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Dense gas in low-metallicity galaxies

Abstract : Stars form out of the densest parts of molecular clouds. Far-IR emission can be used to estimate the star formation rate (SFR) and high dipole moment molecules, typically HCN, trace the dense gas. A strong correlation exists between HCN and far-IR emission, with the ratio being nearly constant, over a large range of physical scales. A few recent observations have found HCN to be weak with respect to the far-IR and CO in subsolar metallicity (low-Z) objects. We present observations of the Local Group galaxies M 33, IC 10, and NGC 6822 with the IRAM 30 m and NRO 45 m telescopes, greatly improving the sample of low-Z galaxies observed. HCN, HCO+, CS, C2H, and HNC have been detected. Compared to solar metallicity galaxies, the nitrogen-bearing species are weak (HCN, HNC) or not detected (CN, HNCO, N2H+) relative to far-IR or CO emission. HCO+ and C2H emission is normal with respect to CO and far-IR. While 13CO is the usual factor 10 weaker than 12CO, C18O emission was not detected down to very low levels. Including earlier data, we find that the HCN/HCO+ ratio varies with metallicity (O/H) and attribute this to the sharply decreasing nitrogen abundance. The dense gas fraction, traced by the HCN/CO and HCO+/CO ratios, follows the SFR but in the low-Z objects the HCO+ is much easier to measure. Combined with larger and smaller scale measurements, the HCO+ line appears to be an excellent tracer of dense gas and varies linearly with the SFR for both low and high metallicities.
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Contributor : Marie-Paule Pomies <>
Submitted on : Tuesday, January 19, 2021 - 2:21:23 PM
Last modification on : Monday, January 25, 2021 - 4:00:50 PM


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J. Braine, Y. Shimajiri, P. André, Sylvain Bontemps, Yu Gao, et al.. Dense gas in low-metallicity galaxies. Astronomy and Astrophysics - A&A, EDP Sciences, 2017, 597, pp.A44. ⟨10.1051/0004-6361/201629781⟩. ⟨hal-01414137⟩



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