Multi-wavelength emission from leptonic processes in ageing galaxy bubbles.

The evolutionary behavior and multi-wavelength emission properties of bubbles around galaxies, such as the Fermi bubbles of the Milky Way, is unsettled. We perform 3D magnetohydrodynamical simulations to investigate the evolution of leptonic galaxy bubbles driven by a 0.3 Myr intense explosive outburst from the nucleus of Milky Way-like galaxies. Adopting an ageing model for their leptonic cosmic rays, we post-process our simulations to compute the multi-wavelength emission properties of these bubbles. We calculate the resulting spectra emitted from the bubbles from radio frequencies to $\gamma$-rays, and construct emission maps in four energy bands to show the the development of the spatial emission structure of the bubbles. The simulated bubbles show a progression in their spectral properties as they age. In particular, the TeV $\gamma$-ray emission is initially strong and dominated by inverse Compton scattering, but falls rapidly after $\sim$ 1 Myr. By contrast, the radio synchrotron emission remains relatively stable and fades slowly over the lifetime of the bubble. Based on the emission properties of our post-processed simulations, we demonstrate that $\gamma$-ray observations will be limited in their ability to detect galaxy bubbles, with only young bubbles around nearby galaxies being within reach. However, radio observations with e.g. the up-coming Square Kilometer Array, would be able to detect substantially older bubbles at much greater distances, and would be better placed to capture the evolutionary progression and diversity of galaxy bubble populations.