The WMAP haze from the Galactic Center region due to massive star explosions and a reduced cosmic ray scale height

One important prediction of acceleration of particles in the supernova caused shock in the magnetic wind of exploding Wolf Rayet and Red Super Giant stars is the production of an energetic particle component with an E^-2 spectrum, at a level of a few percent in flux at injection. After allowing for transport effects, so steepening the spectrum to E^-7/3, this component of electrons produces electromagnetic radiation and readily explains the WMAP haze from the Galactic Center region in spectrum, intensity and radial profile. This requires the diffusion time scale for cosmic rays in the Galactic Center region to be much shorter than in the Solar neighborhood: the energy for cosmic ray electrons at the transition between diffusion dominance and loss dominance is shifted to considerably higher particle energy. We predict that more precise observations will find a radio spectrum of \nu^-2/3, at higher frequencies \nu^-1, and at yet higher frequencies finally \nu^-3/2.

💡 Research Summary

The authors propose a physically motivated explanation for the anomalous microwave emission known as the WMAP haze that surrounds the Galactic Center. They start from the well‑established picture that supernova shocks propagating through the magnetised winds of massive Wolf‑Rayet (WR) and Red Super‑Giant (RSG) stars can accelerate charged particles. Theory predicts that the freshly injected cosmic‑ray component should have a hard power‑law spectrum dN/dE ∝ E⁻² and that this component constitutes only a few percent of the total cosmic‑ray flux at the source.

When these electrons travel outward from the star‑forming region in the inner Galaxy, they experience both spatial diffusion and radiative energy losses (synchrotron radiation in a magnetic field of order 10–100 µG and inverse‑Compton scattering on the intense infrared and optical photon fields). The authors adopt a diffusion coefficient that is significantly smaller in the Galactic Center than in the solar neighbourhood, implying a much shorter diffusion time scale. Under these conditions the competition between diffusion and losses shifts the transition energy—where loss‑dominated cooling overtakes diffusion—from the usual ∼GeV range up to several hundred GeV. Consequently the steady‑state electron spectrum steepens from the injected E⁻² to roughly E⁻⁷/³ (≈ E⁻²·³³).

Electrons with this spectrum emit synchrotron radiation that reproduces the observed haze in three key aspects: (1) the overall intensity matches the few‑percent excess over the expected Galactic foreground; (2) the spectral shape in the WMAP frequency band (23–94 GHz) is reproduced by the ν⁻¹ dependence that follows from the E⁻⁷/³ electron distribution; (3) the radial profile, which falls off roughly as r⁻¹·⁵ within a few kiloparsecs of the centre, is naturally obtained because the short diffusion length confines the electrons close to their sources.

A distinctive prediction of the model is a piecewise synchrotron spectrum: at low radio frequencies (≲ 100 MHz) the spectrum should follow ν⁻²/³, steepening to ν⁻¹ in the WMAP band, and further to ν⁻³/₂ at higher microwave or sub‑millimetre frequencies. The authors argue that forthcoming low‑frequency radio surveys (e.g., LOFAR, SKA‑Low) and high‑frequency measurements (e.g., Planck, future CMB experiments) will be able to test these slopes.

Importantly, the explanation does not invoke exotic physics such as dark‑matter annihilation or non‑standard particle acceleration mechanisms; it relies solely on known supernova shock acceleration in massive‑star winds combined with a reduced cosmic‑ray scale height in the inner Galaxy. If confirmed, the model would not only solve the haze problem but also provide valuable constraints on the diffusion coefficient and magnetic environment of the Galactic Center, with broader implications for the origin of high‑energy cosmic rays and the interpretation of other diffuse Galactic emissions.