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Resolve the distance ambiguity of Cloud 2

Determine whether the head–tail molecular Cloud 2, centered near Galactic coordinates (l ≈ 331.85°, b ≈ +0.20°), is located at the same distance as Cloud 1 (2.46 ± 0.18 kpc) or at a foreground distance of 1.44 ± 0.09 kpc by resolving the ambiguity indicated by Gaia DR3 G-band extinction breakpoints and CO radial-velocity information.

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Background

The paper identifies two head–tail molecular clouds (Cloud 1 and Cloud 2) with radial velocities around −35 km s−1 and interprets them as falling onto the Galactic disk. Using Gaia DR3 extinction breakpoints and morphological correspondence with CO emission, the authors infer a most likely distance of 2.46 ± 0.18 kpc for Cloud 1.

For Cloud 2, although its radial velocity matches Cloud 1, the Gaia-based extinction analysis yields a possible foreground distance at 1.44 ± 0.09 kpc. The authors therefore state a distance ambiguity for Cloud 2 that needs to be resolved to refine mass and size estimates and to clarify its relationship to the proposed falling-cloud scenario.

References

This result shows that Cloud 2 is likely to be {at} the same distance $2.46 \pm 0.18$ kpc estimated {for} Cloud 1, while we cannot exclude that Cloud 2 possibly exists with a foreground distance of $1.44 \pm 0.09$ kpc.

— Head-tail molecular clouds falling onto the Milky Way disk (2510.18399 - Kohno et al., 21 Oct 2025) in Section 4.1 (Distance to the head-tail molecular clouds)