Large-Scale Structure in Mixed Dark Matter Models with a Nonthermal Volatile Component

Abstract

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, z nr , can be varied almost independently of the volatile fraction, Ω X . We compute transfer functions for a selection of such models, having 0.1 ≤ Ω X ≤ 0.5 and different values of z nr . Using linear theory and assuming a scalefree primordial spectrum, we compare such models with observational constraints on largescale 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 Ω X and z nr ; within the range of the models inspected, those which can be most easily accommodated by the data correspond to the parameter choice Ω X ≃ 0.2 and z nr ≃ 2 × 10 4 Ω X .Subject headings: Cosmology: large-scale structure of the universe -galaxies: clusters: general 1

Light gravitinos as mixed dark matter

“…Therefore, as far as the cosmological implications are concerned, this model behaves like the one based on gravitinos, at least on mass scales larger than that of a galaxy. As a main result of their analysis, Pierpaoli et al [12] pointed out that the simultaneous request of reproducing the observed abundance of galaxy clusters [9] and of high-redshift (Z ≃ 4) damped Ly-α systems (DLAS) [20] requires Ω non-th ≃ 0.2 and Z nr∼ < 10 4 . As for the cluster abundance, since it is determined by the fluctuation amplitude on scales ∼ 10 h −1 Mpc, we expect similar predictions when replacing the cold component with the warm one.…”

“…However, it should be kept in mind that the cold+hot DM (CHDM) scenario is only one of the possibilities to implement the MDM idea. Another option that has been thoroughly investigated recently [12] is that the hot thermal component is replaced by a volatile component made of particles with high rms velocity, which derive from the decay of a heavier particle. Such models based on cold+volatile DM (CVDM) differ from the more conventional CHDM schemes in that they involve a component which has a nonthermal phase space distribution function.…”

CMB and large-scale structure as a test of mixed models with n > 1

“…Let us now compare these features with those of a volatile model, where the hot component originates in the decay of heavier particles X (mass m X ), decoupled since some early time t dg (see also Pierpaoli & Bonometto 1995 and Pierpaoli et al 1996). If the temperature T dg ≪ m X , such a hot component may have a number density much smaller than the massive neutrinos.…”

Mixed models with n > 1 and large-scale structure constraints

“…Particle models leading to such hot DM component were discussed in several papers (e.g., Bonometto, Gabbiani & Masiero 1994, Ghizzardi & Bonometto 1996 and the evolution of density fluctuations in such context was discussed by Pierpaoli & Bonometto (1995) and Pierpaoli et al (1996). Hereafter we shall distinguish these models from models with phase-space particle distribution of thermal origin, calling volatile their non-cold component.…”

“…A particle model allowing a continuous selection of [Ω h , d] pairs is obtained if heavier particles (X) decay into lighter massive particles (v), whose phasespace distribution turns out to be quite different from usual massive ν's. Particle models leading to such hot DM component were discussed in several papers (e.g., Bonometto, Gabbiani & Masiero 1994, Ghizzardi & Bonometto 1996 and the evolution of density fluctuations in such context was discussed by Pierpaoli & Bonometto (1995) and Pierpaoli et al (1996). Hereafter we shall distinguish these models from models with phase-space particle distribution of thermal origin, calling volatile their non-cold component.…”