IceCube's neutrinos: galactic or extra-galactic?

Abstract

The exact origin and production method of the astrophysical neutrino, a subatomic particle that is very difficult to detect, is yet to be confirmed. Here, two source scenarios for the origin of the neutrino are considered: Galactic and extra-galactic. In the Galactic scenario, neutrinos are searched for from the disk and the halo of the Milky Way, whereas in the extra-galactic case neutrinos might be coming from Active Galactic Nuclei, Starburst Galaxies and other highly energetic regions of the Universe. The IceCube Neutrino Observatory has detected an astrophysical neutrino intensity which may reveal the origin of these neutrinos. The Milky Way is not a unique galaxy. If it were to produce some fraction of the neutrinos that IceCube detects then there must be other similar spiral galaxies in the Universe also contributing to the intensity. This could create a contradiction of how many other Milky Way-like galaxies there would be allowed in the rest of the Universe if the assumption is made that the Milky Way produces nearly all of IceCube’s neutrinos. The overall number density of Milky Way-like objects in the Universe can be calculated for different Galactic source distributions. The neutrino sources could be distributed throughout the halo of the Galaxy or confined to the Galactic disk. By considering various models and calculating the number density of equivalent Milky Way-like galaxies in the rest of the Universe, constraints are placed on the fraction of the IceCube intensity that could be coming purely from the Milky Way. According to the results of this research it is ultimately found that, under the simplifying assumption that the halo is spherical, the halo of the Milky Way cannot account for all of IceCube’s neutrinos and under certain assumptions the disks of Milky Way-like galaxies cannot be the sole origin of neutrinos in light of experimental observations.