arXiv subjects

Anatoly Klypin

Publications and source records attributed to Anatoly Klypin.

116 records · Page 7Linked to original sources

arXiv1995

Filament Statistics: A Quantitative Comparison of Cold + Hot and Cold Dark Matter Cosmologies with CfA1 Data

A new class of geometric statistics for analyzing galaxy catalogs is presented. {\it Filament statistics} quantify filamentarity and planarity in large scale structure in a manner consistent with catalog visualizations. These statistics are based on sequences of spatial links which follow local high-density structures. From these link sequences we compute the discrete curvature, planarity, and torsion. Filament statistics are applied to CDM and CHDM ($Ω_ν= 0.3$) simulations of Klypin \etal (1995), the CfA1-like sky catalogs of Nolthenius, Klypin and Primack (1994, 1995), and the CfA1 catalog. For 100 Mpc periodic simulation boxes ($H_0 = 50$ km s$^{-1}$ Mpc$^{-1}$), we find robust discrimination of over 4$σ$ (where $σ$ represents resampling errors) between CHDM and CDM. The {\it reduced filament statistics} show that CfA1 data is intermediate between CHDM and CDM, but more consistent with the CHDM models. Filament statistics provide robust and discriminatory shape statistics with which to test cosmological simulations of various models against present and future redshift survey data.

astro-ph↗

arXiv1994

Cold + Hot Dark Matter Cosmology with $m(ν_μ) \approx m(ν_τ) \approx 2.4$ ev

Cold + Hot Dark Matter (CHDM) $Ω=1$ cosmological models require a total neutrino mass $\sim 5$ eV. Because recent data support the $ν_μ\to ν_τ$ oscillation explanation of the cosmic ray $ν_μ$ deficit, which requires that $m(ν_μ) \approx m(ν_τ)$, this suggests that $m(ν_μ) \approx m(ν_τ) \approx 2.4$ eV. The linear calculations and N-body simulation reported here indicate that an $Ω=1$ CHDM model with two 2.4 eV neutrinos (designated C$ν^2$DM) agrees remarkably well with all available observations, but only if the Hubble parameter $h \approx 0.5$. We also show that even one 2.4 eV neutrino raises serious difficulties for low-$Ω$ flat CDM models.

astro-ph↗

arXiv1994

Damped Lyman Alpha Systems vs. Cold + Hot Dark Matter

Damped Ly$α$ systems provide possibly the most significant evidence for early structure formation, and thus a stringent constraint on the Cold + Hot Dark Matter (CHDM) cosmology. Using the numbers of halos in N-body simulations to normalize Press-Schechter (PS) estimates of the number densities of protogalaxies as a function of redshift, we find that CHDM with $Ω_c/Ω_ν/Ω_b = 0.6/0.3/0.1$ is compatible with the damped Ly$α$ data only at $z<2.5$, but that it is probably incompatible with data at $z>3$. The predictions of CHDM are quite sensitive to the neutrino fraction. We find that $Ω_c/Ω_ν/Ω_b = 0.725/0.20/0.075$ is compatible with the $z>3$ data. With one massive neutrino species, this corresponds to lowering the neutrino mass from 7.0 to 4.7 eV, for $H_0=50\kmsMpc$ and $T=2.726$ K. By analysing our numerical simulations with different resolutions and box sizes as well as those of Ma \& Bertchinger (1994), we show that for the CHDM models with $Ω_ν$=0.2--0.3 the PS approximation should be used with Gaussian filter with $δ_c=1.3-1.4$ if one tries to recover the total mass of a collapsed halo and to include nonlinear effects, due to waves both longer and shorter than those within the simulation box.

astro-ph↗

arXiv1994

The Evolution of X-Ray Clusters in a Cold Plus Hot Dark Matter Universe

We present the first self-consistently computed results on the evolution of X-ray properties of galaxy clusters in a Cold + Hot Dark Matter (CHDM) model. We have performed a hydrodynamic plus N-body simulation for the COBE-compatible CHDM model with standard mass components: Omega(hot) = 0.3, Omega(cold) = 0.6 and Omega(baryon) = 0.1 (h = 0.5). In contrast with the CDM model, which fails to reproduce the observed temperature distribution function dN/dT (Bryan et al. 1994b), the CHDM model fits the observational dN/dT quite well. Our results on X-ray luminosity are less firm but even more intriguing. We find that the resulting X-ray luminosity functions at redshifts z = 0.0, 0.2, 0.4, 0.7 are well fit by observations, where they overlap. The fact that both temperatures and luminosities provide a reasonable fit to the available observational data indicates that, unless we are missing some essential physics, there is neither room nor need for a large fraction of gas in rich clusters: 10% (or less) in baryons is sufficient to explain their X-ray properties. We also see a tight correlation between X-ray luminosity and gas temperature.

astro-ph↗

arXiv1994

Damped Lyman Alpha Systems vs. Cold + Hot Dark Matter

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, CHDM produces structure at relatively low redshifts and thus could be ruled out if there were evidence for formation of massive objects at high redshifts. Damped Ly$α$ systems are abundant in quasar absorption spectra and thus provide possibly the most significant evidence for early structure formation, and thus perhaps the most 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 find that CHDM with $Ω_c/Ω_ν/Ω_b = 0.6/0.3/0.1$ is compatible with the damped Ly$α$ data at $\le 2.5$, but that it is probably incompatible with the limited $z>3$ damped Ly$α$ data. The situation is uncertain because there is very little data for $z>3$, and also it is unclear whether all damped Ly$α$ systems are associated with collapsed protogalaxies. The predictions of CHDM are quite sensitive to the hot (neutrino) fraction, and we find that $Ω_c/Ω_ν/Ω_b = 0.675/0.25/0.075$ is compatible even with the $z>3$ data. This corresponds to lowering the neutrino mass from 6.8 to 5.7 eV, for $H_0=50\kmsMpc$. In CHDM, the higher redshift damped Ly$α$ systems are predicted to have lower masses, which can be checked by measuring the velocity widths of the associated metal line systems.

astro-ph↗

arXiv1993

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

This letter presents results of new high resolution $Ω=1$ 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 $v_{\rmgr}(0.45) \approx 125 \pm 25 \kms$ in agreement with the CfA data, while CDM with bias b=1.0 (COBE-compatible) or b=1.5, both giving $v_{\rm gr}(0.45) \approx 400 \pm 25 \kms$, can be virtually ruled out. Using median $M/L$, the observed value of $Ω$ is $0.10$ (CHDM) to $0.38$ (CDM).

astro-ph↗

arXiv1993

The Zero-Point of the Cluster-Cluster Correlation Function: A Key Test of Cosmological Power Spectra

We propose the zero-point of the cluster-cluster correlation function as a sensitive test for the shape of the power spectrum of initial fluctuations. It is now possible to go beyond the power law description to measure the point at which the correlation function becomes zero. Four independent measurements indicate that the zero point, $r_0$, should be in the range (40-60)$h^{-1}$Mpc. The large value of $r_0$ at which the zero-point occurs rules out conventional CDM models independently of the assumed amplitude. Models with $Ω< 0.25$ should be rejected because they predict too large $r_0$. If the age of the Universe is assumed to be larger than 15 Gyr, models with either $Ω<0.5$ or $h> 0.55$ are rejected. We present the results of numerical simulations of clusters in an $Ω=1$ cosmological model with a mixture of cold plus hot dark matter (CHDM). The correlation function we determined for the simulated clusters has a zero-point, $r_0=55h^{-1}$Mpc that accurately matches the zero point of the observed function.

astro-ph↗

arXiv1993

Structure Formation with Cold + Hot Dark Matter

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 $Ω_{\rm cold}=0.6$, $Ω_ν=0.3$, and $Ω_{\rm baryon}=0.1$. We present analytic fits to the C+HDM power spectra for both cold and hot ($ν$) 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.

astro-ph↗