Cold Dark Matter-dominated Universe Research Articles

Linear growth of structure in massive gravity

We study background dynamics and the growth of matter perturbations in the extended quasidilaton setup of massive gravity. For the analysis of perturbations, we first choose a scalar field matter component and obtain the conditions under which all scalar perturbations are stable. We work in unitary gauge for the matter field, which allows us to directly map to known results in the limit of general relativity. By performing a parameter search, we find that the perturbations are unstable in general, while a particular choice of potential, where the scalar field effectively behaves like pressureless matter, allows for stable perturbations. We next consider the growth of matter perturbations in a cold dark matter-dominated Universe. Working in conformal Newtonian gauge, we obtain evolution equations for various observables including the growth factor and growth rate, and find scale-independent growth in the quasistatic and subhorizon approximations. We finally show how the Hubble parameter and matter perturbations evolve in massive gravity for a specific choice of parameter values, and how this evolution compares to the standard cosmological model consisting of a cosmological constant and cold dark matter.

The hierarchical origin of galaxy morphologies

We report first results from a series of N-body/gasdynamical simulations designed to study the origin of galaxy morphologies in a cold dark matter-dominated universe. The simulations include star formation and feedback and have numerical resolution sufficiently high to allow for a direct investigation of the morphology of simulated galaxies. We find, in agreement with previous theoretical work, that the presence of the main morphological components of galaxies—disks, spheroids, bars—is regulated by the mode of gas accretion and intimately linked to discrete accretion events. In the case we present, disks arise from the smooth deposition of cooled gas at the center of dark halos, spheroids result from the stirring of preexisting disks during mergers, and bars are triggered by tides generated by satellites. This demonstrates that morphology is a transient phenomenon within the lifetime of a galaxy and that the Hubble sequence reflects the varied accretion histories of galaxies in hierarchical formation scenarios. In particular, we demonstrate directly that disk/bulge systems can be built and rebuilt by the smooth accretion of gas onto the remnant of a major merger and that the present-day remnants of late dissipative mergers between disks are spheroidal stellar systems with structure resembling that of field ellipticals. The perplexing variety of galaxy morphologies is thus highly suggestive of—and may actually even demand—a universe where structures have evolved hierarchically.

A unified model for the evolution of galaxies and quasars

We incorporate a simple scheme for the growth of supermassive black holes into semi-analytic models that follow the formation and evolution of galaxies in a cold dark matter-dominated Universe. We assume that supermassive black holes are formed and fuelled during major mergers. If two galaxies of comparable mass merge, their central black holes coalesce and a few per cent of the gas in the merger remnant is accreted by the new black hole over a time-scale of a few times 107 yr. With these simple assumptions, our model not only fits many aspects of the observed evolution of galaxies, but also reproduces quantitatively the observed relation between bulge luminosity and black hole mass in nearby galaxies, the strong evolution of the quasar population with redshift, and the relation between the luminosities of nearby quasars and those of their host galaxies. The strong decline in the number density of quasars from z∼2 to z=0 is a result of the combination of three effects: (i) a decrease in the merging rate; (ii) a decrease in the amount of cold gas available to fuel black holes, and (iii) an increase in the time-scale for gas accretion. The predicted decline in the total content of cold gas in galaxies is consistent with that inferred from observations of damped Lyα systems. Our results strongly suggest that the evolution of supermassive black holes, quasars and starburst galaxies is inextricably linked to the hierarchical build-up of galaxies.

Evolution of X‐Ray Clusters of Galaxies and Shock Heating of the Intracluster Medium

Evolutions of spherical X-ray clusters of galaxies are studied by using an N-body+total variation diminishing (TVD) mesh code. We consider a growth of density perturbation of 1015 M☉ composed of dark matter and gas in a cold dark matter-dominated universe with Ω0 = 1 or 0.2. When the perturbation collapsed at z ~ 1, a shock front appears at r ~ 0.1 Mpc, moving outward as ambient gas accretes toward cluster center. The shock front separates the inner X-ray-emitting, hot region from the outer cool region. In the former, gas is almost in hydrostatic equilibrium but with small radial infall (~100 km s-1) being left, while in the latter, gas falls almost freely and emits no X-rays. Gas inside the shock is strongly compressed and heated by shock so that X-ray luminosity rapidly rises in the early stage (until temperature reaches about virial). In the late stage, on the other hand, the X-ray luminosity rises only gradually due partly to the expansion of the inner high-temperature region and partly to the increase of X-ray emissivity of gas as the result of continuous adiabatic compression inside the shock. We also find that the density distribution is generally less concentrated in a lower density universe and, hence, X-ray luminosity rises more slowly than in a higher density universe. The shock front structure, which was not clearly resolved in the previous SPH simulations, is clearly captured by the present simulations. Our results confirm that shock heating plays an important role in the heating process of the intracluster medium. In addition, we find that a sound wave propagates outward, thereby producing modulations with amplitudes of ~10% in the radial temperature and density profiles which, in turn, cause time variations in the strength of the shock. Such modulations, if observed, could be used as a probe to investigate the internal structure of clusters and the initial temperature of gas.

Constraints on cosmological models with the decaying cosmological constant from cosmic background radiation anisotropies

The existence of a scalar field is favorable in the early universe. After the epoch of the inflation, the energy density of the scalar field should decrease and might behave as a time-variable cosmological constant. Using recent observations for the isotropy of the cosmic background radiation, constraints are obtained on cosmological models with the time-variable cosmological constant. It is shown that there is a possibility for the cold dark matter-dominated universe models with initially scale-invariant adiabatic perturbations to realize a spatially flat, low-density universe

The morphology-density relation for galaxies in a cold dark matter-dominated universe

A model is explored in which the observed galaxy morphology-density relation is intrinsically related to the initial fluctuations from which galaxies formed in a universe dominated by cold dark matter (CDM). A direct mapping between initial peak height and final galaxy morphology with sharp thresholds dividing the principal Hubble types is assumed. Using a simplified, fully analytic formalism appropriate to Gaussian models, it is shown how a background cluster or group of galaxies may modulate the abundances of different morphological types. The model accounts for the observed power-law relation between the elliptical fraction and the combined fraction of elliptical and lenticular galaxies. This analytic approach is then extended using N-body simulations. The results support the validity of this prescription for differentiating disk from spheroidal galaxies. A power-law segregation between luminous and dark matter densities is predicted by the simulations. 20 refs.

Formation and evolution of X-ray clusters - A hydrodynamic simulation of the intracluster medium

view Abstract Citations (360) References (57) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Formation and Evolution of X-Ray Clusters: A Hydrodynamic Simulation of the Intracluster Medium Evrard, August E. Abstract The X-ray-emitting intracluster medium (ICM) in clusters of galaxies is studied using a combined three-dimensional hydrodynamic and N-body simulation algorithm with the assumption that clusters formed via hierarchical clustering in a cold dark matter-dominated universe with ICM fraction {OMEGA}_ICM_ = 0.1. The ICM is treated as an ideal γ = 5/3 gas undergoing shock heating within the self-consistency evolving dark matter potential. The evolution of a single, Coma-richness cluster is examined in detail. A core of ~ 10^13^ M_sun_ of gas collapses by z = 1 and grows through mergers and accretion of surrounding material. The process can be crudely defined by a shock front moving outward at ~ 400 km s^-1^ reaching a radius ~ 5h^-1^_50_ Mpc at the present epoch. The 2 x 10^14^h^-1^_50_ M_sun_ of gas within an Abell radius at z = 0 is close to isothermal at T = 7.2 keV with evidence for a modest temperature inversion in the cluster center. Polytropic models provide a poor description of the ICM thermodynamic state. The density profile of the cluster shows no resolved core radius, although a flattening of the logarithmic slope is apparent at radii r ~< 500h^-1^_50_ kpc. The total 2-10 keV luminosity of 7.5 x 10^44^h_50_ ergs s^-1^ is therefore subject to an uncertain core contribution. A Sunyaev-Zel'dovich central decrement of roughly 0.5 mK would be expected from this cluster at redshifts z ~< 0.25 at 1' resolution. The signal drops below 0.1 mK at z = 0.5 Investigation of the standard hydrostatic isothermal B-model shows that estimates of the specific energy ratio β = σ^2^/(kT/micron_p_) based on surface brightness profiles are systematically biased. The discrepancy arises from the existence of residual kinetic energy in the shock-heated gas and poor modeling of the underlying binding mass distribution. Simple binding mass estimates based on this model underestimate the actual binding mass within an Abell radius by ~ 30%. The mass contained within an Abell radius for Coma and A2256 is estimated to be 2.4 and 2.6 x 10^15^h^- 1^_50_ M_sun_, respectively, after correcting for these effects. Publication: The Astrophysical Journal Pub Date: November 1990 DOI: 10.1086/169350 Bibcode: 1990ApJ...363..349E Keywords: Cosmology; Galactic Clusters; Galactic Evolution; Interstellar Gas; X Ray Sources; Brightness Distribution; Dark Matter; Hydrodynamics; Many Body Problem; Universe; Astrophysics; GALAXIES: CLUSTERING; GALAXIES: INTERGALACTIC MEDIUM; GALAXIES: X-RAYS; HYDRODYNAMICS full text sources ADS | data products SIMBAD (2)

Evolution of the intergalactic medium in a cold dark matter-dominated universe

view Abstract Citations (25) References (24) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Evolution of the Intergalactic Medium in a Cold Dark Matter--dominated Universe Chiang, Wei-Hwan ; Ryu, Dongsu ; Vishniac, Ethan T. Abstract We study the evolution of the intergalactic medium (IGM) in a cold dark matter-dominated universe. The dark matter is approximated as a pressureless ideal fluid. The IGM is approximated as an ordinary fluid. Several three-dimensional numerical simulations are performed to follow the evolution of the model universe from z ~ 100 to the present time. Galaxies are assumed to form from z = 10 to z = 1 at the highest density peaks. This leads to a burst of galaxy formation. They are subsequently assumed to release energy back into the IGM over a period of 10^8^ yr. The IGM is allowed to cool by Compton and radiative cooling. The model simulates a comoving volume of (9.6h^-1^)^3^ Mpc^3^ in current units on a grid of 32^3^ cells for h = 1 and h = 0.5. The major results are the following: (1) The growth of the root mean square density fluctuations in the IGM is greatly reduced during the period of heating, but it picks up at late times as gravity dominates again. (2) The density contour plots show large connected structure in the IGM. (3) The power spectrum for the IGM on smaller scales grows slower than the power spectrum of the dark matter. The slope of the two-point correlation function for the IGM is similar to that of the observed galaxy-galaxy correlation function between separations 1h^-1^ Mpc < r < 3h^-1^ Mpc. (4) The amount of gas with temperatures below 10^4^ K is negligible, that between 10^4^ and 10^5^ K is ~ 10%-20%, that between 10^5^ and 10^6^ K is >= 40%, and that above 10^6^ K is ~ 40%. The fraction of neutral hydrogen is very small. (5) The variation in pressure is ~10 times that in the temperature, suggesting no pressure equilibrium. Pressure is generally higher in regions of higher densities. (6) Optically thin lines computed show a variety of profiles. (7) The IGM has peculiar velocities of ~150 km s^-1^. The possible relevance of the model results to the forest of the Lyα absorption lines observed in quasars is discussed. Publication: The Astrophysical Journal Pub Date: April 1989 DOI: 10.1086/167321 Bibcode: 1989ApJ...339..603C Keywords: Computational Astrophysics; Dark Matter; Evolution (Development); Galaxies; Intergalactic Media; Universe; Absorption Spectra; Density Distribution; Galactic Evolution; Ideal Fluids; Lyman Alpha Radiation; Power Spectra; Astrophysics; COSMOLOGY; DARK MATTER; GALAXIES: INTERGALACTIC MEDIUM full text sources ADS |

Upper limit on the cosmological baryon density from the cosmic background radiation anisotropies in the cold dark matter-dominated universe

Upper limit on the cosmological baryon density from the cosmic background radiation anisotropies in the cold dark matter-dominated universe

Cold dark matter dominated, inflationary universe with Omega(0) less than 1 and N less than 1

The theoretical prejudice for a flat universe with an initially scale-invariant power spectrum has restricted the number of cosmological scenarios investigated for studying the formation of structure in the universe. A cold dark matter-dominated universe with a density parameter Omega(0) and a primordial spectral index n different from unity is considered, and its possible consistency with the inflationary model is discussed. It is shown that some of the difficulties of a flat cold dark matter scenario can be avoided by having Omega(0) less than 1 and n less than 1. For Omega(0) roughly 0.4 and n roughly 0.75 a good agreement is obtained with the large-scale drifts, the bounds on the cosmic microwave background smoothness, the Abell cluster abundance, and their correlation function. 85 references.