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It remains unclear what sets the efficiency with which molecular gastransforms into stars. Here we present a new VLA map of the spiral galaxy M51in 33GHz radio continuum, an extinction-free tracer of star formation, at 3"scales (~100pc). We combined this map with interferometric PdBI/NOEMAobservations of CO(1-0) and HCN(1-0) at matched resolution for three regions inM51 (central molecular ring, northern and southern spiral arm segments). Whileour measurements roughly fall on the well-known correlation between totalinfrared and HCN luminosity, bridging the gap between Galactic andextragalactic observations, we find systematic offsets from that relation fordifferent dynamical environments probed in M51, e.g. the southern arm segmentis more quiescent due to low star formation efficiency (SFE) of the dense gas,despite having a high dense gas fraction. Combining our results withmeasurements from the literature at 100pc scales, we find that the SFE of thedense gas and the dense gas fraction anti-correlate and correlate,respectively, with the local stellar mass surface density. This is consistentwith previous kpc-scale studies. In addition, we find a significantanti-correlation between the SFE and velocity dispersion of the dense gas.Finally, we confirm that a correlation also holds between star formation ratesurface density and the dense gas fraction, but it is not stronger than thecorrelation with dense gas surface density. Our results are hard to reconcilewith models relying on a universal gas density threshold for star formation andsuggest that turbulence and galactic dynamics play a major role in setting howefficiently dense gas converts into stars.

Dense gas is not enough: environmental variations in the star formation efficiency of dense molecular gas at 100pc scales in M51