arXiv subjects

Michael L. Norman

Publications and source records attributed to Michael L. Norman.

At least 55 records · Page 3Linked to original sources

arXiv2012

Comparisons of Cosmological MHD Galaxy Cluster Simulations to Radio Observations

Radio observations of galaxy clusters show that there are $μ$G magnetic fields permeating the intra-cluster medium (ICM), but it is hard to accurately constrain the strength and structure of the magnetic fields without the help of advanced computer simulations. We present qualitative comparisons of synthetic VLA observations of simulated galaxy clusters to radio observations of Faraday Rotation Measure (RM) and radio halos. The cluster formation is modeled using adaptive mesh refinement (AMR) magneto-hydrodynamic (MHD) simulations with the assumption that the initial magnetic fields are injected into the ICM by active galactic nuclei (AGNs) at high redshift. In addition to simulated clusters in Xu et al. (2010, 2011), we present a new simulation with magnetic field injections from multiple AGNs. We find that the cluster with multiple injection sources is magnetized to a similar level as in previous simulations with a single AGN. The RM profiles from simulated clusters, both $|RM|$ and the dispersion of RM ($σ_{RM}$), are consistent at a first-order with the radial distribution from observations. The correlations between the $σ_{RM}$ and X-ray surface brightness from simulations are in a broad agreement with the observations, although there is an indication that the simulated clusters could be slightly over-dense and less magnetized with respect to those in the observed sample. In addition, the simulated radio halos agree with the observed correlations between the radio power versus the cluster X-ray luminosity and between the radio power versus the radio halo size. These studies show that the cluster wide magnetic fields that originate from AGNs and are then amplified by the ICM turbulence (Xu et al. 2010) match observations of magnetic fields in galaxy clusters.

astro-ph.CO↗

arXiv2012

Flux Correlations in Supersonic Isothermal Turbulence

Using data from a large-scale three-dimensional simulation of supersonic isothermal turbulence, we have tested the validity of an exact flux relation derived analytically from the Navier--Stokes equation by Falkovich, Fouxon and Oz [2010 New relations for correlation functions in Navier--Stokes turbulence. J. Fluid Mech. 644, 465]. That relation, for compressible barotropic fluids, was derived assuming turbulence generated by a large-scale force. However, compressible turbulence in simulations is usually initialized and maintained by a large-scale acceleration, as in gravity-driven astrophysical flows. We present a new approximate flux relation for isothermal turbulence driven by a large-scale acceleration, and find it in reasonable agreement with the simulation results.

physics.flu-dyn↗

arXiv2012

The Birth of a Galaxy. II. The Role of Radiation Pressure

Massive stars provide feedback that shapes the interstellar medium of galaxies at all redshifts and their resulting stellar populations. Here we present three adaptive mesh refinement radiation hydrodynamics simulations that illustrate the impact of momentum transfer from ionising radiation to the absorbing gas on star formation in high-redshift dwarf galaxies. Momentum transfer is calculated by solving the radiative transfer equation with a ray tracing algorithm that is adaptive in spatial and angular coordinates. We find that momentum input partially affects star formation by increasing the turbulent support to a three-dimensional rms velocity equal to the circular velocity of early haloes. Compared to a calculation that neglects radiation pressure, the star formation rate is decreased by a factor of five to 1.8 x 10^{-2} Msun/yr in a dwarf galaxy with a dark matter and stellar mass of 2.0 x 10^8 and 4.5 x 10^5 solar masses, respectively, when radiation pressure is included. Its mean metallicity of 10^{-2.1} Z_sun is consistent with the observed dwarf galaxy luminosity-metallicity relation. However, what one may naively expect from the calculation without radiation pressure, the central region of the galaxy overcools and produces a compact, metal-rich stellar population with an average metallicity of 0.3 Z_sun, indicative of an incorrect physical recipe. In addition to photo-heating in HII regions, radiation pressure further drives dense gas from star forming regions, so supernovae feedback occurs in a warmer and more diffuse medium, launching metal-rich outflows. Capturing this aspect and a temporal separation between the start of radiative and supernova feedback are numerically important in the modeling of galaxies to avoid the "overcooling problem". We estimate that dust in early low-mass galaxies is unlikely to aid in momentum transfer from radiation to the gas.

astro-ph.CO↗

arXiv2012

What Shapes the Structure of Molecular Clouds: Turbulence or Gravity?

We revisit the origin of Larson's scaling relations, which describe the structure and kinematics of molecular clouds, based on recent observations and large-scale simulations of supersonic turbulence. Using dimensional analysis, we first show that both linewidth-size and mass-size correlations observed on scales 0.1-50 pc can be explained by a simple conceptual theory of compressible turbulence without resorting to the often assumed virial equilibrium or detailed energy balance condition. The scaling laws can be consistently interpreted as a signature of supersonic turbulence with no need to invoke gravity. We then show how self-similarity of structure established by the turbulence breaks in star-forming clouds through development of gravitational instabilities in the vicinity of the sonic scale, l_s~0.1 pc, leading to the formation of prestellar cores.

astro-ph.GA↗

arXiv2012

The Two States of Star Forming Clouds

We examine the effects of self-gravity and magnetic fields on supersonic turbulence in isothermal molecular clouds with high resolution simulations and adaptive mesh refinement. These simulations use large root grids (512^3) to capture turbulence and four levels of refinement to capture high density, for an effective resolution of 8,196^3. Three Mach 9 simulations are performed, two super-Alfvénic and one trans-Alfvénic. We find that gravity splits the clouds into two populations, one low density turbulent state and one high density collapsing state. The low density state exhibits properties similar to non-self-gravitating in this regime, and we examine the effects of varied magnetic field strength on statistical properties: the density probability distribution function is approximately lognormal; velocity power spectral slopes decrease with field strength; alignment between velocity and magnetic field increases with field; the magnetic field probability distribution can be fit to a stretched exponential. The high density state is characterized by self-similar spheres; the density PDF is a power-law; collapse rate decreases with increasing mean field; density power spectra have positive slopes, P(ρ,k) \propto k; thermal-to-magnetic pressure ratios are unity for all simulations; dynamic-to-magnetic pressure ratios are larger than unity for all simulations; magnetic field distribution is a power-law. The high Alfvén Mach numbers in collapsing regions explain recent observations of magnetic influence decreasing with density. We also find that the high density state is found in filaments formed by converging flows, consistent with recent Herschel observations. Possible modifications to existing star formation theories are explored.

astro-ph.SR↗

arXiv2011

The Birth of a Galaxy: Primordial Metal Enrichment and Stellar Populations

By definition, Population III stars are metal-free, and their protostellar collapse is driven by molecular hydrogen cooling in the gas-phase, leading to large characteristic masses. Population II stars with lower characteristic masses form when the star-forming gas reaches a critical metallicity of 10^{-6} - 10^{-3.5} Z_\odot. We present an adaptive mesh refinement radiation hydrodynamics simulation that follows the transition from Population III to II star formation. The maximum spatial resolution of 1 comoving parsec allows for individual molecular clouds to be well-resolved and their stellar associations to be studied in detail. We model stellar radiative feedback with adaptive ray tracing. A top-heavy initial mass function for the Population III stars is considered, resulting in a plausible distribution of pair-instability supernovae and associated metal enrichment. We find that the gas fraction recovers from 5 percent to nearly the cosmic fraction in halos with merger histories rich in halos above 10^7 solar masses. A single pair-instability supernova is sufficient to enrich the host halo to a metallicity floor of 10^{-3} Z_\odot and to transition to Population II star formation. This provides a natural explanation for the observed floor on damped Lyman alpha (DLA) systems metallicities reported in the literature, which is of this order. We find that stellar metallicities do not necessarily trace stellar ages, as mergers of halos with established stellar populations can create superpositions of t-Z evolutionary tracks. A bimodal metallicity distribution is created after a starburst occurs when the halo can cool efficiently through atomic line cooling.

astro-ph.CO↗

arXiv2011

Evolution and Distribution of Magnetic Fields from AGNs in Galaxy Clusters II. The Effects of Cluster Size and Dynamical State

Theory and simulations suggest that magnetic fields from radio jets and lobes powered by their central super massive black holes can be an important source of magnetic fields in the galaxy clusters. This is paper II in a series of studies where we present self-consistent high-resolution adaptive mesh refinement cosmological magnetohydrodynamic (MHD) simulations that simultaneously follow the formation of a galaxy cluster and evolution of magnetic fields ejected by an active galactic nucleus (AGN). We studied 12 different galaxy clusters with virial masses ranging from 1 $\times$ 10$^{14}$ to 2 $\times$ 10$^{15}$ M$_{\odot}$. In this work we examine the effects of the mass and merger history on the final magnetic properties. We find that the evolution of magnetic fields is qualitatively similar to those of previous studies. In most clusters, the injected magnetic fields can be transported throughout the cluster and be further amplified by the intra-cluster medium (ICM) turbulence during the cluster formation process with hierarchical mergers, while the amplification history and the magnetic field distribution depend on the cluster formation and magnetism history. This can be very different for different clusters. The total magnetic energies in these clusters are between 4 $\times$ 10$^{57}$ and $10^{61}$ erg, which is mainly decided by the cluster mass, scaling approximately with the square of the total mass. Dynamically older relaxed clusters usually have more magnetic fields in their ICM. The dynamically very young clusters may be magnetized weakly since there is not enough time for magnetic fields to be amplified.

astro-ph.CO↗

arXiv2011

Turbulent Clustering of Protoplanetary Dust and Planetesimal Formation

We study clustering of inertial particles in turbulent flows and discuss its applications to dust particles in protoplanetary disks. Using numerical simulations, we compute the radial distribution function (RDF), which measures the probability of finding particle pairs at given distances, and the probability density function of the particle concentration. The clustering statistics depend on the Stokes number, $St$, defined as the ratio of the particle friction timescale, $τ_{\rm p} $, to the Kolmogorov timescale in the flow. In the dissipation range, the clustering intensity strongly peaks at $St \simeq 1$, and the RDF for $St \sim 1$ shows a fast power-law increase toward small scales, suggesting that turbulent clustering may considerably enhance the particle collision rate. Clustering at inertial-range scales is of particular interest to the problem of planetesimal formation. At these scales, the strongest clustering is from particles with $τ_{\rm p}$ in the inertial range. Clustering of these particles occurs primarily around a scale where the eddy turnover time is $\simτ_{\rm p}$. Particles of different sizes tend to cluster at different locations, leading to flat RDFs between different particles at small scales. In the presence of multiple particle sizes, the overall clustering strength decreases as the particle size distribution broadens. We discuss particle clustering in recent models for planetesimal formation. We point out that, in the model based on turbulent clustering of chondrule-size particles, the probability of finding strong clusters that can seed planetesimals may have been significantly overestimated. We discuss various clustering mechanisms in simulations of planetesimal formation by gravitational collapse of dense clumps of meter-size particles, in particular the contribution from turbulent clustering due to the limited numerical resolution.

astro-ph.EP↗

arXiv2011

Comparing Numerical Methods for Isothermal Magnetized Supersonic Turbulence

We employ simulations of supersonic super-Alfvenic turbulence decay as a benchmark test problem to assess and compare the performance of nine astrophysical MHD methods actively used to model star formation. The set of nine codes includes: ENZO, FLASH, KT-MHD, LL-MHD, PLUTO, PPML, RAMSES, STAGGER, and ZEUS. We present a comprehensive set of statistical measures designed to quantify the effects of numerical dissipation in these MHD solvers. We compare power spectra for basic fields to determine the effective spectral bandwidth of the methods and rank them based on their relative effective Reynolds numbers. We also compare numerical dissipation for solenoidal and dilatational velocity components to check for possible impacts of the numerics on small-scale density statistics. Finally, we discuss convergence of various characteristics for the turbulence decay test and impacts of various components of numerical schemes on the accuracy of solutions. We show that the best performing codes employ a consistently high order of accuracy for spatial reconstruction of the evolved fields, transverse gradient interpolation, conservation law update step, and Lorentz force computation. The best results are achieved with divergence-free evolution of the magnetic field using the constrained transport method, and using little to no explicit artificial viscosity. Codes which fall short in one or more of these areas are still useful, but they must compensate higher numerical dissipation with higher numerical resolution. This paper is the largest, most comprehensive MHD code comparison on an application-like test problem to date. We hope this work will help developers improve their numerical algorithms while helping users to make informed choices in picking optimal applications for their specific astrophysical problems.

astro-ph.SR↗

arXiv2011

Dispersal of Galactic Magnetic Fields into Intracluster Space

Little is known about the origin and basic properties of magnetic fields in clusters of galaxies. High conductivity in magnetized interstellar plasma suggests that galactic magnetic fields are (at least partly) ejected into intracluster (IC) space by the same processes that enrich IC gas with metals. We explore the dispersal of galactic fields by hydrodynamical simulations with our new {\em Enzo-Galcon} code, which is capable of tracking a large number galaxies during cluster assembly, and modeling the processes that disperse their interstellar media. Doing so we are able to describe the evolution of the mean strength of the field and its profile across the cluster. With the known density profile of dispersed gas and an estimated range of coherence scales, we predict the spatial distribution of Faraday rotation measure and find it to be consistent with observational data.

astro-ph.CO↗

arXiv2011

On the Density Distribution in Star-forming Interstellar Clouds

We use deep adaptive mesh refinement simulations of isothermal self-gravitating supersonic turbulence to study the imprints of gravity on the mass density distribution in molecular clouds. The simulations show that the density distribution in self-gravitating clouds develops an extended power-law tail at high densities on top of the usual lognormal. We associate the origin of the tail with self-similar collapse solutions and predict the power index values in the range from -7/4 to -3/2 that agree with both simulations and observations of star-forming molecular clouds.

astro-ph.GA↗

arXiv2010

Radiation Hydrodynamical Instabilities in Cosmological and Galactic Ionization Fronts

Ionization fronts, the sharp radiation fronts behind which H/He ionizing photons from massive stars and galaxies propagate through space, were ubiquitous in the universe from its earliest times. The cosmic dark ages ended with the formation of the first primeval stars and galaxies a few hundred Myr after the Big Bang. Numerical simulations suggest that stars in this era were very massive, 25 - 500 solar masses, with H II regions of up to 30,000 light-years in diameter. We present three-dimensional radiation hydrodynamical calculations that reveal that the I-fronts of the first stars and galaxies were prone to violent instabilities, enhancing the escape of UV photons into the early intergalactic medium (IGM) and forming clumpy media in which supernovae later exploded. The enrichment of such clumps with metals by the first supernovae may have led to the prompt formation of a second generation of low-mass stars, profoundly transforming the nature of the first protogalaxies. Cosmological radiation hydrodynamics is unique because ionizing photons coupled strongly to both gas flows and primordial chemistry at early epochs, introducing a hierarchy of disparate characteristic timescales whose relative magnitudes can vary greatly throughout a given calculation. We describe the adaptive multistep integration scheme we have developed for the self-consistent transport of both cosmological and galactic ionization fronts.

astro-ph.CO↗

arXiv2010

Historical perspective on astrophysical MHD simulations

This contribution contains the introductory remarks that I presented at IAU Symposium 270 on ``Computational Star Formation" held in Barcelona, Spain, May 31 -- June 4, 2010. I discuss the historical development of numerical MHD methods in astrophysics from a personal perspective. The recent advent of robust, higher order-accurate MHD algorithms and adaptive mesh refinement numerical simulations promises to greatly improve our understanding of the role of magnetic fields in star formation.

astro-ph.IM↗

arXiv2010

Pop III Stellar Masses and IMF

We provide a status report on our current understanding of the mass scales for Pop III.1 and Pop III.2 stars. Since the last review (Norman 2008), substantial progress has been made both numerically and analytically on the late stages of protostellar cloud core collapse, protostar formation and accretion, and stellar evolution taking into account cloud core properties and radiative feedback effects. Based on this, there are growing indications that primordial stars forming from purely cosmological initial conditions (Pop III.1) were substantially more massive than stars forming in preionized gas (Pop III.2) where HD cooling is important. Different stellar endpoints are predicted for these two types of Pop III stars with different chemical enrichment signatures: the former die as pair instability supernovae or intermediate mass black holes, whereas the latter die as iron core-collapse supernovae, leaving behind neutron star and stellar black hole remnants. We review recent simulations which show evidence for binary fragmentation at high densities, and comment on the significance of these results. We then summarize an attempt to directly calculate the Pop III.1 IMF taking into account the latest numerical and analytical models. We conclude with suggestions for the kind of simulations needed next to continue improving our understanding of Pop III star formation, which is a necessary input to understanding high redshift galaxy formation.

astro-ph.CO↗

arXiv2010

A Multi-Code Analysis Toolkit for Astrophysical Simulation Data

The analysis of complex multiphysics astrophysical simulations presents a unique and rapidly growing set of challenges: reproducibility, parallelization, and vast increases in data size and complexity chief among them. In order to meet these challenges, and in order to open up new avenues for collaboration between users of multiple simulation platforms, we present yt (available at http://yt.enzotools.org/), an open source, community-developed astrophysical analysis and visualization toolkit. Analysis and visualization with yt are oriented around physically relevant quantities rather than quantities native to astrophysical simulation codes. While originally designed for handling Enzo's structure adaptive mesh refinement (AMR) data, yt has been extended to work with several different simulation methods and simulation codes including Orion, RAMSES, and FLASH. We report on its methods for reading, handling, and visualizing data, including projections, multivariate volume rendering, multi-dimensional histograms, halo finding, light cone generation and topologically-connected isocontour identification. Furthermore, we discuss the underlying algorithms yt uses for processing and visualizing data, and its mechanisms for parallelization of analysis tasks.

astro-ph.IM↗

arXiv2010

Interstellar Turbulence and Star Formation

We provide a brief overview of recent advances and outstanding issues in simulations of interstellar turbulence, including isothermal models for interior structure of molecular clouds and larger-scale multiphase models designed to simulate the formation of molecular clouds. We show how self-organization in highly compressible magnetized turbulence in the multiphase ISM can be exploited in simple numerical models to generate realistic initial conditions for star formation.

astro-ph.GA↗

arXiv2010

Evolution and Distribution of Magnetic Fields from AGNs in Galaxy Clusters. I. The Effect of Injection Energy and Redshift

We present a series of cosmological magnetohydrodynamic (MHD) simulations that simultaneously follow the formation of a galaxy cluster and evolution of magnetic fields ejected by an Active Galactic Nucleus (AGN). Specifically, we investigate the influence of both the epoch of AGN (z $\sim$ 3-0.5) and the AGN energy ($\sim$ 3 $\times$ 10$^{57}$ - 2 $\times$ 10$^{60}$ ergs)on the final magnetic field distribution in a relatively massive cluster (M$_{vir}$ $\sim$10$^{15}$ M$_\odot$). We find that as long as the AGN magnetic fields are ejected before the major mergers in the cluster formation history, magnetic fields can be transported throughout the cluster and can be further amplified by the intra-cluster medium (ICM) turbulence cause by hierarchical mergers during the cluster formation process. The total magnetic energy in the cluster can reach $\sim$ $10^{61}$ ergs, with micro Gauss fields distributed over $\sim$ Mpc scale. The amplification of the total magnetic energy by the ICM turbulence can be significant, up to $\sim$1000 times in some cases. Therefore even weak magnetic fields from AGNs can be used to magnetize the cluster to the observed level. The final magnetic energy in the ICM is determined by the ICM turbulent energy, with a weak dependence on the AGN injection energy. We discuss the properties of magnetic fields throughout the cluster and the synthetic Faraday rotation measure maps they produce. We also show that high spatial resolution over most of the magnetic regions of the cluster is very important to capture the small scale dynamo process and maintain the magnetic field structure in our simulations.

astro-ph.CO↗

arXiv2010

High-Entropy Polar Regions Around the First Protostars

We report on simulations of the formation of the first stars in the Universe, where we identify regions of hot atomic gas (fH2 < 10-6) at densities above 10-14 g/cc, heated to temperatures ranging between 3000 and 8000 K. Within this temperature range atomic hydrogen is unable to cool effectively. We describe the kinetic and thermal characteristics of these regions and investigate their origin. We find that these regions, while small in total mass fraction of the cloud, may be dynamically important over the accretion timescale for the central clump in the cloud, particularly as a chemical, rather than radiative, mechanism for clearing the polar regions of the accretion disk of material and terminating accretion along these directions. These inherently three-dimensional effects stress the need for multi-dimensional calculations of protostellar accretion for reliable predictions of the masses of the very first stars.

astro-ph.CO↗