Morphology and angular scale of the diffuse inner CGM

Disentangle whether the diffuse neutral hydrogen detected in the inner circumgalactic media of NGC 891 and NGC 4565 consists of smoothed-out filamentary or cloud structures or genuinely diffuse, warped gas, and determine its characteristic angular scale or morphology.

Background

The GBT observations establish excess neutral-hydrogen emission relative to the WSRT data, but their single-dish resolution cannot determine whether the emission is beam-filling diffuse gas or unresolved collections of clouds and filaments. The authors explicitly state that the observations alone cannot establish the characteristic size scale. Higher-angular-resolution interferometric observations or a deep map are required to resolve the morphology, an issue with implications for models of galaxy evolution, dark-matter halos, and star formation.

References

We emphasize that we cannot concretely claim the size scale of the { from these observations alone. High angular resolution observations from interferometers would be required to disentangle the dominant angular scale, or morphology, of the detected {.

— 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”; reiterated in Section 5, Conclusions