Damped LymanAlpha Systems versus Cold + Hot Dark Matter
Abstract
Although the cold + hot dark matter (CHDM) cosmology provides perhaps the best fit 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 Lyα systems are abundant in quasar absorption spectra and provide possibly the most significant evidence for early structure formation, and thus a stringent constraint on CHDM. Using the numbers of halos in Nbody simulations to normalize PressSchechter estimates of the number densities of protogalaxies as a function of redshift, we find that CHDM with {OMEGA}_c_/{OMEGA}_{nu}_/{OMEGA}_b_ = 0.6/0.3/0.1 is compatible with the damped Lyα data only at < 2.5, but that it is probably incompatible with the z > 3 damped Lyα 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 find that {OMEGA}_c_/{OMEGA}_{nu}_/{OMEGA}_b_ = 0.725/0.20/0.075 (and possibly even {OMEGA}_c_/{OMEGA}_{nu}_/{OMEGA}_b_ = 0.675/0.25/0.075) is compatible with the z > 3 data. With one massive neutrino species, using {OMEGA}_{nu}_ = 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 Lyα systems are predicted to have lower masses (~3 x 10^10^ M_sun_ at z = 3), a prediction which can be checked by measuring the velocity widths of the associated metalline systems. Predictions for highz objects crucially depend on the effects of limited resolution and the finite box size in Nbody simulations or on the parameters of the PressSchechter approximation, if it is used. By analyzing our numerical simulations with vastly different resolutions and box sizes as well as those of Ma & Bertschinger (1994), we show that for the CHDM models with {OMEGA}_{nu}_ = 0.20.3 the PressSchechter approximation should be used with Gaussian filter with δ_c_ = 1.5 if halos are defined with the mean overdensity larger than 200. If one tries to recover the total mass of a collapsed halo, a better value for the collapse parameter is δ_c_ = 1.40. We argue that nonlinear effects due to waves both longer and shorter than those considered in numerical simulations could probably result in δ_c_ as low as δ_c_ = 1.3.
 Publication:

The Astrophysical Journal
 Pub Date:
 May 1995
 DOI:
 10.1086/175578
 arXiv:
 arXiv:astroph/9405003
 Bibcode:
 1995ApJ...444....1K
 Keywords:

 Computational Astrophysics;
 Cosmology;
 Damping;
 Dark Matter;
 Lyman Alpha Radiation;
 Universe;
 Absorption Spectra;
 Approximation;
 Galactic Evolution;
 Galactic Halos;
 Many Body Problem;
 Neutrinos;
 Quasars;
 Simulation;
 Space Density;
 Astrophysics;
 COSMOLOGY: DARK MATTER;
 COSMOLOGY: LARGESCALE STRUCTURE OF UNIVERSE;
 COSMOLOGY: THEORY;
 GALAXIES: FORMATION;
 GALAXIES: QUASARS: ABSORPTION LINES;
 Astrophysics;
 High Energy Physics  Phenomenology
 EPrint:
 submitted to ApJ 16 pages including 3 figures, uuencoded compressed postscript, SCIPP 94/09