A constant N$_2$H$^+$(1-0)-to-HCN(1-0) ratio on kiloparsec scales

TL;DR

This study demonstrates a consistent ratio between N$_2$H$^+$(1-0) and HCN(1-0) emissions across kiloparsec scales in an external galaxy, supporting the use of HCN as a proxy for dense gas in extragalactic observations.

Contribution

First systematic measurement of N$_2$H$^+$(1-0) across different environments in an external galaxy, establishing a stable ratio with HCN(1-0) at kiloparsec scales.

Findings

Strong correlation ($r>0.98$) between HCN and N$_2$H$^+$ intensities.

Average N$_2$H$^+$/HCN ratio of 0.15±0.02 across observed regions.

Herschel's HCN emission can predict N$_2$H$^+$ emission at 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, NGC6946. 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 $\sim8\mathrm{kpc}$ 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 ($f_\mathrm{dense}$). We measure an average intensity ratio of N$_2$H$^+$(1-0)/HCN(1-0)$=0.15\pm0.02$ 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 $f_\mathrm{dense}$ and the efficiency of dense gas (SFE$_\mathrm{dense}$) 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.