NRO M 33 All-Disk Survey of Giant Molecular Clouds (NRO MAGiC). I. H I to H2 Transition

We present the results of the Nobeyama Radio Observatory (NRO) M 33 All-Disk (30 0 � 30 0 or 7.3 kpc � 7.3 kpc) Survey of Giant Molecular Clouds (NRO MAGiC) based on 12 CO (J = 1–0) observations using the NRO 45 m telescope. The spatial resolution of the resultant map is 19: 3, corresponding to 81 pc, which is sufficient to identify each Giant Molecular Cloud (GMC) in the disk. We found clumpy structures with a typical spatial scale of � 100 pc, corresponding to GMCs, and no diffuse, smoothly distributed component of molecular gas at this sensitivity. The overall distribution of molecular gas roughly agrees with that of H I. However, a closer inspection of the CO and H I maps suggests that not every CO emission is associated with local H I peaks, particularly in the inner portion of the disk ( r< 2 kpc), although most of the CO emission is located at the local H I peaks in the outer radii. We found that most uncovered GMCs are accompanied by massive star-forming regions, although the star-formation rates (SFRs) vary widely from cloud to cloud. The surface density of the azimuthally averaged H I gas exhibits a flat radial distribution. However, the CO radial distribution shows a significant enhancement within the central 1–2 kpc region, which is very similar to that of the SFR. We obtained a map of the molecular fraction, fmol =Σ H2 =(ΣH I +Σ H2 ), with a resolution of 100 pc. This is the first fmol map covering an entire galaxy with a GMC-scale resolution. We found that fmol tends to be high near the center. The correlation between fmol and gas surface density shows two distinct sequences. The presence of two correlation sequences can be explained by a difference of metallicity; i.e., higher (� 2-fold) metallicity in the central region ( r< 1.5 kpc) than in the outer parts. Alternatively, difference in the scale height can also account for the two sequences; i.e., the scale height increases at the outer disk.

[1]  J. Austermann,et al.  Temperature Variations of Cold Dust in the Triangulum Galaxy M 33 , 2011, 1106.2166.

[2]  S. Okumura,et al.  APERTURE SYNTHESIS OBSERVATIONS OF CO, HCN, AND 89 GHz CONTINUUM EMISSION TOWARD NGC 604 IN M33: SEQUENTIAL STAR FORMATION INDUCED BY A SUPERGIANT H ii REGION , 2010, 1009.4082.

[3]  D. Calzetti,et al.  PACS and SPIRE photometer maps of M33: First results of the Herschel M33 extended survey (HERM33ES) , 2010, 1005.2563.

[4]  D. Calzetti,et al.  Molecular and atomic gas in the Local Group galaxy M 33 , 2010, 1003.3222.

[5]  A. Kawamura,et al.  THE SECOND SURVEY OF THE MOLECULAR CLOUDS IN THE LARGE MAGELLANIC CLOUD BY NANTEN. II. STAR FORMATION , 2009, 0908.1168.

[6]  Coleman Krawczyk,et al.  RE-EXAMINING LARSON'S SCALING RELATIONSHIPS IN GALACTIC MOLECULAR CLOUDS , 2008, 0809.1397.

[7]  T. Sawada,et al.  On-The-Fly Observing System of the Nobeyama 45-m and ASTE 10-m Telescopes , 2007, 0712.1283.

[8]  J. Simon,et al.  The M33 Metallicity Project: Resolving the Abundance Gradient Discrepancies in M33 , 2007, 0711.4351.

[9]  Astronomy,et al.  The Calibration of Mid-Infrared Star Formation Rate Indicators , 2007, 0705.3377.

[10]  E. Rosolowsky,et al.  High-Resolution Molecular Gas Maps of M33 , 2007, astro-ph/0703006.

[11]  Y. Sofue,et al.  Three-Dimensional Distribution of the ISM in the Milky Way Galaxy: II. The Molecular Gas Disk , 2006, astro-ph/0610769.

[12]  A. Hirota,et al.  Environmental Effects on Gaseous Disks of the Virgo Spiral Galaxies , 2006, astro-ph/0610820.

[13]  Paul S. Smith,et al.  The Multiband Imaging Photometer for Spitzer (MIPS) , 2004 .

[14]  S. Schneider,et al.  The Molecular Gas Distribution and Schmidt Law in M33 , 2003, astro-ph/0311226.

[15]  The Contribution of Field OB Stars to the Ionization of the Diffuse Ionized Gas in M33 , 2000, astro-ph/0007309.

[16]  Nobuharu Ukita,et al.  BEARS: SIS 25-beam array receiver system for the NRO 45-m telescope , 2000, Astronomical Telescopes and Instrumentation.

[17]  Caltech,et al.  Bar-driven Transport of Molecular Gas to Galactic Centers and Its Consequences , 1999, astro-ph/9906454.

[18]  Jr.,et al.  The Global Schmidt law in star forming galaxies , 1997, astro-ph/9712213.

[19]  N. Kuno,et al.  Distribution and Dynamics of Molecular Gas in the Galaxy M51. III. Kinematics of Molecular Gas , 1997 .

[20]  S. Vogel,et al.  The Near-Infrared Structure of M33 , 1994 .

[21]  B. Elmegreen.  The H to H2 transition in galaxies - Totally molecular galaxies , 1993 .

[22]  B. Madore,et al.  New Cepheid distances to nearby galaxies based on BVRI CCD photometry. II, The Local Group galaxy M33 , 1991 .

[23]  N. Scoville,et al.  The properties of individual giant molecular clouds in M33 , 1990 .

[24]  R. Rand,et al.  M51: molecular spiral arms, giant molecular associations, and superclouds , 1990 .

[25]  N. Scoville,et al.  The molecular gas content of the nuclear region of M33 , 1989 .

[26]  B. Elmegreen.  A pressure and metallicity dependence for molecular cloud correlations and the calibration of mass , 1989 .

[27]  E. Terlevich,et al.  The chemical composition gradient across M 33 , 1988 .

[28]  N. Scoville,et al.  Where massive stars form - Associated radio H II regions and CO clouds in the northern Milky Way , 1987 .

[29]  N. Scoville,et al.  Giant molecular clouds in the galaxy. II. Characteristics of discrete features , 1985 .