GIANT MOLECULAR CLOUD EVOLUTIONS IN THE NEARBY SPIRAL GALAXY M33

We present a giant molecular cloud (GMC) catalog of M33, containing 71 GMCs in total, based on wide-field and high-sensitivity CO(J = 3–2) observations with a spatial resolution of 100 pc using the ASTE 10 m telescope. Employing archival optical data, we identify 75 young stellar groups (YSGs) from the excess of the surface stellar density, and estimate their ages by comparing with stellar evolution models. A spatial comparison among the GMCs, YSGs, and H ii regions enable us to classify GMCs into four categories: Type A, showing no sign of massive star formation (SF); Type B, being associated only with H ii regions; Type C, with both H ii regions and <10 Myr old YSGs; and Type D, with both H ii regions and 10–30 Myr YSGs. Out of 65 GMCs (discarding those at the edges of the observed fields), 1 (1%), 13 (20%), 29 (45%), and 22 (34%) are Types A, B, C, and D, respectively. We interpret these categories as stages in a GMC evolutionary sequence. Assuming that the timescale for each evolutionary stage is proportional to the number of GMCs, the lifetime of a GMC with a mass >105 M☉ is estimated to be 20–40 Myr. In addition, we find that the dense gas fraction as traced by the CO(J = 3–2)/CO(J = 1–0) ratio is enhanced around SF regions. This confirms a scenario where dense gas is preferentially formed around previously generated stars, and will be the fuel for the next stellar generation. In this way, massive SF gradually propagates in a GMC until gas is exhausted.

[1]  C. Kramer,et al.  Giant molecular clouds in the Local Group galaxy M 33 , 2011, 1111.4320.

[2]  N. Kuno,et al.  13CO(J = 1–0) On-the-Fly Mapping of the Giant H II Region NGC 604: Variation in Molecular Gas Density and Temperature due to Sequential Star Formation , 2011, 1108.1448.

[3]  L. Hunt,et al.  The population of young stellar clusters throughout the disk of M 33 , 2011, 1109.4760.

[4]  A. Hirota,et al.  NRO M 33 All-Disk Survey of Giant Molecular Clouds (NRO MAGiC). I. H I to H2 Transition , 2011, 1106.4115.

[5]  D. Calzetti,et al.  THE SUPER-LINEAR SLOPE OF THE SPATIALLY RESOLVED STAR FORMATION LAW IN NGC 3521 AND NGC 5194 (M51a) , 2011, 1104.4122.

[6]  F. Bresolin.  THE ABUNDANCE SCATTER IN M33 FROM H ii REGIONS: IS THERE ANY EVIDENCE FOR AZIMUTHAL METALLICITY VARIATIONS? , 2011, 1102.0044.

[7]  K. Kohno,et al.  DENSE CLUMPS IN GIANT MOLECULAR CLOUDS IN THE LARGE MAGELLANIC CLOUD: DENSITY AND TEMPERATURE DERIVED FROM 13CO(J = 3–2) OBSERVATIONS , 2010, 1012.5037.

[8]  R. Klessen,et al.  HIERARCHICAL STELLAR STRUCTURES IN THE LOCAL GROUP DWARF GALAXY NGC 6822 , 2010, 1010.1940.

[9]  Benjamin F. Williams,et al.  THE ACS NEARBY GALAXY SURVEY TREASURY. IX. CONSTRAINING ASYMPTOTIC GIANT BRANCH EVOLUTION WITH OLD METAL-POOR GALAXIES , 2010, 1009.4618.

[10]  A. Kawamura,et al.  Molecular Clouds in Nearby Galaxies , 2010 .

[11]  Di Li,et al.  THE RELATION BETWEEN GAS AND DUST IN THE TAURUS MOLECULAR CLOUD , 2010, 1007.5060.

[12]  I. S. Roman,et al.  PHOTOMETRIC PROPERTIES OF THE M33 STAR CLUSTER SYSTEM , 2010, 1007.1042.

[13]  A. Hirota,et al.  BREAKDOWN OF KENNICUTT–SCHMIDT LAW AT GIANT MOLECULAR CLOUD SCALES IN M33 , 2010, 1006.5764.

[14]  P. Scowen,et al.  MAPPING THE RECENT STAR FORMATION HISTORY OF THE DISK OF M51 , 2010, 1006.1954.

[15]  M. Grossi,et al.  Young stellar clusters and associations in M 33 , 2010, 1006.1281.

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

[17]  L. Hunt,et al.  Star formation in M 33: the radial and local relations with the gas , 2009, 0912.2015.

[18]  J. Ott,et al.  Warm and Dense Molecular Gas in the N 159 Region: ^(12)CO J = 4–3 and ^(13)CO J = 3–2 Observations with NANTEN2 and ASTE , 2009, 0910.0309.

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

[20]  Robert C. Kennicutt,et al.  DUST-CORRECTED STAR FORMATION RATES OF GALAXIES. I. COMBINATIONS OF Hα AND INFRARED TRACERS , 2009, 0908.0203.

[21]  J. Holtzman,et al.  NEWLY IDENTIFIED STAR CLUSTERS IN M33. II. RADIAL HUBBLE SPACE TELESCOPE/ADVANCED CAMERA FOR SURVEYS FIELDS , 2009 .

[22]  W. Park,et al.  AGES OF M33 STAR CLUSTERS BASED ON THE HUBBLE SPACE TELESCOPE/WFPC2 PHOTOMETRY , 2009, 0905.2121.

[23]  L. Hunt,et al.  Star formation in M 33: multiwavelength signatures across the disk , 2008, 0810.0473.

[24]  J. Leech,et al.  THE JAMES CLERK MAXWELL TELESCOPE NEARBY GALAXIES LEGACY SURVEY. I. STAR-FORMING MOLECULAR GAS IN VIRGO CLUSTER SPIRAL GALAXIES , 2008, 0812.1718.

[25]  Itziar Aretxaga,et al.  New achievements of ASTE: the Atacama Submillimeter Telescope Experiment , 2008, Astronomical Telescopes + Instrumentation.

[26]  U. Chile,et al.  The Second Survey of the Molecular Clouds in the Large Magellanic Cloud by NANTEN. I. Catalog of Molecular Clouds , 2008, 0804.1458.

[27]  H. Yamamoto,et al.  Submillimeter line emission from LMC N159W: a dense, clumpy PDR in a low metallicity environment , 2008, 0802.1929.

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

[29]  Belgium,et al.  Evolution of asymptotic giant branch stars. II. Optical to far-infrared isochrones with improved TP- , 2007, 0711.4922.

[30]  G. Garay,et al.  Submillimeter Observations of Giant Molecular Clouds in the Large Magellanic Cloud: Temperature and Density as Determined from J = 3-2 and J = 1-0 Transitions of CO , 2007, 0710.4202.

[31]  N. Hwang,et al.  A CATALOG OF BRIGHT STAR CLUSTERS IN THE INTERACTING GALAXY M51 , 2007, 0712.1420.

[32]  L. Hunt,et al.  Star formation in M 33: Spitzer photometry of discrete sources , 2007, 0709.2601.

[33]  J. G. Mangum,et al.  The On The Fly Imaging Technique , 2007, 0709.0553.

[34]  W. Park,et al.  A Catalog of New M33 Star Clusters Based on Hubble Space Telescope WFPC2 Images , 2007, 0708.1055.

[35]  D. Calzetti,et al.  Star Formation in NGC 5194 (M51a). II. The Spatially Resolved Star Formation Law , 2007, 0708.0922.

[36]  D. Calzetti,et al.  The Incidence of Highly Obscured Star-forming Regions in SINGS Galaxies , 2007, 0706.3501.

[37]  T. Sawada,et al.  Arclike Distribution of High CO (J = 3-2)/CO (J = 1-0) Ratio Gas Surrounding the Central Star Cluster of the Supergiant H II Region NGC 604 , 2007, 0706.1430.

[38]  M. Gieles,et al.  Hierarchical star formation in M33: fundamental properties of the star-forming regions , 2007, 0706.0495.

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

[40]  C. Mancone,et al.  A Catalog of Star Cluster Candidates in M33 , 2007, 0704.1576.

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

[42]  R. C. Smith,et al.  A Survey of Local Group Galaxies Currently Forming Stars. II. UBVRI Photometry of Stars in Seven Dwarfs and a Comparison of the Entire Sample , 2007, astro-ph/0702236.

[43]  A. Endo,et al.  ASTE CO (3-2) Observations of the Barred Spiral Galaxy M 83: I. Correlation between CO (3-2)/ CO (1-0) Ratios and Star Formation Efficiencies , 2007, astro-ph/0701759.

[44]  R. C. Smith,et al.  A Survey of Local Group Galaxies Currently Forming Stars. I. UBVRI Photometry of Stars in M31 and M33 , 2006, astro-ph/0602128.

[45]  Erik Rosolowsky,et al.  Bias‐free Measurement of Giant Molecular Cloud Properties , 2006, astro-ph/0601706.

[46]  C. Lidman,et al.  The Cool Universe: Observing Cosmic Dawn , 2005 .

[47]  L. Kewley,et al.  Star Formation in NGC 5194 (M51a): The Panchromatic View from GALEX to Spitzer , 2005, astro-ph/0507427.

[48]  D. Schaerer,et al.  A new calibration of stellar parameters of Galactic O stars , 2005, astro-ph/0503346.

[49]  Satoshi Yamamoto,et al.  The Atacama Submillimeter Telescope Experiment (ASTE) , 2004, SPIE Astronomical Telescopes + Instrumentation.

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

[51]  P. Massey.  Massive Stars in the Local Group: Implications for Stellar Evolution and Star Formation , 2003 .

[52]  E. Rosolowsky,et al.  Giant Molecular Clouds in M33. I. BIMA All-Disk Survey , 2003, astro-ph/0308388.

[53]  T. J. Dupuy,et al.  Cluster Mass Functions in the Large and Small Magellanic Clouds: Fading and Size-of-Sample Effects , 2003, astro-ph/0306528.

[54]  F. Bruhweiler,et al.  STIS Spectral Imagery of the OB Stars in NGC 604. II. The Most Luminous Stars , 2003, astro-ph/0303442.

[55]  C. Lada,et al.  Embedded Clusters in Molecular Clouds , 2003, astro-ph/0301540.

[56]  M. Rubio,et al.  A CO Survey of the LMC with NANTEN: III. Formation of Stellar Clusters and Evolution of Molecular Clouds , 2001 .

[57]  M. Rubio,et al.  A CO Survey of the LMC with NANTEN: II. Catalog of Molecular Clouds , 2001 .

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

[59]  Atsushi Tanaka,et al.  Digital spectrometers for the Nobeyama 45-m telescope , 2000, Astronomical Telescopes and Instrumentation.

[60]  M. Kontizas,et al.  OB Stellar Associations in the Large Magellanic Cloud: Identification Method , 2000 .

[61]  R. Delgado,et al.  The massive stellar content of the H ii region NGC 604 and its evolutionary state , 2000, astro-ph/0003067.

[62]  P. Salucci,et al.  The extended rotation curve and the dark matter halo of M33 , 1999, astro-ph/9909252.

[63]  S. Asayama,et al.  First Results of a CO Survey of the Large Magellanic Cloud with NANTEN; Giant Molecular Clouds as Formation Sites of Populous Clusters , 1999 .

[64]  H. Ford,et al.  Star Clusters in M33. II. Global Properties , 1999 .

[65]  T. Wyder,et al.  The Hii Regions of M33. II. A Photometric Catalog of 1272 Newly Identified Emission Regions , 1999 .

[66]  W. Goss,et al.  A High-Resolution VLA and Optical Study of NGC 604 , 1999 .

[67]  D. Schlegel,et al.  Maps of Dust Infrared Emission for Use in Estimation of Reddening and Cosmic Microwave Background Radiation Foregrounds , 1998 .

[68]  H. Ford,et al.  Final Results from the Hubble Space Telescope Key Project to Measure the Hubble Constant , 1998, astro-ph/9801080.

[69]  T. Wyder,et al.  THE LUMINOSITY FUNCTION AND SIZE DISTRIBUTION OF THE H II REGIONS IN M33 , 1997 .

[70]  C. Wilson,et al.  The Density and Temperature of Molecular Clouds in M33 , 1997, astro-ph/9701245.

[71]  J. Parker,et al.  The Stellar Initial Mass Function in the Giant HII Region NGC 595 , 1996 .

[72]  T. Prabhu,et al.  Embedded Clusters in Giant Extragalactic HII Regions III. Extinction and Star Formation , 1996, astro-ph/9601001.

[73]  D. Hunter,et al.  The Intermediate Stellar Mass Population in NGC 604 Determined from Hubble Space Telescope Images , 1996 .

[74]  H. Schmitt,et al.  A Revised and Extended Catalog of Magellanic System Clusters, Associations, and Emission Nebulae. I. Small Magellanic Cloud and Bridge , 1995 .

[75]  E. Wilcots.  Classical H II regions in the Magellanic Clouds. 2: Stellar content , 1994 .

[76]  M. Shara,et al.  A Hubble Space Telescope planetary camera view of giant H II regions - The Wolf-Rayet content of NGC 595 and NGC 604 in M33 , 1993 .

[77]  M. Shara,et al.  The Wolf-Rayet Star Population in the Most Massive Giant H II Regions of M33 , 1990 .

[78]  R. Kennicutt.  The Star Formation Law in Galactic Disks , 1989 .

[79]  A. R. Rivolo,et al.  Mass, luminosity, and line width relations of Galactic molecular clouds , 1987 .

[80]  R. Humphreys,et al.  On the stellar content and structure of the spiral galaxy M33. , 1980 .

[81]  Blair D. Savage,et al.  Observed Properties of Interstellar Dust , 1979 .

[82]  M. Schmidt.  The Rate of Star Formation , 1959 .