arXiv 1706.095612017-06astro-ph.GA

快速分子云破坏需要快速云形成

Fast Molecular Cloud Destruction Requires Fast Cloud Formation

Mordecai-Mark Mac Low, Andreas Burkert, Juan C. Ibáñez-Mejía

备注：: 7 pages, no figures, ApJL accepted; v3: corrected several errors, added discussion, no change in conclusions
DOI：: 10.3847/2041-8213/aa8a61

摘要：

银河系中很大一部分气体是冷的、致密的和分子的。如果所有这些气体在引力的影响下坍缩，并在局部自由落体时间内形成恒星，银河系中的恒星形成率将超过观测到的数量级。其他正在形成恒星的星系也有类似的行为。然而，观测和模拟都表明，分子气体确实是在引力作用下坍缩的，尽管是分层次的。如果在引力坍缩过程中迅速破坏恒星形成云，及时的恒星反馈为观测到的低恒星形成率提供了一个潜在的解决方案。然而，这要求分子云必须是短命的物体，这就提出了一个问题，即如何在分子阶段观察到如此多的气体。只有当分子云形成的速度和它们被破坏的速度一样快时，才能发生这种情况，从而保持致密气体的全球平衡比例。因此，我们研究云的形成时间尺度。我们首先证明，超新星和超级气泡的清扫不能产生致密气体的速度所需的云破坏率相匹配。另一方面，Toomre引力不稳定性可以达到所需的生产率。因此，我们认为，虽然致密的恒星形成气体可能只持续一个单一的全球自由落体时间，恒星形成星系中的致密气体可以在全球范围内存在于引力不稳定性形成和恒星反馈破坏之间的动态平衡状态。在红移z >~ 2时，Toomre不稳定性时间尺度减小，导致在早期预测高分子气体分数，与观测一致。

英文摘要：

A large fraction of the gas in the Galaxy is cold, dense, and molecular. If all this gas collapsed under the influence of gravity and formed stars in a local free-fall time, the star formation rate in the Galaxy would exceed that observed by more than an order of magnitude. Other star-forming galaxies behave similarly. Yet observations and simulations both suggest that the molecular gas is indeed gravitationally collapsing, albeit hierarchically. Prompt stellar feedback offers a potential solution to the low observed star formation rate if it quickly disrupts star-forming clouds during gravitational collapse. However, this requires that molecular clouds must be short-lived objects, raising the question of how so much gas can be observed in the molecular phase. This can occur only if molecular clouds form as quickly as they are destroyed, maintaining a global equilibrium fraction of dense gas. We therefore examine cloud formation timescales. We first demonstrate that supernova and superbubble sweeping cannot produce dense gas at the rate required to match the cloud destruction rate. On the other hand, Toomre gravitational instability can reach the required production rate. We thus argue that, although dense, star-forming gas may last only around a single global free-fall time, the dense gas in star-forming galaxies can globally exist in a state of dynamic equilibrium between formation by gravitational instability, and disruption by stellar feedback. At redshift z >~ 2, the Toomre instability timescale decreases, resulting in a prediction of higher molecular gas fractions at early times, in agreement with observations.

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