Astronomers have for the first time built a three-dimensional map of the thermal state of interstellar gas around the Sun. Previously these gas phases were described only statistically, without any spatial structure.
The map is called P3D. It covers a region about 1 kiloparsec across (roughly 3260 light years), centred on the Sun. The data are sampled on a grid with 2-parsec spacing (about 6.5 light years).
To reconstruct the thermal state of the gas, the researchers combined 3D maps of dust extinction and interstellar far-ultraviolet radiation with a model of the chemistry and heating of neutral gas. They found cold clouds surrounded by thermally unstable envelopes and embedded in a pervasive warm phase.
Within a layer up to 150 parsecs (about 490 light years) from the Galactic plane, around 41% of the neutral mass traced by dust is in a thermally unstable state. This means the gas cycles between phases over 2.9–6 million years. At the same time the cold phase has a narrow density distribution — internal motions do not exceed about 1.5 times the speed of sound.
The authors favour a picture of a multiphase medium that continuously changes state. This calls into question star-formation models that assume single-phase cold gas with strongly supersonic motions.