One interesting product of cosmic-ray interactions with interstellar medium is lithium, which is one of the few elements which was produced during the Big Bang. Until recently measured lithium abundances were only available inside Milky Way, but first measurements outside our galaxy are available for small neighbouring galaxy – Small Magellanic Cloud (SMC), which was also detected in gamma rays. The observed gamma-ray flux of the SMC describes its present day cosmic-ray activity, while its lithium abundances are the result of all cosmic-ray activity in this galaxy during its lifetime.
It turns out that the observed lithium abundances in the SMC are much higher than in the MW halo stars, and are in fact just slightly below the expected primordial abundances. Trying to explain the observed lithium abundances in the SMC is an important issue, since it can shed light on our understanding of lithium abundances in the Universe, which proved to be very problematic until now. In order to link these two observations, we have linked the production of lithium in SMC and the pionic gamma-ray intensity produced by all SMC-like galaxies over the cosmic history (normalised to the present day SMC gamma-ray flux). We have shown that galactic cosmic rays produced in supernova remnants (considered to be the dominant cosmic-ray population in the SMC) can only explain a very small part of the observed abundance of lithium (less than 1%), if we assume that the entire present gamma-ray emissivity of the SMC that we observe also originates from the interaction of galactic cosmic rays with gas within the galaxy.
This leaves room for the possible existence of other interesting sources of lithium in the SMC, like other cosmic-ray populations which were present at some point in SMCs history, for example during close fly–bys of SMC and MW. This CR population would not be present for a long time compared to the life of the galaxy, so they could produce more lithium without violating the present day gamma-ray flux of the SMC. Comparing SMC’s lithium abundances and its gamma-ray flux suggests a possible important role of galaxy–galaxy interactions for production of cosmic rays inside galaxies, both in tidal shocks, as well as through triggering new star formation, and subsequent supernovae explosions.
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