Do Dark Halos Form Too Late in a Cold+Hot Dark Matter Universe?

Details Do Dark Halos Form Too Late in a Cold+Hot Dark Matter Universe? Do Dark Halos Form Too Late in a Cold+Hot Dark Matter Universe?

Dec 1993

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993aas...18311501m&link_type=abstract

American Astronomical Society, 183rd AAS Meeting, #115.01D; Bulletin of the American Astronomical Society, Vol. 25, p.1464

Astronomy and Astrophysics

Astronomy

Scientific paper

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_cdm =0.65, Omega_hdm =0.30, Omega_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 ~ 2.1 million cold and ~ 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_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_sun; 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_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.

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