Presentation #120.05 in the session The Diffuse Gas in the Milky Way II: The Atomic Gas across the Milky Way.
Zeeman splitting is the only direct probe of the magnetic field strength in the diffuse ISM. Sensitive to the line-of-sight component of the magnetic field, Zeeman measurements place only lower limits on the total magnetic field strength unless additional information about the 3D magnetic field orientation is supplied. The Millennium survey (Heiles and Troland 2004) remains the largest collection of absorption Zeeman measurements in diffuse HI and includes sources with multiple cold gas components along the line of sight. Meanwhile, filaments detected in narrow velocity channel maps of HI emission align with the magnetic field orientation on the plane of the sky, and thus carry information about the component of the magnetic field that is not directly probed by Zeeman observations. This work investigates the connection between the Zeeman field strength and the dispersion of HI filaments associated with the Zeeman absorber. We study the morphology of HI emission structures in narrow-channel 21 cm emission data from the public GALFA-HI DR2 data release at the positions and velocities of the 66 Millennium HI absorption Zeeman measurements. We find tentative evidence for an association between the Zeeman-measured field strength and the plane-of-sky gas distribution and discuss whether it can be attributed to 3D magnetic field geometry. We employ publicly available surveys of molecular gas tracers to investigate whether Zeeman measurements display an environmental dependence. Finally, we discuss the feasibility of constraining the 3D magnetic field strength in the diffuse ISM by future analyses.