2010A&A...522A...3G

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 30m IRAM telescope in the CO(2-1) line, achieving a resolution of 12"x2.6km/s, 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 8.5kpc, and covers entirely the 2'x40' 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 20-50mK at 2.6km/s velocity resolution. The CO(2-1) luminosity of the observed region is 1.7±0.1x10⁷K.km/s.pc² and is estimated to be 2.8±0.3x10⁷K.km/s.pc² for the entire galaxy, corresponding to H₂ masses of 1.9x10⁸M_☉ and 3.3x10⁸M_☉ 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 HI 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 HI mass within a radius of 8.5kpc is estimated to be 1.4x10⁹M_☉. The azimuthally averaged CO surface brightness decreases exponentially with a scale length of 1.9±0.1kpc whereas the atomic gas surface density is constant at Σ_HI=6±2M_☉/pc2 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±0.2M_☉/pc2. 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 8kpc from the galaxy center, are presented.