arXiv subjects

Michael L. Norman

Publications and source records attributed to Michael L. Norman.

At least 145 records · Page 8Linked to original sources

arXiv2002

Numerical Simulations of High Redshift Star Formation in Dwarf Galaxies

We present first results from three-dimensional hydrodynamic simulations of the high redshift formation of dwarf galaxies. The simulations use an Eulerian adaptive mesh refinement technique to follow the non-equilibrium chemistry of hydrogen and helium with cosmological initial conditions drawn from a popular Lambda-dominated CDM model. We include the effects of reionization using a uniform radiation field, a phenomenological description of the effect of star formation and, in a separate simulation, the effects of stellar feedback. The results highlight the effects of stellar feedback and photoionization on the baryon content and star formation of galaxies with virial temperatures of approximately 10^4K. Dwarf sized dark matter halos that assemble prior to reionization are able to form stars. Most halos of similar mass that assemble after reionization do not form stars by redshift of three. Dwarf galaxies that form stars show large variations in their gas content because of stellar feedback and photoionization effects. Baryon-to-dark matter mass ratios are found to lie below the cosmic mean as a result of stellar feedback. The supposed substructure problem of CDM is critically assessed on the basis of these results. The star formation histories modulated by radiative and stellar feedbacks are discussed. In addition, metallicities of individual objects are shown to be naturally correlated with their mass-to-light ratios as is also evident in the properties of local dwarf galaxies.

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arXiv2002

Interstellar Phase Transitions Stimulated by Time-dependent Heating

We use three-dimensional hydrodynamic numerical simulations to study phase transformations occurring in a clumpy interstellar gas exposed to time-dependent volumetric heating. To mimic conditions in the Galactic interstellar medium, we take a numerical model of a turbulent multiphase medium from Kritsuk & Norman (2002) computed in a periodic box with mean density n_0=0.25 cm^-3 and mean pressure P_0/k~10^3.4 K cm^-3. A second model with n_0=1 cm^-3 is also considered. Variations of the heating rate on a timescale of 1-10 Myr applied thereafter cause pressure variations in the gas and shifting of the thermal equilibrium curve in the phase plane. This stimulates mass transfer between the gas phases via thermal instability, converting 5-10% of the thermal energy into kinetic energy of gas motions. The experiments demonstrate that recurrent substantial heating episodes can maintain turbulence at this level. Possible applications to the interstellar gas heated by variable far-ultraviolet background radiation produced by short-living massive stars are discussed.

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arXiv2002

Beyond Flux-Limited Diffusion: Parallel Algorithms for Multidimensional Radiation Hydrodynamics

This paper presents a new code for performing multidimensional radiation hydrodynamic (RHD) simulations on parallel computers involving anisotropic radiation fields and nonequilibrium effects. The radiation evolution modules described here encapsulate the physics provided by the serial algorithm of Stone et. al (ApJSupp, vol 80, pp. 819-845), but add new functionality with regard to physics and numerics. Physics enhancments include the addition of time dependence to the computation of the variable tensor Eddington factor (VTEF) closure term, and a matter-radiation coupling scheme which is particularly robust for nonequilibrium problems. Numerical highlights include a discussion of how our code is implemented for parallel execution and a description of our scalable linear solver module. We present a suite of numerical tests from which the virtues and vices of our method may be gleaned; these include nonequilibrium Marshak waves, 2-D "shadow" tests showing the one-sided illumination of an opaque cloud, and full RHD+VTEF simulations of radiating shocks. We conclude that radiation moment solutions closed with variable tensor Eddington factors show a dramatic qualitative improvement over results obtained with flux-limited diffusion, and further that this approach has a bright future in the context of parallel RHD simulations in astrophysics.

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arXiv2002

Thermal Instability-Induced Interstellar Turbulence

We study the dynamics of phase transitions in the interstellar medium by means of three-dimensional hydrodynamic numerical simulations. We use a realistic cooling function and generic nonequilibrium initial conditions to follow the formation history of a multiphase medium in detail in the absence of gravity. We outline a number of qualitatively distinct stages of this process, including a linear isobaric evolution, transition to an isochoric regime, formation of filaments and voids (also known as "thermal" pancakes), the development and decay of supersonic turbulence, an approach to pressure equilibrium, and final relaxation of the multiphase medium. We find that 1%-2% of the initial thermal energy is converted into gas motions in one cooling time. The velocity field then randomizes into turbulence that decays on a dynamical timescale E_k ~ t^-n, 1 < n < 2. While not all initial conditions yield a stable two-phase medium, we examine such a case in detail. We find that the two phases are well mixed with the cold clouds possessing a fine-grained structure near our numerical resolution limit. The amount of gas in the intermediate unstable phase roughly tracks the rms turbulent Mach number, peaking at 25% when M_rms ~ 8, decreasing to 11% when M_rms ~ 0.4.

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arXiv2002

Cosmological Hydrogen Reionization with Three Dimensional Radiative Transfer

We present new calculations of the inhomogeneous process of cosmological reionization by carefully following the radiative transfer in pre-computed hydrodynamical simulations of galaxy formation. These new computations represent an important step on the way towards fully self-consistent and adaptive calculations which will eventually cover the enormous range of scales from the sizes of individual mini-halos to the mean free path of ionizing photons in the post-overlap Universe. The goal of such simulations is to include enough realistic physics to accurately model the formation of early structures and the end of the `dark ages'. Our new calculations demonstrate that the process by which the ionized regions percolate the Universe is complex, and that the idea of voids being ionized before overdense regions is perhaps too simplistic. It seems that observational information pertaining to the reionization epoch may now be in our grasp, through the detection of Gunn-Peterson troughs at z~6. If so, then the comparison of information from many lines of sight with simulations such as ours may allow us to disentangle details of the ionization history and trace the early formation of structure.

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arXiv2001

Achieving Extreme Resolution in Numerical Cosmology Using Adaptive Mesh Refinement: Resolving Primordial Star Formation

As an entry for the 2001 Gordon Bell Award in the "special" category, we describe our 3-d, hybrid, adaptive mesh refinement (AMR) code, Enzo, designed for high-resolution, multiphysics, cosmological structure formation simulations. Our parallel implementation places no limit on the depth or complexity of the adaptive grid hierarchy, allowing us to achieve unprecedented spatial and temporal dynamic range. We report on a simulation of primordial star formation which develops over 8000 subgrids at 34 levels of refinement to achieve a local refinement of a factor of 10^12 in space and time. This allows us to resolve the properties of the first stars which form in the universe assuming standard physics and a standard cosmological model. Achieving extreme resolution requires the use of 128-bit extended precision arithmetic (EPA) to accurately specify the subgrid positions. We describe our EPA AMR implementation on the IBM SP2 Blue Horizon system at the San Diego Supercomputer Center.

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arXiv2001

The Formation of the First Star in the Universe

We describe results from a fully self-consistent three dimensional hydrodynamical simulation of the formation of one of the first stars in the Universe. Dark matter dominated pre-galactic objects form because of gravitational instability from small initidal density perturbations. As they assemble via hierarchical merging, primordial gas cools through ro-vibrational lines of hydrogen molecules and sinks to the center of the dark matter potential well. The high redshift analog of a molecular cloud is formed. When the dense, central parts of the cold gas cloud become self-gravitating, a dense core of approximately 100 solar mass undergoes rapid contraction. At densities n>10^9 cm^-3 a one solar mass proto-stellar core becomes fully molecular due to three-body H_2 formation. Contrary to analytical expectations this process does not lead to renewed fragmentation and only one star is formed. The calculation is stopped when optical depth effects become important, leaving the final mass of the fully formed star somewhat uncertain. At this stage the protostar is acreting material very rapidly (~0.01 solar masses per year). Radiative feedback from the star will not only halt its growth but also inhibit the formation of other stars in the same pre-galactic object (at least until the first star ends its life, presumably as a supernova). We conclude that at most one massive (M >> 1 solar mass) metal free star forms per pre-galactic halo, consistent with recent abundance measurements of metal poor galactic halo stars.

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arXiv2000

Baryons in the Warm-Hot Intergalactic Medium

Approximately 30-40% of all baryons in the present day universe reside in a warm-hot intergalactic medium (WHIM), with temperatures between 10^5<T<10^7 K. This is a generic prediction from six hydrodynamic simulations of currently favored structure formation models having a wide variety of numerical methods, input physics, volumes, and spatial resolutions. Most of these warm-hot baryons reside in diffuse large-scale structures with a median overdensity around 10-30, not in virialized objects such as galaxy groups or galactic halos. The evolution of the WHIM is primarily driven by shock heating from gravitational perturbations breaking on mildly nonlinear, non-equilibrium structures such as filaments. Supernova feedback energy and radiative cooling play lesser roles in its evolution. WHIM gas is consistent with observations of the 0.25 keV X-ray background without being significantly heated by non-gravitational processes because the emitting gas is very diffuse. Our results confirm and extend previous work by Cen & Ostriker and Dave' et al.

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arXiv2000

First Structure Formation and the First Stars

We discuss the results of recent 3D simulations of first structure formation in relationship to the formation of the first stars. On the basis of a new, high-resolution AMR simulation (spatial dynamic range = 30,000,000), we conclude that the first stars are likely to be massive.

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arXiv2000

Simulated Cluster Archive: A Computational Catalog of X-Ray Clusters in a Lambda-CDM Universe

We have simulated the evolution of a large sample of X-ray clusters in a Lambda-CDM universe at high spatial resolution using adaptive mesh refinement and placed the results in an online archive for public access. The Simulated Cluster Archive website http://sca.ncsa.uiuc.edu provides tools for interactive 2D and 3D analysis of gas and dark matter fields, X-ray and SZ imaging, and data export. We encourage community use and solicit their feedback.

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arXiv2000

Introducing ZEUS-MP: A 3D, Parallel, Multiphysics Code for Astrophysical Fluid Dynamics

We describe ZEUS-MP: a Multi-Physics, Massively-Parallel, Message-Passing code for astrophysical fluid dynamics simulations in 3 dimensions. ZEUS-MP is a follow-on to the sequential ZEUS-2D and ZEUS-3D codes developed and disseminated by the Laboratory for Computational Astrophysics (lca.ncsa.uiuc.edu) at NCSA. V1.0 released 1/1/2000 includes the following physics modules: ideal hydrodynamics, ideal MHD, and self-gravity. Future releases will include flux-limited radiation diffusion, thermal heat conduction, two-temperature plasma, and heating and cooling functions. The covariant equations are cast on a moving Eulerian grid with Cartesian, cylindrical, and spherical polar coordinates currently supported. Parallelization is done by domain decomposition and implemented in F77 and MPI. The code is portable across a wide range of platforms from networks of workstations to massively parallel processors. Some parallel performance results are presented as well as an application to turbulent star formation.

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arXiv2000

Formation of Galactic Bulges

We use cosmological hydrodynamic simulations to investigate formation of galactic bulges within the framework of hierarchical clustering in a representative CDM cosmological model. We show that largest objects forming at cosmological redshifts z~4 resemble observed bulges of spiral galaxies or moderate size ellipticals in their general properties like sizes, shapes, and density profiles. This is consistent with observational data indicating the existence of ``old'' bulges and ellipticals at more moderate redshifts. These bulges are gas dominated at redshift z=3, with high rates of star formation and would appear to be good candidates for small blue galaxies seen in the Hubble Deep Field.

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arXiv2000

The Formation and Fragmentation of Primordial Molecular Clouds

Many questions in physical cosmology regarding the thermal history of the intergalactic medium, chemical enrichment, reionization, etc. are thought to be intimately related to the nature and evolution of pregalactic structure. In particular the efficiency of primordial star formation and the primordial IMF are of special interest. We present results from high resolution three--dimensional adaptive mesh refinement simulations that follow the collapse of primordial molecular clouds and their subsequent fragmentation within a cosmologically representative volume. Comoving scales from 128 kpc down to 1 pc are followed accurately. Dark matter dynamics, hydrodynamics and all relevant chemical and radiative processes (cooling) are followed self-consistently for a cluster normalized CDM structure formation model. Primordial molecular clouds with ~10^5 solar masses are assembled by mergers of multiple objects that have formed hydrogen molecules in the gas phase with a fractional abundance of ~10^-4. As the subclumps merge cooling lowers the temperature to ~200 Kelvin in a `cold pocket' at the center of the halo. Within this cold pocket, a quasi-hydrostatically contracting core with ~200 solar mass and number densities > 10^5 cm^-3 is found. We find that less than 1% of the primordial gas in such small scale structures cools and collapses to sufficiently high densities to be available for primordial star formation. Furthermore, it is worthwhile to note that this study achieved the highest dynamic range covered by structured adaptive mesh techniques in cosmological hydrodynamics to date.

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arXiv1998

Resolving the Lyman-alpha Forest

In this paper we critically examine predictions of the Lya forest within the standard cold dark matter (SCDM) model, paying particular attention to the low end of the column-density distribution. We show in particular that the width of these lines, typically measured by the b-parameter of a Voigt profile, is sensitive to spatial resolution in numerical simulations and has previously been overestimated. The new result, which predicts a distribution with a median b of around 20-22 km/s at z=3, is substantially below that observed. We examine a number of possible causes of this discrepancy and argue that it is unlikely to be rectified by an increase in the thermal broadening of the absorbing gas, but is instead telling us something about the distribution of matter on these scales. Although the median differs, the shape of the b-parameter distribution agrees quite well with that observed, and the high-end tail is naturally produced by the filamentary nature of gravitational collapse in these models. In particular, we demonstrate that lines of sight which obliquely intersect a filament or sheet tend to produce absorption lines with larger b parameters. We also examine the physical nature of the gas which is responsible for the forest, showing that for lines with neutral column densities below N_HI ~ 10^14 cm^-2 (for this model at z=3), the peculiar infall velocity is actually slower than the Hubble flow, while larger lines have, on average, turned around and are collapsing.

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arXiv1998

Photon Conserving Radiative Transfer around Point Sources in multi-dimensional Numerical Cosmology

Many questions in physical cosmology regarding the thermal and ionization history of the intergalactic medium are now successfully studied with the help of cosmological hydrodynamical simulations. Here we present a numerical method that solves the radiative transfer around point sources within a three dimensional cartesian grid. The method is energy conserving independently of resolution: this ensures the correct propagation speeds of ionization fronts. We describe the details of the algorithm, and compute as first numerical application the ionized region surrounding a mini-quasar in a cosmological density field at z=7.

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arXiv1998

The Formation and Fragmentation of Primordial Molecular Clouds

Many questions in physical cosmology regarding the thermal history of the intergalactic medium, chemical enrichment, reionization, etc. are thought to be intimately related to the nature and evolution of pregalactic structure. In particular the efficiency of primordial star formation and the primordial IMF are of special interest. We present results from high resolution three-dimensional adaptive mesh refinement simulations that follow the collapse of primordial molecular clouds and their subsequent fragmentation within a cosmologically representative volume. Comoving scales from 128 kpc down to 0.5 pc are followed accurately. Dark matter dynamics, hydrodynamics and all relevant chemical and radiative processes (cooling) are followed self-consistently for a cluster normalized CDM structure formation model. Primordial molecular clouds with ~100,000 solar masses are assembled by mergers of multiple objects that have formed hydrogen molecules in the gas phase with a fractional abundance of <~ 10^-4. As the subclumps merge cooling decreases the temperature to ~200 Kelvin in multiple "cold pockets" of the merger product. Within these cold pockets, quasi-hydrostatically contracting cores with masses \~ 100 M_sun and number densities > 100,000/cm^3 are found. We find that less than 1% of the primordial gas in such small scale structures cools and collapses to sufficiently high densities to be available for primordial star formation. Furthermore, our results indicate that the formation of very massive objects, massive black holes, fragmentation of a large fraction of baryons into brown dwars or Jupiter size fragments seems, in contrast to various claims in the literature, very unlikely. The expected escape fraction of UV photons with (h nu) > 11eV is very small.

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arXiv1998

Simulating the Lyman Alpha Forest

In this paper we review the importance of the Lyman alpha forest as a probe of structure formation in the universe. We first discuss the statistics used to describe the Lyman alpha forest and the numerical techniques used to produce simulated spectra of the forest from a given cosmological model. We then discuss the physical picture of the absorbing structures that emerges from these numerical simulations. Finally, we comment on how two of the statistics, the slope of the column density distribution and the b parameter distribution, may be used to constrain competing cosmologies.

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arXiv1998

Simulating Inhomogeneous Reionization

We describe an approach for incorporating radiative transfer into 3D hydrodynamic cosmological simulations. The method, while approximate, allows for a self-consistent treatment of self-shielding and shadowing, diffuse and point sources of radiation, and frequency dependent transfer. Applications include photodissociation, photoheating, and photoionization of the IGM.

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