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The alignment of molecular cloud magnetic fields with the spiral arms in M33


The formation of molecular clouds, which serve as stellar nurseries in galaxies, is poorly understood. A class of cloud formation models suggests that a large-scale galactic magnetic field is irrelevant at the scale of individual clouds, because the turbulence and rotation of a cloud may randomize the orientation of its magnetic field1,2. Alternatively, galactic fields could be strong enough to impose their direction upon individual clouds3,4, thereby regulating cloud accumulation and fragmentation5, and affecting the rate and efficiency of star formation6. Our location in the disk of the Galaxy makes an assessment of the situation difficult. Here we report observations of the magnetic field orientation of six giant molecular cloud complexes in the nearby, almost face-on, galaxy M33. The fields are aligned with the spiral arms, suggesting that the large-scale field in M33 anchors the clouds.

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Figure 1: The optical spiral arms and the locations of the six most massive GMCs in M33.
Figure 2: CO (2–1) maps and polarization vectors.
Figure 3: Distribution of the CO polarization-arm offsets.
Figure 4: Likelihood of obtaining simulated angle dispersions Σ and offsets Δ of CO polarization within the observed 90% confidence intervals.


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We thank E. Rosolowsky, R. Shetty, T. K. Sridharan, M. Houde, S. Paine, H.-H. Wang, A. Karim, S. Ragan, K. Smith, P. Boley and T. Wu for comments. We appreciate the help of D. Marrone, G. Petitpas and R. Rao with the observations. We are grateful for the Herschel maps of M33 offered by C. Kramer. This research is supported by the Max-Planck-Institut für Astronomie and Harvard-Smithsonian Center for Astrophysics. The Submillimeter Array is a joint project between the Smithsonian Astrophysical Observatory and the Academia Sinica Institute of Astronomy and Astrophysics and is funded by the Smithsonian Institution and the Academia Sinica.

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H.L. designed and executed the experiment. H.L. and T.H. contributed jointly to the manuscript.

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Correspondence to Hua-bai Li.

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The authors declare no competing financial interests.

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Li, Hb., Henning, T. The alignment of molecular cloud magnetic fields with the spiral arms in M33. Nature 479, 499–501 (2011).

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