Giant molecular clouds in the Local Group galaxy M 33^⋆,⋆⋆

¹ Laboratoire d’Astrophysique de Bordeaux, Université de Bordeaux, OASU, CNRS/INSU, 33271 Floirac, France
e-mail: gratier@obs.u-bordeaux1.fr
² IRAM, 300 Rue de la piscine, 38406 St Martin d’Hères, France
³ Instituto Radioastronomia Milimetrica (IRAM), Av. Divina Pastora 7, Nucleo Central, 18012 Granada, Spain
⁴ INAF – Osservatorio Astrofisico di Arcetri, L.E. Fermi 5, 50125 Firenze, Italy
⁵ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
⁶ Observatoire de Paris, LERMA, CNRS, 61 Av. de l’Observatoire, 75014 Paris, France
⁷ Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany

Received: 31 January 2011
Accepted: 14 November 2011

Abstract

We present an analysis of a systematic CO(2–1) survey at 12′′ resolution covering most of the Local Group spiral M 33, which, at a distance of 840 kpc, is close enough for individual giant molecular clouds (GMCs) to be identified. The goal of this work is to study the properties of the GMCs in this subsolar metallicity galaxy. The CPROPS (Cloud PROPertieS) algorithm was used to identify 337 GMCs in M 33, the largest sample to date for an external galaxy. The sample is used to study the GMC luminosity function, or mass spectrum under the assumption of a constant N(H₂) / I_CO ratio. We find that n(L)dL ∝ L^{−2.0 ± 0.1} for the entire sample. However, when the sample is divided into inner and outer disk samples, the exponent changes from 1.6 ± 0.2 in the center 2 kpc to 2.3 ± 0.2 for galactocentric distances larger than 2 kpc. On the basis of the emission in the FUV, Hα, 8 μm, and 24 μm bands, each cloud was classified in terms of its star-forming activity – no star formation or either embedded or exposed star formation (visible in FUV and Hα). At least one sixth of the clouds had no (massive) star formation, suggesting that the average time required for star formation to start is about one sixth of the total time for which the object is identifiable as a GMC. The clouds without star formation have significantly lower CO luminosities than those with star formation, whether embedded or exposed, a result that is presumably related to the lack of heating sources. Taking the cloud sample as a whole, the main non-trivial correlation is the decrease in cloud CO brightness (or luminosity) with galactocentric radius. The complete cloud catalog, including the CO and HI spectra and the CO contours overlaid on the FUV, Hα, 8 μm, and 24 μm images is presented in the appendix.