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We present the first results from "Surveying the Whirlpool at Arcseconds withNOEMA" (SWAN), an IRAM Northern Extended Millimetre Array (NOEMA)+30m largeprogram that maps emission from several molecular lines at 90 and 110 GHz inthe iconic nearby grand-design spiral galaxy M~51 at cloud-scale resolution($\sim$3\arcsec=125\,pc). As part of this work, we have obtained the firstsensitive cloud-scale map of N$_2$H$^+$(1-0) of the inner $\sim5\,\times7\,$kpc of a normal star-forming galaxy, which we compare to HCN(1-0) andCO(1-0) emission to test their ability in tracing dense, star-forming gas. Theaverage N$_2$H$^+$-to-HCN line ratio of our total FoV is $0.20\pm0.09$, withstrong regional variations of a factor of $\gtrsim 2$ throughout the disk,including the south-western spiral arm and the center. The central $\sim1\,$kpcexhibits elevated HCN emission compared to N$_2$H$^+$, probably caused byAGN-driven excitation effects. We find that HCN and N$_2$H$^+$ are stronglysuper-linearily correlated in intensity ($\rho_\mathrm{Sp}\sim 0.8$), with anaverage scatter of $\sim0.14\,$dex over a span of $\gtrsim 1.5\,$dex inintensity. When excluding the central region, the data is best described by apower-law of exponent $1.2$, indicating that there is more N$_2$H$^+$ per unitHCN in brighter regions. Our observations demonstrate that the HCN-to-CO lineratio is a sensitive tracer of gas density in agreement with findings of recentGalactic studies which utilize N$_2$H$^+$. The peculiar line ratios presentnear the AGN and the scatter of the power-law fit in the disk suggest that inaddition to a first-order correlation with gas density, second-order physics(such as optical depth, gas temperature) or chemistry (abundance variations)are encoded in the N$_2$H$^+$/CO, HCN/CO and N$_2$H$^+$/HCN ratios.

Surveying the Whirlpool at Arcseconds with NOEMA (SWAN)- I. Mapping the HCN and N$_2$H$^+$ 3mm lines