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We present the first results from the EMPIRE survey, an IRAM large programthat is mapping tracers of high density molecular gas across the disks of ninenearby star-forming galaxies. Here, we present new maps of the 3-mm transitionsof HCN, HCO+, and HNC across the whole disk of our pilot target, M51. Asexpected, dense gas correlates with tracers of recent star formation, fillingthe "luminosity gap" between Galactic cores and whole galaxies. In detail, weshow that both the fraction of gas that is dense, f_dense traced by HCN/CO, andthe rate at which dense gas forms stars, SFE_dense traced by IR/HCN, depend onenvironment in the galaxy. The sense of the dependence is that high surfacedensity, high molecular gas fraction regions of the galaxy show high dense gasfractions and low dense gas star formation efficiencies. This agrees withrecent results for individual pointings by Usero et al. 2015 but using unbiasedwhole-galaxy maps. It also agrees qualitatively with the behavior observedcontrasting our own Solar Neighborhood with the central regions of the MilkyWay. The sense of the trends can be explained if the dense gas fraction tracksinterstellar pressure but star formation occurs only in regions of high densitycontrast.

The EMPIRE Survey: Systematic Variations in the Dense Gas Fraction and Star Formation Efficiency from Full-Disk Mapping of M51