2023A&A...676L..11J

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 (f_dense). 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 f_dense and the efficiency of dense gas (SFE_dense) 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.