Zoom Region Research Articles

Representing low-mass black hole seeds in cosmological simulations: A new sub-grid stochastic seed model

ABSTRACT The physical origin of the seeds of supermassive black holes (SMBHs), with postulated initial masses ranging from ∼105 M⊙ to as low as ∼102 M⊙, is currently unknown. Most existing cosmological hydrodynamic simulations adopt very simple, ad hoc prescriptions for BH seeding and seed at unphysically high masses ∼105–106 M⊙. In this work, we introduce a novel sub-grid BH seeding model for cosmological simulations that is directly calibrated to high-resolution zoom simulations that explicitly resolve ∼103 M⊙ seeds forming within haloes with pristine, dense gas. We trace the BH growth along galaxy merger trees until their descendants reach masses of ∼104 or 105 M⊙. The results are used to build a new stochastic seeding model that directly seeds these descendants in lower resolution versions of our zoom region. Remarkably, we find that by seeding the descendants simply based on total galaxy mass, redshift and an environmental richness parameter, we can reproduce the results of the detailed gas-based seeding model. The baryonic properties of the host galaxies are well reproduced by the mass-based seeding criterion. The redshift-dependence of the mass-based criterion captures the combined influence of halo growth, dense gas formation, and metal enrichment on the formation of ∼103 M⊙ seeds. The environment-based seeding criterion seeds the descendants in rich environments with higher numbers of neighbouring galaxies. This accounts for the impact of unresolved merger dominated growth of BHs, which produces faster growth of descendants in richer environments with more extensive BH merger history. Our new seed model will be useful for representing a variety of low-mass seeding channels within next-generation larger volume uniform cosmological simulations.

Mapping Structural Connectivity Using Diffusion MRI: Challenges and Opportunities

The effects of the misalignment between image-intensity-based tissue segmentation and brain parcellation on the assignment of streamlines to network nodes: Upper panel: (a) With the application of anatomical constraints (e.g., Ref. 35), streamline endpoints (colored in purple) occur at the GM-WM interface or within the subcortical GM. (b) Due to factors such as discretization of structural labels, many of these endpoints (purple points) do not locate inside the, for example, FreeSurfer parcellation image (blue ribbon)48 and thus are not assigned to a label. (c) The T1 image shown in (a,b) is replaced by the GM partial volume maps derived from intensity-based tissue segmentation.80 (d) A zoom region of (c) illustrates the discrepancy (pointed by arrows) between tissue segmentation and brain parcellation, revealing that streamlines cannot be assigned purely based on the voxels where the endpoints reside. Bottom panel: The discrepancies also present in subcortical GM, and in fact could be even crucial when parcellation images are prepared by transforming an atlas to an individual's space (i.e., the left branch of Fig. 5). This is because the degree of misalignment between subcortical GM segmentations and brain parcellations could be increased by the registration error. As an example: (e) The AAL atlas is coregistered to individuals' data via linear and nonlinear transformations. The background shows a T1-weighted image slice of the subject. (f) The background image is replaced by the GM partial volume map obtained from tissue segmentation. (g) The red arrows point to the considerable misalignment between two images at the thalami regions by Chun-Hung 1666–1682.

I – A hydrodynamical clone of the Virgo cluster of galaxies to confirm observationally driven formation scenarios

ABSTRACT At ∼16–17 Mpc from us, the Virgo cluster is a formidable source of information to study cluster formation and galaxy evolution in rich environments. Several observationally driven formation scenarios arose within the past decade to explain the properties of galaxies that entered the cluster recently and the nature of the last significant merger that the cluster underwent. Confirming these scenarios requires extremely faithful numerical counterparts of the cluster. This paper presents the first clone, Constrained LOcal and Nesting Environment, simulation of the Virgo cluster within a ∼15 Mpc radius sphere. This cosmological hydrodynamical simulation, with feedback from supernovae and active galactic nuclei, with a ∼3 × 107 M⊙ dark matter particle mass and a minimum cell size of 350 pc in the zoom region, reproduces Virgo within its large-scale environment unlike a random cluster simulation. Overall the distribution of the simulated galaxy population matches the observed one including M87. The simulated cluster formation reveals exquisite agreements with observationally driven scenarios: within the last Gyr, about 300 small galaxies (M* > 107 M⊙) entered the cluster, most of them within the last 500 Myr. The last significant merger event occurred about 2 Gyr ago: a group with a tenth of the mass of today’s cluster entered from the far side as viewed from the Milky Way. This excellent numerical replica of Virgo will permit studying different galaxy type evolution (jellyfish, backsplash, etc.) as well as feedback phenomena in the cluster core via unbiased comparisons between simulated and observed galaxies and hot gas phase profiles to understand this great physics laboratory.

Improving performance of zoom-in cosmological simulations using initial conditions with customized grids

We present a method for customizing the root grid of zoom-in initial conditions used for simulations of galaxy formation. Starting from the white noise used to seed the structures of an existing initial condition, we cut out a smaller region of interest and use this trimmed white noise cube to create a new root grid. This new root grid contains similar structures as the original, but allows for a smaller box volume and different grid resolution that can be tuned to best suit a given simulation code. To minimally disturb the zoom region, the dark matter particles and gas cells from the original zoom region are placed within the new root grid, with no modification other than a bulk velocity offset to match the systemic velocity of the corresponding region in the new root grid. We validate this method using a zoom-in initial condition containing a Local Group analog. We run collisionless simulations using the original and modified initial conditions, finding good agreement. The dark matter halo masses of the two most massive galaxies at z=0 match the original to within 15%. The times and masses of major mergers are reproduced well, as are the full dark matter accretion histories. While we do not reproduce specific satellite galaxies found in the original simulation, we obtain qualitative agreement in the distributions of the maximum circular velocity and the distance from the central galaxy. We also examine the runtime speedup provided by this method for full hydrodynamic simulations with the ART code. We find that reducing the root grid cell size improves performance, but the increased particle and cell numbers can negate some of the gain. We test several realizations, with our best runs achieving a speedup of nearly a factor of two.

Shattering of Cosmic Sheets due to Thermal Instabilities: A Formation Channel for Metal-free Lyman Limit Systems

Abstract We present a new cosmological zoom-in simulation, where the zoom region consists of two halos with virial mass M v ∼ 5 × 1012 M ⊙ and an approximately megaparsec long cosmic filament connecting them at z ∼ 2. Using this simulation, we study the evolution of the intergalactic medium in between these two halos at unprecedented resolution. At 5 ≳ z ≳ 3, the two halos are found to lie in a large intergalactic sheet, or “pancake,” consisting of multiple coplanar dense filaments along which nearly all halos with M v > 109 M ⊙ are located. This sheet collapses at z ∼ 5 from the merger of two smaller sheets. The strong shock generated by this merger leads to thermal instabilities in the postshock region, and to a shattering of the sheet resulting in ≲ kiloparsec-scale clouds with temperatures of T ≳ 2 × 104 K and densities of n ≳ 10−3 cm−3, which are pressure confined in a hot medium with T ∼ 106 K and n ≳ 10−5 cm−3. When the sheet is viewed face-on, these cold clouds have neutral hydrogen column densities of N H i > 1017.2 cm−2, making them detectable as Lyman limit systems, though they lie well outside the virial radius of any halo and even well outside the dense filaments. Their chemical composition is pristine, having zero metallicity, similar to several recently observed systems. Since these systems form far from any galaxies, these results are robust to galaxy formation physics, resulting purely from the collapse of large-scale structure and radiative cooling, provided sufficient spatial resolution is available.

Identifying visual targets amongst interfering distractors: Sorting out the roles of perceptual load, dilution, and attentional zoom.

Visual selection is imperfect; whenever a complex array of objects is processed, representations of multiple objects are likely to be active simultaneously. A full account of attentional processing must explain how these representations affect one another and how they interact to produce a response. Evidence on these interactions comes from measures of distractor interference and from dilution of distractor effects by other nontargets. Based on these data, different principles have been proposed to help understand target-distractor interactions, including accounts based on perceptual load and on dilution among nontargets. We review evidence from a number of experiments, including some using Yantis and Jonides's (Journal of Experimental Psychology: Human Perception and Performance, 10, 601-621, 1984, Journal of Experimental Psychology: Human Perception and Performance, 16, 121-134, 1990) methods for preventing abrupt onsets, which can disrupt spatial attention. The results underscore spatial constraints on the allocation of attention to include targets and exclude distractors. Selection is most effective when a single region can be selected that includes all possible target locations and excludes possible distractor locations. This region can be expanded or contracted as needed for the task, as suggested by C. W. Eriksen and St. James's (Perception & Psychophysics, 40, 225-240, 1986) zoom lens model. This attentional zoom setting is probably affected by a number of factors, including the number of nontargets, the similarity among stimulus elements, the discriminability of the possible targets, and the discrimination difficulty of a concurrent task. A narrower attentional zoom setting that excludes a distractor will prevent interference from that distractor. Interference from a distractor will be diluted by nontargets, but only if they are within the attentional zoom region.

How to zoom: bias, contamination and Lagrange volumes in multimass cosmological simulations

We perform a suite of multimass cosmological zoom simulations of individual dark matter halos and explore how to best select Lagrangian regions for resimulation without contaminating the halo of interest with low-resolution particles. Such contamination can lead to significant errors in the gas distribution of hydrodynamical simulations, as we show. For a fixed Lagrange volume, we find that the chance of contamination increases systematically with the level of zoom. In order to avoid contamination, the Lagrangian volume selected for resimulation must increase monotonically with the resolution difference between parent box and the zoom region. We provide a simple formula for selecting Lagrangian regions (in units of the halo virial volume) as a function of the level of zoom required. We also explore the degree to which a halo's Lagrangian volume correlates with other halo properties (concentration, spin, formation time, shape, etc.) and find no significant correlation. There is a mild correlation between Lagrange volume and environment, such that halos living in the most clustered regions have larger Lagrangian volumes. Nevertheless, selecting halos to be isolated is not the best way to ensure inexpensive zoom simulations. We explain how one can safely choose halos with the smallest Lagrangian volumes, which are the least expensive to resimulate, without biasing one's sample.

The two-way nested global chemistry-transport zoom model TM5: algorithm and applications

Abstract. This paper describes the global chemistry Transport Model, version 5 (TM5) which allows two-way nested zooming. The model is used for global studies which require high resolution regionally but can work on a coarser resolution globally. The zoom algorithm introduces refinement in both space and time in some predefined regions. Boundary conditions of the zoom region are provided by a coarser parent grid and the results of the zoom area are communicated back to the parent. A case study using 222Rn measurements that were taken during the MINOS campaign reveals the advantages of local zooming. As a next step, it is investigated to what extent simulated concentrations over Europe are influenced by using an additional zoom domain over North America. An artificial ozone-like tracer is introduced with a lifetime of twenty days and simplified non-linear chemistry. The concentration differences at Mace Head (Ireland) are generally smaller than 10%, much smaller than the effects of the resolution enhancement over Europe. Thus, coarsening of resolution at some distance of a sampling station seems allowed. However, it is also noted that the budgets of the tracers change considerably due to resolution dependencies of, for instance, vertical transport. Due to the two-way nested algorithm, TM5 offers a consistent tool to study the effects of grid refinement on global atmospheric chemistry issues like intercontinental transport of air pollution.

Accuracy estimation of the 3-D BEM analysis of elastostatic problems by the zooming method

A p-version zooming method for BEM is developed for 3-D elastostatic problems. The objective of this report is to develop an accuracy guarantee method for 3-D BEM analysis. First, the relations between the element degree and the analysis error of the displacement and of the traction are investigated by the numerical experiments of a thick-walled spheroid subjected to the internal or external pressure. Next, on the basis of these relations an accuracy guarantee method for the 3-D zooming method is proposed. In this method, the existence range of the analysis error of the displacement or of the traction at the point of particular interest is given by the sum of two errors. One is the analysis error of the displacement on the zooming boundary and the other is the analysis error of the traction at the point of particular interest in the zooming region. Finally, an adaptive analysis program using this method is developed and applied to the analysis of various 3-D elastostatic problems. The usefulness of this method is illustrated by these application results.

A basic study of the accuracy estimation of structural analysis by the zooming method. (3rd report. Two dimensional elastostatic stress analysis by the finite element method).

As a basic study for the establishment of an accuracy estimation method in the structural analysis using the zooming method, this third report deals with the problems of two dimensional elastostatic stress analysis by the finite element method. From numerical experiments, the following relation was deduced, ε∝ (h/a)k, k>1, where ε is the error of the value by the finite element method relative to the exact value and h/a is the dimensionless mesh diameter. Using this relation, the errors of displacement on zooming boundary and that of stress at the selected points in the zooming region can be estimated by the recursive analyses of the whole region or of the zooming region. The error of stress at the selected points is expected to be smaller than the sum of these errors. A computer program using this error estimation method was developed and applied to several problems of various shapes. The usefulness of this accuracy estimation method was illustrated by these application results.

A basic study of the accuracy estimation of structural analysis by the zooming method. (1st report Finite element analysis of the transverse bending of thin flat plates)

As a basic study for the establishment of an accuracy estimation method in structural analysis using the zooming method, this report deals with the finite element analysis of the problems of transverse bending of thin, flat plates. From numerical experiments for uniform mesh divisions, the following relation was deduced, e∝(h/a)k, kg-1, where e is the error of the value by the finite element method relative to exact value and h/a is the dimensionless mesh diameter. Using this relation, the errors of gradients on a zooming boundary and that of the bending moment at the selected points in the zooming region can be estimated by the recursive analyses of the whole region or of the zooming region. The error of the bending moment at the selected points is expected to be smaller than the sum of these two errors. A computer program using this error estimation method was developed and applied to sixteen problems of various shapes. The usefulness of this accuracy estimation method was illustrated by these application results.

A basic study of the accuracy estimation of structural analysis by the zooming method. Finite element analysis of the transverse bending of thin flat plates.

As a basic study for the establishment of an accuracy estimation method in structural analysis using the zooming method, this report deals with the finite element analysis of the problems of transverse bending of thin, flat plates. From numerical experiments for uniform mesh divisions, the following expression was deduced, ε∝(h/a)k, k≥1, where ε is the error of the value by the finite element method relative to the exact value, and h/a is the dimensionless mesh diameter. Using this relations, the errors of gradients on a zooming boundary, and that of the bending moment at selected points in the zooming region can be estimated by the recursive analyses of the whole region or of the zooming region. The error of the bending moment at the selected points is expected to be smaller than the sum of these two errors. A computer program using this errors estimation method was developed and applied to sixteen problems of various shapes. The usefulness of this accuracy estimation method was illustrated by these application results.

A Zooming Method for Dynamic Contact Stress Analysis by Finite Element Method

A dynamic zooming analysis method (ZAM) is presented, to calculate efficiently dynamic contact stresses and their distributions by the finite element method (FEM). The zooming region is determined by the same method as the static ZAM. However, displacements and velocities of nodal points in the region are iteratively computed by use of boundary values until their values approach the correct ones at a specific discrete time. this method is used to evaluate correctly the approximated initial values of the equation of motion. To verify this dynamic ZAM, we analyze the dynamic contact stresses of contact-impact problems under a condition starting from very roughly approximated initial values. The results calculated by this ZAM agree with those directly obtained by the elastodynamic contact FEM. The dynamic ZAM is efficient to analyze the dynamic contact problems and the dynamic stress concentrations by a small machine memory. Then, this method can be applied to the elastodynamic contact analysis of structures consisting of complicated configurations such as mating gears.