Using the Five-hundred-meter Aperture Spherical radio Telescope (FAST), a team of international astronomers detected a giant filamentary H I structure “Cattail”, which is possibly the furthest (Rgc∼22 kpc) and largest (∼1.1 kpc) filament to date. Their study recently appeared in Arxiv.
Gas filaments are the largest known structures in the universe, consisting of walls of gravitationally bound galaxy superclusters. The largest elongated molecular cloud structures are called giant molecular filaments with length greater than 10 pc. Compared to gaint molecular filaments, H I filaments are not well studied. Most of the H I filaments are aligned with the Galactic plane, which is similar to the situation of giant molecular filaments. The H I filaments are normally cold with a typical excitation temperature Tex∼50 K and often associated with CO dark molecular gas. However, detailed physical properties of H I filaments, as well as their distribution in the Galaxy, are not well characterized.
Now, a team of international astronomers led by Keping Qiu, with the help of FAST, observed the sky region of Right Ascension of 307°.7 < α < 311°.0 and Declination of 40°.9 < δ < 43°.4 on 2019 August 24. This sky region covers the main part of the Cygnus-X North molecular cloud, which has a velocity range of – 30 km s¯1 to 20 km s¯1 and is located 1.4 kpc away from the Sun.
They detected a gaint filamentary H I structure having a velocity between –170 km s¯1 to –130 km s¯1, and a mean velocity of – 150 km s¯1 at a Galactocentric distance of 22 kpc.
This has a length of 1.1 kpc, which appears to be so far the furthest and largest giant filament in the Galaxy. They named it Cattail. 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, they found that Cattail could have an even larger length, up to 5 kpc. They also identified 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.
“Based on the above analysis, we suggest two possible explanations for Cattail: it is a giant filament with a length of ∼5 kpc, or part of a new arm in the Extreme Outer Galaxy (EOG) ”— they wrote.
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.
Featured image: Artist’s conception view of the Milky Way (R. Hurt: NASA/JPL-Caltech/SSC) . The new part of the OSC and the Cattail identified in this work are indicated with the green and blue dashed line, respectively. © Keping Qiu et al.
Reference: Chong Li, Keping Qiu, Bo Hu, Yue Cao, “The discovery of the largest gas filament in our Galaxy, or a new spiral arm?”, Arxiv, 2021. https://arxiv.org/abs/2108.01905
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