Mapping the Thermal Condensation of Diffuse H I in the North Celestial Pole Loop

TL;DR

This study maps the multiphase neutral hydrogen in the North Celestial Pole Loop using archival 21 cm data, revealing coherent components, a dynamic arch, and turbulence characteristics, advancing understanding of early star formation processes.

Contribution

It introduces a detailed multiphase mapping of the NCPL using Gaussian decomposition with spatial regularization, identifying coherent structures and their dynamics.

Findings

Identified four coherent components in the NCPL.

Discovered a well-defined arch moving at 14 km/s.

Cold phase shows slightly supersonic turbulence.

Abstract

The North Celestial Pole Loop (NCPL) provides a unique laboratory for studying the early stages of star formation, in particular the condensation of the neutral interstellar medium (ISM). Understanding the physical properties that control the evolution of its contents is key to uncovering the origin of the NCPL. Archival data from the NCPL region of the GHIGLS 21 cm line survey (9'4) are used to map its multiphase content with ${\tt ROHSA}$, a Gaussian decomposition tool that includes spatial regularization. Column density and mass fraction maps of each phase were extracted along with their uncertainties. Archival data from the DHIGLS 21 cm (1') survey are used to further probe the multiphase content of the NCPL. We have identified four spatially (and dynamically) coherent components in the NCPL, one of which is a remarkably well-defined arch moving at about $14\ {\rm km s^{-1}}$ away from us that could be a relic of the large-scale organized dynamical process at the origin of the phase transition. The cold and lukewarm phases together dominate the mass content of the neutral gas along the loop. Using absorption measurements, we find that the cold phase exhibits slightly supersonic turbulence.