Chemistry in isolation: High CCH/HCO⁺ line ratio in the AMIGA galaxy CIG 638^⋆

¹ Institut de Radio Astronomie Millimétrique, 300 rue de la Piscine, Dom. Univ., 38406 St. Martin d’Hères, France
e-mail: smartin@iram.fr
² Instituto de Astrofísica de Andalucía, Granada, IAA-CSIC Apdo. 3004, 18080 Granada, Spain
³ European Southern Observatory, Alonso de Córdova 3107, Vitacura, Casilla 19001 Santiago 19, Chile
⁴ Joint ALMA Observatory (JAO), Alonso de Córdova 3107, 763 0355 Vitacura, Santiago, Chile
⁵ National Astronomical Observatory of Japan (NAOJ), 2-21-1 Osawa, Mitaka, 181-8588 Tokyo, Japan
⁶ Department of Astronomical Science, The Graduate University for Advanced Studies (SOKENDAI), 2-21-1 Osawa, Mitaka, 181-8588 Tokyo, Japan
⁷ Instituto Radioastronomía Milimétrica, Av. Divina Pastora 7, 18012 Granada, Spain

Received: 24 January 2014
Accepted: 14 February 2014

Abstract

Context. Multi-molecule observations towards an increasing variety of galaxies have been showing that the relative molecular abundances are affected by the type of activity. However, these studies are biased towards bright active galaxies, which are typically in interaction.

Aims. We study the molecular composition of one of the most isolated galaxies in the local Universe where the physical and chemical properties of their molecular clouds have been determined by intrinsic mechanisms.

Methods. We present 3 mm broad band observations of the galaxy CIG 638, extracted from the AMIGA sample of isolated galaxies. The emission of the J = 1–0 transitions of CCH, HCN, HCO⁺, and HNC are detected. Integrated intensity ratios between these line are compared with similar observations from the literature towards active galaxies including starburst galaxies (SB), active galactic nuclei, luminous infrared galaxies (LIRG), and GMCs in M 33.

Results. A significantly high ratio of CCH with respect to HCN, HCO⁺, and HNC is found towards CIG 638 when compared with all other galaxies where these species have been detected. This points to either an overabundance of CCH or to a relative lack of dense molecular gas as supported by the low HCN/CO ratio, or both.

Conclusions. The data suggest that the CIG 638 is naturally a less perturbed galaxy where a lower fraction of dense molecular gas, as well as a more even distribution could explain the measured ratios. In this scenario the dense gas tracers would be naturally dimmer, while the UV enhanced CCH, would be overproduced in a less shielded medium.