Einstein-de Sitter Research Articles

Restrictions to the galaxy evolutionary models from the Hawaiian Deep Fields SSA13 and SSA22

Quantitative structural analysis of the galaxies present in the Hawaiian Deep Fields SSA13 and SSA22 is reported. The structural parameters of the galaxies have been obtained automatically by fitting a two-component model (Sérsic r1/n bulge and exponential disc) to the surface brightness of the galaxies. The galaxies were classified on the basis of the bulge-to-total luminosity ratio (B/T). The magnitude selection criteria and the reliability of our method have been checked by using Monte Carlo simulations. A complete sample of objects up to redshift 0.8 has been achieved. Spheroidal objects (E/S0) represent ≈33 per cent and spirals ≈41 per cent of the total number of galaxies, while mergers and unclassified objects represent ≈26 per cent. We have computed the comoving space density of the different kinds of object. In an Einstein–de Sitter universe, a decrease in the comoving density of E/S0 galaxies is observed as redshift increases (≈30 per cent less at z=0.8), while for spiral galaxies a relatively quiet evolution is reported. The framework of hierarchical clustering evolution models of galaxies seems to be the most appropriate to explain our results.

XMM-Newton observation of the distant ($\vec{z=0.6}$) galaxy cluster RX J1120.1+4318

We report on a 20 ks XMM observation of the distant cluster RX J1120.1+4318, discovered at z = 0.6 in the SHARC survey. The cluster has a regular spherical morphology, suggesting it is in a relaxed state. The combined fit of the EPIC/MOS&pn camera gives a cluster mean temperature of kT = 5.3 +/- 0.5 keV with an iron abundance of 0.47 +/- 0.19. The temperature profile, measured for the first time at such a redshift, is consistent with an isothermal atmosphere up to half the virial radius. The surface brightness profile, measured nearly up to the virial radius, is well fitted by a beta-model, with beta = 0.78(-0.04)(+0.06) and a core radius of theta(c) = 0.44(-0.04)(+0.06) arcmin. We compared the properties of RX J1120.1+4318 with the properties of nearby clusters for two cosmological models. an Einstein-de Sitter Universe and a flat low density Universe with Omega(0) = 0.3. For both models, the scaled emission measure profile beyond the core, the gas mass fraction and luminosity are consistent with the expectations of the self-similar model of cluster formation, although a slightly better agreement is obtained for a low density Universe. There is no evidence of a central cooling flow, in spite of the apparent relaxed state of the cluster. This is consistent with its estimated cooling time, larger than the age of the Universe at the cluster redshift. The entropy profile shows a flat core with a central entropy of similar to140 keV cm(2), remarkably similar to the entropy floor observed in nearby clusters, and a rising profile beyond typically 0.1 virial radius. Implications of our results, in terms of non-gravitational physics in cluster formation, are discussed.

Influence of expansion on hierarchical structure.

We study a one-dimensional model of gravitational instability in an Einstein-de Sitter universe. Scaling in both space and time results in an autonomous set of coupled Poisson-Vlasov equations for both the field and phase space density, and the N-body problem. Using dynamical simulation, we find direct evidence of hierarchical clustering. A multifractal analysis reveals a bifractal geometry similar to that observed in the distribution of galaxies. To demonstrate the role of scaling, we compare the system to other one-dimensional models recently employed to study structure formation. Finally we show that the model yields an estimate of the time of galaxy formation of the correct order.

Observations of the Hubble Deep Field South with the Infrared Space Observatory- II. Associations and star formation rates

We present results from a deep mid-infrared survey of the Hubble Deep Field South (HDF-S) region performed at 6.7 and 15 μm with the ISOCAM instrument on board the Infrared Space Observatory (ISO). We find reliable optical/near-infrared associations for 32 of the 35 sources detected in this field by Oliver et al. (Paper I): eight of them are identified as stars, one is definitely an active galactic nucleus (AGN), a second seems likely to be an AGN too, while the remaining 22 appear to be normal spiral or starburst galaxies. Using model spectral energy distributions (SEDs) of similar galaxies, we compare methods for estimating the star formation rates (SFRs) in these objects, finding that an estimator based on integrated (3–1000 μm) infrared luminosity reproduces the model SFRs best. Applying this estimator to model fits to the SEDs of our 22 spiral and starburst galaxies, we find that they are forming stars at rates of ∼1–100 M⊙ yr-1, with a median value of ∼40 M⊙ yr−1, assuming an Einstein–de Sitter universe with a Hubble constant of 50 km s−1 Mpc−1, and star formation taking place according to a Salpeter initial mass function (IMF) across the mass range 0.1–100 M⊙. We split the redshift range 0.0≤z≤0.6 into two equal-volume bins to compute raw estimates of the star formation rate density, ρ˙∗, contributed by these sources, assuming the same cosmology and IMF as above and computing errors based on estimated uncertainties in the SFRs of individual galaxies. We compare these results with other estimates of ρ˙∗ made with the same assumptions, showing them to be consistent with the results of Flores et al. from their ISO survey of the CFRS 1415+52 field. However, the relatively small volume of our survey means that our ρ˙∗ estimates suffer from a large sampling variance, implying that our results, by themselves, do not place tight constraints on the global mean star formation rate density.

Morphological number counts and redshift distributions to I = 25 from the Hubble Deep Fields: constraints on cosmological models from early-type galaxies

We combine magnitude and photometric redshift data on galaxies in the Hubble Deep Fields with morphological classifications in order to separate out the distributions for early-type galaxies. The updated morphological galaxy number counts down to and the corresponding redshift distributions are used as joint constraints on cosmological models; in particular, on the values of the density parameter Ω0 and the normalized cosmological constant λ0. We find that an Einstein-de Sitter universe with simple passive evolution gives an excellent fit to the counts and redshift data at all magnitudes. An open, low-Ω0 model with no net evolution (and conservation of the number of ellipticals), which fits the counts equally well, is somewhat less successful, predicting slightly lower mean redshifts and, more significantly, the lack of a high-z tail. A number-conserving model with a dominant contribution from λ0, on the other hand, is far less successful, predicting a much narrower distribution than is seen. More complex models are obviously possible, but we conclude that if large-scale transmutation between types does not occur, then the lambda-dominated models provide a very poor fit to the current data.

Statistical characteristics of a stochastic background of gravitational waves from neutron star formation

By using a recent model for the evolving star formation rate, we investigate the statistical distribution of gravitational wave amplitudes due to supernovae that result in neutron star formation in the Einstein–de Sitter cosmology. To account for the uncertainty in gravitational wave emission for this source type, we use a random mixture of three simulated waveform types computed by Zwerger and Müller. We investigate statistical parameters of the resulting gravitational wave amplitude distribution in our frame.

A new standardmodel of the universe

Up to about four years ago the dynamical properties of theevolution of the universe were assumed to be well described bythe Einstein-de Sitter universe model, which is a flat universemodel dominated by cold matter. However, the discovery that thecosmic expansion is accelerating made it clear that, at thepresent time, the evolution of the universe is dominated by somesort of vacuum energy with repulsive gravitation. The mostsimple type of vacuum energy is the Lorentz invariant vacuumenergy which has constant energy density during theexpansion of the universe. This type of vacuum may berepresented mathematically by including a cosmological constantin Einstein's field equations. Hence, the flatFriedmann-Lemaître model, which is a universe model withcold matter and vacuum energy, has replaced theEinstein-de Sitter model as the standard model of the universe.In this paper we give a pedagogical presentation of thismodel, including, among others, a new formula for the point oftime when decelerated expansion changed into acceleratedexpansion.

Simulating a stochastic background of gravitational waves from neutron star formation at cosmological distances

We develop a temporal simulation of the potentially detectable gravitational wave background from neutron star formation at cosmological distances. By using a recent model for the evolving star formation rate, we investigate the statistical distribution of gravitational wave amplitudes due to supernovae that result in neutron star formation in the Einstein–de Sitter cosmology. We find that the gravitational wave amplitude distribution in our frame is highly skewed, with skewness related to the distribution of sources, and that the potentially detectable gravitational wave strain is dominated by sources at a redshift of . Time traces of the simulation, using selected waveforms, are presented graphically and are also made available as web based audio files. The method developed can readily be extended to different cosmologies, as well as to incorporate other waveforms and source types. This type of simulation will be useful in testing and optimizing detection strategies for gravitational wave backgrounds due to various types of individually undetectable astrophysical sources.

Infrared magnitude–redshift relations for luminous radio galaxies

Infrared magnitude–redshift relations relations) for the 3CR and 6C samples of radio galaxies are presented for a wide range of plausible cosmological models, including those with non zero cosmological constant ΩΛ. Variations in the galaxy formation redshift, metallicity and star formation history are also considered. The results of the modelling are displayed in terms of magnitude differences between the models and no evolution tracks, illustrating the amount of K band evolution necessary to account for the observational data. Given a number of plausible assumptions, the results of these analyses suggest that: (i) cosmologies which predict (where T0 denotes the current age of the Universe) can be excluded; (ii) the star formation redshift should lie in the redshift interval , values towards the lower end of the range being preferred in cosmologies with larger values of T0H0; (iii) the Einstein–de Sitter model provides a reasonable fit to the data; and (iv) models with finite values of ΩΛ can provide good agreement with the observations only if appropriate adjustments of other parameters such as the galaxy metallicities and star formation histories are made. Without such modifications, even after accounting for stellar evolution, the high redshift radio galaxies are more luminous (that is, more massive) than those nearby in models with finite ΩΛ, including the favoured model with . For cosmological models with larger values of T0H0, the conclusions are the same regardless of whether any adjustments are made or not. The implications of these results for cosmology and models of galaxy formation are discussed.

Cosmological implications of old galaxies at high redshifts

Old high-z galaxies are important tools for understanding the structure formation problem and may become the key to determine the ultimate fate of the Universe. In this paper, the inferred ages of the three oldest galaxies at high redshifts reported in the literature are used to constrain the first epoch of galaxy formation and to reanalyse the high-z time scale crisis. The lower limits on the formation redshift z f depends on the quantity of cold dark matter in the Universe. In particular, if omegam > or = 0.37 these galaxies are not formed in FRW cosmologies with no dark energy. This result is in line with the Supernovae type Ia measurements which suggest that the bulk of energy in the Universe is repulsive and appears like an unknown form of dark energy component. In a complementar analysis, unlike recent claims favoring the end of the age problem, it is shown that the Einstein-de Sitter model is excluded at high-z by 3sigma.

SCALE INVARIANCE IN A PERTURBED EINSTEIN-DE SITTER COSMOLOGY

This paper seeks to check the validity of the apparent fractal conjecture,1,2 which states that the observed power-law behavior for the average density of large-scale distribution of galaxies arises when some observational quantities, selected by their relevance in average density profile determination, are calculated along the past light cone. Since general relativity states that astronomical observations are carried out in this spacetime hypersurface, observables necessary for direct comparison with astronomical data must be calculated along it. Implementing this condition in the proposed set of observational relations profoundly changes the behavior of many observables in the standard cosmological models. In particular, the average density becomes inhomogeneous, even in the spatially homogeneous spacetime of standard cosmology, change which was already analyzed by Ribeiro (1992b, 1993, 1994, 1995)3–6 for a non-perturbed model. Here we derive observational relations in a perturbed Einstein-de Sitter cosmology by means of the perturbation scheme proposed by Abdalla and Mohayaee (1999),7 where the scale factor is expanded in power series to yield perturbative terms. The differential equations derived in this perturbative context, and other observables necessary in our analysis, are solved numerically. The results show that our perturbed Einstein-de Sitter cosmology can be approximately described by a decaying power-law like average density profile, meaning that the dust distribution of this cosmology has a scaling behavior compatible with the power-law profile of the density-distance correlation observed in the galaxy catalogues. These results show that, in the context of this work, the apparent fractal conjecture is correct.

Cosmology from cosmic microwave background and galaxy clusters

We present the results of analysis of constraints on cosmological parameters from cosmic microwave background (CMB) alone and in combination with the galaxy cluster baryon fraction assuming inflation-generated adiabatic scalar fluctuations. The CMB constraints are obtained using our likelihood approximation method (Bartlett et al. 2000; Douspis et al. 2001). In the present analysis we use the new data coming from MAXIMA and BOOMERanG balloon-borne experiments and the first results of the DASI interferometer together with the COBE/DMR data. The quality of these independent data sets implies that the $C_\ell$ are rather well known, and allow reliable constraints. We found that the constraints in the $\Omega-H_0$ plane are very tightened, favouring a flat Universe, that the index of the primordial fluctuations is very close to one, that the primordial baryon density is now in good agreement with primordial nucleosynthesis. Nevertheless degeneracies between several parameters still exist, and for instance the constraint on the cosmological constant or the Hubble constant are very weak, preferred values being low. A way to break these degeneracies is to cross-constrain the parameters by combining them with constraints from other independent data. We use the baryon fraction determination from X-ray clusters of galaxies as an additional constraint and show that the combined analysis leads to strong constraints on all cosmological parameters. Using a high baryon fraction ( ∼ 15% for $h = 0.5$) we found a rather low Hubble constant, values around 80 km s -1 /Mpc being ruled out. Using a recent and low baryon fraction estimation ( ∼ 10% for $h = 0.5$) we found a preferred model with a low Hubble constant and a high density content ($\Omega_{\rm m}$), an Einstein-de Sitter model being only weakly ruled out.

Measuring Ωmwith theROSATDeep Cluster Survey

We analyze the ROSAT Deep Cluster Survey (RDCS) to derive cosmological constraints from the evolution of the cluster X-ray luminosity distribution. The sample contains 103 galaxy clusters out to z 0.85 and flux limit Flim = 3 × 10-14 ergs s-1 cm-2 (RDCS-3) in the [0.5-2.0] keV energy band, with a high-redshift extension containing four clusters at 0.90 ≤ z ≤ 1.26 and brighter than Flim = 1 × 10-14 ergs s-1 cm-2 (RDCS-1). We assume cosmological models to be specified by the matter density parameter Ωm, the rms fluctuation amplitude at the 8 h-1 Mpc scale σ8, and the shape parameter for the cold dark matter-like power spectrum Γ. Model predictions for the cluster mass function are converted into the X-ray luminosity function in two steps. First, we convert mass into intracluster gas temperature by assuming hydrostatic equilibrium. Then, temperature is converted into X-ray luminosity by using the most recent data on the LX-TX relation for nearby and distant clusters. These include the Chandra data for six distant clusters at 0.57 ≤ z ≤ 1.27. From RDCS-3 we find Ωm = 0.35 and σ8 = 0.66 for a spatially flat universe with a cosmological constant, with no significant constraint on Γ (errors correspond to 1 σ confidence levels for three fitting parameters). Even accounting for both theoretical and observational uncertainties in the mass-X-ray luminosity conversion, an Einstein-de Sitter model is always excluded at far more than the 3 σ level. We also show that the number of X-ray-bright clusters in RDCS-1 at z > 0.9 is expected from the evolution inferred at z < 0.9 data.

Constraining cosmological parameters with the clustering properties of galaxy clusters in optical and X-ray bands

We use a theoretical model to predict the clustering properties of galaxy clusters. Our technique accounts for past light-cone effects on the observed clustering and follows the non-linear evolution in redshift of the underlying dark matter correlation function and cluster bias factor. A linear treatment of redshift-space distortions is also included. We perform a maximum-likelihood analysis by comparing the theoretical predictions to a set of observational data, in both the optical (two different subsamples of the APM catalogue and the EDCC catalogue) and X-ray band (RASS1 bright sample, BCS, XBACs, REFLEX). In the framework of cold dark matter models, we compute the constraints on cosmological parameters, such as the matter density Ω0m, the cosmological constant Ω0Λ, the power-spectrum shape parameter Γ and normalization σ8. Our results show that X-ray data are more powerful than optical ones, allowing smaller regions in the parameter space. If we fix Γ and σ8 to the values suggested by different observational data sets, we obtain strong constraints on the matter density parameter, and , for the optical and X-ray data, respectively. Allowing the shape parameter to vary, we find that the clustering properties of clusters are almost independent of the matter density parameter and of the presence of a cosmological constant, while they appear to be strongly dependent on the shape parameter. Using the X-ray data only, we obtain and for the Einstein–de Sitter model, while and for open and flat models with . Finally, we use our model to make predictions on the correlation length of galaxy clusters expected in future surveys. In particular, we show the results for an optical catalogue with characteristics similar to the EIS project and for a very deep X-ray catalogue with the characteristics of the XMM/LSS survey. We find that clusters at high redshifts are expected to have a larger correlation length than local ones.

Quasi–Self‐Similar Evolution of the Two‐Point Correlation Function: Strongly Nonlinear Regime in Ω0&lt; 1 Universes

The well-known self-similar solution for the two-point correlation function of the density field is valid only in an Einstein-de Sitter universe. We attempt to extend the solution for non-Einstein-de Sitter universes. For this purpose we introduce an idea of quasi-self-similar evolution; this approach is based on the assumption that the evolution of the two-point correlation is a succession of stages of evolution, each of which spans a short enough period to be considered approximately self-similar. In addition, we assume that clustering is stable on scales where a physically motivated virialization condition is satisfied. These assumptions lead to a definite prediction for the behavior of the two-point correlation function in the strongly nonlinear regime. We show that the prediction agrees well with N-body simulations in non-Einstein-de Sitter cases and discuss some remaining problems.

On de sitter deflationary cosmology from the spin-torsion primordial fluctuations and cobe data

Abstract Fluctuations on de Sitter metric solution of Einstein-Cartan field equations are obtained in terms of the matter and spin-torsion density fluctuations. A proof for the gravitational instability of the de Sitter metric in spaces with torsion from COBE data is obtained. In fact Einstein-de Sitter solution is shown to be unstable even in the absence of torsion.

Space and observers in cosmology

I provide a prescription to define space, at a given moment, for an arbitrary observer in an arbitrary (sufficiently regular) curved space-time. This prescription, based on synchronicity (simultaneity) arguments, defines a foliation of space-time, which corresponds to a family of canonically associated observers. It provides also a natural global reference frame (with space and time coordinates) for the observer, in space-time (or rather in the part of it which is causally connected to him), which remains Minkowskian along his world-line. This definition intends to provide a basis for the problem of quantization in curved space-time, and/or for non inertial observers. Application to Minkowski spacetime illustrates clearly the fact that different observers see different spaces. For example, it allows one to define space everywhere without ambiguity, for the Langevin observer (involved in the Langevin pseudoparadox of twins). Applied to the Rindler observer (with uniform acceleration) it leads to the Rindler coordinates, whose choice is so justified with a physical basis. This leads to an interpretation of the Unruh effect, as due to the observer-dependence of the definition of space (and time). This prescription is also applied in cosmology, for inertial observers in the Friedmann-Lemaître models: space for the observer appears to differ from the hypersurfaces of homogeneity, which do not obey the simultaneity requirement. I work out two examples: the Einstein-de Sitter model, in which space, for an inertial observer, is not flat nor homogeneous, and the de Sitter case.

The 2dF QSO Redshift Survey - VI. Measuring Λ andβfrom redshift-space distortions in the power spectrum

When the 2dF QSO Redshift Survey (2QZ) is complete, a powerful geometric test for the cosmological constant will be available. By comparing the clustering along and across the line of sight and modelling the effects of peculiar velocities and bulk motions in redshift space, geometric distortions, which occur if the wrong cosmology is assumed, can be detected. In this paper we investigate the effect of geometric and redshift-space distortions in the power spectrum parallel and perpendicular to the observer's line of sight, . Ballinger et al. developed a model to estimate the cosmological constant, Λ, and the important parameter from these distortions. We apply this model to a detailed simulation of the final 25k 2QZ, produced using the Virgo Consortium's huge Hubble Volume N-body ΛCDM (cold dark matter) light-cone simulation. We confirm the conclusions of Ballinger et al.: the shapes of the redshift-space and geometric distortions are very similar, and discriminating between the two to produce a purely geometric constraint on Λ is difficult. When all the uncertainties in measuring for the 2QZ are taken into account, we find that only a joint constraint is possible. By combining this result with a second constraint based on mass clustering evolution, however, we can make significant progress. We predict that this method should allow us to constrain β to approximately ±0.1, and Ωm to ±0.25, using the final catalogue. We apply the method to the 2QZ catalogue of 10 000 quasars (QSOs) and find that this incomplete catalogue marginally favours a Λ cosmology, obtaining best-fitting values of and . However, Einstein–de Sitter models are only rejected at the 1.4σ level in the current survey. The rejection of lambda-dominated models is stronger at ∼2σ.

The post-collapse equilibrium structure of cosmological haloes in a low-density universe

An analytical model is presented for the post-collapse equilibrium structure of virialized objects that condense out of a low-density cosmological background universe, either matter-dominated or flat with a cosmological constant. This generalizes the model we derived previously for an Einstein–de Sitter (EdS) universe. The model is based upon the assumption that cosmological haloes form from the collapse and virialization of ‘top-hat’ density perturbations, and are spherical, isotropic and isothermal. This leads to the prediction of a unique, non-singular, truncated isothermal sphere (TIS), a particular solution of the Lane–Emden equation (suitably modified when Λ≠0). The size and virial temperature are unique functions of the mass and redshift of formation of the object for a given background universe. The central density is roughly proportional to the critical density of the universe at the epoch of collapse. This TIS model is in good agreement with observations of the internal structure of dark-matter-dominated haloes on scales ranging from dwarf galaxies to X-ray clusters. It also reproduces many of the average properties of haloes in simulations of the cold dark matter (CDM) model to good accuracy, suggesting that it is a useful analytical approximation for haloes that form from realistic initial conditions. Our TIS model matches the density profiles of haloes in CDM N-body simulations outside the innermost region, while avoiding the steep central cusp of the latter which is in apparent conflict with observations. The TIS model may also be relevant to non-standard CDM models, such as that for self-interacting dark matter, recently proposed to resolve this conflict.

Simulations of deep pencil-beam redshift surveys

We create mock pencil-beam redshift surveys from very large cosmological N-body simulations of two cold dark matter (CDM) cosmogonies, an Einstein-de Sitter model (τCDM) and a flat model with Ω0 =0.3 and a cosmologial constant ΛCDM).