Fermi Bubbles: Giant, Multibillion-Year-Old Reservoirs of Galactic Center Cosmic Rays

Recently evidence has emerged for enormous features in the $\gamma$-ray sky observed by the Fermi-LAT instrument: bilateral ”bubbles” of emission centered on the core of the Galaxy and extending to around $\pm$ 10 kpc from the Galactic plane. These structures are coincident with a nonthermal microwave ”haze” and an extended region of x-ray emission. The bubbles’ $\gamma$-ray emission is characterized by a hard and relatively uniform spectrum, relatively uniform intensity, and an overall luminosity $4\times {10}^{37}\mathrm{erg}/\mathrm{s}$, around 1 order of magnitude larger than their microwave luminosity while more than order of magnitude less than their x-ray luminosity. Here we show that the bubbles are naturally explained as due to a population of relic cosmic ray protons and heavier ions injected by processes associated with extremely long time scale ($\gtrsim$ 8 Gyr) and high areal density star formation in the Galactic center.

Figure 1

Time scales for CR protons and electrons in the bubble medium. ”$t_p^{\max }$”: maximum cooling time, $1/(d\ln E/dt)$, for protons given ionization and $pp$ collisions in the $n_H\simeq 0.005{\mathrm{cm}}^{-3}$ bubble plasma. The vertical solid lines show the approximate mean proton primary energy for daughter $\gamma$ rays of 1 GeV and 100 GeV. “$t_{\mathrm{inj}}^{\min }$”: minimum formation time ($\sim 8\mathrm{Gyr}$) for the bubbles in order that the system reach saturation. “$t_e^{\max }$”: maximum time scale (adopting $U_B\ll U_{\mathrm{CMB}}$ or $B\ll 3\times {10}^{-6}\mathrm{G}$) of electron cooling due to the combination of ionization and bremsstrahlung in the plasma and IC scattering on the CMB. The solid horizontal line shows the time scale for transport of electrons out to the full extent of the bubbles on a (very fast) $1000\mathrm{km}/\mathrm{s}$ wind. The vertical dashed lines show the energies required for an electron to IC scatter a CMB photon to 1 and 100 GeV. Much higher energy electrons than protons are required to produce a daughter $\gamma$ ray of the same energy.

Figure 2

Fermi bubble $\gamma$-ray spectrum [1]. The error bars are $1\sigma$. The solid curve shows the best-fit spectrum ($\propto E_p^{-2.1}$), the dotted curve shows the steepest reasonable spectrum ($\propto E_p^{-2.3}$), and, for comparison, the dashed curve is the spectrum expected were the bubbles suffused with protons having the $\propto E_p^{-2.7}$ distribution of the galactic disk.