Giant Molecular Filaments in the Milky Way
  Throughout the Milky Way, molecular clouds typically appear filamentary, and
mounting evidence indicates that this morphology plays an important role in
star formation. What is not known is to what extent the dense filaments most
closely associated with star formation are connected to the surrounding diffuse
clouds up to arbitrarily large scales. How are these cradles of star formation
linked to the Milky Way's spiral structure? Using archival Galactic plane
survey data, we have used multiple datasets in search of large-scale,
velocity-coherent filaments in the Galactic plane. In this paper, we present
our methods employed to identify coherent filamentary structures first in
extinction and confirmed using Galactic Ring Survey data. We present a sample
of seven Giant Molecular Filaments (GMFs) that have lengths of order $\sim$100
pc, total masses of 10$^4$ - 10$^5$ M$_{\odot}$, and exhibit velocity coherence
over their full length. The GMFs we study appear to be inter-arm clouds and may
be the Milky Way analogues to spurs observed in nearby spiral galaxies. We find
that between 2 and 12% of the total mass (above $\sim$10$^{20}$ cm$^{-2}$) is
"dense" (above 10$^{22}$ cm$^{-2}$), where filaments near spiral arms in the
Galactic midplane tend to have higher dense gas mass fractions than those
further from the arms.
