arXiv subjects

Ben Moore

Publications and source records attributed to Ben Moore.

At least 73 records · Page 4Linked to original sources

arXiv2008

Multimass spherical structure models for N-body simulations

We present a simple and efficient method to set up spherical structure models for N-body simulations with a multimass technique. This technique reduces by a substantial factor the computer run time needed in order to resolve a given scale as compared to single-mass models. It therefore allows to resolve smaller scales in N-body simulations for a given computer run time. Here, we present several models with an effective resolution of up to 1.68 x 10^9 particles within their virial radius which are stable over cosmologically relevant time-scales. As an application, we confirm the theoretical prediction by Dehnen (2005) that in mergers of collisonless structures like dark matter haloes always the cusp of the steepest progenitor is preserved. We model each merger progenitor with an effective number of particles of approximately 10^8 particles. We also find that in a core-core merger the central density approximately doubles whereas in the cusp-cusp case the central density only increases by approximately 50%. This may suggest that the central region of flat structures are better protected and get less energy input through the merger process.

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arXiv2008

Reply to Melott's Comment on ``Discreteness Effects in Lambda Cold Dark Matter Simulations: A Wavelet-Statistical View'' by Romeo et al

Melott has made pioneering studies of the effects of particle discreteness in N-body simulations, a fundamental point that needs careful thought and analysis since all such simulations suffer from numerical noise arising from the use of finite-mass particles. Melott (arXiv:0804.0589) claims that the conclusions of our paper (arXiv:0804.0294) are essentially equivalent to those of his earlier work. Melott is wrong: he has jumped onto one of our conclusions and interpreted that in his own way. Here we point out the whys and the wherefores.

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arXiv2008

The formation of ultra-compact dwarf galaxies and nucleated dwarf galaxies

Ultra compact dwarf galaxies (UCDs) have similar properties as massive globular clusters or the nuclei of nucleated galaxies. Recent observations suggesting a high dark matter content and a steep spatial distribution within groups and clusters provide new clues as to their origins. We perform high-resolution N-body / smoothed particle hydrodynamics simulations designed to elucidate two possible formation mechanisms for these systems: the merging of globular clusters in the centre of a dark matter halo, or the massively stripped remnant of a nucleated galaxy. Both models produce density profiles as well as the half light radii that can fit the observational constraints. However, we show that the first scenario results to UCDs that are underluminous and contain no dark matter. This is because the sinking process ejects most of the dark matter particles from the halo centre. Stripped nuclei give a more promising explanation, especially if the nuclei form via the sinking of gas, funneled down inner galactic bars, since this process enhances the central dark matter content. Even when the entire disk is tidally stripped away, the nucleus stays intact and can remain dark matter dominated even after severe stripping. Total galaxy disruption beyond the nuclei only occurs on certain orbits and depends on the amount of dissipation during nuclei formation. By comparing the total disruption of CDM subhaloes in a cluster potential we demonstrate that this model also leads to the observed spatial distribution of UCDs which can be tested in more detail with larger data sets.

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arXiv2007

Dependence of the Local Reionization History on Halo Mass and Environment: Did Virgo Reionize the Local Group?

The reionization of the Universe has profound effects on the way galaxies form and on their observed properties at later times. Of particular importance is the relative timing of the reionization history of a region and its halo assembly history, which can affect the nature of the first stars formed in that region, the properties and radial distribution of its stellar halo, globular cluster population and its satellite galaxies. We distinguish two basic cases for the reionization of a halo - internal reionization, whereby the stars forming in situ reionize their host galaxy, and external reionization, whereby the progenitor of a galaxy is reionized by external radiation before its own stars are able to form in sufficient numbers. We use a set of large-scale radiative transfer and structure formation simulations, based on cosmologies derived from both WMAP 1-year and WMAP 3-year data, to evaluate the mean reionization redshifts and the probability of internal/external reionization for Local Group-like systems, galaxies in the field and central cD galaxies in clusters. We find that these probabilities are strongly dependent on the underlying cosmology and the efficiency of photon production, but also on the halo mass. There is a rapid transition between predominantly external and predominantly internal reionization at a mass scale of 1.0e12 Msun (corresponding roughly to L*galaxies), with haloes less massive than this being reionized preferentially from distant sources. We provide a fit for the reionization redshift as a function of halo mass, which could be helpful to parameterize reionization in semi-analytical models of galaxy formation on cosmological scales. We find no statistical correlation between the reionization history of field galaxies and their environment.

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arXiv2007

Influence of Orbital Behaviour and Chaos on the Shape of Dark Matter Halos

It has been shown that the dissipative gas infall during galaxy formation has the capability to modify the shape of dark matter halos. In this paper we perform the first detailed analysis of particle orbits in a cosmological dark matter halos to understand {\it how} and {\it why} baryons alter its shape. We perform a series of numerical experiments where we grow a baryonic core inside a live dark matter halo extracted from a cosmological simulation. We follow the orbits of more than 300 particles with more that 50000 timesteps. Our results clearly show that the dissipational component is responsible for repeatedly deflecting orbits which visit often the center of the system. Moreover the gravitational potential time dependence associated with the growth of the baryonic mass, shifts the frequencies of the orbits, making them extremely chaotic. This randomization makes the orbits explore a large phase space. When this effect takes place for a significant number of orbits it will be manifested in the density distribution as an approach to a rounder configuration. As a consequence, the influence of the central mass on the shape of the phase space decreases with increasing distance from the center. We discuss the importance of future analysis of controlled experiments (i.e. using analytic potentials instead of live DM halos) to better decipher the dynamics of this phenomenon.

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arXiv2007

Concentration, Spin and Shape of Dark Matter Haloes: Scatter and the Dependence on Mass and Environment

We use a series of cosmological N-body simulations for a flat LCDM cosmology to investigate the properties of dark matter haloes in the mass range 3.0e9-3.0e13 Msun. These properties include the concentration parameter (c), the spin parameter (lambda) and the mean axis ratio (q). For the concentration-mass relation we find c~M^(-0.11) in agreement with the model proposed by Bullock et al. even if we find a lower normalization (15%). The results for lambda and q are in good agreement with previous studies, while c and lambda are anti-correlated. In an attempt to remove unrelaxed haloes, we use the offset parameter (xoff), defined as the distance between the most bound particle and the center of mass. Removing haloes with large xoff increases the c by ~10%, lowers the lambda by ~15%, and removes the most prolate haloes. In addition, it largely removes the anti-correlation between c and lambda though not entirely. We also investigate the effects of the large-scale environment. We find that more concentrated haloes live in denser environments. Note, however, that the trend is weak compared to the scatter. For the spin parameters we find no environment dependence, while there is a weak indication that the most spherical haloes reside in denser region. Finally, using a simple model for disk galaxy formation we show that haloes that host low surface brightness galaxies are expected to be hosted by a biased sub-set of haloes. Not only do these haloes have spin parameters that are larger than average, they also have c that are 15% percent lower than the average at a given halo mass. We discuss the implications of all these findings for the claimed disagreement between halo concentrations inferred from LSB rotation curves, and those expected for a LCDM cosmology. (abridged)

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arXiv2006

An alternative to grids and glasses: Quaquaversal pre-initial conditions for N-body simulations

N-body simulations sample their initial conditions on an initial particle distribution, which for cosmological simulations is usually a glass or grid, whilst a Poisson distribution is used for galaxy models, spherical collapse etc. These pre-initial conditions have inherent correlations, noise due to discreteness and preferential alignments, whilst the glass distribution is poorly defined and computationally expensive to construct. We present a novel particle distribution which can be useful as a pre-initial condition for N-body simulations, using a simple construction based on a ``quaquaversal'' tiling of space. This distribution has little preferred orientation (i.e. is statistically isotropic), has a rapidly vanishing large scale power-spectrum (P(k) ~ k^4), and is trivial to create. It should be particularly useful for warm dark matter and cold collapse simulations.

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arXiv2006

Angular momentum transport and disk morphology in SPH simulations of galaxy formation

We perform controlled N-Body/SPH simulations of disk galaxy formation by cooling a rotating gaseous mass distribution inside equilibrium cuspy spherical and triaxial dark matter halos. We systematically study the angular momentum transport and the disk morphology as we increase the number of dark matter and gas particles from 10^4 to 10^6, and decrease the gravitational softening from 2 kpc to 50 parsecs. The angular momentum transport, disk morphology and radial profiles depend sensitively on force and mass resolution. At low resolution, similar to that used in most current cosmological simulations, the cold gas component has lost half of its initial angular momentum via different mechanisms. The angular momentum is transferred primarily to the hot halo component, by resolution-dependent hydrodynamical and gravitational torques, the latter arising from asymmetries in the mass distribution. In addition, disk-particles can lose angular momentum while they are still in the hot phase by artificial viscosity. In the central disk, particles can transfer away over 99% of their initial angular momentum due to spiral structure and/or the presence of a central bar. The strength of this transport also depends on force and mass resolution - large softening will suppress the bar instability, low mass resolution enhances the spiral structure. This complex interplay between resolution and angular momentum transfer highlights the complexity of simulations of galaxy formation even in isolated haloes. With 10^6 gas and dark matter particles, disk particles lose only 10-20% of their original angular momentum, yet we are unable to produce pure exponential profiles.

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arXiv2006

Dynamical friction in constant density cores: a failure of the Chandrasekhar formula

Using analytic calculations and N-body simulations we show that in constant density (harmonic) cores, sinking satellites undergo an initial phase of very rapid (super-Chandrasekhar) dynamical friction, after which they experience no dynamical friction at all. For density profiles with a central power law profile of log-slope, $-α$, the infalling satellite heats the background and causes $α$ to decrease. For $α< 0.5$ initially, the satellite generates a small central constant density core and stalls as in the $α= 0$ case. We discuss some astrophysical applications of our results to decaying satellite orbits, galactic bars and mergers of supermassive black hole binaries. In a companion paper we show that a central constant density core can provide a natural solution to the timing problem for Fornax's globular clusters.

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arXiv2006

Fundamental differences between SPH and grid methods

We have carried out a hydrodynamical code comparison study of interacting multiphase fluids. The two commonly used techniques of grid and smoothed particle hydrodynamics (SPH) show striking differences in their ability to model processes that are fundamentally important across many areas of astrophysics. Whilst Eulerian grid based methods are able to resolve and treat important dynamical instabilities, such as Kelvin-Helmholtz or Rayleigh-Taylor, these processes are poorly or not at all resolved by existing SPH techniques. We show that the reason for this is that SPH, at least in its standard implementation, introduces spurious pressure forces on particles in regions where there are steep density gradients. This results in a boundary gap of the size of the SPH smoothing kernel over which information is not transferred.

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arXiv2006

The survival and disruption of CDM micro-haloes: implications for direct and indirect detection experiments

If the dark matter particle is a neutralino then the first structures to form are cuspy cold dark matter (CDM) haloes collapsing after redshifts z ~ 100 in the mass range 10^{-6} - 10^{-3} Msun. We carry out a detailed study of the survival of these micro-haloes in the Galaxy as they experience tidal encounters with stars, molecular clouds, and other dark matter substructures. We test the validity of analytic impulsive heating calculations using high resolution N-body simulations. A major limitation of analytic estimates is that mean energy inputs are compared to mean binding energies, instead of the actual mass lost from the system. This energy criterion leads to an overestimate of the stripped mass and underestimate of the disruption timescale since CDM haloes are strongly bound in their inner parts. We show that a significant fraction of material from CDM micro-haloes can be unbound by encounters with Galactic substructure and stars, however the cuspy central regions remain relatively intact. Furthermore, the micro-haloes near the solar radius are those which collapse significantly earlier than average and will suffer very little mass loss. Thus we expect a fraction of surviving bound micro-haloes, a smooth component with narrow features in phase space, which may be uncovered by direct detection experiments, as well as numerous surviving cuspy cores with proper motions of arc-minutes per year, which can be detected indirectly via their annihilation into gamma-rays.

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arXiv2006

Dark Satellites and Cosmic Reionization

A possible explanation of the present discrepancy between the abundance of galactic subhaloes predicted by N-Body simulations with those observed in the Local Group is presented. We study the impact of an early reionization on the baryonic component of the Universe using SPH simulations on group and galactic scales. We use a simplified model for reionization described as an instantaneous increment of the IGM temperature (with 1e5 8). We find that a key role is played by compton cooling (interaction between hot electrons and the CMB photons); at high redshift (z_r>10) this cooling is very efficient and it is able to counteract any heating of the gas within few Myrs. This means that a late reionization is needed (z_r<9) to sufficiently reduce the number of luminous dwarf satellites around our Galaxy. For a reionization redshift z_r=8 and a reionization temperature of T_{IGM}~1e5 K we are able to reproduce the observed number of Local Group dwarf galaxies in our simulations.

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arXiv2006

Empirical models for Dark Matter Halos. I. Nonparametric Construction of Density Profiles and Comparison with Parametric Models

We use techniques from nonparametric function estimation theory to extract the density profiles, and their derivatives, from a set of N-body dark matter halos. We consider halos generated from LCDM simulations of gravitational clustering, as well as isolated, spherical collapses. The logarithmic density slopes gamma = d(log rho)/d(log r) of the LCDM halos are found to vary as power-laws in radius, reaching values of gamma ~ -1 at the innermost resolved radii (~0.01 r_virial). This behavior is significantly different from that of broken power-law models like the NFW profile, but similar to that of models like de Vaucouleurs'. Accordingly, we compare the N-body density profiles with various parametric models to find which provide the best fit. We consider an NFW-like model with arbitrary inner slope; Dehnen & McLaughlin's anisotropic model; Einasto's model (identical in functional form to Sersic's model but fit to the space density); and the density model of Prugniel & Simien that was designed to match the deprojected form of Sersic's R^{1/n} law. Overall, the best-fitting model to the LCDM halos is Einasto's, although the Prugniel-Simien and Dehnen-McLaughlin models also perform well. With regard to the spherical collapse halos, both the Prugniel-Simien and Einasto models describe the density profiles well, with an rms scatter some four times smaller than that obtained with either the NFW-like model or the 3-parameter Dehnen-McLaughlin model. Finally, we confirm recent claims of a systematic variation in profile shape with halo mass.

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arXiv2006

Empirical Models for Dark Matter Halos. II. Inner profile slopes, dynamical profiles, and rho/sigma^3

We have recently shown that both the Prugniel-Simien model and Sersic's function (hereafter referred to as the Einasto model when applied to internal density profiles) describe simulated dark matter halos better than an NFW-like model with equal number of parameters. Here we provide analytical expressions for the logarithmic slopes of these models, and compare them with data from real galaxies. Depending on the Einasto parameters of the dark matter halo, one can expect an extrapolated, inner (0.01--1 kpc), logarithmic profile slope ranging from -0.2 to -1.5, with a typical value at 0.1 kpc around -0.7. Application of this (better fitting) model therefore alleviates some of the past disagreement with observations on this issue. We additionally provide useful expressions for the concentration and assorted scale radii: r_s, r_{-2}, r_e, R_e, r_virial, and r_max -- the radius where the circular velocity profile has its maximum value. We also present the circular velocity profiles and the radial behavior of rho(r)/sigma(r)^3 for both the Einasto and Prugniel-Simien models. We find this representation of the phase-space density profile to be well approximated by a power-law with slope slightly shallower than -2 near r=r_{-2}.

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arXiv2006

Empirical Models for Dark Matter Halos. III. The Kormendy relation and the log(rho_e)-log(R_e) relation

We have recently shown that the 3-parameter density-profile model from Prugniel & Simien provides a better fit to simulated, galaxy- and cluster-sized, dark matter halos than an NFW-like model with arbitrary inner profile slope gamma (Paper I). By construction, the parameters of the Prugniel-Simien model equate to those of the Sersic R^{1/n} function fitted to the projected distribution. Using the Prugniel-Simien model, we are therefore able to show that the location of simulated (10^{12} M_sun) galaxy-sized dark matter halos in the _e-log(R_e) diagram coincides with that of brightest cluster galaxies, i.e., the dark matter halos appear consistent with the Kormendy relation defined by luminous elliptical galaxies. These objects are also seen to define the new, and equally strong, relation log(rho_e) = 0.5 - 2.5log(R_e), in which rho_e is the internal density at r=R_e. Simulated (10^{14.5} M_sun) cluster-sized dark matter halos and the gas component of real galaxy clusters follow the relation log(rho_e) = 2.5[1 - log(R_e)]. Given the shapes of the various density profiles, we are able to conclude that while dwarf elliptical galaxies and galaxy clusters can have dark matter halos with effective radii of comparable size to the effective radii of their baryonic component, luminous elliptical galaxies can not. For increasingly large elliptical galaxies, with increasingly large profile shapes `n', to be dark matter dominated at large radii requires dark matter halos with increasingly large effective radii compared to the effective radii of their stellar component.

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arXiv2006

High resolution SPH simulations of disk formation in CDM halos; resolution tests

We perform N-Body/SPH simulations of disk galaxy formation inside equilibrium spherical and triaxial cuspy dark matter halos. We systematically study the disk properties and morphology as we increase the numbers of dark matter and gas particles from 10^4 to 10^6 and change the force resolution. The force resolution influences the morphological evolution of the disk quite dramatically. Unless the baryon fraction is significantly lower than the universal value, with high force resolution a gaseous bar always forms within a billion years after allowing cooling to begin. The bar interacts with the disk, transferring angular momentum and increasing its scale length. In none of the simulations does the final mass distribution of the baryons obey a single exponential profile. Indeed within a few hundred parsecs to a kiloparsec from the center the density rises much more steeply than in the rest of the disk, and this is true irrespective of the presence of the bar.

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arXiv2006

Cooling flows within galactic haloes: the kinematics and properties of infalling multi-phase gas

We study the formation of disks via the cooling flow of gas within galactic haloes using smoothed particle hydrodynamics simulations. These simulations resolve mass scales of a few thousand solar masses in the gas component for the first time. Thermal instabilities result in the formation of numerous warm clouds that are pressure confined by the hot ambient halo gas. The clouds fall slowly onto the disk through non-spherical accretion from material flowing preferentially down the angular momentum axis. The rotational velocity of the infalling cold gas decreases as a function of height above the disk, closely resembling that of the extra-planar gas recently observed around the spiral galaxy NGC 891.

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arXiv2006

The Secular Evolution of Disk Structural Parameters

We present a comprehensive series of $N$-body as well as $N$-body + SPH simulations to study the secular evolution of the structure of disk galaxies. Our simulations are organized in a hierarchy of increasing complexity, ranging from rigid-halo collisionless simulations to fully live simulations with gas and star formation. Comparisons between the different types of simulations allow us to isolate the role of various physical mechanisms. We focus on the evolution of systems expected in a LCDM universe. Our goal is to examine which structural properties of disk galaxies may result from secular evolution rather than from direct hierarchical assembly. In the vertical direction, we find that various mechanisms can lead to heating. The strongest heating occurs during the vertical buckling instability of a bar. Among the consequences of this instability is the formation of peanut-shaped bulges which produce clear kinematic signatures when observed face-on. We find that bars are robust structures that are not destroyed by buckling. They can be destroyed instead by a central mass concentration but we find that this mass needs to be a large fraction of the total mass of the disk. We then study the evolution of stellar surface density profiles showing how angular momentum redistribution leads to increasing central densities and disk scale lengths and to profile breaks at large radii. The breaks in these simulations are in excellent agreement with observed breaks, even when the evolution is purely collisionless. Disk scale-lengths increase even when the total disk angular momentum is conserved; thus mapping halo angular momenta to scale-lengths is non-trivial. [Abridged]

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