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 H i filamentary structure in the sky region of 307fdg7 < α < 311fdg0 and 40fdg9 < δ < 43fdg4. 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 × 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.