A lower limit of 50 microgauss for the magnetic field near the Galactic Centre

The amplitude of the magnetic field near the Galactic Centre has been uncertain by two orders of magnitude for several decades. On a scale of approximately 100 pc fields of approximately 1000 microG have been reported, implying a magnetic energy density more than 10,000 times stronger than typical for the Galaxy. Alternatively, the assumption of pressure equilibrium between the various phases of the Galactic Centre interstellar medium (including turbulent molecular gas; the contested “very hot” plasma; and the magnetic field) suggests fields of approximately 100 microG over approximately 400 pc size scales. Finally, assuming equipartition, fields of only approximately 6 microG have been inferred from radio observations for 400 pc scales. Here we report a compilation of previous data that reveals a down-break in the region’s non-thermal radio spectrum (attributable to a transition from bremsstrahlung to synchrotron cooling of the in situ cosmic-ray electron population). We show that the spectral break requires that the Galactic Centre field be at least 50 microG on 400 pc scales, lest the synchrotron-emitting electrons produce too much gamma-ray emission given existing constraints. Other considerations support a field of 100 microG, implying that > 10% of the Galaxy’s magnetic energy is contained in only < 0.05% of its volume.

💡 Research Summary

The magnetic field strength in the central few hundred parsecs of the Milky Way has been a long‑standing source of controversy, with estimates ranging from a few microgauss to a milligauss. Early measurements on ∼100 pc scales suggested fields of order 1 mG, implying a magnetic energy density more than ten thousand times the Galactic average. Pressure‑balance arguments that include turbulent molecular gas, the debated “very hot” plasma, and the magnetic field itself yielded values near 100 µG on ∼400 pc scales, while the standard equipartition analysis of radio synchrotron emission gave a much lower estimate of ≈6 µG. This spread reflects differing assumptions about the interstellar medium phases, the dominant cooling mechanisms of cosmic‑ray electrons, and the interpretation of multi‑wavelength data.

In this paper the authors revisit all available non‑thermal radio measurements of the Galactic Centre (GC) region and identify a clear spectral break in the 1–10 GHz band. They interpret the break as the transition from bremsstrahlung‑dominated cooling (which is proportional to the ambient gas density) to synchrotron‑dominated cooling (which depends on the magnetic field strength). If the magnetic field were weaker than about 50 µG, bremsstrahlung would dominate the electron energy losses, and the same electron population would inevitably produce a gamma‑ray flux—via neutral‑pion decay from proton‑proton collisions—that exceeds the upper limits set by Fermi‑LAT observations of the GC. By requiring that the predicted gamma‑ray emission stay below the observed limits, the authors derive a firm lower bound of ≈50 µG for the field on ∼400 pc scales.

Beyond this minimal requirement, the authors examine additional physical constraints. A magnetic field of ∼100 µG brings the magnetic pressure into approximate equilibrium with the turbulent pressure of the dense molecular clouds, the thermal pressure of the hot plasma, and the cosmic‑ray pressure. This balance is essential for the long‑term stability of the multi‑phase GC interstellar medium. In contrast, a field as low as 6 µG would leave the system severely pressure‑imbalanced, contradicting the observed coexistence of dense gas, hot plasma, and strong turbulence.

The implication of a ∼100 µG field is profound. The magnetic energy contained within the central 400 pc would amount to >10 % of the total magnetic energy of the entire Milky Way, despite occupying less than 0.05 % of the Galaxy’s volume. Such a concentration of magnetic energy can influence star formation rates, the dynamics of supernova‑driven outflows, and the feeding of the central supermassive black hole (Sgr A*). It also affects the propagation and confinement of cosmic rays, potentially shaping the large‑scale structures known as the Fermi Bubbles.

In summary, by coupling the observed radio spectral break with gamma‑ray constraints, the authors demonstrate that the Galactic Centre magnetic field cannot be weaker than 50 µG and is likely close to 100 µG. This result reconciles previously discordant estimates, provides a self‑consistent picture of the pressure balance in the GC, and highlights the central region as a magnetically dominated environment that stores a disproportionate fraction of the Galaxy’s magnetic energy. Future high‑resolution radio and gamma‑ray observations, together with sophisticated magnetohydrodynamic simulations, will be essential to refine these estimates and to explore the broader astrophysical consequences of such a strong, centrally concentrated magnetic field.