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In galactic nuclei, stellar densities are so high that stars can physicallycollide with each other. In this work we focus on the collision of red giantsand in particular on the formation of non-thermal processes through collisionsand their properties. We analytically address these points by evaluatinghead-on collisions but also take into account scenarios with a deviation fromthe radial orbit, which we treat in a perturbative fashion. The collisionsproduce internal shocks with supersonic Mach numbers. Almost immediately,jet-like structures with important Lorentz factors form. The debris from thecollision produces another shock wave which, when interacting with theinterstellar medium of a galactic nucleus, leads to particle acceleration. Weestimate the background flux in X- and gamma rays created by the background ofthese collisions by deriving the spectral index within a radius of 100 Mpc andfind that they are high. Additionally, we make an estimate of the neutrinoproduction and find about $10^{11}$ neutrinos per square meter per second for acollision at 100 Mpc from Earth. Also, we derive that there is a non-negligiblechance to ignite fusion during the collision, due to the squeezing of thematerial. We investigate the possibility that the degenerate cores collide witheach other, leading to a high afterglow luminosity, and find that it isnon-negligible, although this should be addressed with dedicated numericalsimulations. Colliding red giants in galactic nuclei trigger a plethora ofhigh-energy phenomena, and have a particular gravitational wave emissionassociated, as shown by us, so that their detection will allow us to rule outalternatives.

Colliding red giants in galactic nuclei: Shocks, jets, impact on the ISM, X- and gamma-rays, neutrinos, fusion ignition and afterglow