1 Instituto Radioastronomía Milimétrica, Av. Divina Pastora 7, Nucleo Central, 18012 Granada, Spain
2 Sterrewacht Leiden, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
3 Observatorio Astronómico Nacional (OAN)-Observatorio de Madrid, Alfonso XII 3, 28014 Madrid, Spain
4 Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
5 Laboratoire d’Astrophysique de Bordeaux, Université de Bordeaux, OASU, CNRS/INSU, 33271 Floirac, France
6 University of British Columbia Okanagan, 3333 University Way, Kelowna BC V1V 1V7, Canada
7 Department of Astronomy & Astrophysics, Tata Institute of Fundamental Research, Homi Bhabha Road, Mumbai 400005, India
8 Department of Earth and Space Sciences, Chalmers University of Technology, Onsala Observatory, 439 94 Onsala, Sweden
9 Laboratoire d’Astrophysique de Marseille – LAM, Université Aix-Marseille, CNRS, UMR 7326, 38 rue F. Joliot-Curie, 13388 Marseille Cedex 13, France
10 IRAM, 300 rue de la Piscine, 38406 Saint Martin d’Hères, France
11 Max-Planck Institut für Radioastronomie (MPIfR), Auf dem Hügel 69, 53121 Bonn, Germany
12 Departamento de Astrofísica, Centro de Astrobiología, CSIC-INTA, Ctra. de Torrejón a Ajalvir km 4, 28850 Madrid, Spain
13 Dept. Física Teórica y del Cosmos, Universidad de Granada, Spain
14 SRON Netherlands Institute for Space Research, Landleven 12, 9747 AD Groningen, The Netherlands
15 Astron. Dept., King Abdulaziz University, PO Box 80203, Jeddah, Saudi Arabia
Received: 18 April 2012
Accepted: 20 September 2012
Aims. We aim to better understand the emission of molecular tracers of the diffuse and dense gas in giant molecular clouds and the influence that metallicity, optical extinction, density, far-UV field, and star formation rate have on these tracers.
Methods. Using the IRAM 30 m telescope, we detected HCN, HCO+, 12CO, and 13CO in six GMCs along the major axis of M 33 at a resolution of ~114 pc and out to a radial distance of 3.4 kpc. Optical, far-infrared, and submillimeter data from Herschel and other observatories complement these observations. To interpret the observed molecular line emission, we created two grids of models of photon-dominated regions, one for solar and one for M 33-type subsolar metallicity.
Results. The observed HCO+/HCN line ratios range between 1.1 and 2.5. Similarly high ratios have been observed in the Large Magellanic Cloud. The HCN/CO ratio varies between 0.4% and 2.9% in the disk of M 33. The 12CO/13CO line ratio varies between 9 and 15 similar to variations found in the diffuse gas and the centers of GMCs of the Milky Way. Stacking of all spectra allowed HNC and C2H to be detected. The resulting HCO+/HNC and HCN/HNC ratios of ~8 and 6, respectively, lie at the high end of ratios observed in a large set of (ultra-)luminous infrared galaxies. HCN abundances are lower in the subsolar metallicity PDR models, while HCO+ abundances are enhanced. For HCN this effect is more pronounced at low optical extinctions. The observed HCO+/HCN and HCN/CO line ratios are naturally explained by subsolar PDR models of low optical extinctions between 4 and 10 mag and of moderate densities of n 3 × 103–3 × 104 cm-3, while the FUV field strength only has a small effect on the modeled line ratios. The line ratios are almost equally well reproduced by the solar-metallicity models, indicating that variations in metallicity only play a minor role in influencing these line ratios.
Key words: galaxies: individual: M 33 / galaxies: ISM / ISM: molecules / ISM: clouds / photon-dominated region (PDR)
Based on observations with the IRAM 30m telescope, Herschel, and other observatories. IRAM is supported by CNRS/INSU (France), the MPG (Germany), and the IGN (Spain). Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.
Appendices are available in electronic form at http://www.aanda.org
FITS files of the presented spectra of the ground-state transitions of HCN, HCO+, 12CO and 13CO are available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (188.8.131.52) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/549/A17
© ESO, 2012