Phase Separation of HI in the Draco Nebula: CNM formation Triggered by Colliding WNM Flows
Abstract
We investigate the thermal phase structures and kinematics of atomic hydrogen (H I) gas in the Draco nebula to clarify how infalling H I evolves toward cold atomic and molecular clouds. Using the DHIGLS H I 21 cm data, we apply the Regularized Optimization for Hyper-Spectral Analysis (ROHSA) algorithm to decompose the emission into spatially coherent Gaussian components, and classify them as warm neutral medium (WNM), lukewarm neutral medium (LNM), and cold neutral medium (CNM). We identify two kinematically distinct WNM components: WNM1 near −3 km s−1 and WNM2 near −30 km s−1. The LNM forms a broad velocity bridge between two WNMs, whereas the CNM is concentrated within a narrower intermediate velocity range. CO emission is also confined to this interval and is associated with CNM-rich regions. The H I mass fractions are ∼ 41%, ∼ 23%, ∼ 17%, and ∼ 19% for WNM1, WNM2, LNM, and CNM, respectively, with the LNM and CNM together accounting for ∼ 36% of the total H I mass. No comparably independent WNM component is recovered at intermediate velocities, although a weak broad component may remain unresolved. The observed phase structures are consistent with a dynamically compressed multiphase interaction layer in which compression increases the gas density, shortens the radiative cooling time, and drives the evolution from WNM through thermally unstable gas toward the CNM. The present interaction may both create new CNM, and further compress or grow pre-existing dense structures. Our results provide the missing link between previous evidence for dynamical interaction and molecular cloud formation in Draco, and identify CNM formation as a key step in transforming gas accreting toward the Galactic disk into cold atomic and molecular clouds.
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BibTeX
@misc{naruse2026phase,
title = {{Phase Separation of HI in the Draco Nebula: CNM formation Triggered by Colliding WNM Flows}},
author = {Yuhi Naruse and Hiroaki Yamamoto and Yamato Matsuzuki and Rin Yamada and Atsushi Nishimura and Toshikazu Onishi},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.17753},
eprint = {2610.17753},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.17753},
}
RIS
TY - UNPB
TI - Phase Separation of HI in the Draco Nebula: CNM formation Triggered by Colliding WNM Flows
AU - Yuhi Naruse
AU - Hiroaki Yamamoto
AU - Yamato Matsuzuki
AU - Rin Yamada
AU - Atsushi Nishimura
AU - Toshikazu Onishi
PY - 2026
DA - 2026/10/13
PB - arXiv
DO - 10.48550/arxiv.2610.17753
UR - https://arxiv.org/abs/2610.17753
AB - We investigate the thermal phase structures and kinematics of atomic hydrogen (H I) gas in the Draco nebula to clarify how infalling H I evolves toward cold atomic and molecular clouds. Using the DHIGLS H I 21 cm data, we apply the Regularized Optimization for Hyper-Spectral Analysis (ROHSA) algorithm to decompose the emission into spatially coherent Gaussian components, and classify them as warm neutral medium (WNM), lukewarm neutral medium (LNM), and cold neutral medium (CNM). We identify two kinematically distinct WNM components: WNM1 near −3 km s^−1 and WNM2 near −30 km s^−1. The LNM forms a broad velocity bridge between two WNMs, whereas the CNM is concentrated within a narrower intermediate velocity range. CO emission is also confined to this interval and is associated with CNM-rich regions. The H I mass fractions are ∼ 41%, ∼ 23%, ∼ 17%, and ∼ 19% for WNM1, WNM2, LNM, and CNM, respectively, with the LNM and CNM together accounting for ∼ 36% of the total H I mass. No comparably independent WNM component is recovered at intermediate velocities, although a weak broad component may remain unresolved. The observed phase structures are consistent with a dynamically compressed multiphase interaction layer in which compression increases the gas density, shortens the radiative cooling time, and drives the evolution from WNM through thermally unstable gas toward the CNM. The present interaction may both create new CNM, and further compress or grow pre-existing dense structures. Our results provide the missing link between previous evidence for dynamical interaction and molecular cloud formation in Draco, and identify CNM formation as a key step in transforming gas accreting toward the Galactic disk into cold atomic and molecular clouds.
ER -