Observations of Interstellar Magnetic Fields

Abstract

This article describes how interstellar magnetic fields are detected, measured, and mapped, the results of such observations, and the role played by interstellar magnetic fields in the physics of the interstellar medium. A goal of the observations is the measurement of the morphology and strengths of the uniform (B u) and random (B r) components of magnetic fields. Observational techniques probe either the component of B parallel to the line of sight (B‖) or in the plane of the sky (B⊥). Tracers of B‖ are Faraday rotation of the position angle of linearly polarized radiation and Zeeman splitting of spectral lines. Tracers of B⊥ are the strength of synchrotron radiation and linear polarization of synchrotron radiation and of emission or absorption from dust and spectral lines. Starlight polarization shows that on large spatial scales the Galactic magnetic field is not heavily tangled (B u/B r ≈ 0.7-1.0), that the field is generally parallel to the Galactic plane near the plane, that the local field points approximately along the local spiral arm (pitch angle 9.4°, center of curvature 7.8 kpc distant towards ℓ≈15.4;°), and that off the Galactic plane there is considerable small-scale structure to the field. Galactic synchrotron emission shows magnetic spiral arms with a total strength Bt ≈6 μG and B u ≈ 4 μG. Pulsar data show evidence for reversals of the field direction with Galactic radius and yield Br ≈ 5 μG and B u ≈ 1.5 μG; the morphology of the largescale mean field is consistent with dynamo generation. H I Zeeman detections for diffuse clouds yield B‖ ∼ 5;–20 μG with many limits B‖< 5 μG. A recent survey of Galactic H I in absorption against extragalactic sources confirms the result that the fields in diffuse clouds are often quite weak. The critical parameter for evaluating the importance of magnetic fields in star formation is the ratio of the mass to the magnetic flux, M/ΦB; observations focus on measuring both this quantity and the morphology of fields in dense regions. Zeeman observations of molecular lines are consistent with B ασv√n, which is the theoretical prediction for flattened cores supported by a combination of a uniform magnetic field pressure and turbulence. In cores, motions are approximately Alfvénic, and M/ΦB has a critical to slightly supercritical value. The ratio of B r/B u appears to decline with density. In some molecular cores there is evidence for the “hourglass” pinch that would be produced by cloud contraction with the magnetic field frozen into the matter.