Galactic centre star formation writ large in gamma-rays

TL;DR

This paper models the high-energy processes in the Galactic center, explaining diffuse gamma-ray and microwave emissions through cosmic ray interactions, magnetic fields, and galactic winds, suggesting a long-standing energetic source since the Galaxy's early days.

Contribution

It provides a comprehensive model linking cosmic rays, magnetic fields, and galactic winds to observed gamma-ray and microwave emissions in the Galactic center and Fermi bubbles, highlighting a persistent energetic source.

Findings

Magnetic field in GC is 100-300 microG.

Super-wind removes >95% of cosmic rays from the region.

Steady-state cosmic ray power is ~10^39 erg/s, explaining Fermi bubbles.

Abstract

We have modelled the high-energy astrophysics of the inner 200 pc of the Galaxy with a view to explaining the diffuse, broad-band (radio continuum to TeV gamma-ray), non-thermal signal detected from this region. Our modelling pins down the ISM parameters for the environment wherein cosmic ray (CR) electrons and ions reside in the Galactic centre (GC). We find that the magnetic field in this region is 100-300 microG, the gas density < 60 cm^-3, and that a powerful (> 200 km/s) 'super'-wind acts to remove > 95% of the cosmic rays accelerated in the region before they have time to lose their energy in situ. The ~ 10^39 erg/s carried away by the GC cosmic ray protons is precisely enough to energise the ~GeV gamma-ray emission from the Fermi 'bubbles' recently found to extend north and south of the GC out to distances of ~10 kpc, provided that the bubbles constitute thick targets to the GC protons and that the situation has reached steady state. In such a situation of 'saturation' the hard, uniform spectrum of the bubbles are explained and secondary electron synchrotron explains the non-thermal microwave emission found in WMAP data mirroring the bubbles. Given the very low density of the bubble plasma (<0.01 cm^-3), the pp loss time in the Bubbles is > 5 Gyr. Our scenario thus has the startling implication that a GC source of nonthermal particles of time-averaged power 10^39 erg/s has persisted since the youth of the Galaxy.