tokens: K km s −1 , or 2.3 Jy km s −1 . The flux of the NOEMA observation is 1.1 Jy km s −1 in the same region. The difference could be due to missing short spacings, such that larger structures are resolved out, or to the residual interferometric “bowl”, which has not been completely eliminated by the cleaning. 2.2. CO and IR Data The CO J = 1–0 data were taken with the CARMA array combined with zero spacings from the 5 × 5 Beam Array Receiver System on the Nobeyama Radio Observatory 45M telescope (Koda et al. 2011 ). Schinnerer et al. ( 2013 ) observed M51 with the Plateau de Bure interferometer at high resolution, but their map does not extend to regions as far out as this one. In order to compare with sites of recent star formation, we use the 70 μ m image from the Very Nearby Galaxy Survey, (Mentuch et al. 2012 ) which was accessed through the Herschel Database in Marseille (HeDaM 10 10 http://hedam.lam.fr , 11 11 The Herschel Database in Marseille (HeDaM) is operated by CeSAM and hosted by the Laboratoire d’Astrophysique de Marseille. ). The resolution is only slightly poorer than ours, so we can use the Herschel 70 μ m images to estimate the SFRs (Boquien et al. 2011 ; Galametz et al. 2013 ). 3. Results Figure 2 shows the integrated intensity maps of the HCN, HCO + , and HNC emission lines along with the CO(1−0) image obtained by Koda et al. ( 2011 ). The integrated intensities are measured as , and the emission velocity ranges ( V ) are defined from the data cube and shown in Table 1 . Uncertainties are calculated as ( is the data cube rms, and W and are the velocity range and velocity resolution of the integrated intensity map) and are 5.6, 0.15, 0.14, and 0.13 K km s −1 for CO, HCN, HCO + , and HNC, respectively. Because the HCN, HCO + , and HNC emissions show similar but clearly different morphologies, we choose not to bias our results on any of these tracers and instead use the higher signal-to-noise ratio CO map with slightly better angular resolution to define the giant molecular associations (GMAs). A total of six GMAs have been identified (black numbered polygons in Figure 2 ) by the ClumpFind 12 12 http://www.ifa.hawaii.edu/users/jpw/clumpfind.shtml algorithm (Williams et al. 1994 ). The HCN and HCO + emission peaks are similar at about 1.2 and 1.4 K km s −1 , and the peak positions are consistent with each other at the center of GMA 2. The HNC line is only detected in GMAs 1 and 2, and the emission region is displaced to the south compared to the other lines. GMAs 3, 4, and 6 are weak in HCN but strong in HCO + (and CO). The HCO + distribution is generally broader than that of CO, HCN, and HNC. Figure 2 also shows the H α emission regions (white contours) in order to allow a comparison with the sites of massive star formation. As can be seen from the detail in Figure 1 , the 70 μ m emission distribution is quite similar to that of H α , so the white contours provide a good reference for the sites of star formation in these GMAs.
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