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

Abstract

Two unresolved questions at galaxy centers, namely the formation of the nuclear star cluster (NSC) and the origin of the gamma-ray excess in the Milky Way (MW) and Andromeda (M31), are both related to the formation and evolution of globular clusters (GCs). They migrate towards the galaxy center due to dynamical friction, and get tidally disrupted to release the stellar mass content including millisecond pulsars (MSPs), which contribute to the NSC and gamma-ray excess.

In this study, we propose a semi-analytical model of GC formation and evolution that utilizes the Illustris cosmological simulation to accurately capture the formation epochs of GCs and simulate their subsequent evolution. Our analysis confirms that our GC properties at z=0 are consistent with observations, and our model naturally explains the formation of a massive NSC in a galaxy similar to theMW and M31. We also find a remarkable similarity in our model prediction with the gamma-ray excess signal in the MW. However, our predictions fall short by approximately an order of magnitude in M31, indicating distinct origins for the two gamma-ray excesses. Meanwhile, we utilize the catalog of Illustris halos to investigate the influence of galaxy assembly history. We find that the earlier a galaxy is assembled, the heavier and spatially more concentrated its GC system behaves at z=0. This results in a larger NSC mass and brighter gamma-ray emission from deposited MSPs.