Do Dark Halos Form Too Late in a Cold+Hot Dark Matter Universe?
Session 115 -- Cosmology and Dark Matter
Oral presentation, Saturday, January 15, 10:15-11:45, Salon III Room (Crystal Gateway)

## [115.01] Do Dark Halos Form Too Late in a Cold+Hot Dark Matter Universe?

Chung-Pei Ma (Caltech), Edmund Bertschinger (MIT)

Replacing some of the cold dark matter (CDM) with massive neutrinos appears to be a viable way to alleviate some of the structure-formation problems found in CDM models. We investigate a cold+hot dark matter (CDM+HDM) model with $\Omega_{\rm cdm}=0.65$, $\Omega_{\rm hdm}=0.30$, $\Omega_{\rm baryon}=0.05$, $H_0=50$ km s$^{-1}$ Mpc$^{-1}$, and an isentropic, scale-invariant ($n=1$) primordial power spectrum normalized to COBE. We report results from a high-resolution P$^3$M $N$-body simulation with $\sim$ 2.1 million cold and $\sim$ 21 million hot particles. The simulation is performed in a 100 Mpc comoving cube with a 50 kpc comoving Plummer force-softening distance starting at redshift $z=13.55$.

Special care is taken to obtain an accurate sampling of the neutrino phase space for the HDM initial conditions. We have done this by integrating the trajectories of individual HDM particles using linearized general relativity including all relevant particle species (CDM, photons, baryons, massless neutrinoss, and massive neutrinos) from $z = 10^9$ (shortly after decoupling, when we sample the Fermi-Dirac distribution) to $z=13.55$. The nonlinear Newtonian integration begins then and proceeds to $z=0$. At several epochs we identify dark matter halos using a density-maximum (DENMAX) algorithm that finds all particles within closed contours of the smoothed density field surrounding a peak.

We examine whether the CDM+HDM model with $\Omega_{\rm hdm}=0.3$ leads to the formation of too few high-redshift objects due to the suppression of small-scale power by the massive neutrinos. In our simulation volume at $z=2.3$ we find that only 2 galactic-scale ($<100$ kpc) objects have collapsed to mean overdensity exceeding 200 with mass exceeding $10^{12}$ $M_\odot$; at $z=4$ there are none. Unless the efficiency of quasar formation is greater than previously estimated, this model cannot account for the abundance of high-redshift objects. However, small-scale power can be increased by decreasing $\Omega_{\rm hdm}$. It remains unclear whether, by adjusting this parameter, we may arrive at a model with sufficient power on large scales and sufficiently early galaxy formation.