Impact of galactic shear and stellar feedback on star formation
  A numerical shearing box is used to perform three-dimensional simulations of
a 1 kpc stratified cubic box of turbulent and self-gravitating interstellar
medium (in a rotating frame) with supernovae and HII feedback. We vary the
value of the velocity gradient induced by the shear and the initial value of
the galactic magnetic field. Finally the different star formation rates and the
properties of the structures associated with this set of simulations are
computed. We first confirm that the feedback has a strong limiting effect on
star formation. The galactic shear has also a great influence: the higher the
shear, the lower the SFR. Taking the value of the velocity gradient in the
solar neighbourhood, the SFR is too high compared to the observed Kennicutt
law, by a factor approximately three to six. This discrepancy can be solved by
arguing that the relevant value of the shear is not the one in the solar
neighbourhood, and that in reality the star formation efficiency within
clusters is not 100%. Taking into account the fact that star-forming clouds
generally lie in spiral arms where the shear can be substantially higher (as
probed by galaxy-scale simulations), the SFR is now close to the observed one.
Different numerical recipes have been tested for the sink particles, giving a
numerical incertitude of a factor of about two on the SFR. Finally we have also
estimated the velocity dispersions in our dense clouds and found that they lie
below the observed Larson law by a factor of about two. Conclusions. In our
simulations, magnetic field, shear, HII regions, and supernovae all contribute
significantly to reduce the SFR. In this numerical setup with feedback from
supernovae and HII regions and a relevant value of galactic shear, the SFRs are
compatible with those observed, with a numerical incertitude factor of about
two.
