A constant N2H+(1-0)-to-HCN (1-0) ratio on kiloparsec scales

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Astronomy and Astrophysics, Volume 676
Nitrogen hydrides such as NH3 and N2H+ are widely used by Galactic observers to trace the cold dense regions of the interstellar medium. In external galaxies, because of limited sensitivity, HCN has become the most common tracer of dense gas over large parts of galaxies. We provide the first systematic measurements of N2H+ (1-0) across different environments of an external spiral galaxy, NGC 6946. We find a strong correlation (r > 0.98, p < 0.01) between the HCN (1-0) and N2H+ (1-0) intensities across the inner 8 kpc of the galaxy, at kiloparsec scales. This correlation is equally strong between the ratios N2H+ (1-0)/CO (1-0) and HCN (1-0)/CO (1-0), tracers of dense gas fractions (fdense). We measure an average intensity ratio of N2H+ (1-0)/HCN (1-0) = 0.15 ± 0.02 over our set of five IRAM-30m pointings. These trends are further supported by existing measurements for Galactic and extragalactic sources. This narrow distribution in the average ratio suggests that the observed systematic trends found in kiloparsec-scale extragalactic studies of fdense and the efficiency of dense gas (SFEdense) would not change if we employed N2H+ (1-0) as a more direct tracer of dense gas. At kiloparsec scales our results indicate that the HCN (1-0) emission can be used to predict the expected N2H+ (1-0) over those regions. Our results suggest that, even if HCN (1-0) and N2H+ (1-0) trace different density regimes within molecular clouds, subcloud differences average out at kiloparsec scales, yielding the two tracers proportional to each other.
Funding Information: This work is based on observations carried out with the IRAM-30m telescope. IRAM is supported by INSU/CNRS (France), MPG (Germany) and IGN (Spain). The authors would like to thank E. Pellegrini, D. Harsono and S. Ellison for useful discussions, and the referee, Neal Evans, for a constructive and helpful report. A.H. acknowledges funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (Grant agreement Nos. 851435). R.S.K. and S.C.O.G. acknowledge financial support from the ERC via the ERC Synergy Grant “ECOGAL” (project ID 855130), from the German Excellence Strategy via the Heidelberg Cluster of Excellence (EXC 2181 – 390900948) “STRUCTURES”, and from the German Ministry for Economic Affairs and Climate Action in project “MAINN” (funding ID 50OO2206). R.S.K. also thanks for computing resources provided bwHPC and DFG through grant INST 35/1134-1 FUGG and for data storage at SDS@hd through grant INST 35/1314-1 FUGG. M.J.J.D. and M.Q. acknowledge support from the Spanish grant PID2019-106027GA-C44, funded by MCIN/AEI/10.13039/501100011033. M.C. gratefully acknowledges funding from the DFG through an Emmy Noether Research Group (grant number CH2137/1-1). L.N. acknowledges funding from the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) – 516405419. A.U. acknowledges support from the Spanish grants PGC2018-094671-B-I00, funded by MCIN/AEI/10.13039/501100011033 and by “ERDF A way of making Europe”, and PID2019-108765GB-I00, funded by MCIN/AEI/10.13039/501100011033. Publisher Copyright: © 2023 EDP. All rights reserved.
Galaxies: ISM, ISM: molecules, Radio lines: galaxies
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Jimenez-Donaire, M J, Usero, A, Beslia, I, Tafalla, M, Chacon-Tanarro, A, Salome, Q, Eibensteiner, C, Garcia-Rodriguez, A, Hacar, A, Barnes, A T, Bigiel, F, Chevance, M, Colombo, D, Dale, D A, Davis, T A, Glover, S C O, Kauffmann, J, Klessen, R S, Leroy, A K, Neumann, L, Pan, H, Pety, J, Querejeta, M, Saito, T, Schinnerer, E, Stuber, S & Williams, T G 2023, ' A constant N 2 H + (1-0)-to-HCN (1-0) ratio on kiloparsec scales ', Astronomy and Astrophysics, vol. 676, L11 . https://doi.org/10.1051/0004-6361/202347050