Optically thick HI does not dominate dark gas in the local ISM
  The local interstellar medium (ISM) is suffused with "dark" gas, identified
by excess infrared and gamma ray emission, yet undetected by standard ISM
tracers such as neutral hydrogen (HI) or carbon monoxide emission. Based on
observed dust properties from Planck, recent studies have argued that HI mixed
with dust is strongly saturated and that dark gas is dominated by
optically-thick HI. We test this hypothesis by reproducing this model using
data from Planck and new 21 cm emission maps from GALFA-HI -- the first
large-area 21cm emission survey with comparable angular resolution to Planck.
We compare the results with those from a large sample of HI column densities
based on direct observations of HI optical depth, and find that the inferred
column density corrections are significantly lower than those inferred by the
Planck-based model. Further, we rule out the hypothesis that the pencil-beam HI
absorption sight lines preferentially miss opaque "blobs" with small covering
fraction, as these structures require densities and pressures which are
incompatible with ISM conditions. Our results support the picture that excess
dust emission in the local ISM is not dominated by optically-thick HI, but is
rather a combination of intrinsic changes in dust grain emissivities and H2
missed by CO observations.
