ΛCDM Cosmological Simulations Research Articles

Modelling the outskirts of galaxies in a cosmological context

AbstractCurrent data broadly support trends of galaxy surface brightness profile amplitude and shape with total stellar mass predicted by state-of-the-art ΛCDM cosmological simulations, although recent results show signs of interesting discrepancies, particularly for galaxies less massive than the Milky Way. Here I discuss how perhaps the largest contribution to such discrepancies can be inferred almost directly from how well a given model agrees with the observed present-day galaxy stellar mass function.

Effects of cosmological parameters and star formation models on the cosmic star formation history in ΛCDM cosmological simulations

We investigate the effects of the change of cosmological parameters and star formation (SF) models on the cosmic SF history using cosmological smoothed particle hydrodynamics (SPH) simulations based on the cold dark matter (CDM) model. We vary the cosmological parameters within 1-sigma error from the WMAP best-fit parameters, and find that such changes in cosmological parameters mostly affect the amplitude of the cosmic SF history. At high redshift (hereafter high-z), the star formation rate (SFR) is sensitive to the cosmological parameters that control the small-scale power of the primordial power spectrum, while the cosmic matter content becomes important at lower redshifts. We also test two new SF models: 1) the `Pressure' model based on the work by Schaye & Dalla Vecchia (2008), and 2) the `Blitz' model that takes the effect of molecular hydrogen formation into account, based on the work by Blitz & Rosolowsky (2006). Compared to the previous conventional SF model, the Pressure model reduces the SFR in low-density regions and shows better agreement with the observations of the Kennicutt-Schmidt law. This model also suppresses the early star formation and shifts the peak of the cosmic SF history toward lower redshift, more consistently with the recent observational estimates of cosmic SFR density. The simulations with the new SF model also predict lower global stellar mass densities at high-z, larger populations of low-mass galaxies and a higher gas fraction in high-z galaxies. Our results suggest that there is room left in the model uncertainties to reconcile the discrepancy that was found between the theory and observations of cosmic SF history and stellar mass density. Nevertheless, our simulations still predict higher stellar mass densities than most of the observational estimates.

Mass and Spatial Distributions of Subhaloes in ΛCDM Cosmological Simulations

AbstractWe study subhalo populations surrounding massive dark matter haloes by using three AMIGA simulations each having different mass and spatial resolutions. Our analysis shows that the slope of the subhalo mass function has a value 0.9, which agrees with previous studies. The dependence of mass functions on redshift is the same for subhaloes and main haloes. In all simulations, combined subhalo masses are about 0.1-0.2 of main halo masses and this mass fraction increases slightly with redshift and the mass of the main halo. The distribution of mass fractions for subhaloes is close to Gaussian at z = 0 and differs slightly at earlier epochs. Spatial distribution of subhaloes as measured in units of virial radius Rvir of the main halo does not depend on redshift and follows r1/3 rule. Spatial distribution of all haloes surrounding main haloes continues up to 3 times Rvir with equal slope but lower amplitude. Beyond 16 times Rvir, the average distribution of haloes becomes uniform.

Gravitational Mesoscopic Constraints in Cosmological Dark Matter Halos

We present an analysis of the behaviour of the ‘coarse-grained’ (‘mesoscopic’) rank partitioning of the mean energy of collections of particles composing virialized dark matter halos in a Λ-CDM cosmological simulation. We find evidence that rank preservation depends on halo mass, in the sense that more massive halos show more rank preservation than less massive ones. We find that the most massive halos obey Arnold’s theorem (on the ordering of the characteristic frequencies of the system) more frequently than less massive halos. This method may be useful to evaluate the coarse-graining level (minimum number of particles per energy cell) necessary to reasonably measure signatures of ‘mesoscopic’ rank orderings in a gravitational system.

Density-potential pairs for spherical stellar systems with Sérsic light profiles and (optional) power-law cores

Popular models for describing the luminosity-density profiles of dynamically hot stellar systems (e.g. Jaffe, Hernquist, Dehnen) were constructed with the desire to match the deprojected form of an R1/4 light profile. Real galaxies, however, are now known to have a range of different light-profile shapes that scale with mass. Consequently, although highly useful, the above models have implicit limitations, and this is illustrated here through their application to a number of real galaxy density profiles. On the other hand, the analytical density profile given by Prugniel & Simien closely matches the deprojected form of Sérsic R1/n light profiles – including deprojected exponential light profiles. It is thus applicable for describing bulges in spiral galaxies, dwarf elliptical galaxies, and both ordinary and giant elliptical galaxies. Moreover, the observed Sérsic quantities define the parameters of the density model. Here we provide simple equations, in terms of elementary and special functions, for the gravitational potential and force associated with this density profile. Furthermore, to match galaxies with partially depleted cores, and better explore the supermassive black hole/galaxy connection, we have added a power-law core to this density profile and derived similar expressions for the potential and force of this hybrid profile. Expressions for the mass and velocity dispersion, assuming isotropy, are also given. These spherical models may also prove appropriate for describing the dark matter distribution in haloes formed from ΛCDM cosmological simulations.

The Dependence on Environment of Cold Dark Matter Halo Properties

A series of high-resolution ΛCDM cosmological N-body simulations are used to study the properties of galaxy-size dark halos as a function of global environment. We analyze halos in three types of environment: (cluster halos and their surroundings), (large regions with density contrasts -0.85), and (halos not contained within larger halos). We find that halos in clusters have a median spin parameter ~1.3 times lower, a minor-to-major axial ratio ~1.2 times lower (more spherical), and a less aligned internal angular momentum than halos in voids and the field. For masses 5 × 1011 h-1 M☉, halos in cluster regions are on average ~30%-40% more concentrated and have ~2 times higher central densities than halos in voids. While for halos in cluster regions the concentration parameters decrease on average with mass with a slope of ~0.1, for halos in voids these concentrations do not seem to change with mass. When comparing only parent halos from the samples, the differences are less pronounced but still significant. We obtain also the maximum circular velocity-mass and rms velocity-mass relations. These relations are shallower and more scattered for halos in clusters than in voids, and for a given circular velocity or rms velocity, the mass is smaller at z = 1 than at z = 0 for all environments. At z = 1, the differences in the halo properties with environment almost disappear, suggesting that the differences were established mainly after z ~ 1. The halos in the cluster regions undergo more dramatic changes than those in the field or the voids. The differences in halo properties with environment are due to (1) the dependence of halo formation time on global environment and (2) local effects such as tidal stripping and the tumultuous histories that halos suffer in high-density regions. We calculate seminumerical models of disk galaxy evolution using halos with the concentrations and spin parameters found for the different environments. For a given disk mass, the galaxy disks have higher surface density, larger maximum circular velocity and secular bulge-to-disk ratio, lower gas fraction, and are redder as one goes from void to cluster environments. Although all these trends agree with observations, the latter tend to show more differences, suggesting that physical ingredients not considered here, such as misalignment of angular momentum, halo triaxiality, merging, ram pressure stripping, harassment, etc., play an important role for galaxy evolution, especially in high-density environments.

X‐Ray Scaling Relations of Galaxy Groups in a Hydrodynamic Cosmological Simulation

We examine the scalings of X-ray luminosity, temperature, and dark matter or galaxy velocity dispersion for galaxy groups in a ΛCDM cosmological simulation, which incorporates gravity, gas dynamics, radiative cooling, and star formation, but no substantial nongravitational heating. In agreement with observations, the simulated LX-σ and LX-TX relations are steeper than those predicted by adiabatic simulations or self-similar models, with LX ∝ σ4.4 and LX ∝ T for massive groups and significantly steeper relations below a break at σ ≈ 180 km s-1 (TX ≈ 0.7 keV). The TX-σ relation is fairly close to the self-similar scaling relation, with TX ∝ σ1.75, provided that the velocity dispersion is estimated from the dark matter or from 10 galaxies. The of hot gas in low-mass groups is higher than predicted by self-similar scaling or adiabatic simulations, and it agrees with observational data that suggest an entropy floor. The steeper scalings of the luminosity relations are driven by radiative cooling, which reduces the hot (X-ray-emitting) gas fraction from 50% of the total baryons at σ ≈ 500 km s-1 to 20% at σ ≈ 100 km s-1. A secondary effect is that hot gas in smaller systems is less clumpy, further driving down LX. A smaller volume simulation with 8 times higher mass resolution predicts nearly identical X-ray luminosities at a given group mass, demonstrating the insensitivity of the predicted scaling relations to numerical resolution. The higher resolution simulation predicts higher hot gas fractions at a given group mass, and these predicted fractions are in excellent agreement with available observations. There remain some quantitative discrepancies: the predicted mass scale of the LX-TX and LX-σ breaks is somewhat too low, and the luminosity-weighted temperatures are too high at a given σ, probably because our simulated temperature profiles are flat or rising toward small radii, while observed profiles decline at r 0.2Rvir. We conclude that radiative cooling has an important quantitative impact on group X-ray properties and can account for many of the observed trends that have been interpreted as evidence for nongravitational heating. Improved simulations and observations are needed to understand the remaining discrepancies and to decide the relative importance of cooling and nongravitational heating in determining X-ray scalings.

Multi-level adaptive particle mesh (MLAPM): a C code for cosmological simulations

We present a computer code written in c that is designed to simulate structure formation from collisionless matter. The code is purely grid-based and uses a recursively refined Cartesian grid to solve Poisson's equation for the potential, rather than obtaining the potential from a Green's function. Refinements can have arbitrary shapes and in practice closely follow the complex morphology of the density field that evolves. The time-step shortens by a factor of 2 with each successive refinement. Competing approaches to N-body simulation are discussed from the point of view of the basic theory of N-body simulation. It is argued that an appropriate choice of softening length ε is of great importance and that ε should be at all points an appropriate multiple of the local interparticle separation. Unlike tree and P3M codes, multigrid codes automatically satisfy this requirement. We show that at early times and low densities in cosmological simulations, ε needs to be significantly smaller relative to the interparticle separation than in virialized regions. Tests of the ability of the code's Poisson solver to recover the gravitational fields of both virialized haloes and Zel'dovich waves are presented, as are tests of the code's ability to reproduce analytic solutions for plane-wave evolution. The times required to conduct a ΛCDM cosmological simulation for various configurations are compared with the times required to complete the same simulation with the ART, AP3M and GADGET codes. The power spectra, halo mass functions and halo–halo correlation functions of simulations conducted with different codes are compared. The code is available from.