Mass Accretion Histories Research Articles

The Physics of Galaxy Clustering. I. A Model for Subhalo Populations

We present a semianalytic model for cold dark matter halo substructure that can be used as a framework for studying the physics of galaxy formation and as an ingredient in halo models of galaxy clustering. The model has the following main ingredients: (1) extended Press-Schechter mass accretion histories, (2) host halo density profiles computed according to the trends observed in cosmological simulations, (3) distributions of initial orbital parameters of accreting subhalos measured in a high-resolution simulation of three Milky Way-size halos, and (4) integration of the orbital evolution of subhalos including the effects of dynamical friction and tidal mass loss. We perform a comprehensive comparison of the model calculations to the results of a suite of high-resolution cosmological simulations. The comparisons show that subhalo statistics such as the velocity and mass functions, the radial distributions, and the halo occupation distributions agree well over 3 orders of magnitude in host halo mass and at various redshifts. We find that both in the simulations and in our model the radial distributions of subhalos are significantly shallower than that of the dark matter density. The abundance of subhalos in a host is set by competition between tidal disruption and new accretion. Halos of high mass and halos at high redshift tend to host more subhalos because the subhalos have, on average, been accreted more recently. Similarly, at a fixed mass and epoch, halos that formed more recently host a larger number of subhalos. Observed fossil groups may represent an extreme tail of this correlation. We find a related correlation between host halo concentration and satellite abundance at fixed host mass, Nsat ∝ c, where a changes with redshift and host-to-subhalo mass ratio. Lastly, we use our substructure model to populate host halos in one of the high-resolution cosmological simulations, replacing the actual subhalos resolved in this simulation and using the host mass as the only input for the model calculation. We show that the resulting correlation function of such a hybrid halo ensemble is indistinguishable from that measured directly in the simulation. This supports one of the key tenets of the standard halo model, i.e., the assumption that the halo occupation distribution is statistically independent of the host halo environment.

Density Profiles of ΛCDM Clusters

We analyze the mass accretion histories (MAHs) and density profiles of cluster-size halos with virial masses of 0.6 - 2.5 × 10 14 h -1 M⊙ in a flatCDM cosmology. We find that most MAHs have a similar shape: an early, merger-dominated mass increase followed by a more gradual, accretion-dominated growth. For some clusters the intense merger activity and rapid mass growth continue until the present-day epoch. In agreement with previous studies, we find that the concentration of the density distri bution is tightly correlated with the halo's MAH and with its formation redshift. During the period of fast mass growth the concentration remains approximately constant and low cv ≈ 3 - 4, while during the slow accretion stages the concentration increases with decreasing redshift as cv ∝ (1 + z) -1 . We consider fits of three widely discussed analytic density profiles to the simulated clusters focusing on the most relaxed inner regions. We find that there is no unique best fit analytic profile for all the systems. At the same time, if a cluster is best fit by a particul ar analytic profile at z = 0, the same is usually true at earlier epochs out to z ∼ 1 - 2. The local logarithmic slope of the density profiles at 3% of the virial radius ranges from -1.2 to -2.0, a remarkable diversity for the relatively narrow mass range of our cluster sample. Interestingly, for all the studied clusters the logarithmic slope becomes s hallower with decreasing radius without reaching an asymptotic value down to the smallest resolved scale (. 1% of the virial radius). We do not find a clear correlation of the inner slope with the formation redshift or the shape of the halo's MAH. We do find, however, that during the period of rapid mass growth the density profiles can be wel l described by a single power law �(r) ∝ r - with ∼ 1.5 - 2. The relatively shallow power law slopes result in low concentrations at these stages of evolution, as the scale radius where the density profiles reaches the slo pe of -2 is at large radii. This indicates that the inner power law like density distribution of halos is built u p during the periods of rapid mass accretion and active merging, while outer steeper profile is formed when the mass a ccretion slows down. To check the convergence and robustness of our conclusions, we resimulate one of our clusters using eight times more particles and twice better force resolution. We find good agreement between the t wo simulations in all of the results discussed in our study. Subject headings: cosmology: theory - dark matter - clusters: formation - clusters - structure methods: numerical

Substructures in cold dark matter haloes

We analyse the properties of substructures within dark matter haloes (subhaloes) using a set of high-resolution numerical simulations of the formation of structure in a ACDM universe. Our simulation set includes 11 high-resolution simulations of massive clusters as well as a region of mean density, allowing us to study the spatial and mass distribution of substructures down to a mass resolution limit of 10 9 h -1 M ○. . We also investigate how the properties of substructures vary as a function of the mass of the 'parent' halo in which they are located. We find that the substructure mass function depends at most weakly on the mass of the parent halo and is well described by a power law. The radial number density profiles of substructures are steeper in low mass haloes than in high-mass haloes. More massive substructures tend to avoid the centres of haloes and are preferentially located in the external regions of their parent haloes. We also study the mass accretion and merging histories of substructures, which we find to be largely independent of environment. We find that a significant fraction of the substructures residing in clusters at the present day were accreted at redshifts z < 1. This implies that a significant fraction of present-day 'passive' cluster galaxies were still outside the cluster progenitor and were more active at z ∼ 1.

The growth and structure of dark matter haloes

In this paper, we analyse in detail the mass-accretion histories and structural properties of dark haloes in high-resolution N-body simulations. We model the density distribution in individual haloes using the Navarro-Frenk-White (NFW) profile. For a given halo, there is a tight correlation between its inner-scale radius r(s) and the mass within it, M-s, for all its main progenitors. Using this correlation, one can predict quite well the structural properties of a dark halo at any time in its history from its mass-accretion history, implying that the structure properties and the mass-accretion history are closely correlated. The predicted growing rate of concentration c with time tends to increase with decreasing mass-accretion rate. The build-up of dark haloes in cold dark matter (CDM) models generally consists of an early phase of fast accretion (where the halo mass M-h increases with time much faster than the expansion rate of the Universe) and a late phase of slow accretion (where M-h increases with time approximately as the expansion rate). These two phases are separated at a time when c similar to 4 and the typical binding energy of the halo is approximately equal to that of a singular isothermal sphere with the same circular velocity. Haloes in the two accretion phases show systematically different properties, for example, the circular velocity v(h) increases rapidly with time in the fast accretion phase but remains almost constant in the slow accretion phase, the inner properties of a halo, such as r(s) and M-s increase rapidly with time in the fast accretion phase but change only slowly in the slow accretion phase, the inner circular velocity v(s) is approximately equal to v(h) in the fast accretion phase but is larger in the slow accretion phase. The potential well associated with a halo is built up mainly in the fast accretion phase, while a large amount of mass can be accreted in the slow accretion phase without changing the potential well significantly. We discuss our results in connection with the formation of dark haloes and galaxies in hierarchical models.

Concentrations of Dark Halos from Their Assembly Histories

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 ΛCDM cosmology. For each halo at z = 0, we identify its merger history tree and determine concentration parameters cvir 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 ac, defined when the log mass accretion rate d log M/d log a falls below a critical value S. We find that late-forming galaxies tend to be less concentrated, such that cvir observed at any epoch ao is strongly correlated with ac via cvir = c1ao/ac. Scatter about this relation is mostly due to measurement errors in cvir and ac, implying that the actual spread in cvir for halos of a given mass can be mostly attributed to scatter in ac. We demonstrate that this relation can also be used to predict the mass and redshift dependence of cvir and the scatter about the median cvir(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 semianalytic 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 Rvir. 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.

The universal mass accretion history of cold dark matter haloes

We use the extended Press-Schechter formalism to investigate the rate at which cold dark matter haloes accrete mass. We discuss the shortcomings of previous methods that have been used to compute the mass accretion histories of dark matter haloes, and present an improved method based on the N-branch merger tree algorithm of Somerville & Kolatt. We show that this method no longer suffers from inconsistencies in halo formation times, and compare its predictions with high-resolution N-body simulations. Although the overall agreement is reasonable, there are slight inconsistencies which are most easily interpreted as a reflection of ellipsoidal collapse (as opposed to spherical collapse assumed in the Press-Schechter formalism). We show that the average mass accretion histories follow a simple, universal profile, and we present a simple recipe for computing the two scale-parameters which is applicable to a wide range of halo masses and cosmologies. Together with the universal profiles for the density and angular momentum distributions of cold dark matter haloes, these universal mass accretion histories provide a simple but accurate framework for modelling the structure and formation of dark matter haloes. In particular, they can be used as a backbone for modelling various aspects of galaxy formation where one is not interested in the detailed effects of merging. As an example we use the universal mass accretion history to compute the rate at which dark matter haloes accrete mass, which we compare with the cosmic star formation history of the Universe.

Nebular thermal evolution and the properties of primitive planetary materials

Abstract— Models of the solar nebula are constructed to investigate the hypothesis that surviving planetary objects began to form as the nebula cooled from an early, hot epoch. The imprint of such an epoch might be retained in the spatial distribution of planetary material, the systematic deviations of its elemental composition from that of the Sun, chemical indicators of primordial oxidation state, and variations in oxygen and other isotopic compositions. Our method of investigation is to calculate the time‐dependent, two‐dimensional temperature distributions within model nebulas of prescribed dynamical evolution, and to deduce the consequences of the calculated thermal histories for coagulated solid material. The models are defined by parameters which characterize nebular initial states (mass and angular momentum), mass accretion histories, and coagulation rates and efficiencies. It is demonstrated that coagulation during the cooling of the nebula from a hot state is expected to produce systematic heterogeneities which affect the chemical and isotopic compositions of planetary material. The radial thermal gradient at the midplane results in delayed coagulation of the more volatile elements. Vertical thermal gradients isolate the most refractory material and concentrate evaporated heavy elements in the gas phase. It is concluded that these effects could be responsible for the distribution of terrestrial planetary masses, the systematic depletion patterns of the moderately volatile elements in chondritic meteorites and the Earth, the range of oxygen isotopic compositions exhibited by calcium‐aluminum‐rich inclusions (CAIs) and other refractory inclusions, and some geochemical evidence for a moderately enhanced oxidation state. However, nebular fractionations on a global scale are unlikely to account for the more oxidizing conditions inferred for some CAIs and chondritic silicates, which require dust enhancements greater than a few hundred. This conclusion, along with the well‐established evidence from studies of chondrules and CAIs for thermal excursions of short duration, make it likely that local environments, unrelated to nebular thermal evolution, were also important.

The Origin of Star Formation Gradients in Rich Galaxy Clusters

We examine the origin of clustercentric gradients in the star formation rates and colors of rich cluster galaxies within the context of a simple model where clusters are built through the ongoing accretion of field galaxies. The model assumes that after galaxies enter the cluster their star formation rates decline on a timescale of a few gigayears, the typical gas consumption timescale of disk galaxies in the field. Such behavior might be expected if tides and ram pressure strip off the gaseous envelopes that normally fuel star formation in spirals over a Hubble time. Combining these timescales with mass accretion histories derived from N-body simulations of cluster formation in a ΛCDM universe, we reproduce the systematic differences observed in the color distribution of cluster and field galaxies, as well as the strong suppression of star formation in cluster galaxies and its dependence on clustercentric radius. The simulations also indicate that a significant fraction of galaxies beyond the virial radius of the cluster may have been within the main body of the cluster in the past, a result that explains naturally why star formation in the outskirts of clusters (and as far out as 2 virial radii) is systematically suppressed relative to the field. The agreement with the data beyond the cluster virial radius is also improved if we assume that stripping happens within lower mass systems, before the galaxy is accreted into the main body of the cluster. We conclude that the star formation rates of cluster galaxies depend primarily on the time elapsed since their accretion onto massive virialized systems and that the cessation of star formation may have taken place gradually over a few gigayears.