Molecules as tracers of galaxy evolution: an EMIR survey

I. Presentation of the data and first results^⋆

¹ Department of Earth and Space SciencesChalmers University of Technology, Onsala Space Observatory, 439 92 Onsala, Sweden
e-mail: francesco.costagliola@chalmers.se
² Instituto de Astrofísica de Andalucía (IAA-CSIC), PO Box 3004, 18080 Granada, Spain
³ Cornell University, Astronomy Department, Ithaca, NY 14853, USA
⁴ European Southern Observarory, Alonso de Córdova 3107, Vitacura, Casilla 19001, Santiago 19, Chile
⁵ Centre for Star and Planet Formation, Natural History Museum of Denmark, University of Copenhagen, Øster Voldgade 5-7, 1350 København K, Denmark
⁶ Astronomisches Institut Ruhr-Universitaet Bochum, Universitaetsstr. 150, 44780 Bochum, Germany
⁷ Leiden Observatory, Leiden University, 2300 RA, Leiden, The Netherlands
⁸ Institute for Astronomy, University of Edinburgh, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, UK
⁹ SRON Netherlands Institute for Space Research, PO Box 800, 9700 AV, Groningen, The Netherlands

Received: 23 August 2010
Accepted: 23 December 2010

Abstract

Aims. We investigate the molecular gas properties of a sample of 23 galaxies in order to find and test chemical signatures of galaxy evolution and to compare them to IR evolutionary tracers.

Methods. Observation at 3 mm wavelengths were obtained with the EMIR broadband receiver, mounted on the IRAM 30 m telescope on Pico Veleta, Spain. We compare the emission of the main molecular species with existing models of chemical evolution by means of line intensity ratios diagrams and principal component analysis.

Results. We detect molecular emission in 19 galaxies in two 8 GHz-wide bands centred at 88 and 112 GHz. The main detected molecules are CO, ¹³CO, HCN, HNC, HCO⁺, CN, and C₂H. We also detect HC₃N J = 10–9 in the galaxies IRAS 17208, IC 860, NGC 4418, NGC 7771, and NGC 1068. The only HC₃N detections are in objects with HCO⁺/HCN  <  1. Galaxies with the highest HC₃N/HCN ratios have warm IRAS colours (60/100 μm  >  0.8). The brightest HC₃N emission is found in IC 860, where we also detect the molecule in its vibrationally excited state. We find low HNC/HCN line ratios (<0.5), that cannot be explained by existing PDR or XDR chemical models. The intensities of HCO+ and HNC appear anti-correlated. No correlation is found between the HNC/HCN line ratio and dust temperature. All HNC-bright objects are either luminous IR galaxies (LIRG) or Seyferts. Galaxies with bright polycyclic aromatic hydrocarbons (PAH) emission show low HNC/HCO⁺ ratios. The CO/¹³CO ratio is positively correlated with the dust temperature and is generally higher than in our galaxy. The emission of CN and C¹⁸O is correlated.

Conclusions. Bright HC₃N emission in HCO⁺-faint objects may imply that these are not dominated by X-ray chemistry. Thus the HCN/HCO⁺ line ratio is not, by itself, a reliable tracer of XDRs. Bright HC₃N and faint HCO⁺ could be signatures of embedded star-formation, instead of AGN activity. Mechanical heating caused by supernova explosions may be responsible for the low HNC/HCN and high HCO⁺/HCN ratios in some starbursts. We cannot exclude, however, that the discussed trends are largely caused by optical depth effects or excitation. Chemical models alone cannot explain all properties of the observed molecular emission. Better constraints to the gas spacial distribution and excitation are needed to distinguish abundance and excitation effects.