2021ApJ...918L...2L

Using the Five-hundred-meter Aperture Spherical radio Telescope (FAST), we detect a giant H I filamentary structure in the sky region of 307.°7 < α < 311.°0 and 40.°9 < δ < 43.°4. The structure has a velocity range of -170 to -130 km s^–1, and a mean velocity of -150 km s^–1, putting it at a Galactocentric distance of 22 kpc. The H I structure, which we name Cattail, has a length of 1.1 kpc, which so far appears to be the furthest and largest giant filament in the Galaxy. Its mass is calculated to be 6.5 x 10⁴ M_☉ and the linear mass density is 60 M_☉ 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 be even longer, up to 5 kpc. We also identify another new elongated structure as the extension into the Galactic first quadrant of the Outer Scutum-Centaurus (OSC) arm, and Cattail appears to be located far behind the OSC arm. The question about how such a huge filament is produced at this extreme Galactic location remains open. Alternatively, Cattail might be part of a new arm beyond the OSC arm, though it is puzzling that the structure does not fully follow the warp of the Galactic disk.