Dense gas is not enough: environmental variations in the star formation
  efficiency of dense molecular gas at 100pc scales in M51
  It remains unclear what sets the efficiency with which molecular gas
transforms into stars. Here we present a new VLA map of the spiral galaxy M51
in 33GHz radio continuum, an extinction-free tracer of star formation, at 3"
scales (~100pc). We combined this map with interferometric PdBI/NOEMA
observations of CO(1-0) and HCN(1-0) at matched resolution for three regions in
M51 (central molecular ring, northern and southern spiral arm segments). While
our measurements roughly fall on the well-known correlation between total
infrared and HCN luminosity, bridging the gap between Galactic and
extragalactic observations, we find systematic offsets from that relation for
different dynamical environments probed in M51, e.g. the southern arm segment
is more quiescent due to low star formation efficiency (SFE) of the dense gas,
despite having a high dense gas fraction. Combining our results with
measurements from the literature at 100pc scales, we find that the SFE of the
dense gas and the dense gas fraction anti-correlate and correlate,
respectively, with the local stellar mass surface density. This is consistent
with previous kpc-scale studies. In addition, we find a significant
anti-correlation between the SFE and velocity dispersion of the dense gas.
Finally, we confirm that a correlation also holds between star formation rate
surface density and the dense gas fraction, but it is not stronger than the
correlation with dense gas surface density. Our results are hard to reconcile
with models relying on a universal gas density threshold for star formation and
suggest that turbulence and galactic dynamics play a major role in setting how
efficiently dense gas converts into stars.
