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تسجيل مستخدم جديدConcentrations and Assembly Histories of Dark Matter Halos
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Risa H. Wechsler
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We study the relation between the density profiles of dark matter halos and their mass assembly histories, using a statistical sample of halos in a high-resolution N-body simulation of the LCDM cosmology. For each halo at z=0, we identify its merger-history tree, and determine concentration parameters c_vir for all progenitors, thus providing a structural merger tree for each halo. We fit the mass accretion histories by a universal function with one parameter, the formation epoch a_c, defined when the log mass accretion rate dlog M/doga falls below a critical value. We find that late forming galaxies tend to be less concentrated, such that c_vir ``observed at any epoch a_obs is strongly correlated with a_c via c_vir=c_1 a_obs/a_c. Scatter about this relation is mostly due to measurement errors in c_vir and a_c, implying that the actual spread in c_vir for halos of a given mass can be mostly attributed to scatter in a_c. Because of the direct connection between halo concentration and velocity rotation curves, and because of probable connections between halo mass assembly history and star formation history, the tight correlation between these properties provides an essential new ingredient for galaxy formation modeling.
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(abridged) We study the relation between the density profiles of dark matter halos and their mass assembly histories, using a statistical sample of halos in a high-resolution N-body simulation of the LCDM cosmology. For each halo at z=0, we identify
its merger-history tree, and determine concentration parameters c_vir for all progenitors, thus providing a structural merger tree for each halo. We fit the mass accretion histories by a universal function with one parameter, the formation epoch a_c, defined when the log mass accretion rate dlogM/dloga falls below a critical value S. We find that late forming galaxies tend to be less concentrated, such that c_vir ``observed at any epoch a_o is strongly correlated with a_c via c_vir=c_1*a_o/a_c. Scatter about this relation is mostly due to measurement errors in c_v and a_c, implying that the actual spread in c_vir for halos of a given mass can be mostly attributed to scatter in a_c. We demonstrate that this relation can also be used to predict the mass and redshift dependence of c_v, and the scatter about the median c_vir(M,z), using accretion histories derived from the Extended Press-Schechter (EPS) formalism, after adjusting for a constant offset between the formation times as predicted by EPS and as measured in the simulations;this new ingredient can thus be easily incorporated into semi-analytic models of galaxy formation. The correlation found between halo concentration and mass accretion rate suggests a physical interpretation: for high mass infall rates the central density is related to the background density; when the mass infall rate slows, the central density stays approximately constant and the halo concentration just grows as R_vir. The tight correlation demonstrated here provides an essential new ingredient for galaxy formation modeling.
A large amount of observations have constrained cosmological parameters and the initial density fluctuation spectrum to a very high accuracy. However, cosmological parameters change with time and the power index of the power spectrum varies with mass
scale dramatically in the so-called concordance Lambda CDM cosmology. Thus, any successful model for its structural evolution should work well simultaneously for various cosmological models and different power spectra. We use a large set of high-resolution N-body simulations of a variety of structure formation models (scale-free, standard CDM, open CDM, and Lambda CDM) to study the mass accretion histories (MAHs), the mass and redshift dependence of concentrations and the concentration evolution histories of dark matter halos. We find that there is significant disagreement between the much-used empirical models in the literature and our simulations. According to two simple but tight correlations we find from the simulation results, we develop new empirical models for both the MAHs and the concentration evolution histories of dark matter halos, and the latter can also be used to predict the mass and redshift dependence of halo concentrations. These models are accurate and universal: the same set of model parameters works well for different cosmological models and for halos of different masses at different redshifts and the model predictions are highly accurate even when the histories are traced to very high redshift. These models are also simple and easy to implement. A web calculator and a user-friendly code to make the relevant calculations are available from http://www.shao.ac.cn/dhzhao/mandc.html . We explain why Lambda CDM halos on nearly all mass scales show two distinct phases in their evolution histories.
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We use a large $N$-body simulation to study the relation of the structural properties of dark matter halos to their assembly history and environment. The complexity of individual halo assembly histories can be well described by a small number of prin
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entration evolves with time and emerges from the mass assembly history. We also explore the origin of the scatter in the relation between concentration and assembly history. We show that the evolution of halo concentration has two primary modes: (1) smooth increase due to pseudo-evolution; and (2) intense responses to physical merger events. Merger events induce lasting and substantial changes in halo structures, and we observe a universal response in the concentration parameter. We argue that merger events are a major contributor to the uncertainty in halo concentration at fixed halo mass and formation time. In fact, even haloes that are typically classified as having quiescent formation histories experience multiple minor mergers. These minor mergers drive small deviations from pseudo-evolution, which cause fluctuations in the concentration parameters and result in effectively irreducible scatter in the relation between concentration and assembly history. Hence, caution should be taken when using present-day halo concentration parameter as a proxy for the halo assembly history, especially if the recent merger history is unknown.
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