A Constant N2H+(1-0)-to-HCN(1-0) Ratio On Kiloparsec Scales

By Keith Cowing
Status Report
August 3, 2023
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A Constant N2H+(1-0)-to-HCN(1-0) Ratio On Kiloparsec Scales
NGC 6946 with EMPIRE CO (1-0) contours overlaid on an optical image. The contours illustrate the IRAM-30m 12CO (1-0) integrated intensities. The millimeter observations have a resolution of 26′′(∼800 pc), indicated by the orange circle. The contours are drawn at arbitrary intervals between 5 and 200 K km s−1 to highlight the distribution of the molecular gas disk. The yellow circles indicate the locations of our pointed N2H + (1-0) and HCN (1-0) observations, covering a wide range of galactic environmental conditions: center (0), spiral arms (1 and 2), interarm (4), and outskirts (3). The size represents the 28′′ resolution of the IRAM-30m beam at ∼ 86 GHz. — astro-ph.GA

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, NGC6946. We find a strong correlation (r>0.98,p<0.01) between the HCN(1-0) and N2H+(1-0) intensities across the inner ∼8kpc 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.

M. J. Jiménez-Donaire, A. Usero, I. Bešlić, M. Tafalla, A. Chacón-Tanarro, Q. Salomé, C. Eibensteiner, A. García-Rodríguez, 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

Comments: Accepted for publication in Astronomy & Astrophysics
Subjects: Astrophysics of Galaxies (astro-ph.GA)
Cite as: arXiv:2308.01342 [astro-ph.GA] (or arXiv:2308.01342v1 [astro-ph.GA] for this version)
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Submission history
From: Maria Jesus Jimenez-Donaire
[v1] Wed, 2 Aug 2023 18:00:02 UTC (3,339 KB)

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