The smallest dark matter haloes are the first objects to form in the hierarchical structure formation of cold dark matter (CDM) cosmology and are expected to be the densest and most fundamental building blocks of CDM structures in our universe. Nevertheless, the physical characteristics of these haloes have stayed illusive, as they remain well beyond the current resolution of N-body simulations (at redshift zero). However, they dominate the predictions (and uncertainty) in expected dark matter annihilation signal, amongst other astrophysical observables. Using the conservation of total energy and the ellipsoidal collapse framework, we can analytically find the mean and scatter of concentration c and 1-D velocity dispersion σ 1d for haloes of different virial mass M 200 . Both c and σ 1d /M 1/3 200 are in good agreement with numerical results within the regime probed by simulations -slowly decreasing functions of mass that approach constant values at large masses. In particular, the predictions for the 1-D velocity dispersion of cluster mass haloes are surprisingly robust as the inverse heat capacity of cosmological haloes crosses zero at M 200 ∼ 10 14 M . However, we find that current extrapolations from simulations to smallest CDM haloes dramatically depend on the assumed profile (e.g. NFW vs. Einasto) and fitting function, which is why theoretical considerations, such as the one presented here, can significantly constrain the range of feasible predictions.

Standard big bang cosmology predicts a cosmic neutrino background at T ν 1.95 K. Given the current neutrino oscillation measurements, we know most neutrinos move at large, but non-relativistic, velocities. Therefore, dark matter haloes moving in the sea of primordial neutrinos form a neutrino wake behind them, which would slow them down, due to the effect of dynamical friction. In this paper, we quantify this effect for realistic haloes, in the context of the halo model of structure formation, and show that it scales as m 4 ν × relative velocity, and monotonically grows with the halo mass. Galaxy redshift surveys can be sensitive to this effect (at > 3σ confidence level, depending on survey properties, neutrino mass and hierarchy) through redshift space distortions (RSD) of distinct galaxy populations.

Individual extragalactic dark matter halos, such as those associated with nearby galaxies and galaxy clusters, are promising targets for searches for gamma rays from dark matter annihilation. We review the predictions for the annihilation flux from individual halos, focusing on the effect of current uncertainties in the concentration-mass relation and the contribution from halo substructure, and also estimating the intrinsic halo-to-halo scatter expected. After careful consideration of recent simulation results, we conclude that the concentrations of the smallest halos, while well-determined at high redshift, are still uncertain by a factor of 4-6 when extrapolated to low redshift. This in turn produces up to two orders of magnitude uncertainty in the predicted annihilation flux for any halo mass above this scale. Substructure evolution, the small-scale cutoff to the power spectrum, cosmology, and baryonic effects all introduce smaller, though cumulative, uncertainties. We then consider intrinsic variations from halo to halo. These arise from variations in concentration and substructure, leading to a scatter of ∼2.5 in the predicted annihilation luminosity. Finally, we consider the problem of detecting gamma rays from annihilation, given the expected contributions from other sources. We estimate the signal-to-noise ratio for gamma-ray detection as a function of halo mass, assuming that the isotropic gamma-ray background and cosmic rays from star formation are the main noise sources in the detection. This calculation suggests that groupscale halos, individually or in stacks, may be a particularly interesting target for the next generation of annihilation searches.