample. The observable fraction $f_\mathrm{obs} = 50\%$ at $z \sim 7$ and $M_{\star} \sim 10^{8.5}$ to $10^{9}\,M_{\odot}$ for a JWST/NIRCam survey reaching a limiting magnitude of $m^\mathrm{lim}_\mathrm{AB} \sim 29$-$30$, representative of surveys such as JADES and CEERS. JWST-detectable galaxies near the survey limit tend to have properties characteristic of galaxies in the bursty phase: on average, they show approximately 2.5 times higher cold, dense gas fractions and 20 times higher specific star formation rates at a given stellar mass than galaxies below the rest-UV detection threshold. Our study represents a first step in quantifying selection effects and the associated biases due to bursty star formation in studying high-redshift galaxy properties.

研究动机与目标

• 激励在高-redshift星系中研究爆发性星形成及其对JWST调查的观测意义。
• 量化SFR变动性如何影响在JWST/NIRCam观测中的星系可观测性。
• 评估在推断的物理性质（如气体分数、sSFR和金属含量）上的选择效应。
• 为解释JWST样本提供指南，并激励对早期星系爆发性SFH的进一步研究。

提出的方法

• 使用26个FIRE-2高红移zoom-in模拟分析来自 z>5 到 z~12 的主晕中的中心星系。
• 通过星粒子形成时间和质量，使用考古法计算瞬时SFR。
• 生成包含电离云发射和IGM衰减的JWST拟观测光度，以评估在NIRCam 波段的可检测性。
• 将可观测性定义为在Lyman跃迁分界线以远的波段至少有两次>5σ的探测，且在蓝端无探测。
• 使用经验的 Aλ(M⋆) 关系并假设SMC型消减来探索尘衰减。
• 在与JADES-Deep、JADES-Medium和CEERS相似的探测深度下，评估随 M⋆ 与红移变化的可观测分数 f_obs。

Figure 2: Impact of bursty SFH on the observability of two example galaxies, z5m11d and z5m12b from our simulations. Top: Time variability of the SFR evaluated for an averaging timescale of 10 Myr. Highlighted segments show the redshift intervals inspected below for the observability. Middle: Appare

实验结果

研究问题

• RQ1 SFR变动性如何影响 JWST 对高-redshift 星系的可观測性？
• RQ2 由于爆发性星形成，在 UV 选择样本中推断的星系性质会出现哪些选择效应？
• RQ3 在哪些特征的恒星质量尺度上，爆发性最强烈地影响可观测性？
• RQ4 电离云发射和IGM衰减如何调制由爆发性SFH引入的可观测性偏差？
• RQ5 在跨调查按晕质量函数加权时，像 f_gas、CD 和 sSFR 这样的观测偏差是否仍然稳健？

主要发现

• 由于SFR变动性，接近调查探测限的星系的可观测性具有极强的时间依赖性，电离云发射放大了这一效应。
• 在 z~7 时，M⋆ 在10^8.5–10^9 M⊙ 范围内的 JWST/NIRCam 调查达到 m_AB,lim ~ 29–30，可观测分数 f_obs 约为50%。
• 在探测极限附近的 JWST 可检测星系在给定 M⋆ 下，冷致密气体分数约高出~2.5倍，sSFR约高出~20倍，与不可探测的对应物相比。
• 在按晕质量函数加权时，因爆发性仍存在选择偏差，导致 f_gas,CD 的偏移仍然相似（约0.4 dex），sSFR 偏移约为1.2 dex。
• 爆发性SFH会显著影响UV光度函数，从而影响丰度匹配和恒星质量估计的推断；在低质量、高红移星系中偏差尤为显著。

Figure 3: The observable fraction, $f_{\mathrm{obs}}=N_{\mathrm{obs}}/N_{\mathrm{tot}}$ , defined as the ratio of detectable and total numbers of galaxies in each bin, of galaxies at $z=6$ to 12 for the three example JWST survey depths considered (left: CEERS; middle: JADES-Medium; right: JADES-Deep