The first three-dimensional map of thermal phases in the local interstellar medium
The thermal state of interstellar gas controls whether it remains warm and diffuse or cools into dense clouds. Twenty-one centimetre observations have established a statistical picture of neutral-gas phases, but their three-dimensional architecture has not previously been mapped. Here we present $\mathcal{P}_{\rm 3D}$, 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. $\mathcal{P}_{\rm 3D}$ 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 $|z| \leq 150$ pc of the Galactic midplane, $\sim$41 % of the dust-traced neutral mass is thermally unstable, implying phase cycling on a timescale of $\sim$2.9-6 Myr. Yet the cold phase has a narrow density distribution, consistent with internal Mach numbers that are transonic at most ($\mathcal{M}_s \lesssim 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.