The epoch of structure formation in blue mixed dark matter models

Recent data on the high-redshift abundance of damped Ly alpha systems are compared with theoretical predictions for 'blue' (i.e. n > 1) mixed dark matter (MDM) models. The results show that decreasing the hot component fraction Omega(v), and/or increasing the primordial spectral index n leads to an earlier epoch of cosmic structure formation. We also show, however, that varying Omega(v) and n in these ways makes the models barely consistent with the observed abundance of galaxy clusters. Therefore, requiring both the observational constraints on damped Ly alpha systems and the cluster abundance to be satisfied simultaneously represents a challenge for the MDM class of models.