2015ApJ...810..140C

We present the spatially resolved observations of HCN J = 1-0 emission in the nearby spiral galaxy M51 using the IRAM 30 m telescope. The HCN map covers an extent of 4'x5', which is, so far, the largest in M51 with a spatial resolution of 28". There is a correlation between infrared emission (an indication of star formation rate (SFR)) and HCN (1-0) emission (a dense gas tracer) on a kiloparsec scale in M51, a natural extension of the proportionality between the SFR and the dense gas mass established globally in galaxies. Within M51, the relation appears to be sub-linear (with a slope of 0.74±0.16) as L_IR rises less quickly than L_HCN. We attribute this to a difference between center and outer disk such that the central regions have stronger HCN (1-0) emission per unit star formation. The IR-HCN correlation in M51 is further compared with global ones from the Milky Way to high-z galaxies and bridges the gap between giant molecular clouds and galaxies. Like the centers of nearby galaxies, the L_IR/L_HCN ratio measured in M51 (particularly in the central regions) is slightly lower than what is measured globally in galaxies, yet is still within the scatter. This implies that though the L_IR/L_HCN ratio varies as a function of physical environment in the different positions of M51, IR, and HCN indeed show a linear correlation over 10 orders of magnitude.