Origin of major-axis H I truncation

Establish the physical origin of the truncation of neutral hydrogen along the major axis of spiral galaxies, including whether it marks the outer boundary of the gaseous disk.

Background

The authors discuss azimuthal variations in the neutral fraction of circumgalactic gas as one possible explanation for the observed butterfly-like morphology. Along the major axis, neutral hydrogen truncation may define the outer gaseous-disk boundary, but the physical cause of that truncation is not established. Resolving this issue is relevant to interpreting the reduced neutral-gas signal along the major axis relative to the off-axis regions.

References

Although the origin of the truncation along the major axis is still debated, it is largely assumed that it marks the outer boundary of the gaseous disk.

— Diffuse HI emission in the circumgalactic medium of NGC891 and NGC4565 - III: azimuthal profiles  (2608.19186 - Rickel et al., 19 Aug 2026) in Section 4.1, “Assuming true emission” — item 1, Ionization