Gamma-ray Signal from Earth-mass Dark Matter Microhalos

Abstract: Earth-mass dark matter microhalos with size of ~100 AUs are the first structures formed in the universe, if the dark matter of the Universe are made of neutralino. Here, we report the results of ultra-high-resolution simulations of the formation and evolution of these microhalos. We found that microhalos have the central density cusps of the form $\rho \propto r^{-1.5}$, much steeper than the cusps of larger dark halos. The central regions of these microhalos survive the encounters with stars except in very inner region of the galaxy down to the radius of a few hundreds pcs from the galactic center. The annihilation signals from nearest microhalos are observed as gamma-ray point-sources (radius less than 1'), with unusually large proper motions of ~0.2 degree per year. Their surface brightnesses are ~10% of that of the galactic center. Their S/N ratios might be better if they are far from the galactic plane. Luminosities of subhalos are determined only by their mass, and they are more than one order of magnitude luminous than the estimation by Springel et al. (2008): A boost factor can be larger than 1000. Perturbations to the millisecond pulsars by gravitational attractions of nearby earth-mass microhalos can be detected by the observations of Parkes Pulsar Timing Array (PPTA).