Molecular and atomic gas 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 Saint-Martin d’Hères, France
³ Instituto Radioastronomia Milimetrica (IRAM), Av. Divina Pastora 7, Nucleo Central, 18012 Granada, Spain
⁴ Institute of Astronomy & Astrophysics, National Observatory of Athens, I. Metaxa & V. Pavlou, P. Penteli, 15236 Athens, Greece
⁵ Max-Plank-Institut für Radioastronomy (MPIfR), Auf dem Hügel 69, 53121 Bonn, Germany
⁶ INAF - Osservatorio Astrofisico di Arcetri, LE Fermi, 5, 50125 Firenze, Italy
⁷ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
⁸ Department of Astronomy - LGRT, University of Massachusetts - Amherst, 710 north Pleasant Street, Amherst, MA 01002, USA
⁹ Observatorio Astronomico Nacional (OAN) Observatorio de Madrid, Alfonso XII 3, 28014- Madrid, Spain
¹⁰ Observatoire de Paris, LERMA, CNRS, 61 Av. de l’Observatoire, 75014 Paris, France
¹¹ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
¹² Department of Radio and Space Science, Chalmers University of Technology, Onsala Observatory, 439 94 Onsala, Sweden
¹³ CSIRO Astronomy & Space Science, Australia Telescope National Facility, PO Box 76, Epping NSW 1710, Australia
¹⁴ SRON Netherlands Institute for Space Research, Landleven 12, 9747 AD Groningen, The Netherlands
¹⁵ 1.Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany
¹⁶ Department of Astronomy & Astrophysics, Tata Institute of Fundamental Research, Homi Bhabha Road, Mumbai 400005, India

Received: 16 March 2010
Accepted: 11 June 2010

Abstract

We present high-resolution large-scale observations of the molecular and atomic gas in the Local Group galaxy M 33. The observations were carried out using the HEterodyne Receiver Array (HERA) at the 30 m IRAM telescope in the CO(2–1) line, achieving a resolution of 12″ × 2.6   km   s^-1, enabling individual giant molecular clouds (GMCs) to be resolved. The observed region is 650 square arcminutes mainly along the major axis and out to a radius of 8.5 kpc, and covers entirely the 2′ × 40′ radial strip observed with the HIFI and PACS Spectrometers as part of the HERM33ES Herschel key program. The achieved sensitivity in main-beam temperature is 20–50 mK at 2.6      km   s^-1 velocity resolution. The CO(2–1) luminosity of the observed region is 1.7 ± 0.1 × 10⁷   K   km   s^-1   pc² and is estimated to be 2.8 ± 0.3 × 10⁷   K   km   s^-1   pc² for the entire galaxy, corresponding to H₂ masses of 1.9 × 10⁸   M_⊙ and 3.3 × 10⁸   M_⊙ respectively (including He), calculated with N(H₂)/I_{CO(1 − 0)} twice the Galactic value due to the half-solar metallicity of M 33. The H i 21 cm VLA archive observations were reduced, and the mosaic was imaged and cleaned using the multi-scale task in the CASA software package, yielding a series of datacubes with resolutions ranging from 5″ to 25″. The H i mass within a radius of 8.5 kpc is estimated to be 1.4 × 10⁹   M_⊙ . The azimuthally averaged CO surface brightness decreases exponentially with a scale length of 1.9 ± 0.1 kpc whereas theatomic gas surface density is constant at Σ_Hi = 6 ± 2   M_⊙    pc^-2 deprojected to face-on. For an N(H₂)/I_{CO(1 − 0)} conversion factor twice that of the Milky Way, the central kiloparsec H₂ surface density is Σ_H2 = 8.5 ± 0.2   M_⊙    pc^-2. The star formation rate per unit molecular gas (SF efficiency, the rate of transformation of molecular gas into stars), as traced by the ratio of CO to H_α and FIR brightness, is constant with radius. The SFE, with a N(H₂)/I_{CO(1 − 0)} factor twice galactic, appears 2–4 times greater than for large spiral galaxies. A morphological comparison of molecular and atomic gas with tracers of star formation is presented showing good agreement between these maps both in terms of peaks and holes. A few exceptions are noted. Several spectra, including those of a molecular cloud situated more than 8 kpc from the galaxy center, are presented.