A Census of Large-Scale ($\ge$ 10 pc), Velocity-Coherent, Dense
  Filaments in the Northern Galactic Plane: Automated Identification Using
  Minimum Spanning Tree
  Large-scale gaseous filaments with length up to the order of 100 pc are on
the upper end of the filamentary hierarchy of the Galactic interstellar medium.
Their association with respect to the Galactic structure and their role in
Galactic star formation are of great interest from both observational and
theoretical point of view. Previous "by-eye" searches, combined together, have
started to uncover the Galactic distribution of large filaments, yet inherent
bias and small sample size limit conclusive statistical results to be drawn.
Here, we present (1) a new, automated method to identify large-scale
velocity-coherent dense filaments, and (2) the first statistics and the
Galactic distribution of these filaments. We use a customized minimum spanning
tree algorithm to identify filaments by connecting voxels in the
position-position-velocity space, using the Bolocam Galactic Plane Survey
spectroscopic catalog. In the range of $7.^{\circ}5 \le l \le 194^{\circ}$, we
have identified 54 large-scale filaments and derived mass ($\sim 10^3 - 10^5 \,
M_\odot$), length (10-276 pc), linear mass density (54-8625 $M_\odot \,
\rm{pc}^{-1}$), aspect ratio, linearity, velocity gradient, temperature,
fragmentation, Galactic location and orientation angle. The filaments
concentrate along major spiral arms. They are widely distributed across the
Galactic disk, with 50% located within $\pm$20 pc from the Galactic mid-plane
and 27% run in the center of spiral arms (aka "bones"). An order of 1% of the
molecular ISM is confined in large filaments. Massive star formation is more
favorable in large filaments compared to elsewhere. This is the first
comprehensive catalog of large filaments useful for a quantitative comparison
with spiral structures and numerical simulations.
