1,721,047 research outputs found
The rise and fall of satellites in galaxy clusters
No full textWe use N-body simulations to study the infall of dark matter haloes on to rich clusters of galaxies. After identification of all cluster progenitors in the simulations, we select those haloes that accrete directly on to the main cluster progenitor. We construct the mass function of these merging satellites, and calculate the main orbital parameters for the accreted lumps. The average circularity of the orbits is epsilon~=0.5, while either radial or almost circular orbits are equally avoided. More massive satellites move along slightly more eccentric orbits, with lower specific angular momentum and a smaller pericentre. We find that the infall of satellites on to the main cluster progenitor has a very anisotropic distribution. This anisotropy is to a large extent responsible for the shape and orientation of the final cluster and of its velocity ellipsoid. At the end of the simulations, the major axis of the cluster is aligned both with that of its velocity ellipsoid and with the major axis of the ellipsoid defined by the satellite infall pattern, to ~30 deg on average. We also find that, in lower mass clusters, a higher fraction of the final virial mass is provided by small, dense satellites. These sink to the centre of the parent cluster and so enhance its central density. This mechanism is found to be partially responsible for the correlation between halo masses and characteristic overdensities, recently highlighted by Navarro, Frenk & White
The assembly of matter in galaxy clusters
No full textWe study the merging history of dark matter haloes that end up in rich clusters, using N-body simulations of a scale-free universe. We compare the predictions of the extended Press & Schechter (P&S) formalism with several conditional statistics of the protocluster matter: the mass distribution and relative abundance of progenitor haloes at different redshifts, the infall rate of progenitors within the protocluster, the formation redshift of the most massive cluster progenitor, and the accretion rates of other haloes on to it. The high quality of our simulations allows an unprecedented resolution in the mass range of the studied distributions. We also present the global mass function for the same cosmological model. We find that the P&S formalism and its extensions cannot simultaneously describe the global evolution of clustering and its evolution in a protocluster environment. The best-fitting P&S model for the global mass function is a poor fit to the statistics of cluster progenitors. This discrepancy is in the sense of underpredicting the number of high-mass progenitors at high redshift. Although the P&S formalism can provide a good qualitative description of the global evolution of hierarchical clustering, particular attention is needed when applying the theory to the mass distribution of progenitor objects at high redshift
Dynamical evolution of the ICM
No full textWe use numerical simulations of galaxy clusters to study the dynamical and thermal evolution of the ICM from high redshift to the present time. We measure the properties of the satellites accreting on the cluster main progenitor and then measure their self-bound mass fraction as a function of time after the merging. We also calculate the mean properties of their orbits and investigate the time evolution of their internal velocity dispersion and gas temperature. We measure and model the mean pericentric and apocentric times, distances and velocities. Finally we show how the properties of the ICM at redshift zero can be understood by decomposing them as a function of the cluster merging history
Large-scale bias and the peak background split
No full textDark matter haloes are biased tracers of the underlying dark matter distribution. We use a simple model to provide a relation between the abundance of dark matter haloes and their spatial distribution on large scales. Our model shows that knowledge of the unconditional mass function alone is sufficient to provide an accurate estimate of the large-scale bias factor. We then use the mass function measured in numerical simulations of SCDM, OCDM and LambdaCDM to compute this bias. Comparison with these simulations shows that this simple way of estimating the bias relation and its evolution is accurate for less massive haloes as well as massive ones. In particular, we show that haloes that are less/more massive than typical M_* haloes at the time they form are more/less strongly clustered than is predicted by formulae based on the standard Press-Schechter mass function
An excursion set model of hierarchical clustering: ellipsoidal collapse and the moving barrier
No full textThe excursion set approach allows one to estimate the abundance and spatial distribution of virialized dark matter haloes efficiently and accurately. The predictions of this approach depend on how the non-linear processes of collapse and virialization are modelled. We present simple analytic approximations that allow us to compare the excursion set predictions associated with spherical and ellipsoidal collapse. In particular, we present formulae for the universal unconditional mass function of bound objects and the conditional mass function which describes the mass function of the progenitors of haloes in a given mass range today. We show that the ellipsoidal collapse based moving barrier model provides a better description of what we measure in the numerical simulations than the spherical collapse based constant barrier model, although the agreement between model and simulations is better at large lookback times. Our results for the conditional mass function can be used to compute accurate approximations to the local-density mass function, which quantifies the tendency for massive haloes to populate denser regions than less massive haloes. This happens because low-density regions can be thought of as being collapsed haloes viewed at large lookback times, whereas high-density regions are collapsed haloes viewed at small lookback times. Although we have applied our analytic formulae only to two simple barrier shapes, we show that they are, in fact, accurate for a wide variety of moving barriers. We suggest how they can be used to study the case in which the initial dark matter distribution is not completely cold
Formation times and masses of dark matter haloes
Full text linkThe most commonly used definition of halo formation is the time when the most massive progenitor of a halo first contains at least half the final mass of its parent. Reasonably accurate formulae for the distribution of formation times of haloes of fixed mass have been available for some time. We use numerical simulations of hierarchical gravitational clustering to test the accuracy of formulae for the mass at formation. We also derive and test a formula for the joint distribution of formation masses and times. The structure of a halo is expected to be related to its accretion history. Our tests show that our formulae for formation masses and times are reasonably accurate, so we expect that they will aid future analytical studies of halo structure
Adding long-wavelength modes to an N-body simulation
No full textWe present a new method to add long-wavelength power to an evolved N-body simulation, making use of the Zeldovich approximation to change positions and velocities of particles. We describe the theoretical framework of our technique and apply it to a P3M cosmological simulation performed on a cube of 100 Mpc on a side, obtaining a new "simulation" of 800 Mpc on a side. We study the effect of the power added by long waves by means of several statistics of the density and velocity field, and we suggest possible applications of our method to the study of the large-scale structure of the universe
RECALIBRATION OF THE H(-0.5)-MAGNITUDES OF SPIRAL GALAXIES
No full textThe H-magnitude aperture data published by the Aaronson et al. collaboration over a 10 year period is collected into a homogeneous data set of 1731 observations of 665 galaxies. Ninety-six percent of these galaxies have isophotal diameters and axial ratios determined by the Third Reference Cataloque of Bright Galaxies (RC3; de Vaucouleurs et al. 1991), the most self-consistent set of optical data currently available. The precepts governing the optical data in the RC3 are systematically different from those of the Second Reference Catalogue (de Vaucouleurs, de Vaucouleurs, & Corwin 1976), which were used by Aaronson et al. for their original analyses of galaxy peculiar motions. This in turn leads to systematic differences in growth curves and fiducial H-magnitudes, prompting the present recalibration of the near-infrared Tully-Fisher relationship. New optically normalized H-magnitude growth curves are defined for galaxies of types SO to Im, from which new values of fiducial H-magnitudes, Hg-0.5, are measured for the 665 galaxies. A series of internal tests show that these four standard growth curves are defined to an accuracy of 0.05 mag over the interval -1.5 less than or equal to log (A/Dg) less than or equal to -0.2. Comparisons with the Aaronson et al. values of diameters, axial ratios, and fiducial H-magnitudes show the expected differences, given the different definitions of these parameters. The values of Hg-0.5 are assigned quality indices: a quality value of 1 indicates an accuracy of less than 0.2 mag, quality 2 indicates an accuracy of 0.2-0.35 mag, and quality 3 indicates an accuracy of more than 0.35 mag. Revised values of corrected H I velocity widths are also given, based on the new set of axial ratios defiend by the RC3
On the environmental dependence of halo formation
No full textA generic prediction of hierarchical gravitational clustering models is
that the distribution of halo formation times should depend relatively
strongly on halo mass, massive haloes forming more recently, and depend
only weakly, if at all, on the large-scale environment of the haloes. We
present a novel test of this assumption, which uses the statistics of
weighted or `marked' correlations, which prove to be particularly
well-suited to detecting and quantifying weak correlations with
environment. We find that close pairs of haloes form at slightly higher
redshifts than more widely separated halo pairs, suggesting that haloes
in dense regions form at slightly earlier times than haloes of the same
mass in less dense regions. The environmental trends we find are useful
for models that relate the properties of galaxies to the formation
histories of the haloes that surround them
Hydrodynamic Simulations of Galaxy Formation
No full textThis review is a short introduction to numerical hydrodynamics in a cosmological
context, intended for the non specialist. The main processes relevant to galaxy formation
are first presented. The fluid equations are then introduced, and their implementation in
numerical codes by Eulerian grid based methods and by Smooth Particle Hydrodynamics
is sketched. As an application, I finally show some results from an SPH simulation of a
galaxy cluste
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