Magnetic field strengths in the cold neutral medium of the Galaxy

Measurements of magnetic field strengths in the diffuse interstellar medium (ISM) of the Galaxy offer important insights into the physics of the ISM. Here we describe and interpret a new series of field strength measurements in the cold neutral medium (CNM). These measurements have been made via the Zeeman effect in Galactic H I absorption lines toward extra-galactic radio continuum sources. The data come from the Millennium Arecibo H I absorption line survey of Heiles & Troland. The results of this survey establish that the median magnetic field strength in the CNM is about 6 mu G. This value is very similar to field strengths in diffuse gas up to two orders of magnitude less dense. The magnetic field in the CNM is important since its energy density is comparable to turbulent energy densities. Also, the mass-to-flux ratio in the CNM appears magnetically subcritical. If this ratio is preserved as the gas becomes self-gravitating, then the magnetic field will dominate subsequent evolution and star formation within the gas.