arXiv subjects

S. Gottloeber

Publications and source records attributed to S. Gottloeber.

At least 37 records · Page 2Linked to original sources

arXiv2007

Spin alignment of dark matter haloes in the shells of the largest voids

Using the high resolution cosmological N-body simulation MareNostrum Universe we study the orientation of shape and angular momentum of galaxy-size dark matter haloes around large voids. We find that haloes located on the shells of the largest cosmic voids have angular momenta that tend to be preferentially perpendicular to the direction that joins the centre of the halo and the centre of the void. This alignment has been found in spiral galaxies around voids using galaxy redshift surveys. We measure for the first time the strength of this alignment, showing how it falls off with increasing distance to the centre of the void. We also confirm the correlation between the intensity of this alignment and the halo mass. The analysis of the orientation of the halo main axes confirms the results of previous works. Moreover, we find a similar alignment for the baryonic matter inside dark matter haloes, which is much stronger in their inner parts.

astro-ph↗

arXiv2007

Entropy of gas and dark matter in galaxy clusters

On the basis of a large scale 'adiabatic', namely non-radiative and non-dissipative, cosmological smooth particle hydrodynamic simulation we compare the entropy profiles of the gas and the dark matter (DM) in galaxy clusters. The quantity K_g = T_g ρ_g^{-2/3} provides a measure for the entropy of the intra-cluster gas. By analogy with the thermodynamic variables of the gas the velocity dispersion of the DM is associated with a formal temperature and thereby K_DM = σ_DM^2 ρ_DM^{-2/3} is defined. This DM entropy is related to the DM phase space density by K_DM \propto Q_DM^{-2/3}. In accord with other studies the DM phase space density follows a power law behaviour, Q_DM \propto r^{-1.82}, which corresponds to K_DM \propto r^{1.21}. The simulated intra-cluster gas has a flat entropy core within (0.8 \pm 0.4) R_s, where R_s is the NFW scale radius. The outer profile follows the DM behaviour, K_g \propto r^{1.21}, in close agreement with X-ray observations. Upon scaling the DM and gas densities by their mean cosmological values we find that outside the entropy core a constant ratio of K_g / K_{DM} = 0.71 \pm 0.18 prevails. By extending the definition of the gas temperature to include also the bulk kinetic energy the ratio of the DM and gas extended entropy is found to be unity for r > 0.8 R_s. The constant ratio of the gas thermal entropy to that of the DM implies that observations of the intra-cluster gas can provide an almost direct probe of the DM.

astro-ph↗

arXiv2006

Oscillatory relaxation of a merging galaxy cluster

Within the cosmic framework clusters of galaxies are relatively young objects. Many of them have recently experienced major mergers. Here we investigate an equal mass merging event at z = ~0.6 resulting in a dark matter haloe of ~2.2 times 10^{14} Msol/h at z=0. The merging process is covered by 270 outputs of a high resolution cosmological N-body simulation performed with the ART (adaptive refinement tree) code. Some 2 Gyrs elapse between the first peri-centre passage of the progenitor cores and their final coalescence. During that phase the cores experience six peri-centre passages with minimal distances declining from ~30 to ~2 kpc/h. The time intervals between the peri-centre passages continuously decrease from 9 to 1 times 10^8 yrs. We follow the mean density, the velocity dispersion and the entropy of the two progenitors within a set of fixed proper radii (25, 50, 100, 250, 500, 1000 kpc/h). During the peri-centre passages we find sharp peaks of the mean densities within these radii, which exceed the sum of the corresponding progenitor densities. In addition to the intermixing of the merging haloes, the densities increase due to contraction caused by the momentary deepening of the potential well. At the end of the oscillatory relaxation phase the material originating from the less concentrated of the two equal mass progenitors is deposited at larger radii and shows a slightly more radially anisotropic velocity dispersion compared to the material coming from the more concentrated progenitor. Every peri-centre passage is accompanied by a substantial drop of the central potential well. We briefly discuss the possibility that AGN outbursts are triggered by the periodically changing potential.

astro-ph↗

arXiv2006

Dark and baryonic matter in the MareNostrum Universe

We report some results from one of the largest hydrodynamical cosmological simulations of large scale structures that has been done up to date. The MareNostrum Universe SPH simulation consists of 2 billion particles (2 times 1024^3) in a cubic box of 500 h^-1 Mpc on a side. This simulation has been done in the MareNostrum parallel supercomputer at the Barcelona SuperComputer Center. Due to the large simulated volume and good mass resolution, our simulated catalog of dark matter halos comprises more than half a million objects with masses larger than a typical Milky Way galaxy halo. From this dataset we have studied several statistical properties such as the evolution of the halo mass function, the void distribution, the shapes of dark and gas halos and the large scale distribution of baryons.

astro-ph↗

arXiv2006

Secondary infall and dark matter haloes

We test the Secondary Infall Model (SIM) by direct comparison with the results of N-body simulations. Eight cluster-size and six galactic-size dark matter haloes have been selected at $z=0$ and re-simulated with high resolution. Based on their density profiles at the initial redshift, we compute their evolution by the SIM, assuming a simple prescription for the angular momentum. A comparison of the density profiles obtained by the SIM and the numerical experiments at $z=5$, 1 and 0 shows that, for most of the haloes at most epochs, the SIM reproduces the simulated mater distribution with a typical fractional deviation of less than 40 per cent over more than six order of magnitudes in the density. It is also found that, within the SIM framework, most of the diversity in the shape of the density profiles at $z=0$ arises from the scatter in the primordial initial conditions rather than the scatter in the angular momentum distribution. A crude optimization shows that a similar degree of agreement is obtained for galactic and cluster haloes, but the former seem to require slightly higher amounts of angular momentum. Our main conclusion is that the SIM provides a viable dynamical model for predicting the structure and evolution of the density profile of dark matter haloes.

astro-ph↗

arXiv2006

The Dependence of the Mass Assembly History of Cold Dark Matter Halos on Environment

We show by means of a high-resolution N-body simulation how the mass assembly histories of galaxy-size cold dark matter (CDM) halos depend on environment. Halos in high density environments form earlier and a higher fraction of their mass is assembled in major mergers, compared to low density environments. The distribution of the present-day specific mass aggregation rate is strongly dependent on environment. While in low density environments only ~20% of the halos are not accreting mass at the present epoch, this fraction rises to ~80% at high densities. At z=1 the median of the specific aggregation rate is ~4 times larger than at z=0 and almost independent on environment. All the dependences on environment found here are critically enhanced by local processes associated to subhalos because the fraction of subhalos increases as the environment gets denser. The distribution of the halo specific mass aggregation rate as well as its dependence on environment resemble the relations for the specific star formation rate distribution of galaxies. An analogue of the morphology-density relation is also present at the level of CDM halos, being driven by the halo major merging history. Nevertheless, baryonic processes are necessary in order to explain further details and the evolution of the star formation rate-, color- and morphology-environment relations.

astro-ph↗

arXiv2006

Adiabatic scaling relations of galaxy clusters

The aim of the present work is to show that, contrary to popular belief, galaxy clusters are **not** expected to be self-similar, even when the only energy sources available are gravity and shock-wave heating. In particular, we investigate the scaling relations between mass, luminosity and temperature of galaxy groups and clusters in the absence of radiative processes. Theoretical expectations are derived from a polytropic model of the intracluster medium and compared with the results of high-resolution adiabatic gasdynamical simulations. It is shown that, in addition to the well-known relation between the mass and concentration of the dark matter halo, the effective polytropic index of the gas also varies systematically with cluster mass, and therefore neither the dark matter nor the gas profiles are exactly self-similar. It is remarkable, though, that the effects of concentration and polytropic index tend to cancel each other, leading to scaling relations whose logarithmic slopes roughly match the predictions of the most basic self-similar models. We provide a phenomenological fit to the relation between polytropic index and concentration, as well as a self-consistent scheme to derive the non-linear scaling relations expected for any cosmology and the best-fit normalizations of the M-T, L-T and F-T relations appropriate for a Lambda-CDM universe. The predicted scaling relations reproduce observational data reasonably well for massive clusters, where the effects of cooling and star formation are expected to play a minor role.

astro-ph↗

arXiv2005

Tracing the Nature of Dark Energy with Galaxy Distribution

Dynamical Dark Energy (DE) is a viable alternative to the cosmological constant. Yet, constructing tests to discriminate between Lambda and dynamical DE models is difficult because the differences are not large. In this paper we explore tests based on the galaxy mass function, the void probability function (VPF), and the number of galaxy clusters. At high z the number density of clusters shows large differences between DE models, but geometrical factors reduce the differences substantially. We find that detecting a model dependence in the cluster redshift distribution is a hard challenge. We show that the galaxy redshift distribution is potentially a more sensitive characteristics. We do so by populating dark matter halos in Nbody simulations with galaxies using well-tested Halo Occupation Distribution (HOD). We also estimate the Void Probability Function and find that, in samples with the same angular surface density of galaxies in different models, the VPF is almost model independent and cannot be used as a test for DE. Once again, geometry and cosmic evolution compensate each other. By comparing VPF's for samples with fixed galaxy mass limits, we find measurable differences.

astro-ph↗

arXiv2005

The dependence on environment of Cold Dark Matter Halo properties

High-resolution LCDM cosmological N-body simulations are used to study the properties of galaxy-size dark halos in different environments (cluster, void, and "field"). Halos in clusters and their surroundings have a median spin parameter ~1.3 times lower, and tend to be more spherical and to have less aligned internal angular momentum than halos in voids and the field. For halos in clusters the concentration parameters decrease on average with mass with a slope of ~0.1; for halos in voids these concentrations do not change with mass. For masses <5 10^11 M_sh^-1, halos in clusters are on average ~30-40% more concentrated and have ~2 times higher central densities than halos in voids. When comparing only parent halos, the differences are less pronounced but they are still significant. The Vmax-and Vrms-mass relations are shallower and more scattered for halos in clusters than in voids, and for a given Vmax or Vrms, the mass is smaller at z=1 than at z=0 in all the environments. At z=1, the differences in the halo properties with environment almost dissapear, suggesting this that the differences were stablished mainly after z~1. The halos in clusters undergo more dramatic changes than those in the field or the voids. The differences with environment are owing to (i) the dependence of halo formation time on environment, and (ii) local effects as tidal stripping and the tumultuos histories that halos suffer in high-density regions. We calculate seminumerical models of disk galaxy evolution in halos with the properties found for the different environments. For a given disk mass, the galaxy disks have higher surface density, larger Vd,max and secular bulge-to-disk ratio, lower gas fraction, and are redder as one goes from cluster to void environments, in rough agreement with observations. (abridged)

astro-ph↗

arXiv2005

How far do they go? The outer structure of dark matter halos

We study the density profiles of collapsed galaxy-size dark matter halos with masses 1e11-5e12 Msun focusing mostly on the halo outer regions from the formal virial radius Rvir up to 5-7Rvir. We find that isolated halos in this mass range extend well beyond Rvir exhibiting all properties of virialized objects up to 2-3Rvir: relatively smooth density profiles and no systematic infall velocities. The dark matter halos in this mass range do not grow as one naively may expect through a steady accretion of satellites, i.e., on average there is no mass infall. This is strikingly different from more massive halos, which have large infall velocities outside of the virial radius. We provide accurate fit for the density profile of these galaxy-size halos. For a wide range (0.01-2)Rvir of radii the halo density profiles are fit with the approximation rho=rho_s exp(-2n[x^{1/n}-1])+rho_m, where x=r/r_s, rho_m is the mean matter density of the Universe, and the index n is in the range n=6-7.5. These profiles do not show a sudden change of behavior beyond the virial radius. For larger radii we combine the statistics of the initial fluctuations with the spherical collapse model to obtain predictions for the mean and most probable density profiles for halos of several masses. The model give excellent results beyond 2-3 formal virial radii.

astro-ph↗

arXiv2005

Microlensing in the double quasar SBS1520+530

We present the results of a monitoring campaign of the double quasar SBS1520+530 at Maidanak observatory from April 2003 to August 2004. We obtained light curves in V and R filters that show small-amplitude Δm~0.1 mag intrinsic variations of the quasar on time scales of about 100 days. The data set is consistent with the previously determined time delay of Δt=(130+-3) days by Burud et al. (2002). We find that the time delay corrected magnitude difference between the quasar images is now larger by (0.14+-0.03) mag than during the observations by Burud et al. (2002). This confirms the presence of gravitational microlensing variations in this system.

astro-ph↗

arXiv2003

The radial structure of galaxy groups and clusters

Simple self-consistent models of galaxy groups and clusters are tested against the results of high-resolution adiabatic gasdynamical simulations. We investigate two models based on the existence of a 'universal' dark matter density profile and two versions of the beta-model. The mass distribution of relaxed clusters can be fitted by phenomenological formulae proposed in the literature. Haloes that have experienced a recent merging event are systematically less concentrated and show steeper profiles than relaxed objects near the centre. The hot X-ray emitting gas is found to be in approximate hydrostatic equilibrium with the dark matter potential, and it is well described by a polytropic equation of state. Analytic formulae for the gas density and temperature can be derived from these premises. Though able to reproduce the X-ray surface brightness, the beta-model is shown to provide a poor description of our numerical clusters. We find strong evidence of a 'universal' temperature profile that decreases by a factor of 2-3 from the centre to the virial radius, whereas baryon fraction and entropy are monotonically increasing functions. Numerical resolution and entropy conservation play a key role in the shapes of the profiles at small radii.

astro-ph↗

arXiv2003

On the physical origin of dark matter density profiles

The radial mass distribution of dark matter haloes is investigated within the framework of the spherical infall model. We present a new formulation of spherical collapse including non-radial motions, and compare the analytical profiles with a set of high-resolution N-body simulations ranging from galactic to cluster scales. We argue that the dark matter density profile is entirely determined by the initial conditions, which are described by only two parameters: the height of the primordial peak and the smoothing scale. These are physically meaningful quantities in our model, related to the mass and formation time of the halo. Angular momentum is dominated by velocity dispersion, and it is responsible for the shape of the density profile near the centre. The phase-space density of our simulated haloes is well described by a power-law profile, rho/sigma^3 = 10^{1.46\pm0.04} (rho_c/Vvir^3) (r/Rvir)^{-1.90\pm0.05}. Setting the eccentricity of particle orbits according to the numerical results, our model is able to reproduce the mass distribution of individual haloes.

astro-ph↗

arXiv2003

Velocity dispersion profile in dark matter halos

Numerous numerical studies indicate that dark matter halos show an almost universal radial density profile. The origin of the profile is still under debate. We investigate this topic and pay particular attention to the velocity dispersion profile. To this end we have performed high-resolution simulations with two independent codes, ART and {\sc Gadget}. The radial velocity dispersion can be approximated as function of the potential by $σ_r^2 = a (Φ/ Φ_{\rm{out}})^κ(Φ_{\rm{out}} - Φ)$, where $Φ_{\rm{out}}$ is the outer potential of the halo. For the parameters $a$ and $κ$ we find $a=0.29\pm0.04$ and $κ=0.41\pm0.03$. We find that the power-law asymptote $σ^2 \propto Φ^κ$ is valid out to much larger distances from the halo center than any power asymptote for the density profile $ρ\propto r^{-n}$. The asymptotic slope $n(r \to 0)$ of the density profile is related to the exponent $κ$ via $n=2κ/(1+κ)$. Thus the value obtained for $κ$ from the available simulation data can be used to obtain an estimate of the density profile below presently resolved scales. We predict a continuously decreasing $n$ towards the halo center with the asymptotic value $n \lesssim 0.58$ at $r=0$.

astro-ph↗

arXiv2002

Numerical simulations of the cosmic star formation history

The cosmic star formation history in Cold Dark Matter dominated cosmological scenarios is studied by means of hydrodynamical numerical simulations. In particular, we explore a low density model with a Lambda-term and two high density models with different power spectra, all of them being spatially flat. Our simulations employ a fully nonlinear N-body and Eulerian hydrodynamics algorithm with a model for star formation and supernovae feedback that depends on two phenomenological parameters determined in agreement with previous papers. We find a nearly constant star formation rate beyond z=1, and we discuss which facts may determine the decrease in the SFR from z=1 to the present epoch. The Lambda-term cosmology with realistic parameters for star formation and feedback best reproduces the observed star formation history.

astro-ph↗

arXiv2000

Determination of cosmological parameters from large scale structure observations

The possibility of determining cosmological parameters on the basis of a wide set of observational data including the Abell-ACO cluster power spectrum and mass function, peculiar velocities of galaxies, the distribution of Ly-$α$ clouds and CMB temperature fluctuations is analyzed. Using a $χ^2$ minimization method, assuming $Ω_Λ+Ω_m =1$ and no contribution from gravity waves, we found that a tilted $Λ$MDM model with one sort of massive neutrinos and the parameters $n\approx 1.12$, $Ω_m\approx 0.4$ ($Ω_Λ\approx 0.6$), $Ω_{cdm}\approx 0.3$, $Ω_ν\approx 0.06$, $Ω_b\approx 0.04$ and $h\approx 0.7$ matches observational data best. $Ω_ν$ is higher for more species of massive neutrinos, $\sim 0.1$ for two and $\sim 0.13$ for three species. $Ω_m$ raises by $\sim 0.08$ and $\sim 0.15$ respectively. Varying only a subset of parameters and fixing the others shows also that the observational data set used here rules out pure CDM models with $h\ge 0.5$, scale invariant primordial power spectrum, zero cosmological constant and spatial curvature at a very high confidence level, $>99.99%$. The corresponding class of MDM models are ruled out at $\sim 95%$ C.L. It is notable also that this data set determines the amplitude of scalar fluctuations approximately at the same level as COBE four-year data. It indicates that a possible tensor component in the COBE data cannot be very substantial.

astro-ph↗

arXiv2000

Cosmological parameters from large scale structure observations

The possibility of determining cosmological parameters on the basis of a wide set of observational data including the Abell-ACO cluster power spectrum and mass function, peculiar velocities of galaxies, the distribution of Ly-$α$ clouds and CMB temperature fluctuations is analyzed. Using a $χ^2$ minimization method, assuming $Ω_Λ+Ω_{\rm{matter}} =1 $ and no contribution from gravity waves, we found that a tilted $Λ$MDM model with one sort of massive neutrinos and the parameters $n=1.12\pm 0.10$, $Ω_m=0.41\pm 0.11$ ($Ω_Λ=0.59\pm0.11$), $Ω_{cdm}=0.31\pm 0.15$, $Ω_ν=0.059\pm 0.028$, $Ω_b=0.039\pm 0.014$ and $h=0.70\pm 0.12$ matches observational data best. The 1$σ$ (68.3%) confidence limits on each cosmological parameter, which are obtained by marginalizing over the other parameters, are $0.82\le n\le1.39$, $0.19\leΩ_m\le 1$ ($0\leΩ_Λ\le 0.81$), $0\leΩ_ν\le 0.17$, $0.021\le Ω_b\le 0.13$ and $0.38\le h\le 0.85$ $1.5\le b_{cl}\le 3.5$. Here $b_{cl}$ is the cluster bias parameter. The best-fit parameters for 31 models which are inside of $1σ$ range of the best model are presented. It is shown also that observational data set used here rules out the class of CDM models with $h\ge 0.5$, scale invariant primordial power spectrum, zero cosmological constant and spatial curvature at very high confidence level, $>99.99%$. The corresponding class of MDM models are ruled out at $\sim 95%$ C.L.

astro-ph↗

arXiv1999

The evolution of groups and clusters

Using high resolution N-body simulations we have studied the formation and evolution of clusters and groups in a LCDM cosmological model. Groups of galaxies have been formed already before z=4. The total number of small bound systems and the total number of galaxies in these small systems rapidly decreases after z=1.5. The fraction of isolated galaxies remains approximately constant after z =1, whereas the fraction of galaxies in groups decreases. Merging of groups and accretion leads to cluster formation at z < 2. Some of the groups merge into large isolated halos.

astro-ph↗