A Revised Nuclear Star Formation Driven, Hadronic Model for the Fermi Bubbles
Year: 2013
Type: preprint
Abstract: The Fermi Bubbles are enigmatic \gamma-ray features of the Galactic bulge. Both putative activity (within $\sim$ few $\times$ Myr) connected to the Galactic center super-massive black hole and, alternatively, nuclear star formation have been claimed as the energising source of the Bubbles. Likewise, both inverse-Compton emission by non-thermal electrons (`leptonic' models) and collisions between non-thermal protons and gas (`hadronic' models) have been advanced as the process supplying the Bubbles' \gamma -ray emission. An issue for any steady state hadronic model is that the very low density of the Bubbles' plasma seems to require that they accumulate protons over a multi-Gyr timescale, much longer than other natural timescales occurring in the problem. Here we present a hadronic model where the timescale for generating the Bubbles' hadronic \gamma -ray emission is $\sim$ few $\times 10^8$ years. Our model invokes collapse of the Bubbles' thermally-unstable plasma, leading to an accumulation of cosmic rays and magnetic field into localised, warm ($\sim 10^4$ K), and likely filamentary condensations of higher density gas. Under the condition that these filaments are supported by non-thermal pressure, we can predict the hadronic emission from the Bubbles to be $L_\gamma \simeq 2 \times 10^{37}$ erg/s $ \dot{M}_\mathrm{in}/(0.1 M_{Sun}/$ year $) T_\mathrm{FB}^2/(3.5 \times 10^7 K) ^2$ ; precisely their observed luminosity (normalizing to the star-formation-driven mass flux into the Bubbles and their measured plasma temperature). The existence of the population of warm gas condensations required by our model is supported by FUV absorption studies of the Galactic halo.
Source: arXiv (Cornell University)
Language: English
Subfield: Nuclear and High Energy Physics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W343607833