arXiv subjects

Ilian T. Iliev

Publications and source records attributed to Ilian T. Iliev.

136 records · Page 8Linked to original sources

arXiv2002

On the Direct Detectability of the Cosmic Dark Ages: 21-cm Emission from Minihalos

In the standard Cold Dark Matter (CDM) theory of structure formation, virialized minihalos (with T_vir}<10^4 K) form in abundance at high redshift (z>6), during the cosmic ``dark ages.'' The hydrogen in these minihalos, the first nonlinear baryonic structures to form in the universe, is mostly neutral and sufficiently hot and dense to emit strongly at the 21-cm line. We calculate the emission from individual minihalos and the radiation background contributed by their combined effect. Minihalos create a ``21-cm forest'' of emission lines. We predict that the angular fluctuations in this 21-cm background should be detectable with the planned LOFAR and SKA radio arrays, thus providing a direct probe of structure formation during the ``dark ages.'' Such a detection will serve to confirm the basic CDM paradigm while constraining the shape of the power-spectrum of primordial density fluctuations down to much smaller scales than have previously been constrained, the onset and duration of the reionization epoch, and the conditions which led to the first stars and quasars. We present results here for the currently-favored, flat $Λ$CDM model, for different tilts of the primordial power spectrum.

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arXiv2001

The Universal Equilibrium of CDM Halos: Making Tracks on the Cosmic Virial Plane

Dark-matter halos are the scaffolding around which galaxies and clusters are built. They form when the gravitational instability of primordial density fluctuations causes regions which are denser than average to slow their cosmic expansion, recollapse, and virialize. Objects as different in size and mass as dwarf spheroidal galaxies and galaxy clusters are predicted by the CDM model to have halos with a universal, self-similar equilibrium structure whose parameters are determined by the halo's total mass and collapse redshift. These latter two are statistically correlated, however, since halos of the same mass form on average at the same epoch, with small-mass objects forming first and then merging hierarchically. The structural properties of dark-matter dominated halos of different masses, therefore, should reflect this statistical correlation, an imprint of the statistical properties of the primordial density fluctuations which formed them. Current data reveal these correlations, providing a fundamental test of the CDM model which probes the shape of the power spectrum of primordial density fluctuations and the cosmological background parameters.

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arXiv2001

The Central Mass and Phase-Space Densities of Dark Matter Halos: Cosmological Implications

Current data suggest that the central mass densities $ρ_0$ and phase-space densities $Q\equivρ_0/σ_V^3$ of cosmological halos in the present universe are correlated with their velocity dispersions $σ_V$ over a very wide range of $σ_V$ from less than 10 to more than 1000 $\rm km s^{-1}$. Such correlations are an expected consequence of the statistical correlation of the formation epochs of virialized objects in the CDM model with their masses; the smaller-mass halos typically form first and merge to form larger-mass halos later. We have derived the $Q-σ_V$ and $ρ_0-σ_V$ correlations for different CDM cosmologies and compared the predicted correlations with the observed properties of a sample of low-redshift halos ranging in size from dwarf spheroidal galaxies to galaxy clusters. Our predictions are generally consistent with the data, with preference for the currently-favored, flat $Λ$CDM model. Such a comparison serves to test the basic CDM paradigm while constraining the background cosmology and the power-spectrum of primordial density fluctuations, including larger wavenumbers than have previously been constrained.

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arXiv2001

The Equilibrium Structure of Cosmological Halos

We have derived an analytical model for the postcollapse equilibrium structure of cosmological halos as nonsingular truncated isothermal spheres (TIS) and compared this model with observations and simulations of cosmological halos on all scales. Our model is in good agreement with the observations of the internal structure of dark-matter-dominated halos from dwarf galaxies to X-ray clusters. It reproduces many of the average properties of halos in CDM simulations to good accuracy, including the density profiles outside the central region, while avoiding the possible discrepancy at small radii between observed galaxy and cluster density profiles and the singular density profiles predicted by N-body simulations of the CDM model. While much attention has been focused lately on this possible discrepancy, we show that the observed galaxy rotation curves and correlations of halo properties nevertheless contain valuable additional information with which to test the theory, despite this uncertainty at small radii. The available data allows us to constrain the fundamental cosmological parameters and also to put a unique constraint on the primordial density fluctuation power spectrum at large wavenumbers (i.e. small mass scale).

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arXiv2001

The Postcollapse Equilibrium Structure of Cosmological Haloes in a Low-Density Universe

An analytical model is presented for the postcollapse equilibrium structure of virialized objects which 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, nonsingular, truncated isothermal sphere (TIS), a particular solution of the Lane-Emden equation (modified for nonzero cosmological constant). 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 which 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 nonstandard CDM models, like self-interacting dark matter, recently proposed to resolve this conflict.

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arXiv2000

On the Origin of the Rotation Curves of Dark-Matter-Dominated Galaxies

Rotation curves of dark-matter-dominated galaxies measure the mass profiles of galactic halos and thereby test theories of their cosmological origin. While attention has focused lately on the possible discrepancy at small galactocentric radii between observed rotation curves and the singular density profiles predicted by N-body simulations of the Cold Dark Matter (CDM) model, the observed rotation curves nevertheless contain valuable additional information with which to test the theory and constrain the fundamental cosmological parameters, despite this uncertainty at small radii. An analytical model we derived elsewhere for the postcollapse equilibrium of cosmological halos as truncated, nonsingular, isothermal spheres (TIS) reproduces many of the average properties of halos in CDM simulations to good accuracy, including the density profiles outside the central region. The circular velocity profile of this TIS model is, moreover, in excellent agreement with the observed ones and yields the mass and formation epoch of an observed halo from the parameters of its rotation curve. This allows us to predict correlations amongst rotation curve parameters, such as the maximum velocity and the radius at which it occurs, for different mass halos forming at different epochs in the CDM model. As an example, we derive the observed v_max-r_max relation analytically, with preference for the flat Lambda-CDM model.

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arXiv2000

On the Mass Profile of Galaxy Cluster CL 0024+1654 Inferred from Strong Lensing

Observations of a flat density profile in the cores of dark-matter-dominated halos on the two extremes of mass for virialized objects in the universe, dwarf galaxies and galaxy clusters, present a serious challenge to the current standard theory of structure formation involving Cold Dark Matter (CDM). By contrast, N-body simulations of halo formation in the latter indicate density profiles which are singular and steeply rising towards the center. A flat-density core on the cluster scale is indicated by gravitational lensing observations, most significantly by the strong-lensing measurements of CL 0024+1654 by the Hubble Space Telescope. A recent re-analysis of this cluster has suggested that a uniform-density core is not demanded by the data, thereby eliminating a significant piece of the conflict between the observations and the CDM theoretical predictions. We show here, however, that the singular mass profile which that analysis reports as consistent with the lensing measurements of CL 0024+1654 implies a velocity dispersion which is much higher than the measured value for this cluster.

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arXiv2000

The Equilibrium Structure of Cosmological Halos: Dwarf Galaxies to X-ray Clusters

An analytical model for the postcollapse equilibrium structure of virialized objects which condense out of the cosmological background universe is described and compared with observations and simulations of cosmological halos. The model is based upon the assumption that virialized halos are isothermal, which leads to a prediction of a unique nonsingular isothermal sphere for the equilibrium structure, with a core density which is proportional to the mean background density at the epoch of collapse. These predicted nonsingular isothermal spheres are in good agreement with observations of the internal structure of dark-matter-dominated halos from dwarf galaxies to X-ray clusters. Our model also reproduces many of the average properties of halos in CDM simulations to good accuracy, suggesting that it is a useful analytical approximation for halos which form from realistic initial conditions. While CDM N-body simulations find profiles with a central cusp, our nonsingular model matches the simulated halos outside the innermost region well. This model may also be of interest as a description of halos in nonstandard CDM models like self-interacting dark matter, which have been proposed to eliminate the discrepancy between the cuspy halos of standard CDM simulations and observed halos with uniform-density cores.

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arXiv2000

The Equilibrium Structure of Cosmological Halos: From Dwarf Galaxies to X-ray Clusters

A new model for the postcollapse equilibrium structure of virialized objects which condense out of the cosmological background universe is described and compared with observations and simulations of cosmological halos from dwarf galaxies to X-ray clusters. The model is based upon the assumption that virialized halos are isothermal, which leads to a prediction of a unique nonsingular isothermal sphere for the equilibrium structure, with a core radius which is approximately 1/30 times the size and a core density which is proportional to the mean background density at the epoch of collapse. These predicted nonsingular isothermal spheres are in good agreement with the observations of the internal structure of dark-matter-dominated halos from dwarf galaxies to X-ray clusters.

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arXiv1999

A model for the postcollapse equilibrium of cosmological structure: truncated isothermal spheres from top-hat density perturbations

The postcollapse structure of objects which form by gravitational condensation out of the expanding cosmological background universe is a key element in the theory of galaxy formation. Towards this end, we have reconsidered the outcome of the nonlinear growth of a uniform, spherical density perturbation in an unperturbed background universe - the cosmological ``top-hat'' problem. We adopt the usual assumption that the collapse to infinite density at a finite time predicted by the top-hat solution is interrupted by a rapid virialization caused by the growth of small-scale inhomogeneities in the initial perturbation. We replace the standard description of the postcollapse object as a uniform sphere in virial equilibrium by a more self-consistent one as a truncated, nonsingular, isothermal sphere in virial and hydrostatic equilibrium, including for the first time a proper treatment of the finite-pressure boundary condition on the sphere. The results differ significantly from both the uniform sphere and the singular isothermal sphere approximations for the postcollapse objects. These results will have a significant effect on a wide range of applications of the Press-Schechter and other semi-analytical models to cosmology. The truncated isothermal sphere solution presented here predicts the virial temperature and integrated mass distribution of the X-ray clusters formed in the CDM model as found by detailed, 3D, numerical gas and N-body dynamical simulations remarkably well. This solution allows us to derive analytically the numerically-calibrated mass-temperature and radius-temperature scaling laws for X-ray clusters which were derived empirically by Evrard, Metzler and Navarro from simulation results for the CDM model. (Shortened)

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