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

Abstract

We present the first results from "Surveying the Whirlpool at Arcseconds with NOEMA" (SWAN), an IRAM Northern Extended Millimetre Array (NOEMA)+30m large program that maps emission from several molecular lines at 90 and 110 GHz in the iconic nearby grand-design spiral galaxy M~51 at cloud-scale resolution ($\sim$3\arcsec=125\,pc). As part of this work, we have obtained the first sensitive cloud-scale map of N${}_2$H${}^{+}$(1-0) of the inner $\sim 5\times 7$kpc of a normal star-forming galaxy, which we compare to HCN(1-0) and CO(1-0) emission to test their ability in tracing dense, star-forming gas. The average N${}_2$H${}^{+}$-to-HCN line ratio of our total FoV is $0.20\pm 0.09$, with strong regional variations of a factor of $\gtrsim 2$ throughout the disk, including the south-western spiral arm and the center. The central $\sim 1$kpc exhibits elevated HCN emission compared to N${}_2$H${}^{+}$, probably caused by AGN-driven excitation effects. We find that HCN and N${}_2$H${}^{+}$ are strongly super-linearily correlated in intensity ($\ddot{I}{}_{\mathrm{Sp}}\sim 0.8$), with an average scatter of $\sim 0.14$dex over a span of $\gtrsim 1.5$dex in intensity. When excluding the central region, the data is best described by a power-law of exponent $1.2$, indicating that there is more N${}_2$H${}^{+}$ per unit HCN in brighter regions. Our observations demonstrate that the HCN-to-CO line ratio is a sensitive tracer of gas density in agreement with findings of recent Galactic studies which utilize N${}_2$H${}^{+}$. The peculiar line ratios present near the AGN and the scatter of the power-law fit in the disk suggest that in addition 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.

Accepted for publication in A&A. 6 pages, 3 figures (+ Appendix 4 pages, 2 figures)