Halo concentrations and weak-lensing number counts in dark energy cosmologies

We study the effects of a dark energy component with equation of state p = wrho with constant w greater than or equal to -1 on the formation of Cold Dark Matter (CDM) haloes. We find two main effects: first, haloes form earlier as w increases, and second, the amplitude of the dark-matter power spectrum gets reduced in order to remain compatible with the large scale Cosmic Microwave Background (CMB) anisotropies. These effects counteract. Using recipes derived from numerical simulations, we show that haloes are expected to be up to similar to50% more concentrated in CDM models with quintessence compared to LambdaCDM models, the maximum increase being reached for w similar to -0.6. For larger w, the amplitude of the power spectrum decreases rapidly and makes expected halo concentrations decrease. Halo detections through weak gravitational lensing are highly sensitive to halo concentrations. We show that weak-lensing halo counts with the aperture-mass technique increase by a factor of similar to2 as w is increased from -1 to - 0.6, offering a new method for constraining the nature of dark energy.