A consistent explanation for the unusual initial mass function and star formation rate in the Central Molecular Zone (CMZ)

Abstract: We examine various physical processes that may explain the shallow high-mass slope of the IMF as well as the low SFR in star-forming molecular clouds (MCs) in the Central Molecular Zone (CMZ). We show that the strong tidal field and the tidal shear experienced by the CMZ have opposite effects on the collapse of density fluctuations and nearly compensate, but in any case have a negligible impact and can not explain these unusual properties. Similarly, we show that the intense magnetic field in the CMZ provides a negligible pressure support and, for the high densities at play should not modify the probability density function (PDF) of the turbulent gas flow in the clouds, thus affecting negligibly the slope of the IMF. However, we show that, in contrast to MCs in the Galactic disk, the ones in the CMZ experience only one single episode of turbulence injection at large scale, most likely due dominantly to bar gas inflow. Indeed, their rather short lifetime, due to their high mean densities, is similar to one typical turbulence crossing time. Consequently, according to the Hennebelle-Chabrier theory of star formation, within this 'single turbulence episode' scenario, the cloud experiences one single field of turbulence induced density fluctuations, leading eventually to gravitationally unstable prestellar cores. As shown in Hennebelle & Chabrier (2013}, this yields a flatter IMF than usual and leads to the correct observed slope for the CMZ star-forming clouds. Similarly, this single large scale turbulence event within the cloud lifetime yields a 5 to 6 lower SFR than under usual MW cloud conditions, again in agreement with the observed values. Therefore, we suggest that this 'single large scale turbulence injection' episode can explain both the shallow IMF high-mass slope and low SFR of clouds in the CMZ.