Volume 597, January 2017
|Number of page(s)||8|
|Published online||21 December 2016|
Dense gas in low-metallicity galaxies
1 Laboratoire d’Astrophysique de Bordeaux, Univ. Bordeaux, CNRS, B18N, Allée Geoffroy Saint-Hilaire, 33615 Pessac, France
2 Laboratoire AIM, CEA/DRF – CNRS – Université Paris Diderot, IRFU/Service d’Astrophysique, C.E. Saclay, Orme des Merisiers, 91191 Gif-sur-Yvette, France
3 Purple Mountain Observatory, Chinese Academy of Sciences, 2 West Beijing Road, 210008 Nanjing, PR China
4 Key Laboratory of Radio Astronomy, Chinese Academy of Sciences, 210008 Nanjing, PR China
5 School of Astronomy and Space Science, Nanjing University, 210093 Nanjing, PR China
6 Key Laboratory of Modern Astronomy and Astrophysics, Nanjing University, 210093 Nanjing, PR China
7 Collaborative Innovation Center of Modern Astronomy and Space Exploration, 210093 Nanjing, PR China
8 Instituto Radioastronoma Milimétrica, Av. Divina Pastora 7, Nucleo Central, 18012 Granada, Spain
Received: 23 September 2016
Accepted: 27 October 2016
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.
Key words: galaxies: individual: M 33 / galaxies: individual: IC 10 / galaxies: individual: NGC 6822 / Local Group / galaxies: ISM / stars: formation
© ESO, 2016
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