Large–Scale Structure in Mixed Dark Matter Models with a Non–thermal Volatile Component

We investigate the properties of large–scale structure predicted in a class of mixed dark matter models in which the volatile component (made of particles with high rms velocity) derives from the decay of a heavier particle. Such models based on cold+volatile dark matter (CVDM) differ from the standard mixture of CDM and massive neutrinos, usually known as CHDM, in that they involve a component which has a non–thermal phase space distribution function. As a consequence, and differently from CHDM models, the value of the redshift at which volatile particles become non relativistic, znr, can be varied almost independently of the volatile fraction, ΩX . We compute transfer functions for a selection of such models, having 0.1 ≤ ΩX ≤ 0.5 and different values of znr. Using linear theory and assuming a scale– free primordial spectrum, we compare such models with observational constraints on large– scale galaxy clustering and bulk flows, as well as on the abundance of galaxy clusters and high–redshift damped Lyα systems. We find that these constraints enable us to discriminate between different ΩX and znr; within the range of the models inspected, those which can be most easily accommodated by the data correspond to the parameter choice ΩX ≃ 0.2 and znr ≃ 2× 104ΩX . Subject headings: Cosmology: large-scale structure of the universe galaxies: clusters: general