共查询到20条相似文献，搜索用时 15 毫秒
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Rachel S. Somerville Gerard Lemson Tsafrir S. Kolatt Avishai Dekel 《Monthly notices of the Royal Astronomical Society》2000,316(3):479-490
We study the merging history of dark matter haloes in N -body simulations and semi-analytical 'merger trees' based on the extended Press–Schechter (EPS) formalism. The main focus of our study is the joint distribution of progenitor number and mass as a function of redshift and parent halo mass. We begin by investigating the mean quantities predicted directly by the Press–Schechter (PS) and EPS formalism, such as the halo mass and conditional mass functions, and compare these predictions with the results of the simulations. The higher moments of this distribution are not predicted by the EPS formalism alone and must be obtained from the merger trees. We find that the Press–Schechter model deviates from the simulations at the level of 30–50 per cent on certain mass scales, and that the sense of the discrepancy changes as a function of redshift. We show that this discrepancy is reflected in the higher moments of the distribution of progenitor mass and number. We investigate some related statistics such as the accretion rate and the mass ratio of the largest two progenitors. For galaxy sized haloes ( M ∼1012 M⊙ ), we find that the merging history of haloes, as represented by these statistics, is well reproduced in the merger trees compared with the simulations. The agreement deteriorates for larger mass haloes. We conclude that merger trees based on the extended Press–Schechter formalism provide a reasonably reliable framework for semi-analytical models of galaxy formation. 相似文献
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Amina Helmi 《Astronomy and Astrophysics Review》2008,15(3):145-188
Stellar halos may hold some of the best preserved fossils of the formation history of galaxies. They are a natural product
of the merging processes that probably take place during the assembly of a galaxy, and hence may well be the most ubiquitous
component of galaxies, independently of their Hubble type. This review focuses on our current understanding of the spatial
structure, the kinematics and chemistry of halo stars in the Milky Way. In recent years, we have experienced a change in paradigm
thanks to the discovery of large amounts of substructure, especially in the outer halo. I discuss the implications of the
currently available observational constraints and fold them into several possible formation scenarios. Unraveling the formation
of the Galactic halo will be possible in the near future through a combination of large wide field photometric and spectroscopic
surveys, and especially in the era of Gaia. 相似文献
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Jeannette M. Myers Dieter H. Hartmann Lih-Sin The 《Astrophysics and Space Science》2003,284(2):787-790
In an effort to study the halo globular clusters we perform a simulation of inhomogeneous halo collapse using a SPH/N-body
code. We include dark matter particles, (cooling) gas, and stars. Supernova feedback is included as a heat source for the
gas in the local environment of star forming regions. We find this model cannot reproduce the age distribution and spatial
distribution of halo globular clusters.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
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Using the stellar photometry catalogue based on the latest data release (DR4) of the Sloan Digital Sky Survey (SDSS), a study of the Galactic structure using star counts is carried out for selected areas of the sky. The sample areas are selected along a circle at a Galactic latitude of +60°, and 10 strips of high Galactic latitude along different longitudes. Direct statistics of the data show that the surface densities of ℓ from 180° to 360° are systematically higher than those of ℓ from 0° to 180°, defining a region of overdensity (in the direction of Virgo) and another one of underdensity (in the direction of Ursa Major) with respect to an axisymmetric model. It is shown by comparing the results from star counts in the ( g − r ) colour that the density deviations are due to an asymmetry of the stellar density in the halo. Theoretical models for the surface density profile are built and star counts are performed using a triaxial halo of which the parameters are constrained by observational data. Two possible reasons for the asymmetric structure are discussed. 相似文献
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Hannah Parkinson Shaun Cole John Helly 《Monthly notices of the Royal Astronomical Society》2008,383(2):557-564
We present a new Monte Carlo algorithm to generate merger trees describing the formation history of dark matter haloes. The algorithm is a modification of the algorithm of Cole et al. used in the galform semi-analytic galaxy formation model. As such, it is based on the Extended Press–Schechter theory and so should be applicable to hierarchical models with a wide range of power spectra and cosmological models. It is tuned to be in accurate agreement with the conditional mass functions found in the analysis of merger trees extracted from the Λ cold dark matter Millennium N -body simulation. We present a comparison of its predictions not only with these conditional mass functions, but also with additional statistics of the Millennium Simulation halo merger histories. In all cases, we find it to be in good agreement with the Millennium Simulation and thus it should prove to be a very useful tool for semi-analytic models of galaxy formation and for modelling hierarchical structure formation in general. We have made our merger tree generation code and code to navigate the trees available at http://star-www.dur.ac.uk/~cole/merger_trees . 相似文献
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Xavier Hernandez † Andrea Ferrara 《Monthly notices of the Royal Astronomical Society》2001,324(2):484-490
We explore the predictions of the standard hierarchical clustering scenario of galaxy formation, regarding the numbers and metallicities of PopIII stars that are likely to be found within our Galaxy today. By PopIII we refer to stars formed at large redshift ( z >4), with low metallicities ([ Z /Z⊙ ]<−2.5) and in small systems (total mass ≲ 2×108 M⊙ ) that are extremely sensitive to stellar feedback, and which through a prescribed merging history end up becoming part of the Milky Way today. An analytic, extended Press–Schechter formalism is used to obtain the mass functions of haloes which will host PopIII stars at a given redshift, and which will end up in Milky Way sized systems today. Each of these is modelled as a mini-galaxy, with a detailed treatment of the dark halo structure, angular momentum distribution, final gas temperature and disc instabilities, all of which determine the fraction of the baryons that are subject to star formation. The use of new primordial metallicity stellar evolutionary models allows us to trace the history of the stars formed, and give accurate estimates of their expected numbers today and their location in L /L⊙ versus T /K Hertzsprung–Russell (HR) diagrams. A first comparison with observational data suggests that the initial mass function (IMF) of the first stars was increasingly high-mass weighted towards high redshifts, levelling off at z ≳9 at a characteristic stellar mass scale m s =10–15 M⊙ . 相似文献
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Steffen R. Knollmann Chris Power Alexander Knebe 《Monthly notices of the Royal Astronomical Society》2008,385(2):545-552
We explore the dependence of the central logarithmic slope of dark matter halo density profiles α on the spectral index n of the linear matter power spectrum P ( k ) using cosmological N -body simulations of scale-free models [i.e. P ( k ) ∝ k n ]. These simulations are based on a set of clear, reproducible and physically motivated criteria that fix the appropriate starting and stopping times for runs, and allow one to compare haloes across models with different spectral indices and mass resolutions. For each of our simulations we identify samples of well-resolved haloes in dynamical equilibrium and we analyse their mass profiles. By parametrizing the mass profile using a 'generalized' Navarro, Frenk & White profile in which the central logarithmic slope α is allowed to vary while preserving the r −3 asymptotic form at large radii, we obtain preferred central slopes for haloes in each of our models. There is a strong correlation between α and n , such that α becomes shallower as n becomes steeper. However, if we normalize our mass profiles by r −2 , the radius at which the logarithmic slope of the density profile is −2, we find that these differences are no longer present. This is apparent if we plot the maximum slope as a function of r / r −2 – we find that the profiles are similar for haloes forming in different n models. This reflects the importance of concentration, and reveals that the concentrations of haloes forming in steep- n cosmologies tend to be smaller than those of haloes forming in shallow- n cosmologies. We conclude that there is no evidence for convergence to a unique central asymptotic slope, at least on the scales that we can resolve. 相似文献
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