J. M. Diederik Kruijssen, Steven N. Longmore, Bruce G. Elmegreen, Norman Murray, John Bally, Leonardo Testi, Robert C. Kennicutt Jr.
The star formation rate (SFR) in the Central Molecular Zone (CMZ, i.e. the central 500 pc) of the Milky Way is lower by a factor of >10 than expected for the substantial amount of dense gas it contains, which challenges current star formation theories. In this paper, we quantify which physical mechanisms could be causing this observation. On scales larger than the disc scale height, the low SFR is found to be consistent with episodic star formation due to secular instabilities or variations of the gas inflow along the Galactic bar. The CMZ is marginally Toomre-stable when including gas and stars, but highly Toomre-stable when only accounting for the gas, indicating that the condensation of self-gravitating clouds may be limited. On small scales, we find that the SFR in the CMZ is consistent with an elevated critical density for star formation due to the high turbulent pressure - potentially aided by weak magnetic effects and an underproduction of massive stars due to a bottom-heavy IMF. The existence of a universal density threshold for star formation is ruled out, as well as the importance of the HI-H_2 phase transition of hydrogen, the tidal field, the magnetic field, radiation pressure, and cosmic ray heating. We propose observational and numerical tests to distinguish between the remaining candidate star formation inhibitors, in which ALMA will play a key role. We conclude the paper by proposing a self-consistent cycle of star formation in the CMZ, in which the plausible star formation inhibitors are combined. Their ubiquity suggests that the perception of a lowered central SFR should be a common phenomenon in other galaxies. We discuss the implications for galactic star formation and supermassive black hole growth, including a prediction that the recently reported bimodality of star formation in high-redshift galaxies may emanate from a difference in the gas inflow rates.
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http://arxiv.org/abs/1303.6286
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