Another Look at Gaussian Isocurvature Hot Dark Matter Models For Large- Scale Structure

We examine Gaussian isocurvature hot dark matter (massive neutrino) models for large-scale structure in which the initial density perturbations are produced in the baryons with a power--law spectrum $P_B(k) = Ak^{n_B}$. We calculate the linearly-evolved power spectrum and cosmic microwave fluctuations. We find that models with only isocurvature perturbations are inconsistent with observations of damped Ly$\alpha$ systems and COBE constraints on the power index. However, models which contain a mixture of adiabatic and isocurvature perturbations can be made consistent with COBE, galaxy surveys and damped Ly$\alpha$ systems.Isocurvature hot dark matter models also produce a bias between baryons and neutrinos even in the linear regime. We find that this ``natural bias'' can increase the baryon fraction in small scale objects like damped Ly$\alpha$ systems, but it has no effect on cluster scales.