The fact that observed star-forming galaxies convert their gas into stars inefficiently posits a long-standing theoretical puzzle. Available gas in galaxies is depleted on a timescale of several Gyrs which is orders of magnitude longer than any timescale of the processes driving gas evolution in galaxies. Many galaxy simulations can reproduce observed long depletion times but the physical mechanism controlling their values is not well understood. In addition, some of the simulations show a rather counter-intuitive behavior: global depletion times appear to be almost insensitive to the assumptions about local star formation in individual star-forming regions, a phenomenon described as "self-regulation." Yet another part of the puzzle is the observed tight and near-linear correlation between star formation rates and the amount of molecular gas on kiloparsec and larger scales. A linear correlation implies that the depletion time of molecular gas is almost independent of molecular gas density on >kiloparsec scales, while a strong dependence is expected if, e.g., star formation is controlled by molecular gas self-gravity. We present an intuitive physical model that explains the origin of long gas depletion times in galaxies and the near-linear correlation between star formation rates and molecular gas. Our model is based on mass conservation of gas as the gas cycles between dense star-forming and diffuse states in the interstellar medium. We use simulations of an isolated L* galaxy to illustrate our model and to explore the connection between global depletion times and the timescales of processes driving gas evolution on small scales. In particular, we show that our model can explain the physics of self-regulation of star formation in galaxies with efficient stellar feedback. We also show that a linear correlation between star formation rate and molecular gas emerges when feedback efficiently regulates and stirs the evolution of dense, molecular gas. Our model also provides insights into the likely origin of this relation in real galaxies on different scales.




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