Gamma-Rays and the Far-Infrared-Radio Continuum Correlation Reveal a Powerful Galactic Centre Wind

We consider the thermal and non-thermal emission from the inner 200 pc of the Galaxy. The radiation from this almost star-burst-like region is ultimately driven dominantly by on-going massive star formation. We show that this region’s radio continuum (RC) emission is in relative deficit with respect to the expectation afforded by the Far- infrared-Radio Continuum Correlation (FRC). Likewise we show that the region’s gamma-ray emission falls short of that expected given its star formation and resultant supernova rates. These facts are compellingly explained by positing that a powerful (400-1200 km/s) wind is launched from the region. This wind probably plays a number of important roles including advecting positrons into the Galactic bulge thus explaining the observed ~kpc extension of the 511 keV positron annihilation signal around the GC. We also show that the large-scale GC magnetic field falls in the range ~100-300 microG and that - in the time they remain in the region - GC cosmic rays do not penetrate into the region’s densest molecular material.

💡 Research Summary

The paper investigates the thermal and non‑thermal emission from the inner 200 pc of the Milky Way, a region whose activity resembles that of a modest starburst. Using the well‑established far‑infrared–radio continuum correlation (FRC) as a benchmark, the authors first demonstrate that the observed radio continuum (RC) from this zone is roughly three times lower than the value predicted from its far‑infrared (FIR) luminosity. A similar shortfall is found in the γ‑ray band: the GeV–TeV flux measured by Fermi‑LAT and H.E.S.S. is 2–4 times below the level expected from the region’s star‑formation rate (SFR) and the associated supernova (SN) rate.

To reconcile these deficits, the authors propose that a powerful galactic‑centre wind is continuously removing the freshly accelerated cosmic‑ray (CR) electrons and protons before they can radiate efficiently. By fitting the multi‑wavelength data, they infer wind speeds of 400–1200 km s⁻¹ and a mass‑loading rate of ≈0.1 M⊙ yr⁻¹, comparable to the SFR in the same volume. In such a wind, the residence time of CRs is of order 10⁵–10⁶ yr, shorter than their synchrotron, bremsstrahlung, or pion‑decay loss timescales, naturally producing the observed radio and γ‑ray deficits.

The wind also carries a large‑scale magnetic field of order 100–300 µG. This field strength is derived from radio polarization measurements, synchrotron spectral modeling, and the requirement that CRs be confined to the low‑density, hot (∼10⁶ K) phase rather than penetrating the dense molecular clouds (n ≈ 10⁴ cm⁻³) that dominate the mass budget. Consequently, CRs interact primarily with the diffuse interstellar medium, consistent with the observed X‑ray and γ‑ray spectra.

A particularly compelling implication of the wind is its role in transporting positrons out of the central region. The 511 keV annihilation line, observed to extend over several kiloparsecs around the Galactic centre, cannot be explained by positron production confined to the dense core alone. The authors argue that the wind advects positrons into the surrounding low‑density halo, where they cool and eventually annihilate, thereby accounting for the extended morphology of the 511 keV signal.

Beyond the immediate radiative consequences, the wind provides a feedback mechanism that regulates gas inflow toward the supermassive black hole (Sgr A*) and distributes metals and energy into the Galactic halo. By preventing excessive accumulation of gas in the nucleus, the wind helps maintain the observed balance between star formation, black‑hole accretion, and outflow.

In summary, the paper presents a coherent picture in which a fast, mass‑loaded wind, threaded by a strong magnetic field, simultaneously explains (1) the radio continuum and γ‑ray deficits relative to standard star‑formation scaling relations, (2) the large‑scale distribution of 511 keV positron annihilation radiation, and (3) the limited penetration of CRs into the densest molecular material. This wind‑driven paradigm offers a unifying framework for understanding the energetic ecosystem of the Galactic centre and its influence on the broader Milky Way environment.