Metal Enrichment in the Fermi Bubbles as a Probe of Their Origin
  The Fermi bubbles are gigantic gamma-ray structures in our Galaxy. The
physical origin of the bubbles is still under debate. The leading scenarios can
be divided into two categories. One is the nuclear star forming activity
similar to extragalactic starburst galaxies and the other is the past active
galactic nucleus (AGN) like activity of the Galactic center supermassive black
hole. In this paper, we propose that metal abundance measurements will provide
an important clue to probe their origin. Based on a simple spherically
symmetric bubble model, we find that the generated metallicity and abundance
pattern of the bubbles' gas strongly depend on assumed star formation or AGN
activities. Star formation scenarios predict higher metallicities and abundance
ratios of [O/Fe] and [Ne/Fe] than AGN scenarios do because of supernovae
ejecta. Furthermore, the resultant abundance depends on the gamma-ray emission
process because different mass injection histories are required for the
different gamma-ray emission processes due to the acceleration and cooling time
scales of non-thermal particles. Future X-ray missions such as ASTRO-H and
Athena will give a clue to probe the origin of the bubbles through abundance
measurements with their high energy resolution instruments.
