Gamma rays from ultracompact minihalos: Potential constraints on the primordial curvature perturbation

Abstract

Ultracompact minihalos (UCMHs) are dense dark matter structures which can form from large density perturbations shortly after matter-radiation equality. If dark matter is in the form of Weakly Interacting Massive Particles (WIMPs), then UCMHs may be detected via their gamma-ray emission. We investigate how the Fermi satellite could constrain the abundance of UCMHs and place limits on the power spectrum of the primordial curvature perturbation. Detection by Fermi would put a lower limit on the UCMH halo fraction. The smallest detectable halo fraction, fUCMH > ∼ 10 −7 , is for MUCMH ∼ 10 3 M⊙. If gamma-ray emission from UCMHs is not detected, an upper limit can be placed on the halo fraction. The bound is tightest, fUCMH < ∼ 10 −5 , for MUCMH ∼ 10 5 M⊙. The resulting upper limit on the power spectrum of the primordial curvature perturbation in the event of non-detection is in the range PR < ∼ 10 −6.5 − 10 −6 on scales k ∼ 10 1 − 10 6 Mpc −1 . This is substantially tighter than the existing constraints from primordial black hole formation on these scales, however it assumes that dark matter is in the form of WIMPs and UCMHs are not disrupted during the formation of the Milky Way halo.

New constraints on primordial minihalo abundance using cosmic microwave background observations

“…We found that for the mass range 1 GeV-1 TeV, the current abundance of MCHOs which are produced during the e þ e À annihilation phase transition is f MCHOs $ 10 À4 corresponding to the 2 limitation. This is the first constraint using the WMAP data and it is comparable with the results in paper [9], where they used the Fermi gammaray observations.…”

“…N MCHOs is the number density of MCHOs, f MCHOs ¼ MCHOs = critical and this definition is different from [9]. The limitation of the integral is from r cut to R MCHOs .…”

“…where t 0 % 13:7 Gyr [8,9] is the age of the Universe, t i is the time of the MCHOs formation, and we choose t i ðz eq Þ % 77 kyr also used by [9]. Besides the MCHOs, we consider the dark matter halos in this paper.…”

“…And in the last case, they found that the integrated intensity of gamma-ray flux within the 100 pc has exceeded the threshold of EGRET and Fermi-LAT. In [9], using the Fermi gamma-ray observations, the authors investigated the constraints on the current abundance of MCHOs. They found that for the mass of MCHOs $10 3 M , the fraction is $10 À7 .…”

Contribution of ultracompact dark matter minihalos to the isotropic radio background

“…[11][12][13], the authors discussed the influence of the UCMHs on the cosmic microwave background (CMB) due to the dark matter annihilation within them and obtained the constraints on the abundance of UCMHs. The -ray flux from the UCMHs is also studied by several authors [14][15][16]. Besides the ray, the electrons and positrons can also be produced from the UCMHs due to the dark matter annihilation.…”