Showing papers by "Joel R. Primack published in 1993"

Structure Formation with Cold + Hot Dark Matter

Abstract: We report results from high-resolution particle-mesh (PM) N-body simulations of structure formation in an $\Omega=1$ cosmological model with a mixture of Cold plus Hot Dark Matter (C+HDM) having $\Omega_{\rm cold}=0.6$, $\Omega_ u=0.3$, and $\Omega_{\rm baryon}=0.1$. We present analytic fits to the C+HDM power spectra for both cold and hot ($ u$) components, which provide initial conditions for our nonlinear simulations. In order to sample the neutrino velocities adequately, these simulations included six times as many neutrino particles as cold particles. Our simulation boxes were 14, 50, and 200~Mpc cubes (with $H_0=50$ km s$^{-1}$ Mpc$^{-1}$); we also did comparison simulations for Cold Dark Matter (CDM) in a 50~Mpc box. C+HDM with linear bias factor $b=1.5$ is consistent both with the COBE data and with the galaxy correlations we calculate. We find the number of halos as a function of mass and redshift in our simulations; our results for both CDM and C+HDM are well fit by a Press-Schechter model. The number density of galaxy-mass halos is smaller than for CDM, especially at redshift $z>2$, but the numbers of cluster-mass halos are comparable. We also find that on galaxy scales the neutrino velocities and flatter power spectrum in C+HDM result in galaxy pairwise velocities that are in good agreement with the data, and about 30\% smaller than in CDM with the same biasing factor. On scales of several tens of Mpc, the C+HDM streaming velocities are considerably larger than CDM. Thus C+HDM looks promising as a model of structure formation.

Structure formation with cold plus hot dark matter

Abstract: We report results from high-resolution particle-mesh (PM) N-body simulations of structure formation in an Ω=1 cosmological model with a mixture of cold plus hot dark matter (C+HDM) having Ω cold =0.6, Ω ν =0.3, and Ω baryon =0.1. We present analytic fits to the C+HDM power spectra for both cold and hot (neutrino) components, which provide initial conditions for our nonlinear simulations. In order to sample the neutrino velocities adequately, these simulations included 6 times as many neutrino particles as cold particles. Our simulations boxes were 14, 50, and 200 Mpc cubes (with H 0 =50 km s −1 Mpc −1 ); we also did comparison simulations for cold dark matter (CDM) in a 50 Mpc box

Rotation curves from baryonic infall : dependence on disk-to-halo ratio, initial angular momentum, and core radius, and comparison with data

Abstract: Using a simple analytic model of the response of dark matter halos to the dissipative infall of the luminous material to form an exponential disk, we explore the dependence of the final rotation curves on all the relevant parameters: the ratio F≡M b /M of the dissipative baryonic mass M b to the total galaxy mass M including dark matter; the ratio b/R of the disk exponential scale length b to the truncation radius R (beyond which infall can be neglected); the core radius r core of the isothermal halo in the absence of dissipation; and the dimensionless angular momentum parameter λ≡J|E| 1/2 G −1 M −5/2 (where J and E are the total angular momentum and energy of the galaxy)

Cluster Correlations for Cold + Hot Dark Matter and Other Models

Abstract: Calculations of the autocorrelation function of rich clusters of galaxies are presented for several nonstandard variants of cold dark matter (CDM) that have an enhanced power spectrum on large scales compared to standard Ω=1 CDM. The models considered are (1) cold+hot dark matter (C+HDM) models with Ω CDM +Ω v +Ω b =1 (where Ω v , the fraction of the critical density in one or three species of light neutrinos, is comparable to Ω CDM ), (2) Ω CDM +Ω b =0.2 or 1 (where Ω b , the fraction of critical density in baryonic matter, is comparable to Ω CDM ), and (3) CDM with a non-Zel'dovich fluctuation spectrum that arises in a particular inflationary model

Galaxy Groups in Cold + Hot and CDM Universes: Comparison with CfA

Abstract: This letter presents results of new high resolution $\Omega=1$ Cold + Hot Dark Matter (CHDM) and Cold Dark Matter (CDM) simulations Properties of groups in these simulations reflect the lower small-scale velocities and the greater tendency to form distinct filaments on both small and large scales in CHDM as compared to CDM The fraction of galaxies in groups and the median group rms velocity are found to be powerful discriminators between models We combine these two features into a very robust statistic, median group rms velocity $v_{\rm gr}(f_{\rm gr})$ as a function of the fraction $f_{\rm gr}$ of galaxies in groups Using this statistic, we compare ``observed'' simulations to CfA data in redshift space in a careful and consistent way We find that CHDM remains a promising model, with for example $v_{\rmgr}(045) \approx 125 \pm 25 \kms$ in agreement with the CfA data, while CDM with bias b=10 (COBE-compatible) or b=15, both giving $v_{\rm gr}(045) \approx 400 \pm 25 \kms$, can be virtually ruled out Using median $M/L$, the observed value of $\Omega$ is $010$ (CHDM) to $038$ (CDM)

Significant cluster correlations at 30 h-1 Mpc : can standard models be ruled out ?

Abstract: We compute the cluster autocorrelation function, ξ c (r), of an extended, complete sample of 220 rich (R≥1) Abell clusters with measured redshifts. We correct for projection contamination and pay special attention to estimating the random errors, aiming at rejecting certain theoretical models. While the observed ξ c in the range r<20 h −1 Mpc is in agreement with the predictions of CDM and textures models, we find an excess over both theories in the range 20 h −1 Mpc≤r≤40 h −1 Mpc, significant at beyond the 3 σ level

Cosmology after COBE—Review for particle physicists

Abstract: I summarize the following current topics in cosmology: (1) The near-success of Cold Dark Matter (CDM) in predicting the COBE fluctuation amplitude, which favors the hypothesis that structure formed in the universe through gravitational collapse. (2) The indications that ω ≈ 1 and that the power spectrum has a little more power on supercluster and larger scales than CDM. These are suggested by the IRAS and CfA redshift surveys and POTENT galaxy peculiar velocity analysis, and also by the COBE data. (3) The consequent demise of CDM and the rise of hybrid schemes such as Cold+Hot Dark Matter (C+HDM). (4) The possible implications for neutrino masses and mixings, and for cosmology, of the recent results on solar neutrinos. (5) CERN experiments onv μ -v r oscillations, which may be sufficiently sensitive to detect thev r if its mass lies in the cosmologically interesting mass range 1–102 eV. (6) Dark matter searches, including the searches for WIMPs and axions, and the French, Polish, and Berkeley-Livermore-Mt. Stromlo MACHO searchs.