Letter to the Editor

Surveying the Whirlpool at Arcseconds with NOEMA (SWAN)

I. Mapping the HCN and N₂H⁺ 3mm lines

¹ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
e-mail: stuber@mpia.de
² IRAM, 300 rue de la Piscine, 38400 Saint Martin d’Hères, France
³ Sorbonne Université, Observatoire de Paris, Université PSL, École normale supèrieure, CNRS, LERMA, 75005 Paris, France
⁴ Argelander-Institut für Astronomie, Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
⁵ Observatorio Astronómico Nacional (IGN), C/Alfonso XII 3, Madrid 28014, Spain
⁶ LERMA, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Universités, 75014 Paris, France
⁷ Centro de Desarrollos Tecnológicos, Observatorio de Yebes (IGN), 19141 Yebes, Guadalajara, Spain
⁸ Department of Astronomy, The Ohio State University, 140 West 18th Ave, Columbus, OH 43210, USA
⁹ Center for Astrophysics | Harvard & Smithsonian, 60 Garden St., 02138 Cambridge, MA, USA
¹⁰ Center for Astrophysics and Space Sciences, University of California San Diego, 9500 Gilman Drive, La Jolla, CA 92093, USA
¹¹ European Southern Observatory, Karl-Schwarzschild 2, 85748 Garching bei Muenchen, Germany
¹² Institut für Theoretische Astrophysik, Zentrum für Astronomie der Universität Heidelberg, Albert- Ueberle-Strasse 2, 69120 Heidelberg, Germany
¹³ Cosmic Origins Of Life (COOL) Research DAO, coolresearch.io
¹⁴ Department of Physics & Astronomy, University of Wyoming, Laramie, WY 82071, USA
¹⁵ Max-Planck-Institut für extraterrestrische Physik, Giessenbachstraße 1, 85748 Garching, Germany
¹⁶ Department of Physics, Tamkang University, No.151, Yingzhuan Road, Tamsui District, New Taipei City 251301, Taiwan

Received: 9 October 2023
Accepted: 30 November 2023

Abstract

We present the first results from “Surveying the Whirlpool at Arcseconds with NOEMA” (SWAN), an IRAM Northern Extended Millimetre Array (NOEMA)+30 m 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 a cloud-scale resolution (∼3″ = 125 pc). As part of this work, we have obtained the first sensitive cloud-scale map of N₂H⁺(1–0) of the inner ∼5  × 7 kpc of a normal star-forming galaxy, which we compared to HCN(1–0) and ¹²CO(1–0) emission to test their ability in tracing dense, star-forming gas. The average N₂H⁺-to-HCN line ratio of our total FoV is 0.20 ± 0.09, with strong regional variations of a factor of ≳2 throughout the disk, including the south-western spiral arm and the center. The central ∼1 kpc exhibits elevated HCN emission compared to N₂H⁺, probably caused by AGN-driven excitation effects. We find that HCN and N₂H⁺ are strongly super-linearily correlated in intensity (ρ_Sp ∼ 0.8), with an average scatter of ∼0.14 dex over a span of ≳1.5 dex in intensity. When excluding the central region, the data are best described by a power law of an exponent of 1.2, indicating that there is more N₂H⁺ 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 utilising N₂H⁺. 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 N₂H⁺/¹²CO, HCN/¹²CO, and N₂H⁺/HCN ratios.