Dense gas in low-metallicity galaxies

¹ Laboratoire d’Astrophysique de Bordeaux, Univ. Bordeaux, CNRS, B18N, Allée Geoffroy Saint-Hilaire, 33615 Pessac, France
e-mail: jonathan.braine@u-bordeaux.fr
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

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.