Evidence for a large thermal pressure imbalance in the local interstellar medium

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Abstract

THE interstellar medium (ISM) comprises a number of components at very different densities and temperatures, from dense molecular gas at 10 K to very diffuse plasma at 106 K. It has generally been assumed that the warm (104 K) and hot (106 K) components are in thermal pressure equilibrium1–3, although this has not been universally accepted4. Here we use the discovery of a shadow in the diffuse extreme ultraviolet background radiation5, cast by a nearby cloud of cool hydrogen, to calculate directly the thermal pressure in the nearby ( ≤ 40 pc) hot ISM. We find that it is 20 times greater than in the warm cloud surrounding the Sun. This result directly contradicts the basic assumption of thermal pressure balance in the ISM, indicating that some unidentified force (such as confinement by magnetic fields6) must play an important role in the overall pressure balance.

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Bowyer, S., Lieu, R., Sidher, S. et al. Evidence for a large thermal pressure imbalance in the local interstellar medium. Nature 375, 212–214 (1995) doi:10.1038/375212a0

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