arXiv subjects
Publications and source records attributed to V. Springel.
We use the LUQAS sample of 27 high resolution, high signal-to-noise QSO absorption spectra (Kim et al. 2004) and the results from Croft et al. (2002) together with a suite of high-resolution hydro-dynamical simulations run with the GADGET-II code, to infer the linear dark matter power spectrum on scales of \~ 0.3-30 Mpc at z=2.125 and at z=2.72.
We study the formation of clusters of galaxies using high-resolution hydrodynamic cosmological simulations that include the effect of thermal conduction with an effective isotropic conductivity of 1/3 the classical Spitzer value. We find that, both for a hot ($T_{\rm ew}\simeq 12$ keV) and several cold ($T_{\rm ew}\simeq 2$ keV) galaxy clusters, the baryonic fraction converted into stars does not change significantly when thermal conduction is included. However, the temperature profiles are modified, particularly in our simulated hot system, where an extended isothermal core is readily formed. As a consequence of heat flowing from the inner regions of the cluster both to its outer parts and into its innermost resolved regions, the entropy profile is altered as well. This effect is almost negligible for the cold cluster, as expected based on the strong temperature dependence of the conductivity. Our results demonstrate that while thermal conduction can have a significant influence on the properties of the intra--cluster medium of rich galaxy clusters, it appears unlikely to provide by itself a solution for the overcooling problem in clusters, or to explain the current discrepancies between the observed and simulated properties of the intra--cluster medium.
We describe hydrodynamical simulations of galaxy formation in a Lambda cold dark matter (CDM) cosmology performed using a subresolution model for star formation and feedback in a multiphase interstellar medium (ISM). In particular, we demonstrate the formation of a well-resolved disk galaxy. The surface brightness profile of the galaxy is exponential, with a B-band central surface brightness of 21.0 mag arcsec^-2 and a scale-length of R_d = 2.0 h^-1 kpc. We find no evidence for a significant bulge component. The simulated galaxy falls within the I-band Tully-Fisher relation, with an absolute magnitude of I = -21.2 and a peak stellar rotation velocity of V_rot=121.3 km s^-1. While the total specific angular momentum of the stars in the galaxy agrees with observations, the angular momentum in the inner regions appears to be low by a factor of ~2. The star formation rate of the galaxy peaks at ~7 M_sun yr^-1 between redshifts z=2-4, with the mean stellar age decreasing from \~10 Gyrs in the outer regions of the disk to ~7.5 Gyrs in the center, indicating that the disk did not simply form inside-out. The stars exhibit a metallicity gradient from 0.7 Z_sun at the edge of the disk to 1.3 Z_sun in the center. Using a suite of idealized galaxy formation simulations with different models for the ISM, we show that the effective pressure support provided by star formation and feedback in our multiphase model is instrumental in allowing the formation of large, stable disk galaxies. If ISM gas is instead modeled with an isothermal equation of state, or if star formation is suppressed entirely, growing gaseous disks quickly violate the Toomre stability criterion and undergo catastrophic fragmentation.
We present a determination of the bispectrum of the flux in the Lyman-alpha forest of QSO absorption spectra obtained from LUQAS which consists of spectra observed with the high resolution Echelle spectrograph UVES. Typical errors on the observed bispectrum as obtained from a jack-knife estimator are ~ 50%. For wavenumbers in the range 0.03 s/km 2.
We analyse the properties of substructures within dark matter halos (subhalos) using a set of high-resolution numerical simulations of the formation of structure in a Lambda-CDM 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)Mo. 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 halos than in high mass halos. More massive substructures tend to avoid the centres of halos and are preferentially located in the external regions of their parent halos. 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 should have been still outside the cluster progenitor and more active at z~1.
We present results on the X-ray properties of clusters and groups of galaxies, extracted from a large hydrodynamical simulation. We used the GADGET code to simulate a LambdaCDM model within a box of 192 Mpc/h on a side, with 480^3 dark matter particles and as many gas particles. The simulation includes radiative cooling, star formation and supernova feedback. The simulated M-T relation is consistent with observations once we mimic the procedure for mass estimates applied to real clusters. Also, with the adopted choices of Omega_m=0.3 and sigma_8=0.8 the resulting XTF agrees with observational determinations. The L-T relation also agrees with observations for clusters with T>2 keV, with no change of slope at the scale of groups. The entropy in central cluster regions is higher than predicted by gravitational heating alone, the excess being almost the same for clusters and groups. The simulated clusters appear to have suffered some overcooling, with f*~0.2, thus about twice as large as the value observed. Interestingly, temperature profiles are found to steadily increase toward cluster centers. They decrease in the outer regions, much like observational data do at r>0.2r_vir, while not showing an isothermal regime followed by a smooth temperature decline in the innermost regions.
We use a set of high-resolution cosmological N-body simulations to investigate the inner mass profile of galaxy-sized cold dark matter (CDM) halos. These simulations extend the thorough numerical convergence study presented in Paper I of this series (Power et al. 2003), and demonstrate that the mass profile of CDM halos can be robustly estimated beyond a minimum converged radius of order r_conv ~ 1 kpc/h in our highest resolution runs. The density profiles of simulated halos become progressively shallow from the virial radius inwards, and show no sign of approaching a well-defined power-law behaviour near the centre. At r_conv, the logarithmic slope of the density profile is steeper than the asymptotic ρ\propto r^-1 expected from the formula proposed by Navarro, Frenk, and White (1996), but significantly shallower than the steeply divergent ρ\propto r^-1.5 cusp proposed by Moore et al. (1999). We perform a direct comparison of the spherically-averaged dark matter circular velocity (V_c) profiles with rotation curves of low surface brightness (LSB) galaxies from the samples of de Blok et al. (2001), de Blok and Bosma (2002), and Swaters et al. (2003). Most (about two-thirds) LSB galaxies in this dataset are roughly consistent with CDM halo V_c profiles. However, about one third of LSBs in these samples feature a sharp transition between the rising and flat part of the rotation curve that is not seen in the V_c profiles of CDM halos. This discrepancy has been interpreted as excluding the presence of cusps, but we argue that it might simply reflect the difference between circular velocity and gas rotation speed likely to arise in gaseous disks embedded within realistic, triaxial CDM halos.
Magnetic fields (MF) in the Local Supercluster (LSC) of galaxies may have profound consequences for the propagation of Ultra High Energy Cosmic Rays (UHECR). Faraday rotations measurements provide some informations about MF in compact clusters. However, very few is known about less dense regions and about the global structure of MF in the LSC. In order to get a better knowledge of these fields we are performing constrained magnetohydrodynamical simulations of the LSC magnetic field. We will present the results of our simulation and discuss their implications for the angular distribution of expected UHECR deflections.
We discuss Tree+SPH simulations of galaxy clusters and groups, aimed at studying the effect of cooling and non-gravitational heating on observable properties of the ICM. We simulate at high resolution four halos,with masses in the range (0.2-4)10^{14}M_sol. We discuss the effects of using different SPH implementations and show that high resolution is mandatory to correctly follow the cooling pattern of the ICM. All of our heating schemes which correctly reproduce the X-ray scaling properties of clusters and groups do not succeed in reducing the fraction of collapsed gas below a level of 20 (30) per cent at the cluster (group) scale. Finally, gas compression in cooling cluster regions causes an increase of the temperature and a steepening of the temperature profiles, independent of the presence of non-gravitational heating processes. This is inconsistent with recent observational evidence for a decrease of gas temperature towards the center of relaxed clusters. Provided these discrepancies persist even for a more refined modeling of energy feedback, they may indicate that some basic physical process is still missing in hydrodynamical simulations.
We present a comprehensive set of convergence tests which explore the role of various numerical parameters on the equilibrium structure of a simulated dark matter halo. We report results obtained with two independent, state-of-the-art, multi-stepping, parallel N-body codes: PKDGRAV and GADGET. We find that convergent mass profiles can be obtained for suitable choices of the gravitational softening, timestep, force accuracy, initial redshift, and particle number. For softenings chosen so that particle discreteness effects are negligible, convergence in the circular velocity is obtained at radii where the following conditions are satisfied: (i) the timestep is much shorter than the local orbital timescale; (ii) accelerations do not exceed a characteristic acceleration imprinted by the gravitational softening; and (iii) enough particles are enclosed so that the collisional relaxation timescale is longer than the age of the universe. The most stringent requirement for convergence is typically that imposed on the particle number by the collisional relaxation criterion, which implies that in order to estimate accurate circular velocities at radii where the density contrast may reach $\sim 10^6$, the region must enclose of order 3000 particles (or more than a few times $10^6$ within the virial radius). Applying these criteria to a galaxy-sized $Λ$CDM halo, we find that the spherically-averaged density profile becomes progressively shallower from the virial radius inwards, reaching a logarithmic slope shallower than -1.2 at the innermost resolved point, $r \sim 0.005 r_{200}$, with little evidence for convergence to a power-law behaviour in the inner regions.
We simulate the formation and evolution of the local galaxy population starting from initial conditions with a smoothed linear density field which matches that derived from the IRAS 1.2 Jy galaxy survey. Our simulations track the formation and evolution of all dark matter haloes more massive than 10e+11 solar masses out to a distance of 8000 km/s from the Milky Way. We implement prescriptions similar to those of Kauffmann et al. (1999) to follow the assembly and evolution of the galaxies within these haloes. We focus on two variants of the CDM cosmology: an LCDM and a tCDM model. Galaxy formation in each is adjusted to reproduce the I-band Tully-Fisher relation of Giovanelli et al. (1997). We compare the present-day luminosity functions, colours, morphology and spatial distribution of our simulated galaxies with those of the real local population, in particular with the Updated Zwicky Catalog, with the IRAS PSCz redshift survey, and with individual local clusters such as Coma, Virgo and Perseus. We also use the simulations to study the clustering bias between the dark matter and galaxies of differing type. Although some significant discrepancies remain, our simulations recover the observed intrinsic properties and the observed spatial distribution of local galaxies reasonably well. They can thus be used to calibrate methods which use the observed local galaxy population to estimate the cosmic density parameter or to draw conclusions about the mechanisms of galaxy formation. To facilitate such work, we publically release our z=0 galaxy catalogues, together with the underlying mass distribution.
Among the various phenomena observed in interacting galaxies is the ejection due to tidal forces of material into the intergalactic medium and its subsequent rearranging which can lead to the formation of self-gravitating tidal dwarf galaxies (TDGs). We investigate this process with a detailed multiwavelength study of the interacting system Arp 245 (NGC 2992/3) and a numerical model of the collision. The system is observed at an early stage of the interaction, about 100 Myr after perigalacticon, though at a time when tidal tails have already developed. Our HI map shows prominent counterparts to the optical tails. Whereas the stellar and gaseous components of the plume that originates from NGC 2992 match, the stellar and HI tails emanating from NGC 2993 have a different morphology. In particular, the HI forms a ring, a feature that has been successfully reproduced by our numerical simulations. The HI emission in the system as a whole peaks at the tip of the NGC 2992 tail where a gas reservoir of about 10^9 Msun, about 60% of the HI towards NGC 2992, coincides with a star-forming optical condensation, A245N. The latter tidal object exhibits properties ranging between those of dwarf irregular galaxies (structural parameters, gas content, star formation rate) and those of spiral disks (metallicity, star formation efficiency, stellar population). Prompted by the questions raised for this particular object, we discuss some issues related to the definition and identification of TDGs (abridged)
We study how the internal structure of dark halos is affected if Cold Dark Matter particles are assumed to have a large cross-section for elastic collisions. We identify a cluster halo in a large cosmological N-body simulation and resimulate its formation with progressively increasing resolution. We compare the structure found in the two cases where dark matter is treated as collisionless or as a fluid. For the collisionless case our results agree with those of other workers. Collisional dark matter results in a cluster which is more nearly spherical and has a more singular central density profile. Substructure within the cluster is only weakly suppressed relative to the collisionless case. The observed structure of dwarf galaxies argues against self-interacting dark matter if, as seems likely, intermediate cross-sections produce structure lying between the extremes we have simulated.
Whereas it is well known that gas may be driven into the core of merging disks, fueling a central AGN or a nuclear starburst, recent studies have shown that a significant fraction of the stellar/gaseous components is expelled into the intergalactic medium along tidal tails. The tidal debris might be dispersed in the intergalactic/intracluster medium where it adds to the diffuse background light such as that observed in the Coma cluster, might fall back towards the merger, or regroup to form a new generation of galaxies, the so-called tidal dwarf galaxies (TDGs). We illustrate in this paper these various phenomena with a detailed multi-wavelength study of the interacting system NGC 2992/3 (Arp 245).
We investigate the topology of the new Point Source Catalogue Redshift Survey (PSCz) of IRAS galaxies by means of the genus statistic. The survey maps the local Universe with approximately 15000 galaxies over 84.1 per cent of the sky and provides an unprecedented number of resolution elements for the topological analysis. For comparison with the PSCz data we also examine the genus of large N-body simulations of four variants of the cold dark matter cosmogony. The simulations are part of the Virgo project to simulate the formation of structure in the Universe. We assume that the statistical properties of the galaxy distribution can be identified with those of the dark matter particles in the simulations. We extend the standard genus analysis by examining the influence of sampling noise on the genus curve and introducing a statistic able to quantify the amount of phase correlation present in the density field, the "amplitude drop" of the genus compared to a Gaussian field with identical power spectrum. The results for PSCz are consistent with the hypothesis of random-phase initial conditions. In particular, no strong phase correlation is detected on scales ranging from 10 h^(-1)Mpc to 32 h^(-1)Mpc, whereas there is a positive detection of phase correlation at smaller scales. Among the simulations, phase correlations are detected in all models at small scales, albeit with different strengths. when scaled to a common normalization, the amplitude drop primarily depends on the shape of the power spectrum. We find that the constant bias standard CDM model can be ruled out at high significance because the shape of its power spectrum is not consistent with PSCz. The other CDM models with more large-scale power all fit the PSCz data almost equally well, with a slight preference for a high-density tauCDM model.
We study the topology of the Virgo N-body simulations and compare it to the 1.2-Jy redshift survey of IRAS galaxies by means of the genus statistic. Four high-resolution simulations of variants of the CDM cosmology are considered: a flat standard model (SCDM), a variant of it with more large-scale power (tCDM), and two low density universes, one open (OCDM) and one flat (LCDM). The fully sampled N-body simulations are examined down to strongly nonlinear scales, both with spatially fixed smoothing, and with an adaptive smoothing technique. While the tCDM, LCDM, and OCDM simulations have very similar genus statistics in the regime accessible to fixed smoothing, they can be separated with adaptive smoothing at small mass scales. In order to compare the N-body models with the 1.2-Jy survey, we extract large ensembles of mock catalogues from the simulations. These mock surveys are used to test for systematic effects in the genus analysis and to establish the distribution of errors of the genus curve. We find that a simple multivariate analysis of the genus measurements is compromised both by non-Gaussian distributed errors and by noise that dominates the covariance matrix. We therefore introduce a principal components analysis of the genus curve. With a likelihood ratio test we find that the 1.2-Jy data favours the LCDM, tCDM and OCDM models compared to SCDM. When genus measurements for different smoothing scales are combined, the SCDM model can be excluded at a 99% confidence level, while the other three models fit the 1.2-Jy data well. (abridged)