Results of SPARO 2003: Mapping Magnetic Fields in Giant Molecular Clouds

We present results from the Austral Winter 2003 observing campaign of SPARO, a 450 micron polarimeter used with a two-meter telescope at South Pole. We mapped large-scale magnetic fields in four Giant Molecular Clouds (GMCs) in the Galactic disk: NGC 6334, the Carina Nebula, G333.6-0.2 and G331.5-0.1. We find a statistically significant correlation of the inferred field directions with the orientation of the Galactic plane. Specifically, three of the four GMCs (NGC 6334 is the exception) have mean field directions that are within 15 degrees of the plane. The simplest interpretation is that the field direction tends to be preserved during the process of GMC formation. We have also carried out an analysis of published optical polarimetry data. For the closest of the SPARO GMCs, NGC 6334, we can compare the field direction in the cloud as measured by SPARO with the field direction in a larger region surrounding the cloud, as determined from optical polarimetry. For purposes of comparison, we also use optical polarimetry to determine field directions for other regions of similar size and distance. Overall, the results from this optical polarimetry analysis are consistent with our suggestion that field direction tends to be preserved during GMC formation. Finally, we compare the disorder in our magnetic field maps with the disorder seen in magnetic field maps derived from MHD turbulence simulations. We conclude from these comparisons that the magnetic energy density in our clouds is comparable to the turbulent energy density.