The epoch of structure formation in blue mixed dark matter models

Recent data on the high--redshift abundance of damped Ly$α$ systems are compared with theoretical predictions for `blue' (i.e. $n>1$) Mixed Dark Matter models. The results show that decreasing the hot component fraction $Ω_ν$ and/or increasing the primordial spectral index $n$ implies an earlier epoch of cosmic structure formation. However, we also show that varying $Ω_ν$ and $n$ in these directions makes the models barely consistent with the observed abundance of galaxy clusters. Therefore, requiring at the same time observational constraints on damped Ly$α$ systems and cluster abundance to be satisfied represents a challenge for the Mixed Dark Matter class of models.

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