Giant Molecular Clouds in M33. II. High-Resolution Observations

By Erik Rosolowsky, G. Engargiola, R. Plambeck, L. Blitz. 2003

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We present 12CO (J = 1 → 0) observations of 45 giant molecular clouds (GMCs) in M33 made with the BIMA array. The observations have a linear resolution of 20 pc, sufficient to measure the sizes of most GMCs in the sample. We place upper limits on the specific angular momentum of the GMCs and find the observed values to be nearly an order of magnitude below the values predicted from simple formation mechanisms. The velocity gradients across neighboring, high-mass GMCs appear preferentially aligned on scales less than 500 pc. If the clouds are rotating, 40% are counterrotating with respect to the galaxy. GMCs require a braking mechanism if they form from the large-scale radial accumulation of gas. These observations suggest that molecular clouds form locally out of atomic gas, with significant braking by magnetic fields to dissipate the angular momentum imparted by galactic shear. The observed GMCs share basic properties with those found in the Galaxy, such as similar masses, sizes, and line widths, as well as a constant surface density of 120 M☉ pc-2. The size-line width relationship follows ΔV ∝ r0.45±0.02, consistent with that found in the Galaxy. The cloud virial masses imply that the CO-to-H2 conversion factor has a value of 2 × 1020 H2 cm-2 (K km s-1)-1 and does not change significantly over the disk of M33, despite a change of 0.8 dex in the metallicity.

Subfields: Astronomy and Astrophysics, Instrumentation

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Citations in the corpus

• The CO-H2 Conversion Factor in Disc Galaxies and Mergers (2011, background): “…well (Blitz et al. 2007; Wolfire et al. 2010), though there may be some variations associated with metallicity (Wilson1995; Arimoto et al. 1996; Boselli et al. 2002; Rosolowsky et al. 2003; Israel 2005; Bell et al. 2006; Leroy et al. 2006; Bell et al. 2007; Bolatto et al. 2008; Leroy et al. 2011).”
• Submillimeter Galaxies at z ~ 2: Evidence for Major Mergers and Constraints on Lifetimes, IMF, and CO-H2 Conversion Factor (2008, background): “These considerations may explain the difference between the interferometric measurements of Rosolowsky et al. (2003) and Leroy et al. (2006) and the global measurements of Arimoto, Israel, Boselli, and others.”
• Bias‐free Measurement of Giant Molecular Cloud Properties (2006, methodology): “Figure 2 shows that the measured spatial size, line width, and flux of a real GMC in M 33 (EPRB1 from Rosolowsky et al. 2003) are all strong functions of the sensitivity of the data.”
• A General Theory of Turbulence-regulated Star Formation, from Spirals to Ultraluminous Infrared Galaxies (2005, background): “In M33, Rosolowsky et al. (2003) obtain velocity dispersions, masses, and radii for 36 GMCs.”
• THE CO-TO-H2 CONVERSION FACTOR FROM INFRARED DUST EMISSION ACROSS THE LOCAL GROUP (2011, methodology): “Using virial masses, Wilson (1995), Rosolowsky et al. (2003), and Bolatto et al. (2008) all found weak or absent trends in XCO as a function of metallicity.”
• Luminous Infrared Galaxies with the Submillimeter Array. I. Survey Overview and the Central Gas to Dust Ratio (2008, background): “…factor is now reasonably well determined for normal galaxies (Strong et al. 1988; Wilson & Scoville 1990; Strong & Mattox 1996; Dame et al. 2001; Rosolowsky et al. 2003), it has been clear for many years that the standard conversion factor does not apply to starbursting systems like the…”
• On the relationship between molecular hydrogen and carbon monoxide abundances in molecular clouds (2010, methodology): “…assume that extragalactic GMCs are in virial equilibrium and use a virial analysis to determine the cloud mass generally find values for XCO that are similar to those obtained in the Milky Way, with at most a weak metallicity dependence (Wilson 1995; Rosolowsky et al. 2003; Bolatto et al. 2008).”
• FIRE-2 simulations: physics versus numerics in galaxy formation (2017, background): “…observations and simulations have repeatedly shown that almost all star formation occurs in the largest GMC complexes in galaxies (see Evans 1999; Rosolowsky et al. 2003; Blitz & Rosolowsky 2005; Bolatto et al. 2008; Elmegreen 2007; Tasker & Tan 2009; Feldmann & Gnedin 2011; Murray 2011;…”
• The Molecular Gas Distribution and Schmidt Law in M33 (2003, background): “The spatial resolution of the observations (200 pc) is insufficient to detect individual GMCs if their typical size is 50 pc (Wilson & Scoville 1990; Engargiola et al. 2003; Rosolowsky et al. 2003).”
• The Second Survey of the Molecular Clouds in the Large Magellanic Cloud by NANTEN. I. Catalog of Molecular Clouds (2008, background): “Most of the galaxies have similar mass distributions, α ∼ 0.7; an exception is M33, which is steeper than the rest but the dataset has a higher completeness limit (Rosolowsky et al. 2003; Rosolowsky et al. 2005).”
• Molecular and atomic gas in the Local Group galaxy M 33 (2010, result): “Earlier studies of GMCs in M33 include Wilson et al. (1997) and Rosolowsky et al. (2003) and studies similar to our own of other Local Group galaxies have been made by e.g. Fukui et al. (2008); Israel et al. (2003); Leroy et al. (2006); Nieten et al. (2006).”
• THE PdBI ARCSECOND WHIRLPOOL SURVEY (PAWS). I. A CLOUD-SCALE/MULTI-WAVELENGTH VIEW OF THE INTERSTELLAR MEDIUM IN A GRAND-DESIGN SPIRAL GALAXY (2013, background): “…often lack resolution and/or areal coverage to identify statistically significant numbers of GMCs in different galactic environments (e.g. Bolatto et al. 2008; Gratier et al. 2012; Rosolowsky et al. 2003; Donovan Meyer et al. 2011; Rebolledo et al. 2012; Hirota et al. 2011; Nieten et al. 2006).”
• The Resolved Properties of Extragalactic Giant Molecular Clouds (2008, methodology): “To do so, we analyze the interferometric maps of M 31 (Rosolowsky 2007), M 33 (Rosolowsky et al. 2003), and the sample of Galactic GMCs discussed by Solomon et al. (1987).”
• Theory of Star Formation (2007, background): “(On the other hand, magnetic braking appears to be clearly significant in regulating the spin of GMAs and hence GMCs – Rosolowsky et al., 2003; Kim, Ostriker, & Stone, 2003.)”