scholarly journals Dissecting and modelling galaxy assembly bias

2021 ◽  
Vol 502 (3) ◽  
pp. 3242-3263
Author(s):  
Xiaoju Xu ◽  
Idit Zehavi ◽  
Sergio Contreras
Keyword(s):  
Galaxy Formation ◽  
Theoretical Modelling ◽  
Distribution Model ◽  
Environment Impact ◽  
Galaxy Clustering ◽  
The Galaxy ◽  
Individual Contributions ◽  
Environmental Measures ◽  
Galaxy Assembly ◽  
The Individual

ABSTRACT Understanding the galaxy-halo connection is fundamental for contemporary models of galaxy clustering. The extent to which the haloes’ assembly history and environment impact galaxy clustering (a.k.a. galaxy assembly bias; GAB), remains a complex and challenging problem. Using a semi-analytic galaxy formation model, we study the individual contributions of different secondary halo properties to the GAB signal. These are obtained by comparing the clustering of stellar-mass selected samples to that of shuffled samples where the galaxies are randomly reassigned to haloes of fixed mass and a specified secondary halo property. We explore a large range of internal halo properties and environmental measures. We find that commonly used properties like halo age or concentration amount to only 20–30 per cent of the signal, while the smoothed matter density or the tidal anisotropy can account for the full level of GAB (though care should be given to the specific definition). For the ‘successful’ measures, we examine the occupancy variations and the associated changes in the halo occupation function parameters. These are used to create mock catalogues that reproduce the full level of GAB. Finally, we propose a practical modification of the standard halo occupation distribution model, which can be tuned to any level of assembly bias. Fitting the parameters to our semi-analytic model, we demonstrate that the corresponding mock catalogue recovers the target level of GAB as well as the occupancy variations. Our results enable producing realistic mock catalogues and directly inform theoretical modelling of assembly bias and attempts to detect it in the Universe.

scholarly journals Predicting halo occupation and galaxy assembly bias with machine learning

2021 ◽  
Vol 507 (4) ◽  
pp. 4879-4899 ◽  
Author(s):  
Xiaoju Xu ◽  
Saurabh Kumar ◽  
Idit Zehavi ◽  
Sergio Contreras
Keyword(s):  
Machine Learning ◽  
Galaxy Formation ◽  
Critical Role ◽  
Dark Matter Halo ◽  
Formation Model ◽  
Galaxy Clustering ◽  
The Galaxy ◽  
Galaxy Halo ◽  
Galaxy Assembly ◽  
The Impact

Abstract Understanding the impact of halo properties beyond halo mass on the clustering of galaxies (namely galaxy assembly bias) remains a challenge for contemporary models of galaxy clustering. We explore the use of machine learning to predict the halo occupations and recover galaxy clustering and assembly bias in a semi-analytic galaxy formation model. For stellar mass selected samples, we train a random forest algorithm on the number of central and satellite galaxies in each dark matter halo. With the predicted occupations, we create mock galaxy catalogues and measure the clustering and assembly bias. Using a range of halo and environment properties, we find that the machine learning predictions of the occupancy variations with secondary properties, galaxy clustering, and assembly bias are all in excellent agreement with those of our target galaxy formation model. Internal halo properties are most important for the central galaxies prediction, while environment plays a critical role for the satellites. Our machine learning models are all provided in a usable format. We demonstrate that machine learning is a powerful tool for modelling the galaxy–halo connection, and can be used to create realistic mock galaxy catalogues which accurately recover the expected occupancy variations, galaxy clustering, and galaxy assembly bias, imperative for cosmological analyses of upcoming surveys.

scholarly journals Gas Accretion and Mergers in Massive Galaxies at z ~ 2

2012 ◽  
Vol 8 (S295) ◽  
pp. 45-48
Author(s):  
C. J. Conselice ◽  
Jamie Ownsworth ◽  
Alice Mortlock ◽  
Asa F. L. Bluck ◽  
Keyword(s):  
Galaxy Formation ◽  
Theoretical Models ◽  
Massive Galaxies ◽  
A Value ◽  
The Galaxy ◽  
Hydrodynamical Simulations ◽  
Minor Mergers ◽  
Galaxy Assembly ◽  
Gas Accretion

AbstractGalaxy assembly is an unsolved problem, with ΛCDM theoretical models unable to easily account for among other things, the abundances of massive galaxies, and the observed merger history. We show here how the problem of galaxy formation can be addressed in an empirical way without recourse to models. We discuss how galaxy assembly occurs at 1.5 < z < 3 examining the role of major and minor mergers, and gas accretion from the intergalactic medium in forming massive galaxies with log M* > 11 found within the GOODS NICMOS Survey (GNS). We find that major mergers, minor mergers and gas accretion are roughly equally important in the galaxy formation process during this epoch, with 64% of the mass assembled through merging and 36% through accreted gas which is later converted to stars, while 58% of all new star formation during this epoch arises from gas accretion. We also discuss how the total gas accretion rate is measured as Ṁ = 90±40 M⊙ yr−1 at this epoch, a value close to those found in some hydrodynamical simulations.

scholarly journals Galaxy assembly bias of central galaxies in the Illustris simulation

2020 ◽  
Vol 492 (2) ◽  
pp. 2739-2754 ◽  
Author(s):  
Xiaoju Xu ◽  
Zheng Zheng
Keyword(s):  
Star Formation ◽  
Formation Time ◽  
Star Formation History ◽  
Halo Formation ◽  
Galaxy Clustering ◽  
Central Galaxy ◽  
Formation History ◽  
Two Samples ◽  
The Galaxy ◽  
Galaxy Assembly

ABSTRACT Galaxy assembly bias, the correlation between galaxy properties and halo properties at fixed halo mass, could be an important ingredient in halo-based modelling of galaxy clustering. We investigate the central galaxy assembly bias by studying the relation between various galaxy and halo properties in the Illustris hydrodynamic galaxy formation simulation. Galaxy stellar mass M* is found to have a tighter correlation with peak maximum halo circular velocity Vpeak than with halo mass Mh. Once the correlation with Vpeak is accounted for, M* has nearly no dependence on any other halo assembly variables. The correlations between galaxy properties related to star formation history and halo assembly properties also show a cleaner form as a function of Vpeak than as a function of Mh, with the main correlation being with halo formation time and to a less extent halo concentration. Based on the galaxy–halo relation, we present a simple model to relate the bias factors of a central galaxy sample and the corresponding halo sample, both selected based on assembly-related properties. It is found that they are connected by the correlation coefficient of the galaxy and halo properties used to define the two samples, which provides a reasonable description for the samples in the simulation and suggests a simple prescription to incorporate galaxy assembly bias into the halo model. By applying the model to the local galaxy clustering measurements in Lin et al., we infer that the correlation between star formation history or specific star formation rate and halo formation time is consistent with being weak.

A Handbook to Eddic Poetry: Myths and Legends of Early Scandinavia, ed. Carolyne Larrington, Judy Quinn, and Brittany Schorn. Cambridge: Cambridge University Press, 2016, xii, 413 pp., 12 b/w ill.

Mediaevistik ◽  
2018 ◽  
Vol 31 (1) ◽  
pp. 366-366
Author(s):  
Albrecht Classen
Keyword(s):  
Old Norse ◽  
Entire Volume ◽  
Cambridge University ◽  
Critical Studies ◽  
Individual Contributions ◽  
The Many ◽  
The Individual ◽  
Made In ◽  
Scandinavian Literature

Eddic poetry constitutes one of the most important genres in Old Norse or Scandinavian literature and has been studied since the earliest time of modern-day philology. The progress we have made in that field is impressive, considering the many excellent editions and translations, not to mention the countless critical studies in monographs and articles. Nevertheless, there is always a great need to revisit, to summarize, to review, and to digest the knowledge gained so far. The present handbook intends to address all those goals and does so, to spell it out right away, exceedingly well. But in contrast to traditional concepts, the individual contributions constitute fully developed critical article, each with a specialized topic elucidating it as comprehensively as possible, and concluding with a section of notes. Those are kept very brief, but the volume rounds it all off with an inclusive, comprehensive bibliography. And there is also a very useful index at the end. At the beginning, we find, following the table of contents, a list of the contributors, unfortunately without emails, a list of translations and abbreviations of the titles of Eddic poems in the Codex Regius and then elsewhere, and a very insightful and pleasant introduction by Carolyne Larrington. She briefly introduces the genre and then summarizes the essential points made by the individual authors. The entire volume is based on the Eddic Network established by the three editors in 2012, and on two workshops held at St. John’s College, Oxford in 2013 and 2014.

Employee Surveys and Sensing

2020 ◽  
Keyword(s):  
Organizational Performance ◽  
Employee Engagement ◽  
Computational Linguistics ◽  
Full Range ◽  
Organization Culture ◽  
Employee Survey ◽  
Passive Data ◽  
Individual Contributions ◽  
Action Taking ◽  
The Individual

This volume comprises 27 chapters focused on the design and execution of employee survey programs. These chapters reflect the latest advances in technology and analytics and a pervasive emphasis on driving organizational performance and effectiveness. The individual chapters represent the full range of survey-related topics, including design, administration, analysis, feedback, and action-taking. The latest methodological trends and capabilities are discussed including computational linguistics, applications of artificial intelligence, and the use of qualitative methods such as focus groups. Extending beyond traditional employee surveys, contributions include the role of passive data collection as an alternative or supplement in a comprehensive employee listening system. Unique contextual factors are discussed including the use of surveys in a unionized environment. Individual contributions also reflect increasing stakeholder concerns for the protection of privacy among other ethical considerations. Finally, significant clarifications to the literature are provided on the use of surveys for measuring organization culture, strategic climate, and employee engagement.

Knowledge First

2017 ◽  
Author(s):  
J. Adam Carter ◽  
Emma C. Gordon ◽  
Benjamin W. Jarvis
Keyword(s):  
Philosophy Of Mind ◽  
Theoretical Background ◽  
Contemporary Research ◽  
Timothy Williamson ◽  
Research Themes ◽  
New Directions ◽  
Individual Contributions ◽  
The Individual

In this introductory chapter, the volume’s editors provide a theoretical background to the volume’s topic and a brief overview of the papers included. The chapter is divided into five parts: Section 1 explains the main contours of the knowledge-first approach, as it was initially advanced by Timothy Williamson in Knowledge and its Limits. In Sections 2–3, some of the key philosophical motivations for the knowledge-first approach are reviewed, and several key contemporary research themes associated with this approach in epistemology, the philosophy of mind and elsewhere are outlined and briefly discussed. The volume’s papers are divided into two broad categories: foundational issues and applications and new directions. Section 4 discusses briefly the scope and aim of the volume as the editors have conceived it, and Section 5 offers an overview of each of the individual contributions in the volume.

scholarly journals On particle acceleration and transport in plasmas in the Galaxy: theory and observations

2021 ◽  
Vol 87 (1) ◽  
Author(s):  
Elena Amato ◽  
Sabrina Casanova
Keyword(s):  
Particle Acceleration ◽  
Cosmic Rays ◽  
Galactic Cosmic Rays ◽  
Energetic Particles ◽  
Quality Data ◽  
Current Status ◽  
The Earth ◽  
The Galaxy ◽  
Formation And Evolution ◽  
Galaxy Assembly

Accelerated particles are ubiquitous in the Cosmos and play a fundamental role in many processes governing the evolution of the Universe at all scales, from the sub-AU scale relevant for the formation and evolution of stars and planets to the Mpc scale involved in Galaxy assembly. We reveal the presence of energetic particles in many classes of astrophysical sources thanks to their production of non-thermal radiation, and we detect them directly at the Earth as cosmic rays. In the last two decades both direct and indirect observations have provided us a wealth of new, high-quality data about cosmic rays and their interactions both in sources and during propagation, in the Galaxy and in the Solar System. Some of the new data have confirmed existing theories about particle acceleration and propagation and their interplay with the environment in which they occur. Some others have brought about interesting surprises, whose interpretation is not straightforward within the standard framework and may require a change of paradigm in terms of our ideas about the origin of cosmic rays of different species or in different energy ranges. In this article, we focus on cosmic rays of galactic origin, namely with energies below a few petaelectronvolts, where a steepening is observed in the spectrum of energetic particles detected at the Earth. We review the recent observational findings and the current status of the theory about the origin and propagation of galactic cosmic rays.

scholarly journals Emerge: Empirical predictions of galaxy merger rates since z ∼ 6

2020 ◽  
Author(s):  
Joseph A O’Leary ◽  
Benjamin P Moster ◽  
Thorsten Naab ◽  
Rachel S Somerville
Keyword(s):  
Galaxy Formation ◽  
Power Law ◽  
Stellar Mass ◽  
Direct Result ◽  
Mass Relation ◽  
Massive Galaxies ◽  
The Galaxy ◽  
Major Merger ◽  
Low Mass ◽  
Merger Rate

Abstract We explore the galaxy-galaxy merger rate with the empirical model for galaxy formation, emerge. On average, we find that between 2 per cent and 20 per cent of massive galaxies (log10(m*/M⊙) ≥ 10.3) will experience a major merger per Gyr. Our model predicts galaxy merger rates that do not scale as a power-law with redshift when selected by descendant stellar mass, and exhibit a clear stellar mass and mass-ratio dependence. Specifically, major mergers are more frequent at high masses and at low redshift. We show mergers are significant for the stellar mass growth of galaxies log10(m*/M⊙) ≳ 11.0. For the most massive galaxies major mergers dominate the accreted mass fraction, contributing as much as 90 per cent of the total accreted stellar mass. We reinforce that these phenomena are a direct result of the stellar-to-halo mass relation, which results in massive galaxies having a higher likelihood of experiencing major mergers than low mass galaxies. Our model produces a galaxy pair fraction consistent with recent observations, exhibiting a form best described by a power-law exponential function. Translating these pair fractions into merger rates results in an inaccurate prediction compared to the model intrinsic values when using published observation timescales. We find the pair fraction can be well mapped to the intrinsic merger rate by adopting an observation timescale that decreases linearly with redshift as Tobs = −0.36(1 + z) + 2.39 [Gyr], assuming all observed pairs merge by z = 0.

scholarly journals Realistic mock observations of the sizes and stellar mass surface densities of massive galaxies in FIRE-2 zoom-in simulations

2020 ◽  
Vol 501 (2) ◽  
pp. 1591-1602
Author(s):  
T Parsotan ◽  
R K Cochrane ◽  
C C Hayward ◽  
D Anglés-Alcázar ◽  
R Feldmann ◽  
...  
Keyword(s):  
Galaxy Formation ◽  
Surface Density ◽  
Stellar Mass ◽  
Massive Galaxies ◽  
Star Forming ◽  
Stellar Feedback ◽  
Central Surface ◽  
Stellar Ages ◽  
The Galaxy ◽  
Zoom In

ABSTRACT The galaxy size–stellar mass and central surface density–stellar mass relationships are fundamental observational constraints on galaxy formation models. However, inferring the physical size of a galaxy from observed stellar emission is non-trivial due to various observational effects, such as the mass-to-light ratio variations that can be caused by non-uniform stellar ages, metallicities, and dust attenuation. Consequently, forward-modelling light-based sizes from simulations is desirable. In this work, we use the skirt  dust radiative transfer code to generate synthetic observations of massive galaxies ($M_{*}\sim 10^{11}\, \rm {M_{\odot }}$ at z = 2, hosted by haloes of mass $M_{\rm {halo}}\sim 10^{12.5}\, \rm {M_{\odot }}$) from high-resolution cosmological zoom-in simulations that form part of the Feedback In Realistic Environments project. The simulations used in this paper include explicit stellar feedback but no active galactic nucleus (AGN) feedback. From each mock observation, we infer the effective radius (Re), as well as the stellar mass surface density within this radius and within $1\, \rm {kpc}$ (Σe and Σ1, respectively). We first investigate how well the intrinsic half-mass radius and stellar mass surface density can be inferred from observables. The majority of predicted sizes and surface densities are within a factor of 2 of the intrinsic values. We then compare our predictions to the observed size–mass relationship and the Σ1−M⋆ and Σe−M⋆ relationships. At z ≳ 2, the simulated massive galaxies are in general agreement with observational scaling relations. At z ≲ 2, they evolve to become too compact but still star forming, in the stellar mass and redshift regime where many of them should be quenched. Our results suggest that some additional source of feedback, such as AGN-driven outflows, is necessary in order to decrease the central densities of the simulated massive galaxies to bring them into agreement with observations at z ≲ 2.

scholarly journals Of Visions and Visionaries: Information and Communication Technologies (ICT) in China

2021 ◽  
Vol 50 (1) ◽  
pp. 3-11
Author(s):  
Barbara Schulte ◽  
Marina Svensson
Keyword(s):  
Information And Communication Technologies ◽  
Communication Technologies ◽  
Special Issue ◽  
Empirical Reality ◽  
Information And Communication ◽  
Individual Contributions ◽  
State Governance ◽  
Different Dimensions ◽  
Chinese State ◽  
The Individual

This special issue approaches information and communication technologies (ICT) visions and their realisation/implementation at various levels, among different actors and from various perspectives. Conceptually, we distinguish three different dimensions, even though those overlap in the individual contributions as well as in empirical reality – namely ideational, instrumental, and relational. The different contributions address both visions formulated by the Chinese state and by individual actors such as entrepreneurs. Even though the conditions for the use of ICT in China are deeply affected by state governance, this governance is in no way tantamount to one single government. As this issue’s contributions show, state attempts at building a stable cyber-governance are in need of allies and, depending on the allies’ visions and other, competitive visions, the outcomes of these dynamics are seldom truthful realisations of one original grand masterplan.

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