The discovery of the largest gas filament in our Galaxy, or a new spiral
  arm?
  Using the Five-hundred-meter Aperture Spherical radio Telescope (FAST), we
detect a giant HI filamentary structure in the sky region of
307$.\!\!^{\circ}$7 $<$ $\alpha$ $<$ 311$.\!\!^{\circ}$0 and 40$.\!\!^{\circ}$9
$<$ $\delta$ $<$ 43$.\!\!^{\circ}$4. The structure has a velocity range of
$-$170 km s$^{-1}$ to $-$130 km s$^{-1}$, and a mean velocity of $-$150 km
s$^{-1}$, putting it to a Galactocentric distance of 22 kpc. The HI structure
has a length of 1.1 kpc, which appears to be so far the furthest and largest
giant filament in the Galaxy and we name it Cattail. Its mass is calculated to
be 6.5 $\times$ 10$^4$ M$_{\odot}$ and the linear mass density is 60
M$_{\odot}$ pc$^{-1}$. Its width is 207 pc, corresponding to an aspect ratio of
5:1. Cattail possesses a small velocity gradient (0.02 km s$^{-1}$ pc$^{-1}$)
along its major axis. Together with the HI4PI data, we find that Cattail could
have an even larger length, up to 5 kpc. We also identify another new elongated
structure to be the extension into the Galactic first quadrant of the Outer
Scutum-Centaurus (OSC) arm, and Cattail appears to be located far behind the
OSC. The question about how such a huge filament is produced at the extreme
Galactic location remains open. Alternatively, Cattail might be part of a new
arm beyond the OSC, though it is puzzling that the structure does not fully
follow the warp of the Galactic disk.
