The gas between the stars is not uniform. It exists in several thermal states at once: cold dense clouds, diffuse warm gas, and an intermediate unstable phase that cannot stay in equilibrium for long. This has been known from observations of neutral hydrogen at a wavelength of 21 centimetres, carried out for decades. Until now, though, these phases were described only statistically — what fraction of the gas sits in each state, without any link to specific locations in space.
A team of astronomers led by Jonathan Shelest has now built the first three-dimensional map of the thermal state of the interstellar medium. The work is currently under peer review at Nature Astronomy.
Three gas phases around the Sun
The classical picture of the interstellar medium distinguishes a cold neutral phase at around a hundred kelvin, a warm neutral phase at a few thousand kelvin, and an intermediate thermally unstable state. The latter is not in equilibrium: gas caught in it inevitably keeps cooling or keeps heating, moving toward one of the two stable phases.
Previously, the balance between these phases could only be described as averages over large volumes of space. Where exactly the cold clouds sit, and where the diffuse warm component dominates, remained an open question.
Building the P3D map
The new map is called P3D. It covers a region about 1 kiloparsec across (roughly 3260 light years), centred on the Sun, with data sampled on a cubic grid with 2-parsec spacing (about 6.5 light years).
To build the reconstruction, the researchers combined three ingredients. First, existing 3D maps of dust extinction, which show where matter is concentrated in space. Second, 3D maps of interstellar far-ultraviolet radiation — the very radiation that heats neutral gas and drives its chemistry. Third, a thermochemical model of the neutral interstellar medium, which converts this data into a prediction of temperature and phase for every cell of the grid.
This approach delivered, for the first time, not a statistical but a spatial picture: exactly where, within a few hundred parsecs of the Sun, cold, warm, and unstable gas regions are located.
Cold clouds wrapped in warm envelopes
The map revealed a structure that could previously only be inferred. Cold, dense gas clouds are surrounded by thermally unstable envelopes, and the whole system is embedded in a pervasive warm phase that fills the space between clouds.
Within a layer up to 150 parsecs (about 490 light years) of the Galactic plane, roughly 41% of the dust-traced neutral mass is in this thermally unstable state. That is a lot — nearly half the material is not sitting in stable equilibrium but is mid-transition.
The researchers estimate the timescale of this transition at 2.9 to 6 million years. On cosmic timescales that is fast: gas is constantly cycling between cold, unstable, and warm states rather than settling into one of them for long.
Implications for star-formation models
A separate finding concerns the internal dynamics of the cold phase. Despite this active phase cycling, cold gas has a narrow density distribution. Internal motions do not exceed about 1.5 times the speed of sound — meaning the gas is transonic at most, not strongly supersonic.
This runs counter to an assumption underlying many star-formation models: that cold gas is single-phase and moves at strongly supersonic speeds, producing a broad density distribution driven by shocks.
The authors favour a different picture: the interstellar medium is not a set of frozen phases but a dynamically cycling system that continuously changes state. If this interpretation holds up, models describing how stars form out of diffuse gas may need to be reassessed.
