Relative Alignment Between the Magnetic Field and Molecular Gas
  Structure in the Vela C Giant Molecular Cloud using Low and High Density
  Tracers
  We compare the magnetic field orientation for the young giant molecular cloud
Vela C inferred from 500-$\mu$m polarization maps made with the BLASTPol
balloon-borne polarimeter to the orientation of structures in the integrated
line emission maps from Mopra observations. Averaging over the entire cloud we
find that elongated structures in integrated line-intensity, or zeroth-moment
maps, for low density tracers such as $^{12}$CO and $^{13}$CO $J$ $\rightarrow$
1 - 0 are statistically more likely to align parallel to the magnetic field,
while intermediate or high density tracers show (on average) a tendency for
alignment perpendicular to the magnetic field. This observation agrees with
previous studies of the change in relative orientation with column density in
Vela C, and supports a model where the magnetic field is strong enough to have
influenced the formation of dense gas structures within Vela C. The transition
from parallel to no preferred/perpendicular orientation appears to happen
between the densities traced by $^{13}$CO and by C$^{18}$O $J$ $\rightarrow$ 1
- 0. Using RADEX radiative transfer models to estimate the characteristic
number density traced by each molecular line we find that the transition occurs
at a molecular hydrogen number density of approximately $10^3$ cm$^{-3}$. We
also see that the Centre-Ridge (the highest column density and most active
star-forming region within Vela C) appears to have a transition at a lower
number density, suggesting that this may depend on the evolutionary state of
the cloud.
