Low-density Cosmological Models Research Articles

Flux-limited strong gravitational lensing and dark energy

In the standard flat cosmological constant ($\Lambda$) cold dark matter (CDM) cosmology, a model of two populations of lens halos for strong gravitational lensing can reproduce the results of the Jodrell-Bank VLA Astrometric Survey (JVAS) and the Cosmic Lens All-Sky Survey (CLASS) radio survey. In such a model, lensing probabilities are sensitive to three parameters: the concentration parameter $c_1$, the cooling mass scale $M_\mathrm{c}$ and the value of the CDM power spectrum normalization parameter $\sigma_8$. The value ranges of these parameters are constrained by various observations. However, we found that predicted lensing probabilities are also quite sensitive to the flux density (brightness) ratio $q_{\mathrm{r}}$ of the multiple lensing images, which has been, in fact, a very important selection criterion of a sample in any lensing survey experiments. We re-examine the above mentioned model by considering the flux ratio and galactic central Super Massive Black Holes (SMBHs), in flat, low-density cosmological models with different cosmic equations of state $\omega$, and find that the predicted lensing probabilities without considering $q_{\mathrm{r}}$ are over-estimated. A low value of $q_\mathrm{r}$ can be compensated by raising the cooling mass scale $M_\mathrm{c}$ in fitting the predicted lensing probabilities to JVAS/CLASS observations. In order to determine the cosmic equation of state $\omega$, the uncertainty in $M_\mathrm{c}$ must be resolved. The effects of SMBHs cannot be detected by strong gravitational lensing method when $q_{\mathrm{r}}\leq 10$.

Strong Gravitational Lensing and Dark Energy

We investigate the statistics of gravitational lenses in flat, low-density cosmological models with different cosmic equations of state ω. We compute the lensing probabilities as a function of image separation θ using a lens population described by the mass function of Jenkins et al. and modeled as singular isothermal spheres on galactic scales and as Navarro, Frenk, and White halos on cluster scales. It is found that COBE-normalized models with ω > -0.4 produce too few arcsecond-scale lenses in comparison with the Jodrell-VLA Astrometric Survey (JVAS) and the Cosmic Lens All-Sky Survey (CLASS), a result that is consistent with other observational constraints on ω. The wide-separation (θ ≳ 4'') lensing rate is a particularly sensitive probe of both ω and the halo mass concentration. The absence of these systems in the current JVAS/CLASS data excludes highly concentrated halos in ω ≲ -0.7 models. The constraints can be improved by ongoing and future lensing surveys of greater than 105 sources.

Hydrogen 21-centimeter emission from a galaxy at cosmological distance.

We have detected the neutral atomic hydrogen (HI) emission line at a cosmologically significant distance [redshift (z) = 0.18] in the rich galaxy cluster Abell 2218 with the Westerbork Synthesis Radio Telescope. The HI emission originates in a spiral galaxy 2.0 h65(-1) megaparsecs from the cluster core. No other significant detections have been made in the cluster, suggesting that the mechanisms that remove neutral gas from cluster galaxies are efficient. We infer that fewer than three gas-rich galaxies were accreted by Abell 2218 over the past 10(9) years. This low accretion rate is qualitatively consistent with low-density cosmological models in which clusters are largely assembled at z > 1.

Large-scale galaxy distribution in the Las Campanas Redshift Survey

We make use of three-dimensional clustering analysis, inertia tensor methods, and the minimal spanning tree technique to estimate some physical and statistical characteristics of the large-scale galaxy distribution and, in particular, of the sample of overdense regions seen in the Las Campanas Redshift Survey (LCRS). Our investigation provides additional evidence for a network of structures found in our core sampling analysis of the LCRS: a system of rich sheet-like structures, which in turn surround large underdense regions criss-crossed by a variety of filamentary structures. We find that the overdense regions contain ∼40–50 per cent of LCRS galaxies and have proper sizes similar to those of nearby superclusters. The formation of such structures can be roughly described as a non-linear compression of protowalls of typical cross-sectional size ∼20–25 h−1 Mpc; this scale is ∼5 times the conventional value for the onset of non-linear clustering – to wit, r0, the autocorrelation length for galaxies. The comparison with available simulations and theoretical estimates shows that the formation of structure elements with parameters similar to those observed is presently possible only in low-density cosmological models, Ωmh∼0.2–0.3, with a suitable large-scale bias between galaxies and dark matter.

Quasar Clustering and the Lifetime of Quasars

Although the population of luminous quasars rises and falls over a period of 10^9 years, the typical lifetime of individual quasars is uncertain by several orders of magnitude. We show that quasar clustering measurements can substantially narrow the range of possible lifetimes with the assumption that luminous quasars reside in the most massive host halos. If quasars are long-lived, then they are rare phenomena that are highly biased with respect to the underlying dark matter, while if they are short-lived they reside in more typical halos that are less strongly clustered. For a given quasar lifetime, we calculate the minimum host halo mass by matching the observed space density of quasars, using the Press-Schechter approximation. We use the results of Mo & White to calculate the clustering of these halos, and hence of the quasars they contain, as a function of quasar lifetime. A lifetime of t_Q = 4 x 10^7 years, the e-folding timescale of an Eddington luminosity black hole with accretion efficiency eps=0.1, corresponds to a quasar correlation length r_0 ~ 10 Mpc/h in low-density cosmological models at z=2-3; this value is consistent with current clustering measurements, but these have large uncertainties. High-precision clustering measurements from the 2dF and Sloan quasar surveys will test our key assumption of a tight correlation between quasar luminosity and host halo mass, and if this assumption holds then they should determine t_Q to a factor of three or better. An accurate determination of the quasar lifetime will show whether supermassive black holes acquire most of their mass during high-luminosity accretion, and it will show whether the black holes in the nuclei of typical nearby galaxies were once the central engines of high-luminosity quasars.

Secondary Cosmic Microwave Background Anisotropies from Cosmological Reionization

We use numerical simulations of cosmological reionization to calculate the secondary CMB anisotropies in a representative flat low-density cosmological model. We show that the kinetic Sunyaev-Zeldovich effect (scattering off of moving electrons in the ionized intergalactic medium) is dominated by the nonlinear hydrodynamic and gravitational evolution of the density and velocity fields, rather than the detailed distribution of the ionization fraction (patchy reionization) on all angular scales. Combining our results with the recent calculation of secondary CMB anisotropies by Springel et al., we are able to accurately predict the power spectrum of the kinetic SZ effect on almost all angular scales.

The Structure of Dark Matter Halos in Hierarchical Clustering Theories

During hierarchical clustering, smaller masses generally collapse earlier than larger masses and so are denser on the average. The core of a small-mass halo could be dense enough to resist disruption and survive undigested when it is incorporated into a larger object. We explore the possibility that a nested sequence of undigested cores in the center of the halo that have survived the hierarchical, inhomogeneous collapse to form larger and larger objects determines the halo structure in the inner regions. For a flat universe with P(k) ∝ kn, scaling arguments then suggest that the core density profile is ρ ∝ r-α, with α = (9 + 3n)/(5 + n). For any n < 1, the signature of undigested cores is a core density profile shallower than ρ ∝ 1/r2 and dependent on the power spectrum. For typical objects formed from a cold dark matter (CDM)-like power spectrum, the effective value of n is close to -2, and thus α could typically be near 1, the Navarro, Frenk, & White (NFW) value. Velocity dispersions should also decrease with decreasing radius within the core. However, whether such behavior holds depends on detailed dynamics. We first examine the dynamics using a fluid approach to the self-similar collapse solutions for the dark matter phase-space density, including the effect of velocity dispersions. We highlight the importance of tangential velocity dispersions to obtain density profiles shallower than 1/r2 in the core regions. If tangential velocity dispersions in the core are constrained to be less than the radial dispersion, a cuspy core density profile shallower than 1/r cannot hold in self-similar collapse. We then look at the profiles of the outer halos in low-density cosmological models in which the total halo mass is convergent. We find a limiting r-4 outer profile for the open case and a limiting outer profile for the Λ-dominated case, which approaches the form [1 - (r/λ)-3]1/2, where 3 is the logarithmic slope of the initial density profile. Finally, we analyze a suite of dark halo density and velocity dispersion profiles obtained in cosmological N-body simulations of models with n = 0, -1, and -2. The core-density profiles show considerable scatter in their properties, but nevertheless do appear to reflect a memory of the initial power spectrum, with steeper initial spectra producing flatter core profiles. These results apply as well for low-density cosmological models (Ωmatter = 0.2-0.3), since high-density cores were formed early, where Ωmatter ≈ 1.

The Primeval Mass Fluctuation Spectrum and the Distribution of the Nearby Galaxies

The initial peculiar velocities of tracers of the extragalactic mass distribution computed by a numerical application of the action principle offer a measure of the primeval mass fluctuation power spectrum P(k). Presented here is a method of estimating the spectrum from the fit of the velocity correlation function expressed as an integral over P(k) in linear perturbation theory to the products of pairs of initial velocities of the mass tracers. The method is applied to numerical action solutions for the motions of the galaxies in and near the Local Group. The distributions of products of pairs of the initial velocities are in satisfactory agreement with the predictions from the correlation function. The range of initial separations in the sample is too small to constrain the shape of P(k), but the value is reasonably well determined at wavenumber km ~ 0.4 Mpc–1 in a low-density cosmological model, or km Mpc~ 0.75 Mpc–1 in an Einstein-de Sitter model. In a low-density open or cosmologically flat model, the value of P(km) extrapolated to the present in linear perturbation theory agrees with estimates from the galaxy distribution under the assumption that galaxies trace mass. This adds to the evidence that galaxies are useful mass tracers. In the low-density models, the galaxy distribution near the Local Group grew out of a primeval mass fluctuation δM/M ~ 1 extrapolated to the present in linear theory.

The isotropization of low-density cosmological models

We analyse the evolution of Bianchi type-VII h cosmological models during the lepton era and the subsequent adiabatic epochs. We find that these model cannot generally attain to the low-density regime with a reasonable anisotropy level because of a curvature—anisotropy coupling. The primordial damping due to neutrino viscosity, although very effective for the entropy production, cannot help to match to isotropy requirement at the present epoch.

The Peculiar Velocity Field in the Local Supercluster

view Abstract Citations (188) References (23) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Peculiar Velocity Field in the Local Supercluster Peebles, P. J. E. Abstract A general method of estimating the peculiar velocity field expected, under the gravitational instability picture, around the outer parts of a mass concentration like a cluster of galaxies is derived and applied to a preliminary analysis of the Sandage-Tammann data on the distances and redshifts of nearby spiral galaxies. It is shown that there is evidence of a small peculiar velocity field of the sort expected. The field, if real, is consistent with a cosmologically flat universe, = 1; equally well, with a low-density cosmological model, 0.1. It is concluded that the data on the local peculiar velocity field do not yet offer a strong constraint on the density parameter . Subject headings: cosmology - galaxies: clusters of Publication: The Astrophysical Journal Pub Date: April 1976 DOI: 10.1086/154280 Bibcode: 1976ApJ...205..318P full text sources ADS | data products SIMBAD (3)