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 M33, IC10, and NGC6822 with the IRAM 30meter and NRO 45m telescopes, greatly improving the sample of low-Z galaxies observed. HCN, HCO$^+$, CS, C$_2$H, and HNC have been detected. Compared to solar metallicity galaxies, the Nitrogen-bearing species are weak (HCN, HNC) or not detected (CN, HNCO, N$_2$H$^+$) relative to Far-IR or CO emission. HCO$^+$ and C$_2$H emission is normal with respect to CO and Far-IR. While $^{13}$CO is the usual factor 10 weaker than $^{12}$CO, C$^{18}$O 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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Article dans une revue
Astronomy and Astrophysics - A&A, EDP Sciences, 2017, 597, 〈10.1051/0004-6361/201629781〉
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Contributeur : Marie-Paule Pomies <>
Soumis le : lundi 12 décembre 2016 - 10:17:56
Dernière modification le : lundi 14 janvier 2019 - 12:20:04

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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, 〈10.1051/0004-6361/201629781〉. 〈hal-01414137〉



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