Southern Fermi Bubble vertical cutoff

Determine whether the southern Fermi Bubble exhibits a cutoff of neutral hydrogen clouds around z = -2 kpc that is symmetric with the approximately z = +2 kpc cutoff observed in the northern Fermi Bubble.

Background

The survey finds a sharp upper boundary to the northern neutral-cloud population at approximately 2 kpc above the Galactic plane. Because the southern Fermi Bubble is comparatively undersampled, the paper leaves unresolved whether a corresponding boundary exists at approximately 2 kpc below the Galactic plane.

References

This analysis raises a number of questions, especially about the southern FB, which is relatively undersampled in our data. In particular, does the southern FB show a cutoff of \ clouds around $ z = -2$ kpc, symmetric to that in the north?

— High Velocity Neutral Gas in the Fermi Bubbles: New Kinematic Limits and Spatial Structure  (2608.16754 - Lockman et al., 17 Aug 2026) in Section 8, “Summary and discussion”