[论文解读] Scaling relations and baryonic cycling in local star-forming galaxies

C. Tortora, L. K. Hunt+3 more|arXiv (Cornell University)|Jul 15, 2019

Galaxies: Formation, Evolution, Phenomena参考文献 192被引用 19

一句话总结

本研究提出了包含392个本地星系形成星系的MAGMA样本，其恒星质量、恒星形成率（SFR）和气体质量跨越5个数量级，金属度范围接近2个对数单位。通过多维主成分分析（PCA），研究发现金属度（12+log(O/H)）主要受恒星质量和恒星形成率控制，气体含量（Mgas, MH2）起次要但关键作用，预测精度达到约0.1 dex。

ABSTRACT

Assessments of the cold-gas reservoir in galaxies are a cornerstone for understanding star-formation processes and the role of feedback and baryonic cycling in galaxy evolution. Here we exploit a sample of 392 galaxies (dubbed MAGMA, Metallicity and Gas for Mass Assembly), presented in a recent paper, to quantify molecular and atomic gas properties across a broad range in stellar mass, Mstar, from ∼107 − 1011 M⊙. First, we find the metallicity (Z) dependence of the conversion factor for CO luminosity to molecular H2 mass αCO to be shallower than previous estimates, with αCO ∝ (Z/Z⊙)−1.55. Second, molecular gas mass MH2 is found to be strongly correlated with Mstar and star-formation rate (SFR), enabling predictions of MH2 good to within ∼0.2 dex; analogous relations for atomic gas mass MHI and total gas mass Mgas are less accurate, ∼0.4 dex and ∼0.3 dex, respectively. Indeed, the behavior of atomic gas mass MHI in MAGMA scaling relations suggests that it may be a third, independent variable that encapsulates information about the circumgalactic environment and gas accretion. If Mgas is considered to depend on MHI, together with Mstar and SFR, we obtain a relation that predicts Mgas to within ∼0.05 dex. Finally, the analysis of depletion times and the scaling of MHI/Mstar and MH2/Mstar over three different mass bins suggests that the partition of gas and the regulation of star formation through gas content depends on the mass regime. Dwarf galaxies (Mstar ≲ 3 × 109 M⊙) tend to be overwhelmed by (H <ns0:sc>I</ns0:sc>) accretion, and despite short τH2 (and thus presumably high star-formation efficiency), star formation is unable to keep up with the gas supply. For galaxies in the intermediate Mstar “gas-equilibrium” bin (3 × 109 M⊙ ≲ Mstar ≲3 × 1010 M⊙), star formation proceeds apace with gas availability, and H <ns0:sc>I</ns0:sc> and H2 are both proportional to SFR. In the most massive “gas-poor, bimodality” regime (Mstar ≳ 3 × 1010 M⊙), H <ns0:sc>I</ns0:sc> does not apparently participate in star formation, although it generally dominates in mass over H2. Our results confirm that atomic gas plays a key role in baryonic cycling, and is a fundamental ingredient for current and future star formation, especially in dwarf galaxies.

研究动机与目标

• 研究星系演化过程中气体含量、恒星形成与金属度之间的相互作用。
• 将关系关系扩展至低质量星系，这些星系在以往研究中代表性不足。
• 为宇宙学模拟和高红移演化校准建立基准样本。
• 利用先进PCA技术评估Mstar–SFR–Z–Mgas参数空间的维度。
• 通过考虑低质量星系中的样本构成和噪声偏差，解决PCA结果中的不一致问题。

提出的方法

• 整理了一个由392个本地星形成星系组成的同质样本，包含Mstar、SFR、MHI、MH2和12+log(O/H)的同步测量。
• 利用IRAC、WISE和GALEX测光数据重新计算Mstar和SFR，以确保子样本间的一致性。
• 将所有金属度测量统一转换至PP04N2校准标准。
• 应用4D和分段3D主成分分析（PCA），评估Mstar–SFR–Z–Mgas参数空间的内在维度。
• 采用概率性与自助抽样PCA（带替换）以避免由人工扰动引起的噪声偏差。
• 利用带控制噪声的模拟数据验证结果，评估参数不确定性和可靠性。

实验结果

研究问题

• RQ1 本地星形成星系的Mstar–SFR–Z–Mgas参数空间的真实维度是多少？
• RQ2 在包括低质量矮星系在内的广泛恒星质量范围内，金属度与气体含量如何相关？
• RQ3 当同时包含原子气体和分子气体时，基本金属度关系（FMR）是否仍然成立？与基于SFR的FMR相比有何差异？
• RQ4 PCA结果如何受样本构成影响，特别是低质量星系和高红移系统的影响？
• RQ5 在恒星质量与SFR的主导依赖关系之外，气体含量（Mgas, MH2）在多大程度上影响金属度？

主要发现

• Mstar–SFR–Z–Mgas参数空间可由一个二维超曲面良好描述，证实仅需两个变量即可预测金属度。
• 通过考虑Mstar–Z关系曲率的分段3D PCA，金属度（12+log(O/H)）的预测精度达到约0.1 dex。
• SFR对金属度的影响强于原子气体质量（MHI）或分子气体质量（MH2），与早期认为气体含量是主要驱动力的假设相矛盾。
• 包含低质量星系（Mstar ≤ 10^9.3 M⊙）显著提升了关系的稳健性与代表性，尤其改善了MZR的离散度。
• PCA结果对样本构成敏感：包含高红移星系（如z ~ 2）会扭曲SFR与Mgas项的相对重要性，凸显了本地基准的重要性。
• 本研究表明，向PCA中注入人工噪声可能因对异常值敏感及参数空间覆盖更广而扭曲结果，因此主张采用自助法与概率性PCA。