![(Left) The N2H +/HCN ratio toward the central region of M83 shown in the color scale. The black contours show 3-mm continuum intensities with 0.5 mJy beam−1 increments. The beam size is shown at the bottom left corner. (Right) The same figure as Figure 2(a), but including data points from the galactic center region. The grey dashed lines have slopes of 1, and show the lines of constant N2H +/HCN ratios. -- [astro-ph.GA]](https://i0.wp.com/astrobiology.com/wp-content/uploads/2026/10/ALMA-observations.png?fit=1200%2C677&ssl=1)
[astro-ph.GA] Galactic observations have shown that N2H+ intensities preferentially trace star-forming dense molecular gas compared with HCN or HCO+, which are conventionally used as dense-gas tracers in extragalactic studies.
In this study, we aim to verify this trend in the nearby star-forming galaxy M83. Using ALMA, we observed the J=1-0 transitions of N2H+ and HCN, along with the 1-mm and 3-mm continuum, and analyzed the archival CO(1-0) data.
The data achieved a spatial resolution of 48 pc in the southwestern bar-end region of M83, covering an area of 2.1 kpc x 1.8 kpc. N2H+ and HCN intensities are approximately linearly correlated, indicating that HCN luminosity can still reasonably predict the amount of dense gas as well as N2H+ luminosity does.
However, in regions with high column densities, N2H+ becomes a super-linear function of HCN, suggesting that the N2H+ emission is more sensitive to dense gas than HCN. We employed a radiative transfer model that predicts line intensities as a function of column density, adopting parametric column density probability distribution functions (N-PDFs) to describe the underlying column density distribution.
The model demonstrates that the observed linear or superlinear trend can be explained by a log-normal distribution only or a log-normal and a power-law tail distribution in the N-PDF. Furthermore, the N2H+ intensity is significantly more sensitive to the power-law tail than the HCN intensity.
In addition, our analysis incorporating archival data shows that N2H+ is a less reliable tracer of dense gas in galactic center regions, where warmer temperatures may promote N2H+ destruction.
Nanase Harada, Yuri Nishimura, Toshiki Saito, Mario Tafalla, Jin Koda, Doris Arzoumanian, Sophia K. Stuber, Akihiko Hirota, Yoshimasa Watanabe
Comments: Accepted for publication in the Astrophysical Journal, 27 pages, 12 figures
Subjects: Astrophysics of Galaxies (astro-ph.GA)
Cite as: arXiv:2610.21142 [astro-ph.GA] (or arXiv:2610.21142v1 [astro-ph.GA] for this version)
https://doi.org/10.48550/arXiv.2610.21142
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Submission history
From: Nanase Harada
[v1] Thu, 15 Oct 2026 02:55:33 UTC (1,822 KB)
https://arxiv.org/abs/2610.21142
Astrobiology,
