Zusammenfassung
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.
Nutzer