Are Simulations of CDM Consistent with Galactic-Scale Observations at High Redshift?

We compare new observations on the kinematic characteristics of the damped Lya systems against results from numerical SPH simulations to test the predictions of hierarchical galaxy formation. This exercise is particularly motivated by recent numerical results on the cross-section of damped Lya systems. Our analysis focuses on the velocity widths of ~50 low-ion absorption profiles from our sample of z>1.5 damped Lya systems. The results indicate that current numerical simulations fail to match the damped Lya observations at high confidence levels (>99.9%). Although we do not believe that our results present an insurmountable challenge to the paradigm of hierarchical cosmology, the damped Lya observations suggest that current numerical SPH simulations overlook an integral aspect of galaxy formation.