The IRAM M33 CO(2-1) Survey - A complete census of the molecular gas out to 7 kpc

Abstract

In order to study the ISM and the interplay between the atomic and molecular components in a low-metallicity environment, we present a complete high angular and spectral resolution map and data cube of the 12CO(2-1) emission from the Local Group galaxy M33. Its metallicity is roughly half-solar, such that we can compare its ISM with that of the Milky Way with the main changes being the metallicity and the gas mass fraction. The data have a 12" angular resolution (50pc) with a spectral resolution of 2.6 km/s and a mean noise level of 20 mK per channel in antenna temperature. A radial cut along the major axis was also observed in the 12CO(1-0) line. The CO data cube and integrated intensity map are optimal when using HI data to define the baseline window and the velocities over which the CO emission is integrated. Great care was taken when building these maps, testing different windowing and baseline options and investigating the effect of error beam pickup. The total CO(2-1) luminosity is 2.8e7 K km/s pc2, following the spiral arms in the inner disk. There is no clear variation in the CO(2-1/1-0) intensity ratio with radius and the average value is roughly 0.8. The total molecular gas mass is estimated, using a N(H2)/Ico(1-0)=4e20 cm-2/(K km/s) conversion factor, to be 3.1e8 Msol. The CO spectra in the cube were shifted to zero velocity by subtracting the velocity of the HI peak from the CO spectra. Hence, the velocity dispersion between the atomic and molecular components is extremely low, independently justifying the use of the HI line in building our maps. Stacking the spectra in concentric rings shows that the CO linewidth and possibly the CO-HI velocity dispersion decrease in the outer disk. Using the CO(2-1) emission to trace the molecular gas, the probability distribution function of the H2 column density shows an excess at high column density above a log normal distribution.

arXiv comment14 pages, 14 figures, 2 tables, Accepted for publication in Astronomy & Astrophysics

References in corpus (42)

• Comparing the statistics of interstellar turbulence in simulations and observations: Solenoidal versus compressive turbulence forcing
• The Star Formation Rate of Turbulent Magnetized Clouds: Comparing Theory, Simulations, and Observations
• The Role of Pressure in GMC Formation II: The H_2 - Pressure Relation
• A Molecular Star Formation Law in the Atomic Gas Dominated Regime in Nearby Galaxies
• The Density Probability Distribution in Compressible Isothermal Turbulence: Solenoidal versus Compressive Forcing
• Probing the evolution of molecular cloud structure: From quiescence to birth
• Cluster-formation in the Rosette molecular cloud at the junctions of filaments
• Thermal condensation in a turbulent atomic hydrogen flow
• The Second Survey of the Molecular Clouds in the Large Magellanic Cloud by NANTEN. II. Star Formation
• The Metallicity Dependence of the CO-to-H${}_2$ Conversion Factor from Observations of Local Group Galaxies
• What determines the density structure of molecular clouds ? A case study of Orion B with Herschel
• Modeling Molecular Hydrogen and Star Formation in Cosmological Simulations
• The PdBI Arcsecond Whirlpool Survey (PAWS). I. A Cloud-Scale/Multi-Wavelength View of the Interstellar Medium in a Grand-Design Spiral Galaxy
• Giant Molecular Clouds in M33 - I. BIMA All Disk Survey
• 2MASS wide field extinction maps - I. The Pipe nebula
• Dark matter and visible baryons in M33
• The Atomic to Molecular Transition in Galaxies. I: An Analytic Approximation for Photodissociation Fronts in Finite Clouds
• Molecular and Atomic Gas in the Local Group Galaxy M33
• The M33 Metallicity Project: Resolving the Abundance Gradient Discrepancies in M33
• On the evolution of the density pdf in strongly self-gravitating systems
• GMC formation by agglomeration and self gravity
• Density structure of the interstellar medium and the star formation rate in galactic disks
• How long does it take to form a molecular cloud?
• High Resolution Molecular Gas Maps of M33
• A warped disk model for M33 and the 21-cm line width in spiral galaxies
• Giant Molecular Clouds in the Local Group Galaxy M33
• A high-dispersion molecular gas component in nearby galaxies
• Dust heating sources in galaxies: the case of M33 (HERM33ES)
• Metal production in M33: space and time variations
• Ionization compression impact on dense gas distribution and star formation, Probability density functions around H ii regions as seen by Herschel
• The distance of M33 and the stellar population in its outskirts
• Particularly Efficient Star Formation in M33
• A multi-scale study of infrared and radio emission from Scd galaxy M33
• Probability Distribution Functions OF 12CO(J = 1-0) Brightness and Integrated Intensity in M51: The PAWS View
• PACS and SPIRE photometer maps of M33: First results of the Herschel M33 extended survey (HERM33ES)
• Star formation in M33: Spitzer photometry of discrete sources
• The building up of the disk galaxy M33 and the evolution of the metallicity gradient
• Cool and warm dust emission from M33 (HerM33es)
• The Molecular Interstellar Medium of the Local Group Dwarf NGC6822
• NRO M33 All Disk Survey of Giant Molecular Clouds (NRO MAGiC): I. HI to H_2 Transition
• Molecular Cloud Formation and the Star Formation Efficiency in M~33
• New 20-cm radio-continuum study of the small Magellanic cloud: Part I - Images