Mar 30, 2017
Andrey Kravtsov, The University of Chicago
""Towards understanding the inefficiency of star formation in galaxies""
One of the long-standing puzzles of galaxy formation is the global inefficiency with which galaxies convert baryonic matter available to them into stars. This inefficiency is manifested in 1) the fact that ratio of baryon mass observed within galaxies to the total inferred mass of their host halos is much smaller than the universal baryon fraction and 2) the fact that galaxies convert their observed gas into stars on ~2-10 Gyr time scale (gas depletion time), which is much longer than any dynamical time scale within galaxies.
I will review recent progress in galaxy formation simulations due to improvements in treatment of stellar feedback and star formation, which sheds light into the 1st aspect of inefficiency. I will present a new model, in which local star formation efficiency is modelled "on the fly" using a turbulence-based subgrid model based on results of high-resolution simulations of molecular clouds. The model predicts a wide variation of star formation efficiency per free fall time at odds with the usual assumption of constant efficiency. At the same time, our model predicts distribution of star formation rates in broad agreement with observations of both local and resolved extragalactic GMCs. I will show that with realistic implementation of stellar feedback this modelling can reproduce the basic properties of star formation and the Kennicutt-Schmidt relation in galaxies, such as the Milky Way. Finally, using insights from such simulations I will present a simple model explaining why star formation in galaxies is inefficient and depletion times are long.