The elliptical galaxy colour–magnitude relation as a discriminant between the monolithic and merger paradigms

The colour–magnitude relation (CMR) of cluster elliptical galaxies has been widely used to constrain their star formation histories (SFHs) and to discriminate between the monolithic collapse and merger paradigms of elliptical galaxy formation. We use a Λ cold dark matter hierarchical merger model of galaxy formation to investigate the existence and redshift evolution of the elliptical galaxy CMR in the merger paradigm. We show that the SFH of cluster ellipticals predicted by the model is quasi-monolithic , with only ∼10 per cent of the total stellar mass forming after   z ∼ 1  . The quasi-monolithic SFH results in a predicted CMR that agrees well with its observed counterpart in the redshift range  0 < z < 1.27  . We use our analysis to argue that the elliptical-only CMR can be used to constrain the SFHs of present-day cluster ellipticals only if we believe a priori in the monolithic collapse model. It is not a meaningful tool for constraining the SFH in the merger paradigm, since a progressively larger fraction of the progenitor set of present-day cluster ellipticals is contained in late-type star-forming systems at higher redshift, which cannot be ignored when deriving the SFHs. Hence, the elliptical-only CMR is not a useful discriminant between the two competing theories of elliptical galaxy evolution.     

Signatures of galaxy mergers in the Milky Way: Here, there and everywhere

The hierarchical paradigm predicts that large galaxies like the Milky Way formed through mergers of smaller systems, which are expected to leave behind substructures in the halo of the final product. Recently the first tests of this prediction on galaxies other than the Milky Way have been made (eg NGC 5907 by Sackett et al.; M31 by Ibata et al.), but one should bare in mind that it is extremely difficult to detect halos in external galaxies let alone substructures in those halos. On the other hand, the multi-dimensional phase-space information available for our Galaxy (6d for stars in the vicinity of the Sun, and 4d for more distant ones) enables us to directly search for merger signatures. This revised version was published online in September 2006 with corrections to the Cover Date.     

On the origin of high-eccentricity halo stars

The present-day chemical and dynamical properties of the Milky Way are signatures of the Galaxy's formation and evolution. Using a self consistent chemodynamical evolution code we examine these properties within the currently favoured paradigm for galaxy formation – hierarchical clustering within a CDM cosmology. Our Tree N-body/Smoothed Particle Hydrodynamics code includes a self-consistent treatment of gravity, hydrodynamics, radiative cooling, star formation, supernova feedback and chemical enrichment. Two models are described which explore the role of small-scale density perturbations in driving the evolution of structure within the Milky Way. The relationship between metallicity and kinematics of halo stars are quantified and the implications for galaxy formation discussed. While high-eccentricity halo stars have previously been considered a signature of `rapid collapse', we suggest that many such stars may have come from recently accreted satellites. This revised version was published online in August 2006 with corrections to the Cover Date.     

Mapping the substructure in the Galactic halo with the next generation of astrometric satellites

We run numerical simulations of the disruption of satellite galaxies in a Galactic potential to build up the entire stellar halo, in order to investigate what the next generation of astrometric satellites will reveal by observing the halo of the Milky Way. We generate artificial DIVA , FAME and GAIA halo catalogues, in which we look for the signatures left by the accreted satellites. We develop a method based on the standard Friends-of-Friends algorithm applied to the space of integrals of motion. We find this simple method can recover about 50 per cent of the different accretion events, when the observational uncertainties expected for GAIA are taken into account, even when the exact form of the Galactic potential is unknown. The recovery rate for DIVA and FAME is much smaller, but these missions, like GAIA , should be able to test the hierarchical formation paradigm on our Galaxy by measuring the amount of halo substructure in the form of nearby kinematically cold streams with, for example, a two-point correlation function in velocity space.     

Next-generation robotic planetary reconnaissance missions: A paradigm shift

A fundamentally new scientific mission concept for remote planetary surface and subsurface reconnaissance will soon replace the engineering and safety constrained mission designs of the past, allowing for optimal acquisition of geologic, paleohydrologic, paleoclimatic, and possible astrobiologic information of Mars and other extraterrestrial targets. Traditional missions have performed local ground-level reconnaissance through rovers and immobile landers, or global mapping performed by an orbiter. The former is safety and engineering constrained, affording limited detailed reconnaissance of a single site at the expense of a regional understanding, while the latter returns immense datasets, often overlooking detailed information of local and regional significance. A “tier-scalable” paradigm integrates multi-tier (orbitatmosphereground) and multi-agent (orbiterblimpsrovers/sensorwebs) hierarchical mission architectures, not only introducing mission redundancy and safety, but enabling and optimizing intelligent, unconstrained, and distributed science-driven exploration of prime locations on Mars and elsewhere, allowing for increased science return, and paving the way towards fully autonomous robotic missions.     

On hierarchical cosmology

Progress in laboratory studies of plasmas and in the methods of transferring the results to cosmic conditions, together within situ measurements in the magnetospheres, are now causing a ‘paradigm transition’ in cosmic plasma physics. This involves an introduction ofinhomogeneous models with double layers, filaments, ‘cell walls’, etc. Independently, it has been discovered that the mass distribution in the universe is highly inhomogeneous; indeed,hierarchical. According to de Vaucouleurs, the escape velocity of cosmic structures is 10²–10³ times below the Laplace-Schwarzschild limit, leaving avoid region which is identified as a key problem in cosmology. It is shown that a plasma instability in the dispersed medium of the structures may produce this void and, hence, explain the hierarchical structure. The energy which is necessary may derive either from gravitation or from annihilation caused by a breakdown of cell walls. The latter alternative is discussed in detail. It leads to a ‘Fireworks Model’ of the evolution of the metagalaxy. It is questioned whether the homogeneous four-dimensional big bang model can survive in an universe which is inhomogeneous and three-dimensional.     

等级三星系统的运动学研究

统计分析表明,在目前的观测精度(Imas)下,由于三体效应大多数等级三星系统已不能用经典的双二体运动学模型来描述;即使对于应用需求而言,该模型也不再适用于描述三体视效应较强的等级三星系统.因为在实用星表参考架中不宜采用数据量较大的数值历表来描述子星的运动,所以有必要针对等级三星系统建立尽可能简单实用的运动学模型.借助已有的观测资料和拟合研究结果,给出了6个等级三星系统的质量参数和初始条件,进而根据实用需求讨论了这些系统的运动学描述问题.     

The application of hierarchical relative coordinates to the analysis of the movement of subsystems of many-body problems

The behaviour of a subsystem of a many-body problem may be studied using the hierarchical relative coordinates defined by Nugeyre and Bouvier (1981), The present article employs a notation allowing the position vectors of the particles to be expressed explicity as functions of the hierarchical relative coordinates, This machinery is used to generalize a result of Patnaik's (1975) concerning escape from a many-body system.     

Practical Symplectic Methods with Time Transformation for the Few-Body Problem

The use of the extended phase space and time transformations for constructing efficient symplectic algorithms for the investigation of long term behavior of hierarchical few-body systems is discussed. Numerical experiments suggest that the time-transformed generalized leap-frog, combined with symplectic correctors, is one of the most efficient methods for such studies. Applications extend from perturbed two-body motion to hierarchical many-body systems with large eccentricities. This revised version was published online in July 2006 with corrections to the Cover Date.     

Weak lensing from strong clustering

We investigate the effect of weak gravitational lensing in the limit of small angular scales where projected galaxy clustering is strongly non-linear. This is the regime likely to be probed by future weak lensing surveys. We use well-motivated hierarchical scaling arguments and the plane-parallel approximation to study multi-point statistical properties of the convergence field. These statistics can be used to compute the vertex amplitudes in tree models of hierarchical clustering; these can be compared with similar measurements from galaxy surveys, leading to a powerful probe of galaxy bias.     

Generalized cumulant correlators and hierarchical clustering

The cumulant correlators, C _pq , are statistical quantities that generalize the better-known S _p parameters; the former are obtained from the two-point probability distribution function of the density fluctuations while the latter describe only the one-point distribution. If galaxy clustering develops from Gaussian initial fluctuations and a small-angle approximation is adopted, standard perturbative methods suggest a particular hierarchical relationship of the C _pq for projected clustering data, such as that obtained from the Automatic Plate Measuring (APM) survey. We establish the usefulness of the two-point cumulants for describing hierarchical clustering by comparing such calculations against available measurements from projected catalogues, finding very good agreement. We extend the idea of cumulant correlators to multipoint generalized cumulant correlators (related to the higher-order correlation functions). We extend previous studies in the highly non-linear regime to express the generalized cumulant correlators in terms of the underlying 'tree amplitudes' of hierarchical scaling models. Such considerations lead to a technique for determining these hierarchical amplitudes, to arbitrary order, from galaxy catalogues and numerical simulations. Knowledge of these amplitudes yields important clues about the phenomenology of gravitational clustering. For instance, we show that a three-point cumulant correlator can be used to separate the tree amplitudes up to sixth order. We also combine the particular hierarchical Ansatz of Bernardeau & Schaeffer with extended and hyper-extended perturbation theory to infer values of the tree amplitudes in the highly non-linear regime.     

Structure of the Clusters of Galaxies A 2634, A 1983, and DC 0428-53

The hierarchical and dynamical structure of three clusters of galaxies, A 2634, A 1983, and DC 0428-53, is investigated. Based on their example, we can say that the central cD galaxies play the leading role in clusters, affecting the course of their evolution. Radio and x-ray galaxies are not always part of the substructure. The morphological composition of various hierarchical subgroups depends on the stage of evolution. Clusters have a tendency to be oriented in the direction toward the nearest neighbor.     

Early formation and evolution of galaxies

I review success and failure of the hierarchical galaxy formation model. In this scenario, the morphology of galaxies is regulated by the mode of gas accretion and intimately linked to discrete accretion events. Some of the common misconceptions about hierarchical clustering are discussed. The need of a self-consistent approach that incorporates the chemical and dynamical evolution on small scalessimultaneously with the cosmological framework of structure formation on large scales is emphasized. This revised version was published online in August 2006 with corrections to the Cover Date.     

Understanding dwarf galaxies as galactic building blocks

This is a summary of a general discussion held during the third EuroConference on galaxy evolution. Various observational features of the stellar populations in present-day dwarf galaxies were presented to introduce the discussion on the possibility that these systems be the main building blocks of spiral and elliptical galaxies. Many people in the audience turned out to think that the inconsistencies among the observed properties of large and dwarf galaxies are too many to believe that the former are built up only by means of successive accretions of the latter. However, theorists of hierarchical galaxy formation suggested that present-day dwarfs are not representative of the galactic building blocks, which may be completely invisible nowadays. Some of them suggested that, contrary to what is usually assumed in hierarchical modelling, the actual building blocks were still fully gaseous systems when their major mergers occurred. If this is the case, then most of the inconsistencies can be overcome, and the scenario of hierarchical galaxy formation becomes not too different from that of a slow gas accretion. This revised version was published online in August 2006 with corrections to the Cover Date.     

Galaxy Evolution: An Overview

We review recent ideas on the evolution of galaxies. Two models are competing to explain the observational features: the monolithic model in which galaxies assemble at high redshift, and evolve with little influence from the environment, and the hierarchical model in which small galaxies progressively assemble to form bigger objects. We describe the basic features of the model of hierarchical formation, and we try to valuate how the observations in the optical and infrared/submillimetre fit in the scenario. Finally, we show a few results from a hybrid, N-body simulation + semi-analytic model of hierarchical formation that produces detailed quantitative predictions to be compared to the data. This revised version was published online in September 2006 with corrections to the Cover Date.     

The hierarchical stability of the seven known large size ratio triple asteroids using the empirical stability parameters

In this study, the hierarchical stability of the seven known large size ratio triple asteroids is investigated. The effect of the solar gravity and primary’s J ₂ are considered. The force function is expanded in terms of mass ratios based on the Hill’s approximation and the large size ratio property. The empirical stability parameters are used to examine the hierarchical stability of the triple asteroids. It is found that the all the known large size ratio triple asteroid systems are hierarchically stable. This study provides useful information for future evolutions of the triple asteroids.     

Fractal structure of clouds forming the Lα forest in quasar spectra

It follows from observations that the clouds forming the L_α forest have a hierarchical structure. It is suggested that their structure is not only hierarchical, but also fractal. The relationship between the radial concentration of neutral hydrogen N(HI) and the concentration of clouds having the same value of N(HI), well known from observation, is explained on this basis. The fractal dimension of D = 2.5 ± 0.2 is estimated from observational data and calculations. Translated fromAstrofizika, Vol. 40, No. 1, pp. 29–37, February, 1997.     

Investigation of the substructure of clusters of galaxies by the method ofS-tree diagrams: Non-point approximation

Further development of the method of S-tree diagrams for studying the hierarchical structure of dusters of galaxies is presented. First, we give a generalization for a system of N spheres, i.e., we make the transition to non-point models, and describe a method of constructing a new corresponding matrix D. Second, we give an algorithm for analyzing information obtained by the method of S-tree diagrams. This algorithm makes it possible to determine the hierarchical substructure and composition of each subgroup of the investigated system. Translated from Astrofizika, Vol. 40, No. 3, pp. 425–432, August, 1997.