Year: 2026
Type: article
Abstract: The interplay of magnetic fields, turbulence, and gravity governs the structural evolution of the interstellar medium (ISM) and the initial conditions of star formation, yet observational gaps have long enforced a “broken power-law” description of the magnetic field–density relation. Here, we assemble a unified dataset spanning 10 orders of magnitude in density (10 −26 –10 −16 g cm −3 ) by combining pulsar and Zeeman observations. The unified data are consistent with a single, continuous magnetic-field evolution organised by the Alfvén Mach number M A = E K / E B . Within this interpretation, the low-density gas is magnetically dominated ( M A < 1 ), whereas the high-density gas becomes kinetically dominated ( M A > 1 ) as gravity increasingly contributes to the kinetic-energy budget, with magnetic tension continuing to influence the collapse geometry. Within the Gradual Transition interpretation, the empirical break density traces the vicinity of the trans-Alfvénic equipartition point, M A = 1 . This Gradual Transition model makes three predictions tested here. Its exponent and background field are fixed in advance by turbulent physics and recovered by the fits ( β ≈ 0.15–0.21 against a predicted 0.147; B c ≈ 2.0 μ G). A dense-gas fit, extrapolated blindly across four decades, passes through the diffuse pulsar data. And, under the adopted scale mappings, the implied magnetic-energy spectrum approaches a k −5/3 -like scaling on large scales and departs from this extrapolation on small scales, where gravitational compression amplifies the field. The broken power law can therefore be viewed as a piecewise approximation to the continuous magnetic equation of state, with its fitted transition density potentially retaining a physical connection to the onset of gravity-driven motions.
Source: The Astrophysical Journal
Authors: Mengke Zhao, Guang-Xing Li, Keping Qiu
Institutions: Yunnan University, Nanjing University
Language: English
Subfield: Astronomy and Astrophysics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: gold
Estimated APC paid: $4,499
Awards: G548063342, G5909188296, G5981854034
OpenAlex ID: W7212030488