![Three-dimensional thermal phase architecture of the local ISM. (a) Three-dimensional rendering of the cold (blue) and unstable (yellow) isosurfaces, with warm gas shown transparent. The transparent warm volume traces WNM together with unmodelled hot ionised gas, which carries negligible mass (Methods). An interactive version is available at https://sb2580.github.io/phases 3d/. (b) Slices through the Galactic plane (𝑧 = 0) of gas density 𝑛 (left), FUV radiation intensity 𝐼UV (middle), and phase classification (right). Concentric circles mark heliocentric radii of 150, 300, and 450 pc. -- [astro-ph.GA]](https://i0.wp.com/astrobiology.com/wp-content/uploads/2026/07/The-first-three.png?fit=1200%2C675&ssl=1)
[astro-ph.GA] The thermal state of interstellar gas controls whether it remains warm and diffuse or cools into dense clouds. Twentyone centimetre observations have established a statistical picture of neutral-gas phases, but their three-dimensional architecture has not previously been mapped.
Here we present P3D, a three-dimensional reconstruction of thermal phase in the local interstellar medium (ISM), covering a 1 kpc-diameter region centred on the Sun and sampled on a 2 pc Cartesian grid. P3D combines high-resolution 3D maps of dust extinction and interstellar far-ultraviolet radiation with a neutral-ISM thermochemical model.
The reconstruction reveals cold clouds surrounded by thermally unstable envelopes and embedded in a pervasive warm phase. Within |𝒛| ≤ 150 pc of the Galactic midplane, ∼41% of the dust-traced neutral mass is thermally unstable, implying phase cycling on a timescale of ∼2.9–6 Myr.
Yet the cold phase has a narrow density distribution, consistent with internal Mach numbers that are transonic at most (M𝒔 ≲ 1.5). Together, these findings favour a dynamically cycling multiphase interstellar medium and motivate reassessing star-formation models that assume a single-phase, strongly supersonic cold-gas density field.
Jonathan Shelest, Shmuel Bialy, Troy A. Porter, Mark Wolfire, Matan Berko, Margarita Grinberg
Comments: Under peer review at Nature Astronomy. Constructive comments are welcome
Subjects: Astrophysics of Galaxies (astro-ph.GA)
Cite as: arXiv:2607.15352 [astro-ph.GA](or arXiv:2607.15352v1 [astro-ph.GA] for this version)
https://doi.org/10.48550/arXiv.2607.15352
Focus to learn more
Submission history
From: Jonathan Shelest
https://arxiv.org/abs/2607.15352
Astrobiology,
