Wild at Heart:-The Particle Astrophysics of the Galactic Centre

We treat of the high-energy astrophysics of the inner ~200 pc of the Galaxy. Our modelling of this region shows that the supernovae exploding here every few thousand years inject enough power to i) sustain the steady-state, in situ population of cosmic rays (CRs) required to generate the region’s non-thermal radio and TeV {\gamma}-ray emis-sion; ii) drive a powerful wind that advects non-thermal particles out of the inner GC; iii) supply the low-energy CRs whose Coulombic collisions sustain the temperature and ionization rate of the anomalously warm, envelope H2 detected throughout the Cen-tral Molecular Zone; iv) accelerate the primary electrons which provide the extended, non-thermal radio emission seen over ~150 pc scales above and below the plane (the Galactic centre lobe); and v) accelerate the primary protons and heavier ions which, advected to very large scales (up to ~10 kpc), generate the recently-identified WMAP haze and corresponding Fermi haze/bubbles. Our modelling bounds the average magnetic field amplitude in the inner few degrees of the Galaxy to the range 60 < B/microG < 400 (at 2 sigma confidence) and shows that even TeV CRs likely do not have time to penetrate into the cores of the region’s dense molecular clouds before the wind removes them from the region. This latter finding apparently disfavours scenarios in which CRs - in this star-burst-like environment - act to substantially modify the conditions of star-formation. We speculate that the wind we identify plays a crucial role in advecting low-energy positrons from the Galactic nucleus into the bulge, thereby explaining the extended morphology of the 511 keV line emission. (abridged)

💡 Research Summary

The paper presents a comprehensive model of the high‑energy environment within the inner ~200 pc of the Milky Way, arguing that the mechanical power supplied by supernovae (SNe) occurring every few thousand years is sufficient to explain a suite of observed non‑thermal phenomena. First, the authors show that the SN‑driven energy injection sustains a steady‑state population of cosmic rays (CRs) in situ. These CRs generate the observed synchrotron radio emission and the TeV γ‑ray flux via π⁰ decay and inverse‑Compton scattering, matching the spectra measured by radio telescopes and ground‑based γ‑ray observatories (e.g., H.E.S.S., VERITAS).

Second, the pressure contributed by the CRs and hot gas drives a large‑scale outflow—a wind with speeds of order 300–500 km s⁻¹. This wind advects both low‑energy (MeV) and high‑energy (GeV–TeV) particles out of the central region. The low‑energy CRs collide Coulombically with the molecular gas of the Central Molecular Zone (CMZ), providing the heating (to ≈150 K) and ionization rates (ζ ≈ 10⁻¹⁵ s⁻¹) required to explain the anomalously warm H₂ envelopes observed throughout the CMZ.

Third, the wind accelerates primary electrons to GeV energies, which then produce the extended non‑thermal radio lobes (the Galactic Centre Lobe) that rise ≈150 pc above and below the plane. The synchrotron spectrum and spatial extent of these lobes are reproduced when the magnetic field strength is constrained to 60 µG < B < 400 µG (2σ confidence).

Fourth, the same wind carries primary protons and heavier ions to distances of several kiloparsecs, eventually reaching ∼10 kpc. As these particles interact with the low‑density halo gas, they generate the large‑scale microwave excess (the WMAP haze) and the corresponding γ‑ray bubbles (the Fermi bubbles) via inverse‑Compton scattering and hadronic processes. This provides a unified explanation for the multi‑wavelength haze/bubble phenomenon without invoking exotic dark‑matter annihilation.

A crucial insight concerns the penetration depth of TeV CRs. The wind removal timescale (∼10⁵ yr) is shorter than the diffusion time required for TeV particles to enter the dense cores of molecular clouds (n > 10⁴ cm⁻³). Consequently, the majority of high‑energy CRs are expelled before they can significantly alter the internal conditions of star‑forming clumps. This result weakens the hypothesis that CR feedback in this starburst‑like environment can regulate star formation by heating or ionizing dense gas.

Finally, the authors propose that the wind also transports low‑energy positrons (e⁺) from the Galactic nucleus into the bulge. The advection of these positrons explains the observed spatially extended 511 keV annihilation line, whose emission spans ∼10° on the sky, a morphology difficult to reconcile with purely local sources.

In summary, the study integrates supernova‑driven mechanical power, CR acceleration, magnetic field constraints, and a large‑scale galactic wind into a self‑consistent framework that accounts for the radio, γ‑ray, microwave, and annihilation‑line signatures of the Galactic centre. The model makes testable predictions: (i) a wind‑driven velocity field observable in molecular line kinematics; (ii) a correlation between the radio lobe spectral index and wind speed; (iii) a gradual softening of the γ‑ray spectrum with latitude due to advection of CRs. Future high‑resolution observations with facilities such as the Cherenkov Telescope Array, ALMA, and next‑generation X‑ray missions will be able to confirm or refute these predictions, thereby sharpening our understanding of feedback processes in galactic nuclei.