History Of Halo Research Articles

Simulations of early structure formation: Properties of halos that host primordial star formation

Context. Population III (pop III) stars were born in halos characterised by a pristine gas composition. In such a halo, once the gas density reaches nH ∼ 1 cm−3, molecular cooling leads to the collapse of the gas and the birth of pop III stars. Halo properties, such as the chemical abundances, mass, and angular momentum can affect the collapse of the gas, thereby leading to the pop III initial mass function (IMF) of star formation. Aims. We want to study the properties of primordial halos and how halos that host early star formation differ from other types of halos. The aim of this study is to obtain a representative population of halos at a given redshift hosting a cold and massive gas cloud that enables the birth of the first stars. Methods. We investigated the growth of primordial halos in a ΛCDM Universe in a large cosmological simulation. We used the hydrodynamic code RAMSES and the chemical solver KROME to study halo formation with non-equilibrium thermochemistry. We then identified structures in the dark and baryonic matter fields, thereby linking the presence or absence of dense gas clouds to the mass and the physical properties of the hosting halos. Results. In our simulations, the mass threshold for a halo for hosting a cold dense gas cloud is ≃7 × 105 M⊙ and the threshold in the H2 mass fraction is found to be ≃2 × 10−4. This is in agreement with previous works. We find that the halo history and accretion rate play a minor role. Here, we present halos with higher HD abundances, which are shown to be colder, as the temperature in the range between 102 − 104 cm−3 depends on the HD abundance to a large extent. The higher fraction of HD is linked to the higher spin parameter that is seen for the dense gas.

A deep learning approach to halo merger tree construction

ABSTRACT A key ingredient for semi-analytic models of galaxy formation is the mass assembly history of haloes, encoded in a tree structure. The most commonly used method to construct halo merger histories is based on the outcomes of high-resolution, computationally intensive N-body simulations. We show that machine learning (ML) techniques, in particular Generative Adversarial Networks (GANs), are a promising new tool to tackle this problem with a modest computational cost and retaining the best features of merger trees from simulations. We train our GAN model with a limited sample of merger trees from the Evolution and Assembly of GaLaxies and their Environments (EAGLE) simulation suite, constructed using two halo finders–tree builder algorithms: SUBFIND – D-TREES and ROCKSTAR – ConsistentTrees. Our GAN model successfully learns to generate well-constructed merger tree structures with high temporal resolution, and to reproduce the statistical features of the sample of merger trees used for training, when considering up to three variables in the training process. These inputs, whose representations are also learned by our GAN model, are mass of the halo progenitors and the final descendant, progenitor type (main halo or satellite), and distance of a progenitor to that in the main branch. The inclusion of the latter two inputs greatly improves the final learned representation of the halo mass growth history, especially for SUBFIND-like ML trees. When comparing equally sized samples of ML merger trees with those of the EAGLE simulation, we find better agreement for SUBFIND-like ML trees. Finally, our GAN-based framework can be utilized to construct merger histories of low- and intermediate-mass haloes, the most abundant in cosmological simulations.

The merger rates and mass assembly histories of dark matter haloes in the two Millennium simulations

We construct merger trees of dark matter haloes and quantify their merger rates and mass growth rates using the joint dataset from the Millennium and Millennium-II simulations. The finer resolution of the Millennium-II Simulation has allowed us to extend our earlier analysis of halo merger statistics to an unprecedentedly wide range of descendant halo mass (10^10 < M0 < 10^15 Msun), progenitor mass ratio (10^-5 < xi < 1), and redshift (0 < z < 15). We update our earlier fitting form for the mean merger rate per halo as a function of M_0, xi, and z. The overall behavior of this quantity is unchanged: the rate per unit redshift is nearly independent of z out to z~15; the dependence on halo mass is weak (M0^0.13); and it is nearly a power law in the progenitor mass ratio (xi^-2). We also present a simple and accurate fitting formula for the mean mass growth rate of haloes as a function of mass and redshift. This mean rate is 46 Msun/yr for 10^12 Msun haloes at z=0, and it increases with mass as M^{1.1} and with redshift as (1+z)^2.5 (for z > 1). When the fit for the mean mass growth rate is integrated over a halo's history, we find excellent match to the mean mass assembly histories of the simulated haloes. By combining merger rates and mass assembly histories, we present results for the number of mergers over a halo's history and the statistics of the redshift of the last major merger.

Homogeneity of early-type galaxies across clusters

We studied the homogeneity, across clusters, of the color of the red sequence (the intercept of the color-magnitude relation) of 158 clusters and groups detected in the Early Data Release of the Sloan Digital Sky Survey (EDR–SDSS) in the redshift range . We found a high degree of homogeneity: the color of the red sequence shows an intrinsic scatter of 0.02 mag across clusters, suggesting that either galaxies on the red sequence formed a long time ago () or else their star formation is universally delayed with preservation of a small spread in age formation. The latter possibility is ruled out by the mere existence of galaxies at high redshift. While the old age of ellipticals was already been claimed for a small heterogeneous collection of clusters, most of which are rich ones, we found that it holds for ten to one hundred large sample, representative of all clusters and groups detected on the EDR–SDSS. Hence we suggest the possible universality of the color of the galaxies on the red sequence. Furthermore, the sample includes a large number of very poor clusters (also called groups), not studied in previous works, for which the hierarchical and monolithic scenarios of elliptical formation predict different colors for the brightest ellipticals. The observed red sequence color does not depend on cluster/group richness at a level of 0.02 mag, while a ~0.23 mag effect is expected according to the hierarchical prediction. Therefore, the stellar population of red sequence galaxies is similar in clusters and groups, in spite of different halo histories. Finally, since the observed rest-frame color of the red sequence does not depend on environment and redshift, it can be used as a distance indicator, with an error , a few time better than the precision achieved by other photometric redshift estimates and twice better than the precision of the Fundamental Plane for a single galaxy at the median redshift of the EDR–SDSS.

?It's a Great Song!?Hal�Performance as Literary Production

to the same goal. The definitions invariably retain and stress the essential qualities of halo: the spectacular, the unusual, the precarious, havoc, dan? ger, and challenge. These features and the rules or prescriptions governing the performance are shared and applied in a spontaneous fashion when factions engage in the war of insults and music. Halo can be summed up as a multimedia event, as well as a sociomusical drama that involves songs of insult, dance, drumming, mime, poetry, spoken forms, costume, and a variety of visual icons. Although the music is constructed mainly along Anlo-Ewe models, there are special observances, devices, and techniques that are unique to this genre. As will be elaborated later, these devices and related art forms are employed primarily to effect the purposes of aggres? sion and violence, and to establish musical superiority. A performance usually involves two villages or two wards from one vil? lage, and is characterized by direct or comic forms of provocation, aggravation, and sung and spoken insults, which are sometimes exaggerated through dramatic enactments. Each halo context is a highly emotional one, with the two factions and their supporters competing at physical, verbal, and musical levels. The numerous police arrests that result from the per? formance and its related events are further indications of the grave and wider social ramifications of halo. This search for superiority in both phys? ical and musical domains is also often accompanied by magical practices and related machinations against opponents. These practices also fre? quently involve the acquisition of singing gods (supernatural powers with whom the sources of musical creativity are identified). In addition, individuals or groups also take precautions by fortifying themselves spiritually against enemy attacks (i.e., physical and spiritual attacks). The sociodramatic aspect of halo is thus intensified through the physical confrontations, the musical and verbal exchanges, and the involvement with the supernatural realm. The performance can therefore be described as a unique context for reevaluating and qualifying social and interper? sonal relations. As with many oral genres there is no precise dating of the origins of halo, nor is there is a discernible pattern for tracing its diffusion among the Ewe over time. Statistics resulting from several years of fieldwork show that about 88% of Anlo-Ewe towns have a history of halo, and that about 40% of factions have engaged in the genre more than once. These figures, the official ban, and the lingering of veiled forms of insult in contemporary practices confirm the social significance of the art from in Anlo-Ewe society. The most common precipitates of halo are: (1) the taking of someone's wife from a

The structure and dynamical evolution of dark matter haloes

(Shortened) We use N-body simulations to investigate the structure and dynamical evolution of dark matter halos in galaxy clusters. Our sample consists of nine massive halos from an EdS universe with scale free power spectrum and n = -1. Halos are resolved by ~20000 particles each, with a dynamical resolution of 20-25 kpc. Large scale tidal fields are included up to L=150 Mpc using background particles. The halo formation process can be characterized by the alternation of two dynamical configurations: a merging phase and a relaxation phase, defined by their signature on the evolution of the total mass and rms velocity. Halos spend on average one 1/3 of their evolution in the merging phase and 2/3 in the relaxation phase. Using this definition, we study the density profiles and their change during the halo history. The average density profiles are fitted by the NFW analytical model with an rms residual of 17% between the virial radius Rv and 0.01 Rv. The Hernquist (1990) profiles fits the same halos with an rms residual of 26%. The trend with mass of the scale radius of these fits is marginally consistent with that found by Cole & Lacey (1996): in comparison our halos are more centrally concentrated, and the relation between scale radius and halo mass is slightly steeper. We find a moderately large scatter in this relation, due both to dynamical evolution within halos and to fluctuations in the halo population. We analyze the dynamical equilibrium of our halos using the Jeans' equation, and find that on average they are approximately in equilibrium within their virial radius. Finally, we find that the projected mass profiles of our simulated halos are in very good agreement with the profiles of three rich galaxy clusters derived from strong and weak gravitational lensing observations.