The "Maggie" filament: Physical properties of a giant atomic cloud
  The atomic phase of the interstellar medium plays a key role in the formation
process of molecular clouds. Due to the line-of-sight confusion in the Galactic
plane that is associated with its ubiquity, atomic hydrogen emission has been
challenging to study. Employing the high-angular resolution data from the THOR
survey, we identify one of the largest, coherent, mostly atomic HI filaments in
the Milky Way at the line-of-sight velocities around -54 km/s. The giant atomic
filament "Maggie", with a total length of 1.2 kpc, is not detected in most
other tracers, and does not show signs of active star formation. At a kinematic
distance of 17 kpc, Maggie is situated below (by 500 pc) but parallel to the
Galactic HI disk and is trailing the predicted location of the Outer Arm by
5-10 km/s in longitude-velocity space. The centroid velocity exhibits a smooth
gradient of less than $\pm$3 km/s /10 pc and a coherent structure to within
$\pm$6 km/s. The line widths of 10 km/s along the spine of the filament are
dominated by non-thermal effects. After correcting for optical depth effects,
the mass of Maggie's dense spine is estimated to be $7.2\times10^5\,M_{\odot}$.
The mean number density of the filament is 4$\rm\,cm^{-3}$, which is best
explained by the filament being a mix of cold and warm neutral gas. In contrast
to molecular filaments, the turbulent Mach number and velocity structure
function suggest that Maggie is driven by transonic to moderately supersonic
velocities that are likely associated with the Galactic potential rather than
being subject to the effects of self-gravity or stellar feedback. The column
density PDF displays a log-normal shape around a mean of $N_{\rm HI} =
4.8\times 10^{20}\rm\,cm^{-2}$, thus reflecting the absence of dominating
effects of gravitational contraction.
