Revisiting the interpretation of HI Zeeman measurements in the diffuse interstellar medium
Abstract
The HI Zeeman effect provides one of the few direct probes of magnetic field strengths in the diffuse atomic interstellar medium (ISM), yet the physical quantity recovered by the standard weak-field estimator remains uncertain in multiphase gas. This is especially relevant to the low-column-density branch of the observed magnetic field--column density relation, which is nearly flat below NH~1E22 cm-2 and remains difficult to explain theoretically. We investigate this issue using synthetic HI Zeeman observations of multiphase magneto-hydrodynamic simulations of the atomic ISM performed with RAMSES, complemented by analytical two-phase models of the thermally bistable atomic medium. We find that both emission and absorption Zeeman measurements recover the magnetic field weighted by optical depth along the line of sight, rather than the density-weighted field. The two converge only in limiting cases, such as isothermal gas or homogeneous magnetic fields across phases. In realistic multiphase media, the Zeeman-derived field statistically exceeds the density-weighted field by factors of a few, with the strongest departures when cold and warm atomic gas both contribute significantly to the column density. Our analytical models reproduce this behavior and show that thermal bistability generates both an excess and a large dispersion between Zeeman-derived and density-weighted fields, even when the intrinsic magnetic field increases monotonically with density. The effect is strongest below NH~1E22 cm-2, where the temperature contrast between cold and warm HI causes optical-depth and density weighting to diverge. We conclude that HI Zeeman observations primarily trace the optical-depth-weighted magnetic field. This intrinsic thermal weighting may contribute to the broad distribution, and possibly part of the apparent flattening, of the observed magnetic field-column density relation.
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BibTeX
@misc{barthas2026revisiting,
title = {{Revisiting the interpretation of HI Zeeman measurements in the diffuse interstellar medium}},
author = {Adrien Barthas and Andrea Bracco},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.20374},
eprint = {2609.20374},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.20374},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Revisiting the interpretation of HI Zeeman measurements in the diffuse interstellar medium
AU - Adrien Barthas
AU - Andrea Bracco
PY - 2026
DA - 2026/09/17
PB - arXiv
DO - 10.48550/arxiv.2609.20374
UR - https://arxiv.org/abs/2609.20374
AB - The HI Zeeman effect provides one of the few direct probes of magnetic field strengths in the diffuse atomic interstellar medium (ISM), yet the physical quantity recovered by the standard weak-field estimator remains uncertain in multiphase gas. This is especially relevant to the low-column-density branch of the observed magnetic field--column density relation, which is nearly flat below NH~1E22 cm-2 and remains difficult to explain theoretically. We investigate this issue using synthetic HI Zeeman observations of multiphase magneto-hydrodynamic simulations of the atomic ISM performed with RAMSES, complemented by analytical two-phase models of the thermally bistable atomic medium. We find that both emission and absorption Zeeman measurements recover the magnetic field weighted by optical depth along the line of sight, rather than the density-weighted field. The two converge only in limiting cases, such as isothermal gas or homogeneous magnetic fields across phases. In realistic multiphase media, the Zeeman-derived field statistically exceeds the density-weighted field by factors of a few, with the strongest departures when cold and warm atomic gas both contribute significantly to the column density. Our analytical models reproduce this behavior and show that thermal bistability generates both an excess and a large dispersion between Zeeman-derived and density-weighted fields, even when the intrinsic magnetic field increases monotonically with density. The effect is strongest below NH~1E22 cm-2, where the temperature contrast between cold and warm HI causes optical-depth and density weighting to diverge. We conclude that HI Zeeman observations primarily trace the optical-depth-weighted magnetic field. This intrinsic thermal weighting may contribute to the broad distribution, and possibly part of the apparent flattening, of the observed magnetic field-column density relation.
ER -