Surveying the Whirlpool at Arcseconds with NOEMA (SWAN). I. Mapping the HCN and N2H+ 3mm lines

doi:10.1051/0004-6361/202348205

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Title:	Surveying the Whirlpool at Arcseconds with NOEMA (SWAN). I. Mapping the HCN and N2H+ 3mm lines
Authors:	Stuber, Sophia K.;Pety, Jerome;Schinnerer, Eva;Bigiel, Frank;Usero, Antonio;Bešlić, Ivana;Querejeta, Miguel;María, J. Jiménez-Donaire;Leroy, Adam;Brok, Jakob den;Neumann, Lukas;Eibensteiner, Cosima;Teng, Yu-Hsuan;Barnes, Ashley;Chevance, Mélanie;Colombo, Dario;Dale, Daniel A.;Glover, Simon C. O.;Liu, Daizhong;Pan, Hsi-An
Keywords:	ISM: molecules;galaxies: individual: M 51;galaxies: ISM
Date:	2023-12-20
Issue Date:	2024-03-06 12:05:26 (UTC+8)
Publisher:	EDP Sciences
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 (ρSp ∼ 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.
Relation:	Astronomy & Astrophysics 680
DOI:	10.1051/0004-6361/202348205
Appears in Collections:	[物理學系暨研究所] 期刊論文

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