The JCMT BISTRO Survey: The Distribution of Magnetic Field Strengths
  towards the OMC-1 Region
  Measurement of magnetic field strengths in a molecular cloud is essential for
determining the criticality of magnetic support against gravitational collapse.
In this paper, as part of the JCMT BISTRO survey, we suggest a new application
of the Davis-Chandrasekhar-Fermi (DCF) method to estimate the distribution of
magnetic field strengths in the OMC-1 region. We use observations of dust
polarization emission at 450 $\mu$m and 850 $\mu$m, and C$^{18}$O (3-2)
spectral line data obtained with the JCMT. We estimate the volume density, the
velocity dispersion and the polarization angle dispersion in a box, 40$''$
$\times$ 40$''$ (5$\times$5 pixels), which moves over the OMC-1 region. By
substituting three quantities in each box to the DCF method, we get magnetic
field strengths over the OMC-1 region. We note that there are very large
uncertainties in inferred field strengths, as discussed in detail in this
paper. The field strengths vary from 0.8 to 26.4 mG and their mean value is
about 6 mG. Additionally, we obtain maps of the mass-to-flux ratio in units of
a critical value and the Alfv$\acute{e}$n mach number. The central parts of the
BN-KL and South (S) clumps in the OMC-1 region are magnetically supercritical,
so the magnetic field cannot support the clumps against gravitational collapse.
However, the outer parts of the region are magnetically subcritical. The mean
Alfv$\acute{e}$n mach number is about 0.4 over the region, which implies that
the magnetic pressure exceeds the turbulent pressure in the OMC 1 region.
