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

Author(s)

M. J. Jimenez-Donaire, A. Usero, I. Beslia, M. Tafalla, A. Chacon-Tanarro, Q. Salome, C. Eibensteiner, A. Garcia-Rodriguez, A. Hacar, A. T. Barnes, F. Bigiel, M. Chevance, D. Colombo, D. A. Dale, T. A. Davis, S. C.O. Glover, J. Kauffmann, R. S. Klessen, A. K. Leroy, L. Neumann, H. Pan, J. Pety, M. Querejeta, T. Saito, E. Schinnerer, S. Stuber, T. G. Williams

Abstract

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.

Organisation(s)

Department of Astrophysics

External organisation(s)

Observatorio Astronómico Nacional (OAN-IGN), Observatorio de Yebes (IGN), Université de recherche Paris Sciences et Lettres, University of Turku, Aalto University, Rheinische Friedrich-Wilhelms-Universität Bonn, European Southern Observatory (Germany), Scientific Software Center, University of Wyoming, Cardiff University, Massachusetts Institute of Technology, Ohio State University, Tamkang University, Institut de RadioAstronomie Millimétrique (IRAM), National Astronomical Observatory of Japan, Max-Planck-Institut für Astronomie, University of Oxford

Journal

Astronomy & Astrophysics

Volume

676

No. of pages

8

ISSN

0004-6361

DOI

https://doi.org/10.1051/0004-6361/202347050

Publication date

08-2023

Peer reviewed

Yes

Austrian Fields of Science 2012

103004 Astrophysics, 103003 Astronomy

Keywords

ASJC Scopus subject areas

Astronomy and Astrophysics, Space and Planetary Science

Portal url

https://ucrisportal.univie.ac.at/en/publications/7587a2a6-928e-4179-8077-8732a641a467