Globular clusters contributing to the nuclear star cluster and galaxy center gamma ray excess, moderated by galaxy assembly history

Abstract

Recent observations of gravitational way by the Laser Interferometer Gravitational Wave Observatory and gamma-ray emmission by the Fermi GammaRay Space Telescope have lead to suspicion of interesting roles for two types of dense celestial objects at galactic centers - black holes and millisecond pulsars. While the frequent detection of $\sim 30 M_{\odot}$ binary black hole mergers as gravitational wave emmitters reminds people of the possibility that primoridal black holes constitute dark matter, a diffuse residual gamma ray excess at the center of the Milky Way and Anrdomeda might suggest a considerable population of millisecond pulsars as debris of disrupted globular clusters when they migrate into the centerof galaxy. We have followed up and performed the most up to date investigation of the two scenarios and found very interesting results. Although the gravitational wave signals permit the possibility of dark matter with a primordial black hole origin, it was later refuted by a macro lensing observation featuring caustic crossing. This rules out primordial black holes within a narrow mass window as dominant component of dark matter, and people will need to consider an extended mass spectrum or put more faith in the Weakly Interactting Massive Particles. On the other hand, although current study of the gamma ray excess does not find a consistent millisecond pulsar explanation, they were based on an over-simplified formation model, which we aim to replace with a more physical one from N-body galactic formation simulation. Our result shall answer the mysterious nature of the Milky Way and Andromeda’s gamma ray excess by figuring out the contribution from milisecond pulsars and the rest as the dark matter component. Together, the two parts of research provides new insight into the intertwined physical environment with dark matter and dense celestial objects at the galactic center.