Is the recently discovered large scale filamentary feature Cattail in the cold or unstable phase?

TL;DR

This study analyzes the large-scale filamentary neutral hydrogen feature Cattail, using advanced algorithms to determine its physical state, revealing it is likely in an unstable phase and composed of two turbulent systems.

Contribution

It introduces a novel application of the Velocity Decomposition Algorithm and Force Balancing Model to assess the stability and structure of the Cattail filament.

Findings

Cattail is in an unstable neutral media regime.

Cattail consists of two disjoint turbulent features.

Smaller cold media may be embedded within Cattail.

Abstract

A recent publication (Li et al. 2021) discovered one of the largest filamentary neutral hydrogen features dubbed Cattail from high resolution FAST observations that might be a new galactic arm of our own Milky Way. However in the analysis, it was suggested that this neutral hydrogen feature is cold despite having 12km/s total linewidth. We evaluate the probability whether the Cattail is actually cold neutral media via the newly developed Velocity Decomposition Algorithm (Yuen et al. 2021a) and Force Balancing Model (Ho et al. 2021a). We discovered that even with the inclusion of the galactic shear term, the feature is still at the unstable neutral media regime. Moreover, we also discover that the Cattail is two disjoint features in caustics space, suggesting that the Cattail might have two different turbulent systems. We check the spectra of the individual system separated via VDA to confirm this argument. We do not exclude the existence of smaller scale cold media being embedded within this structure.