arXiv subjects

S. D. M. White

Publications and source records attributed to S. D. M. White.

At least 73 records · Page 4Linked to original sources

arXiv2003

The Inner Structure of LCDM Halos II: Halo Mass Profiles and LSB Rotation Curves

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.

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arXiv2003

Simulating IGM Reionization

We have studied the IGM reionization process in its full cosmological context including structure evolution and a realistic galaxy population. We have used a combination of high-resolution N-body simulations (to describe the dark matter and diffuse gas component), a semi-analytic model of galaxy formation (to track the evolution of the sources of ionizing radiation) and the Monte Carlo radiative transfer code CRASH (to follow the propagation of ionizing photons into the IGM). The process has been followed in the largest volume ever used for this kind of study, a field region of the universe with a comoving length of L~20/h Mpc, embedded in a much larger cosmological simulation. To assess the effect of environment on the reionization process, the same radiative transfer simulations have been performed on a 10/h Mpc comoving box, centered on a clustered region. We find that, to account for the all ionizing radiation, objects with total masses of M~10^9 Msun must be resolved. In this case, the simulated stellar population produces a volume averaged ionization fraction x_v=0.999 by z~8, consistent with observations without requiring any additional sources of ionization. We also find that environment substantially affects the reionization process. In fact, although the simulated proto-cluster occupies a smaller volume and produces a higher number of ionizing photons, it gets totally ionized later. This is because high density regions, which are more common in the proto-cluster, are difficult to ionize because of their high recombination rates.

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arXiv2003

Stable clustering, the halo model and nonlinear cosmological power spectra

We present the results of a large library of cosmological N-body simulations, using power-law initial spectra. The nonlinear evolution of the matter power spectra is compared with the predictions of existing analytic scaling formulae based on the work of Hamilton et al. The scaling approach has assumed that highly nonlinear structures obey `stable clustering' and are frozen in proper coordinates. Our results show that, when transformed under the self-similarity scaling, the scale-free spectra define a nonlinear locus that is clearly shallower than would be required under stable clustering. Furthermore, the small-scale nonlinear power increases as both the power-spectrum index n and the density parameter Omega decrease, and this evolution is not well accounted for by the previous scaling formulae. This breakdown of stable clustering can be understood as resulting from the modification of dark-matter haloes by continuing mergers. These effects are naturally included in the analytic `halo model' for nonlinear structure; using this approach we are able to fit both our scale-free results and also our previous CDM data. This approach is more accurate than the commonly-used Peacock--Dodds formula and should be applicable to more general power spectra. Code to evaluate nonlinear power spectra using this method is available from http://as1.chem.nottingham.ac.uk/~res/software.html Following publication, we will make the power-law simulation data available through the Virgo website http://www.mpa-garching.mpg.de/Virgo

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arXiv2003

Early Reionization by the First Galaxies

Large-scale polarization of the cosmic microwave background measured by the WMAP satellite requires a mean optical depth to Thomson scattering, tau_e ~ 0.17. The reionization of the universe must therefore have begun at relatively high redshift. We have studied the reionization process using supercomputer simulations of a large and representative region of a universe which has cosmological parameters consistent with the WMAP results. Our simulations follow both the radiative transfer of ionizing photons and the formation and evolution of the galaxy population which produces them. A previously published model with a standard stellar IMF, with a moderate photon escape fraction from galaxies and with ionizing photon production as expected for zero metallicity stars produces tau_e = 0.104, which is within 1.0 to 1.5-sigma of the ``best'' WMAP value. Values up to 0.16 can be produced by taking larger escape fractions or a top heavy IMF. The data do not require a separate populations of ``miniquasars'' or of stars forming in objects with total masses below 10^9 M_sun. Reconciling such early reionization with the observed Gunn-Peterson troughs in z>6 quasars may be challenging. Possible resolutions of this problem are discussed.

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arXiv2003

The Inner Structure of LambdaCDM Halos I: A Numerical Convergence Study

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.

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arXiv2002

Smoothed particle hydrodynamics for galaxy formation simulations: improved treatments of multiphase gas, of star formation and of supernovae feedback

We investigate a new implementation of the Smoothed Particle Hydrodynamics technique (SPH) designed to improve the realism with which galaxy formation can be simulated. In situations where cooling leads to the coexistence of phases of very different density and temperature, our method substantially reduces artificial overcooling near phase boundaries, prevents the exclusion of hot gas from the vicinity of cold ``clouds'', and allows relative motion of the two phases at each point. We demonstrate the numerical stability of our scheme in the presence of extremely steep density and temperature gradients, as well as in strong accretion shocks and cooling flows. In addition, we present new implementations of star formation and feedback which simulate the effect of energy injection into multiphase gas more successfully than previous schemes. Our feedback recipes deposit thermal energy separately in cold dense gas and hot diffuse gas, and can explicitly reinject cold gas into the hot phase. They make it possible to damp star formation effectively, to reheat cold gas, and to drive outflows into the galaxy halo and beyond. We show feedback effects to be strongest in small mass objects where much of the gas can be expelled. After idealised tests, we carry out a first low resolution study of galaxy formation in a $Λ$CDM universe. Feedback results in substantial and mass-dependent reductions in the total baryonic mass gathered onto the final object as well as in significant modulation of the star formation history.

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arXiv2002

The abundance and clustering of dark haloes in the standard Lambda CDM cosmogony

Much evidence suggests that we live in a flat Cold Dark Matter universe with a cosmological constant. Accurate analytic formulae are now available for many properties of the dark halo population in such a Universe. Assuming current ``concordance'' values for the cosmological parameters, we plot halo abundance against redshift as a function of halo mass, of halo temperature, of the fraction of cosmic matter in haloes, of halo clustering strength, and of the clustering strength of the z=0 descendants of high redshift haloes. These plots are useful for understanding how nonlinear structure grows in the model. They demonstrate a number of properties which may seem surprising, for example: 10^9 solar mass haloes are as abundant at z=20 as L_* galaxies are today; 10^6K haloes are equally abundant at z=8 and at z=0; 10% of all matter is currently in haloes hotter than 1 keV, while more than half is in haloes too cool to trap photo-ionized gas; 1% of all matter at z=15 is in haloes hot enough to ionise hydrogen; haloes of given mass or temperature are more clustered at higher redshift; haloes with the abundance of present-day L_* galaxies are equally clustered at all z<20; the metals produced by star-formation at z>10 are more clustered at z=0 than are L_* galaxies.

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arXiv2002

Cosmic shear from STIS pure parallels - II. Analysis

We report on the marginal detection of cosmic shear on sub-arcmin scales with archive data from the STIS camera on board HST. For the measurement 121 galaxy fields with a field of view of 51'' x 51'' are used to obtain an rms cosmic shear of ~ 4% with 1.5sigma significance. This value is consistent with groundbased results obtained by other groups on larger scales, and with theoretical predictions for a standard LambdaCDM cosmology. To show the suitability of STIS for weak shear measurements we carefully investigated the stability of the PSF. We demonstrate that small temporal changes do not affect the cosmic shear measurement by more than ~ 10%. We also discuss the influence of various weighting and selection schemes for the galaxy ellipticities.

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arXiv2002

Galaxy Clusters in Hubble Volume Simulations: Cosmological Constraints from Sky Survey Populations

We use giga-particle N-body simulations to study galaxy cluster populations in Hubble Volumes of LCDM (Omega_m=0.3, Omega_Lambda=0.7) and tCDM (Omega_m=1) world models. Mapping past light-cones of locations in the computational space, we create mock sky surveys of dark matter structure to z~1.4 over 10,000 sq deg and to z~0.5 over two full spheres. Calibrating the Jenkins mass function at z=0 with samples of ~1.5 million clusters, we show that the fit describes the sky survey counts to <~20% acccuracy over all redshifts for systems larger than poor groups (M>5e13 Msun/h). Fitting the observed local temperature function determines the ratio beta of specific thermal energies in dark matter and intracluster gas. We derive a scaling with power spectrum normalization beta \propto sigma8^{5/3}, and measure a 4% error on sigma8 arising from cosmic variance in temperature-limited cluster samples. Considering distant clusters, the LCDM model matches EMSS and RDCS X-ray-selected survey observations under economical assumptions for intracluster gas evolution. Using transformations of mass-limited cluster samples that mimic sigma8 variation, we explore SZ search expectations for a 10 sq deg survey complete above 10^{14} Msun/h. Cluster counts are shown to be extremely sensitive to sigma8 uncertainty while redshift statistics, such as the sample median, are much more stable. For LCDM, the characteristic temperature at fixed sky surface density is a weak function of redshift, implying an abundance of hot clusters at z>1. Assuming constant beta, four kT>8 keV clusters lie at z>2 and 40 kT>5 keV clusters lie at z>3 on the whole sky. Detection of Coma-sized clusters at z>1 violate LCDM at 95% confidence if their surface density exceeds 0.003 per sq deg, or 120 on the whole sky.

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arXiv2002

A conspicuous tangential alignment of galaxies in a STIS Parallel Shear Survey field: A new dark-lens candidate ?

We report the serendipituous discovery of a conspicuous alignment of galaxies in a field obtained through the STIS Parallel Shear Survey. This project collects randomly distributed 50'' x 50'' fields to investigate the cosmic shear effect on this scale. Analyzing the parallel observations having the Seyfert galaxy NGC625 as primary target, we recognized over the whole field of view a strong apparent tangential alignment of galaxy ellipticities towards the image center. The field shows several arclet-like features typical for images of massive galaxy clusters, but no obvious over-density of bright foreground galaxies. We also find a multiple image candidate. On the basis of the possible strong and weak lensing effect within the data, we discuss whether this could be compatible with a massive halo with no clear optical counterpart.

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arXiv2002

Voids in the Simulated Local Universe

We use simulations of the formation and evolution of the galaxy population in the Local Universe to address the issue of whether the standard theoretical model succeeds in producing empty regions as large and as dark as the observed nearby ones. We follow the formation of galaxies in an LCDM universe and work with mock catalogues which can resolve the morphology of LMC sized galaxies, and the luminosity of objects 6 times fainter. We look for a void signature in sets of virialized haloes selected by mass, as well as in mock galaxy samples selected according to observationally relevant quantities, like luminosity, colour, or morphology. We find several void regions with diameter 10 Mpc/h in the simulation where gravity seems to have swept away even the smallest haloes we were able to track. We probe the environment density of the various populations and compute luminosity functions for galaxies residing in underdense, mean density and overdense regions. We also use nearest neighbour statistics to check possible void populations, taking $L_{*}$ spirals as reference neighbours. Down to our resolution limits, we find that all types of galaxies avoid the same regions, and that no class appears to populate the voids defined by the bright galaxies.

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arXiv2001

Star Formation, Metallicity and Dust Properties Derived from the SAPM Galaxy Survey Spectra

We have derived star formation rates (SFRs), gas-phase oxygen abundances and effective dust absorption optical depths for a sample of galaxies drawn from the Stromlo-APM redshift survey using the new Charlot and Longhetti (2001; CL01) models, which provide a physically consistent description of the effects of stars, gas and dust on the integrated spectra of galaxies. Our sample consists of 705 galaxies with measurements of the fluxes and equivalent widths of Halpha, [OII], and one or both of [NII] and [SII]. For a subset of the galaxies, 60 and 100 micron IRAS fluxes are available. We compare the star formation rates derived using the models with those derived using standard estimators based on the Halpha, the [OII] and the far-infrared luminosities of the galaxies. The CL01 SFR estimates agree well with those derived from the IRAS fluxes, but are typically a factor of ~3 higher than those derived from the Halpha or the [OII] fluxes, even after the usual mean attenuation correction of A_Halpha=1 mag is applied to the data. We show that the reason for this discrepancy is that the standard Halpha estimator neglects the absorption of ionizing photons by dust in HII regions and the contamination of Halpha emission by stellar absorption. We also use our sample to study variations in star formation and metallicity as a function of galaxy absolute bJ magnitude. For this sample, the star formation rate per unit bJ luminosity is independent of magnitude. The gas-phase oxygen abundance does increase with bJ luminosity, although the scatter in metallicity at fixed magnitude is large.

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arXiv2001

Simulating the Formation of the Local Galaxy Population

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.

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arXiv2001

The cosmic shear STIS parallel program - First results

Since the Universe is inhomogeneous on scales well below the Hubble radius, light bundles from distant galaxies are deflected and distorted by the tidal gravitational field of the large-scale matter distribution as they propagate through the Universe. Two-point statistical measures of the observed ellipticities, like the dispersion within a finite aperture or the ellipticity cross-correlation, can be related to the power spectrum of the large-scale structure. The measurement of cosmic shear (especially on small angular scales) can thus be used to constrain cosmological parameters and to test cosmological structure formation in the non-linear regime, without any assumptions about the relation between luminous and dark matter. In this paper we will present preliminary cosmic shear measurements on sub-arcminute scales, obtained from archival STIS parallel data. The high angular resolution of HST, together with the sensitivity and PSF-stability of STIS, allows us to measure cosmic shear along many independent lines-of-sight. Ongoing STIS parallel observations, currently being carried out in the frame of a big GO program (8562+9248), will greatly increase the number of available useful fields and will enable us to measure cosmic shear with higher accuracy on sub-arcminute scales.

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arXiv2001

Cosmic Shear from STIS Pure Parallels: Analysis

The measurement of cosmic shear requires deep imaging with high image quality on many lines of sight to sample the statistics of large-scale structure. The expected distortion of galaxy images by cosmic shear on the STIS angular scale is a few percent, therefore the PSF anisotropy has to be understood and controlled to an accuracy better than 1%. In this poster we present the analysis of the PSF of STIS and a preliminary cosmic shear measurement using archival data from the STIS pure parallel program to show that the STIS camera on-board HST is well suited for our project. The data reduction and catalog production are described in an accompanying paper (astro-ph/0102330).

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arXiv2001

Cosmic shear from STIS Pure Parallels: I Data

Following the second HST servicing mission in 1997 when the STIS instrument was installed and the capability for parallel observations was enhanced, a substantial archive of non-proprietary parallel data has been accumulating. In this paper, we discuss the use of unfiltered STIS imaging data for a project that requires deep observations along as many independent lines-of-sight as possible. We have developed a technique to determine which datasets in the archive can safely be co-added together and have developed an iterative co-addition technique which enabled us to produce 498 high-quality, deep images. The principal motivation for this work is to measure the Cosmic Shear on small angular scales and a value derived from these data will be presented in a subsequent paper. A valuable by-product of this work is a set of high quality combined fields which can be used for other projects. The data are publicly available at http://www.stecf.org/projects/shear/

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arXiv2001

Simulations of Deep Pencil-Beam Redshift Surveys

We create mock pencil-beam redshift surveys from very large cosmological $N$-body simulations of two Cold Dark Matter cosmogonies, an Einstein-de Sitter model ($τ$CDM) and a flat model with $Ω_0 =0.3$ and a cosmological constant ($Λ$CDM). We use these to assess the significance of the apparent periodicity discovered by Broadhurst et al. (1990). Simulation particles are tagged as `galaxies' so as to reproduce observed present-day correlations. They are then identified along the past light-cones of hypothetical observers to create mock catalogues with the geometry and the distance distribution of the Broadhurst et al. data. We produce 1936 (2625) quasi-independent catalogues from our $τ$CDM ($Λ$CDM) simulation. A couple of large clumps in a catalogue can produce a high peak at low wavenumbers in the corresponding one-dimensional power spectrum, without any apparent large-scale periodicity in the original redshift histogram. Although the simulated redshift histograms frequently display regularly spaced clumps, the spacing of these clumps varies between catalogues and there is no `preferred' period over our many realisations. We find only a 0.72 (0.49) per cent chance that the highest peak in the power spectrum of a $τ$CDM ($Λ$CDM) catalogue has a peak-to-noise ratio higher than that in the Broadhurst et al. data. None of the simulated catalogues with such high peaks shows coherently spaced clumps with a significance as high as that of the real data. We conclude that in CDM universes, the kind of regularity observed by Broadhurst et al. has a priori probability well below $10^{-3}$.

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arXiv2000

Predicting the peculiar velocities of nearby PSC-z galaxies using the Least Action Principle

We use the Least Action Principle to predict the peculiar velocities of PSC-z galaxies inside $cz=2000$\km. Linear theory is used to account for tidal effects to $cz=15000$\km, and we iterate galaxy positions to account for redshift distortions. As the Least Action Principle is valid beyond Linear theory, we can predict reliable peculiar velocities even for very nearby galaxies (ie $cz \leq 500$\km). These predicted peculiar velocities are then compared with the observed velocities of 12 galaxies with Cepheid distances. The combination of the PSC-z galaxy survey (with its large sky coverage and uniform selection), with the accurate Cepheid distances, makes this comparison relatively free from systematic effects. We find that galaxies are good tracers of the mass, even at small ($\leq 10$\mpc) scales; and under the assumption of no biasing, $0.25 \leq β\leq 0.75$ (at 90% confidence). We use the reliable predicted peculiar velocities to estimate the Hubble constant \ho from the local volume without ``stepping up'' the distance ladder, finding a confidence range of 65-75\kmM (at 90% confidence).

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