FIRE 实验：一项星系反馈终止实验对恒星形成率理论提出挑战
Playing with FIRE: A Galactic Feedback-Halting Experiment Challenges Star Formation Rate Theories
AI总结：
本研究通过在FIRE-2模拟中开展银河系质量星系的反馈终止实验，发现恒星反馈可直接调控每自由下落时间的恒星形成效率，现有湍流调控理论低估了无反馈时的SFR增幅，证明星系恒星形成在GMC尺度上由反馈多途径调控。
摘要：
恒星反馈以难以通过数值量化的方式影响星系的恒星形成率（SFR）和星际介质，因为反馈是真实模拟中的核心要素。为解决这一问题，我们在FIRE-2（真实环境中的反馈2）模拟框架中，以一个银河系质量的星系为初始对象开展了反馈终止实验。我们终止反馈过程，并将其与维持反馈的模拟进行对比，监测两组模拟的演化差异。研究发现，没有反馈时，星际湍流速度会衰减；致密物质显著增加，同时恒星形成率（SFR）提升超过一个数量级。重要的是，此次SFR的提升幅度，比由更高密度导致的自由下落速率增加所能解释的程度高出约15-20倍。这表明，反馈对每自由下落时间内恒星形成效率的调控，并非仅通过密度分布实现，而是存在更直接的作用机制。为探究巨分子云（GMCs）尺度上的变化，我们利用密度阈值和位力参数阈值识别GMCs，并随星系演化追踪这些云团。终止反馈会引发快速变化，包括新的束缚云大量涌现、松散束缚云的湍流支撑减弱、云团整体密度上升，以及内部恒星形成活动激增。我们利用多种湍流调控理论计算云团积分SFR，结果显示这些理论对SFR激增幅度的预测至少低了三倍。我们得出结论：星系恒星形成本质上是在包含GMCs的尺度上由反馈调控的，且恒星反馈会通过多种方式影响GMCs。
英文摘要：
Stellar feedback influences the star formation rate (SFR) and the interstellar medium of galaxies in ways that are difficult to quantify numerically, because feedback is an essential ingredient of realistic simulations. To overcome this, we conduct a feedback-halting experiment starting with a Milky Way-mass galaxy in the FIRE-2 simulation framework. Terminating feedback, and comparing to a simulation in which feedback is maintained, we monitor how the runs diverge. We find that without feedback, interstellar turbulent velocities decay. There is a marked increase of dense material, while the SFR increases by over an order of magnitude. Importantly, this SFR boost is a factor of $\sim$15-20 larger than is accounted for by the increased free fall rate caused by higher densities. This implies that feedback moderates the star formation efficiency per free-fall time more directly than simply through the density distribution. To probe changes at the scale of giant molecular clouds (GMCs), we identify GMCs using density and virial parameter thresholds, tracking clouds as the galaxy evolves. Halting feedback stimulates rapid changes, including a proliferation of new bound clouds, a decrease of turbulent support in loosely-bound clouds, an overall increase in cloud densities, and a surge of internal star formation. Computing the cloud-integrated SFR using several theories of turbulence regulation, we show that these theories underpredict the surge in SFR by at least a factor of three. We conclude that galactic star formation is essentially feedback-regulated on scales that include GMCs, and that stellar feedback affects GMCs in multiple ways.
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