arXiv subjects

Paul R. Shapiro

Publications and source records attributed to Paul R. Shapiro.

At least 145 records · Page 8Linked to original sources

arXiv2001

Explosions and Outflows during Galaxy Formation

We consider an explosion at the center of a halo which forms at the intersection of filaments inside a cosmological pancake, a convenient test-bed model for galaxy formation. ASPH/P3M simulations reveal that such explosions are anisotropic. The energy and metals are channeled into the low density regions, away from the pancake. The pancake remains essentially undisturbed, even if the explosion is strong enough to blow away all the gas located inside the halo and reheat the IGM surrounding the pancake. Infall quickly replenishes this ejected gas and gradually restores the gas fraction as the halo continues to grow. Estimates of the collapse epoch and SN energy-release for galaxies of different mass in the CDM model can relate these results to scale-dependent questions of blow-out and blow-away and their implication for early IGM heating and metal enrichment and the creation of gas-poor dwarf galaxies.

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arXiv2001

The Evolution of Dark-Matter Dominated Cosmological Halos

Adaptive SPH and N-body simulations were carried out to study the evolution of the equilibrium structure of dark matter halos that result from the gravitational instability and fragmentation of cosmological pancakes. Such halos resemble those formed by hierarchical clustering from realistic initial conditions in a CDM universe and, therefore, serve as a test-bed model for studying halo dynamics. The dark matter density profile is close to the universal halo profile identified previously from N-body simulations of structure formation in CDM, with a total mass and concentration parameter which grow linearly with scale factor a. When gas is included, this concentration parameter is slightly larger than the pure N-body result. We also find that the dark matter velocity distribution is less isotropic and more radial than found by N-body simulations of CDM.

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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 Reionization of the Universe and the Photoevaporation of Cosmological Minihalos

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arXiv2000

Explosions During Galaxy Formation

As an idealized model of the effects of energy release by supernovae during galaxy formation, we consider an explosion at the center of a halo which forms at the intersection of filaments in the plane of a cosmological pancake by gravitational instability during pancake collapse. Such halos resemble the virialized objects found in N-body simulations in a CDM universe and, therefore, serve as a convenient, scale-free test-bed model for galaxy formation. ASPH/P3M simulations reveal that such explosions are anisotropic. The energy and metals are channeled into the low density regions, away from the pancake plane. The pancake remains essentially undisturbed, even if the explosion is strong enough to blow away all the gas located inside the halo at the onset of the explosion and reheat the IGM surrounding the pancake. Infall quickly replenishes this ejected gas and gradually restores the gas fraction as the halo mass continues to grow. Estimates of the collapse epoch and SN energy-release for galaxies of different mass in the CDM model can relate these results to scale-dependent questions of blow-out and blow-away and their implication for early IGM heating and metal enrichment and the creation of dark-matter-dominated dwarf galaxies.

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arXiv2000

The Effect of Gasdynamics on the Structure of Dark Matter Halos

Adaptive SPH and N-body simulations were carried out to study the effect of gasdynamics on the structure of dark matter halos that result from the gravitational instability and fragmentation of cosmological pancakes. Such halos resemble those formed in a hierarchically clustering CDM universe and serve as a test-bed model for studying halo dynamics. With no gas, the density profile is close to the universal profile identified previously from N-body simulations of structure formation in CDM. When gas is included, the gas in the halo is approximately isothermal, and both the dark matter and the gas have singular central density profiles which are steeper than that of the dark matter with no gas. This worsens the disagreement between observations of constant density cores in cosmological halos and the singular ones found in simulations. We also find that the dark matter velocity distribution is less isotropic than found by N-body simulations of CDM, because of the strongly filamentary substructure.

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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

Reionization of the Universe by Starlight and the Photoevaporation of Cosmological Minihalos

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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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arXiv2000

Reionization of the Universe and the Photoevaporation of Cosmological Minihalos

The first sources of ionizing radiation to condense out of the dark and neutral IGM sent ionization fronts sweeping outward through their surroundings, overtaking other condensed objects and photoevaporating them. This feedback effect of universal reionization on cosmic structure formation is demonstrated here for the case of a cosmological minihalo of dark matter and baryons exposed to an external source of ionizing radiation with a quasar-like spectrum, just after the passage of the global ionization front created by the source. We model the pre-ionization minihalo as a truncated, nonsingular isothermal sphere in hydrostatic equilibrium following its collapse out of the expanding background universe and virialization. Results are presented of the first gas dynamical simulations of this process, including radiative transfer. A sample of observational diagnostics is also presented, including the spatially-varying ionization levels of C, N, and O in the flow if a trace of heavy elements is present and the integrated column densities of H I, He I and II, and C IV thru the photoevaporating gas at different velocities which would be measured in absorption against a background source like that responsible for the ionization.

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arXiv1999

Explosions During Galaxy Formation. Scale-Free Simulations

When density fluctuations collapse gravitationally out of the expanding cosmological background universe to form galaxies, the secondary energy release which results can affect their subsequent evolution profoundly. We focus here on the effects of one form of such energy release - explosions, such as might result from the supernovae which end the lives of the first generation of massive stars to form inside protogalaxies. We are particularly interested in the consequences of the nonspherical geometry and continuous infall which are characteristic of galaxy formation from realistic initial and boundary conditions. As an idealized model which serves to illustrate and quantify the importance of these effects, we study the effect of explosions on the quasi-spherical objects which form at the intersections of filaments in the plane of a cosmological pancake, as a result of gravitational instability and fragmentation of the pancake. We study the formation and evolution of these "galaxies," subject to the explosive injection of energy at their centers, by numerical gas dynamical simulation in 3D utilizing our new, anisotropic version of Smoothed Particle Hydrodynamics, Adaptive SPH ("ASPH"), with a P3M gravity solver.

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arXiv1999

The Asymptotic Collapsed Fraction in an Eternal Universe

We calculate the maximum fraction of matter which is able to condense out of the expanding background universe, for any universe which will expand forever. We use a simple spherical model for the growth of density fluctuations in the universe. This model includes open, matter-dominated universe and universes in which there is an uniform background component (e.g. the cosmological constant or "quintessence"). In these background universes, Omega eventually drops significantly below unity. When this happens, gravitational instability is suppressed and, with it, so is the growth of the collapsed fraction. We identify a limitation of the Press-Schechter (PS) approximation. In this approximation, the mass function determined from the predicted collapse of overdense regions is multiplied by a correction factor of 2, assuming that each bound fluctuation will accrete an equal share of mass from nearby underdense regions. Our model determines the actual value of the correction factor. We show that, while the factor of 2 adopted by the PS approximation is correct for an Einstein-de Sitter universe, it is not correct when the freeze-out of fluctuation growth inherent in the more general class of background universes described above occurs. When freeze-out occurs, the correction factor reduces to unity and the PS approximation must overestimate the collapsed fraction. We apply our model to open CDM and flat Lambda-CDM models. For H0=70 and Omega0=0.3, these models yield asymptotic collapsed fractions of 0.0361 and 0.0562, respectively, on the galaxy cluster mass-scale, only 55% of the values determined by the PS approximation. These results have implications for the use of the latter approximation to compare the observed space density of X-ray clusters today with that predicted by cosmological models.

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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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arXiv1998

Radiative Feedback and the Photoevaporation of Intergalactic Clouds

The first sources of ionizing radiation to condense out of the dark and neutral IGM sent ionization fronts sweeping outward through their surroundings, overtaking other primordial gas-clouds and photoevaporating them. Results are presented of the first gas dynamical simulations of this process, including radiative transfer, along with some observational diagnostics.

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arXiv1997

Adaptive Smoothed Particle Hydrodynamics: Methodology II

This paper presents an alternative formulation of the ASPH algorithm for evolving anisotropic smoothing kernels, in which the geometric approach of Shapiro et al. (1996; Paper I) is replaced by an approach involving a local transformation of coordinates to those in which the underlying anisotropic volume changes appear to be isotropic. The ASPH method is presented in 2D and 3D, including a number of details not previously included in Paper I, some of which represent either advances or different choices with respect to Paper I. Among the advances included here are an asynchronous time-integration scheme with different time steps for different particles and the generalization of the ASPH method to 3D. The shock-tracking algorithm described in Paper I for locally adapting the artificial viscosity to restrict viscous heating just to particles encountering shocks, is not included here. Instead, we adopt a different interpolation kernel for use with the artificial viscosity, which has the effect of spatially localizing effects of the artificial viscosity. This version of the ASPH method in 2D and 3D is then applied to a series of 1D, 2D, and 3D test problems, and the results are compared to those of standard SPH applied to the same problems. These include the problem of cosmological pancake collapse, the Riemann shock tube, cylindrical and spherical Sedov blast waves, the collision of two strong shocks, and problems involving shearing disks intended to test the angular momentum conservation properties of the method. These results further support the idea that ASPH has significantly better resolving power than standard SPH for a wide range of problems, including that of cosmological structure formation. (Abridged)

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arXiv1997

A Convenient Set of Comoving Cosmological Variables and Their Application

We present a set of cosmological variables, called "supercomoving variables," which are particularly useful for describing the gas dynamics of cosmic structure formation. For ideal gas with gamma=5/3, the supercomoving position, velocity, density, temperature, and pressure are constant in time in a uniform, isotropic, adiabatically expanding universe. Expressed in terms of these supercomoving variables, the cosmological fluid conservation equations and the Poisson equation closely resemble their noncosmological counterparts. This makes it possible to generalize noncosmological results and techniques to cosmological problems, for a wide range of cosmological models. These variables were initially introduced by Shandarin for matter-dominated models only. We generalize supercomoving variables to models with a uniform component corresponding to a nonzero cosmological constant, domain walls, cosmic strings, a nonclumping form of nonrelativistic matter (e.g. massive nettrinos), or radiation. Each model is characterized by the value of the density parameter Omega0 of the nonrelativistic matter component in which density fluctuation is possible, and the density parameter OmegaX of the additional, nonclumping component. For each type of nonclumping background, we identify FAMILIES within which different values of Omega0 and OmegaX lead to fluid equations and solutions in supercomoving variables which are independent of Omega0 and OmegaX. We also include the effects of heating, radiative cooling, thermal conduction, viscosity, and magnetic fields. As an illustration, we describe 3 familiar cosmological problems in supercomoving variables: the growth of linear density fluctuations, the nonlinear collapse of a 1D plane-wave density fluctuation leading to pancake formation, and the Zel'dovich approximation.

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