DO GALACTIC SYSTEMS FORM TOO LATE IN COLD PLUS HOT DARK-MATTER MODELS

The abundance of galactic systems at high redshifts can impose a stron g constraint on the cold + dark matter (CDM + HDM) models. The hot com ponent reduces the excessive small-scale power in the COBE-normalized CDM model but also delays the epoch of galaxy formation. We present re sults from the first numerical simulations that have enough dynamic ra nge to address accurately the issue of high-redshift halo abundances i n CDM + HDM models. Equivalent high-resolution particle-particle/parti cle-mesh N-body simulations are performed for spatially flat models wi th Omega(v) = 0.3 and 0.2 (with H-O = 50 km s(-1) Mpc(-1) and Omega(b) = 0.05). We study the constraints placed on the models by the high-re dshift quasar space density and by the mass fraction in neutral dense gas associated with damped Ly alpha systems. We find that even with op timistic assumptions, the much-studied Omega(v) = 0.3 model does not p roduce enough massive halos to account for the observed abundance of q uasars at z > 4. The model passes this test if Omega(v) is decreased t o 0.2. Both models do not produce enough high column density halos to account for the amount of gas in damped Ly alpha systems at z greater than or similar to 3: the Omega(v) = 0.3 model falls short by a factor of similar to 80; the Omega(v) = 0.2 model by a factor similar to 3. We conclude that only CDM + HDM models with Omega(v) less than or simi lar to 0.2 can match observations at high redshift, implying an upper bound of 4.7 eV on the most massive light neutrino (presumably the tau ).