Damped Lyman Alpha Systems vs . Cold + Hot

This figure "fig2-1.png" is available in "png"" format from: Abstract Although the Cold + Hot Dark Matter (CHDM) cosmology provides perhaps the best t of any model to all the available data at the current epoch (z = 0), CHDM produces structure at relatively low redshifts and thus is very sensitive to the observed numbers of massive objects at high redshifts. Damped Ly systems are abundant in quasar absorption spectra and provide possibly the most signiicant evidence for early structure formation, and thus a stringent constraint on CHDM. Using the numbers of halos in N-body simulations to normalize Press-Schechter estimates of the number densities of protogalaxies as a function of redshift, we nd that CHDM with c = = b = 0:6=0:3=0:1 is compatible with the damped Ly data only at 2:5, but that it is probably incompatible with the z > 3 damped Ly data. The situation is uncertain because there is very little data for z > 3. The predictions of CHDM are quite sensitive to the hot (neutrino) fraction, and we nd that c = = b = 0:725=0:20=0:075 (and possibly even c = = b = 0:675=0:25=0:075) is compatible with the z > 3 data. With one massive neu-trino species, using = 0:20 instead of 0.30 corresponds to lowering the neutrino mass from 7.0 to 4.7 eV, for H 0 = 50 km s 1 Mpc 1 and T = 2:726 K. In CHDM, the higher redshift damped Ly systems are predicted to have lower masses (310 10 M at z = 3), a prediction which can be checked by measuring the velocity widths of the associated metal line systems. Predictions for high-z objects crucially depend on the eeects of limited resolution and the nite box size in N-body simulations or on the parameters of the Press-Schechter approximation, if it is used. By analysing our numerical simulations with vastly diierent 1 resolutions and box sizes as well as those of Ma & Bertchinger (1994), we show that for the CHDM models with =0.2{0.3 the Press-Schechter approximation should be used with Gaussian lter with c = 1:5 if halos are deened with the mean overdensity larger then 200. If one tries to recover the total mass of a collapsed halo, a better value for the collapse parameter is c = 1:40. We argue that nonlinear eeects due to waves both longer and shorter than those considered in …