A constant N₂H⁺ (1-0)-to-HCN (1-0) ratio on kiloparsec scales
Abstract
Nitrogen hydrides such as NH 3 and N 2 H + 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 N 2 H + (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 N 2 H + (1-0) intensities across the inner ∼8 kpc of the galaxy, at kiloparsec scales. This correlation is equally strong between the ratios N 2 H + (1-0)/CO (1-0) and HCN (1-0)/CO (1-0), tracers of dense gas fractions ( f dense ). We measure an average intensity ratio of N 2 H + (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 f dense and the efficiency of dense gas (SFE dense ) would not change if we employed N 2 H + (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 N 2 H + (1-0) over those regions. Our results suggest that, even if HCN (1-0) and N 2 H + (1-0) trace different density regimes within molecular clouds, subcloud differences average out at kiloparsec scales, yielding the two tracers proportional to each other.
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BibTeX
@article{jimenezdonaire2026constant,
title = {{A constant N₂H⁺ (1-0)-to-HCN (1-0) ratio on kiloparsec scales}},
author = {M J Jiménez-Donaire and A Usero and I Bešlić and M Tafalla and A Chacón-Tanarro and Q Salomé and C Eibensteiner and A García-Rodríguez and A Hacar and A T Barnes and F Bigiel and M Chevance and D Colombo and D A Dale and T A Davis and S C O Glover and J Kauffmann and R S Klessen and A K Leroy and L Neumann and H Pan and J Pety and M Querejeta and T Saito and E Schinnerer and S Stuber and T G Williams},
journal = {Astronomy \& Astrophysics - A\&A},
year = {2026},
month = sep,
doi = {10.1051/0004-6361/202347050},
url = {https://hal.science/hal-05761301v1},
note = {CC BY 4.0},
}
RIS
TY - JOUR
TI - A constant N₂H⁺ (1-0)-to-HCN (1-0) ratio on kiloparsec scales
AU - M J Jiménez-Donaire
AU - A Usero
AU - I Bešlić
AU - M Tafalla
AU - A Chacón-Tanarro
AU - Q Salomé
AU - C Eibensteiner
AU - A García-Rodríguez
AU - A Hacar
AU - A T Barnes
AU - F Bigiel
AU - M Chevance
AU - D Colombo
AU - D A Dale
AU - T A Davis
AU - S C O Glover
AU - J Kauffmann
AU - R S Klessen
AU - A K Leroy
AU - L Neumann
AU - H Pan
AU - J Pety
AU - M Querejeta
AU - T Saito
AU - E Schinnerer
AU - S Stuber
AU - T G Williams
PY - 2026
DA - 2026/09/23
JO - Astronomy & Astrophysics - A&A
DO - 10.1051/0004-6361/202347050
UR - https://hal.science/hal-05761301v1
AB - Nitrogen hydrides such as NH 3 and N 2 H + 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 N 2 H + (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 N 2 H + (1-0) intensities across the inner ∼8 kpc of the galaxy, at kiloparsec scales. This correlation is equally strong between the ratios N 2 H + (1-0)/CO (1-0) and HCN (1-0)/CO (1-0), tracers of dense gas fractions ( f dense ). We measure an average intensity ratio of N 2 H + (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 f dense and the efficiency of dense gas (SFE dense ) would not change if we employed N 2 H + (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 N 2 H + (1-0) over those regions. Our results suggest that, even if HCN (1-0) and N 2 H + (1-0) trace different density regimes within molecular clouds, subcloud differences average out at kiloparsec scales, yielding the two tracers proportional to each other.
SN - 0004-6361
SN - 1432-0746
ER -