Resolved Magnetic Field Mapping of a Molecular Cloud Using GPIPS
  We present the first resolved map of plane-of-sky magnetic field strength for
a quiescent molecular cloud. GRSMC 45.60+0.30 subtends 40 x 10 pc at a distance
of 1.88 kpc, masses 16,000 M_sun, and exhibits no star formation. Near-infrared
background starlight polarizations were obtained for the Galactic Plane
Infrared Polarization Survey using the 1.8 m Perkins telescope and the Mimir
instrument. The cloud area of 0.78 deg2 contains 2684 significant starlight
polarizations for Two Micron All Sky Survey matched stars brighter than 12.5
mag in the H band. Polarizations are generally aligned with the cloud's major
axis, showing an average position angle dispersion of 15 \pm 2{\deg} and
polarization of 1.8 \pm 0.6%. The polarizations were combined with Galactic
Ring Survey 13CO spectroscopy and the Chandrasekhar-Fermi method to estimate
plane-of-sky magnetic field strengths, with an angular resolution of 100
arcsec. The average plane-of-sky magnetic field strength across the cloud is
5.40 \pm 0.04 {\mu}G. The magnetic field strength map exhibits seven
enhancements or "magnetic cores." These cores show an average magnetic field
strength of 8.3 \pm 0.9 {\mu}G, radius of 1.2 \pm 0.2 pc, intercore spacing of
5.7 \pm 0.9 pc, and exclusively subcritical mass-to-flux ratios, implying their
magnetic fields continue to suppress star formation. The magnetic field
strength shows a power-law dependence on gas volume density, with slope 0.75
\pm 0.02 for n_{H_2} >=10 cm-3. This power-law index is identical to those in
studies at higher densities, but disagrees with predictions for the densities
probed here.
