Large-scale filaments associated with Milky Way spiral arms
  The ubiquity of filamentary structure at various scales through out the
Galaxy has triggered a renewed interest in their formation, evolution, and role
in star formation. The largest filaments can reach up to Galactic scale as part
of the spiral arm structure. However, such large scale filaments are hard to
identify systematically due to limitations in identifying methodology (i.e., as
extinction features). We present a new approach to directly search for the
largest, coldest, and densest filaments in the Galaxy, making use of sensitive
Herschel Hi-GAL data complemented by spectral line cubes. We present a sample
of the 9 most prominent Herschel filaments, including 6 identified from a pilot
search field plus 3 from outside the field. These filaments measure 37-99 pc
long and 0.6-3.0 pc wide with masses (0.5-8.3)$\times10^4 \, M_\odot$, and
beam-averaged ($28"$, or 0.4-0.7 pc) peak H$_2$ column densities of
(1.7-9.3)$\times 10^{22} \, \rm{cm^{-2}}$. The bulk of the filaments are
relatively cold (17-21 K), while some local clumps have a dust temperature up
to 25-47 K. All the filaments are located within <~60 pc from the Galactic
mid-plane. Comparing the filaments to a recent spiral arm model incorporating
the latest parallax measurements, we find that 7/9 of them reside within arms,
but most are close to arm edges. These filaments are comparable in length to
the Galactic scale height and therefore are not simply part of a grander
turbulent cascade.
