The cold dark matter (CDM) scenario generically predicts the existence of triaxial dark matter halos which contain notable amounts of substructure. However, analytical halo models with smooth, spherically symmetric density profiles are routinely adopted in the modelling of light propagation effects through such objects. In this paper, we report the biases introduced by this procedure by comparing the surface mass densities of actual N-body halos against the widely used analytical model suggested by Navarro, Frenk and White (1996) (NFW). We conduct our analysis in the redshift range of 0.0 − 1.5.

In cluster sized halos, we find that triaxiality can cause scatter in the surface mass density of the halos up to σ+ = +60% and σ− = −70%, where the 1-σ limits are relative to the analytical NFW model given value. Subhalos can increase this scatter to σ+ = +70% and σ− = −80%. In galaxy sized halos, the triaxial scatter can be as high as σ+ = +80% and σ− = −70%, and with subhalos the values can change to σ+ = +40% and σ− = −80%.

We have developed an analytical model for the surface mass density scatter as a function of distance to the halo centre, halo redshift and halo mass. The analytical description enables one to investigate the reliability of results obtained with simplified halo models. Additionally, it provides the means to add simulated surface density scatter to analytical density profiles. We have tested our model on the calculation of microlensing optical depths for MACHOs in CDM halos.