Fast Molecular Cloud Destruction Requires Fast Cloud Formation

Journal Article · Wed Sep 20 04:00:00 EDT 2017 · Astrophysical Journal Letters

DOI:https://doi.org/10.3847/2041-8213/AA8A61· OSTI ID:22654383

Mac Low, Mordecai-Mark ^[1]; Burkert, Andreas ^[2]

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1. American Museum of Natural History, 79th Street at Central Park West, New York, NY 10024 (United States)
2. Universitäts Sternwarte München, Ludwigs-Maximilian-Universität, D-81679 München (Germany)

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 the observations.

• OSTI ID: 22654383
• Journal Information: Astrophysical Journal Letters, Journal Name: Astrophysical Journal Letters Journal Issue: 1 Vol. 847; ISSN 2041-8205
• Country of Publication: United States
• Language: English

Related Subjects

79 ASTRONOMY AND ASTROPHYSICS
CLOUDS
EQUILIBRIUM
FEEDBACK
FORECASTING
GALAXIES
GRAVITATIONAL COLLAPSE
GRAVITATIONAL INSTABILITY
MOLECULES
RED SHIFT
SIMULATION
STARS
TURBULENCE