arXiv subjects

Simon D. M. White

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

At least 271 records · Page 15Linked to original sources

arXiv1997

A Universal Density Profile from Hierarchical Clustering

We use high-resolution N-body simulations to study the equilibrium density profiles of dark matter halos in hierarchically clustering universes. We find that all such profiles have the same shape, independent of halo mass, of initial density fluctuation spectrum, and of the values of the cosmological parameters. Spherically averaged equilibrium profiles are well fit over two decades in radius by a simple formula originally proposed to describe the structure of galaxy clusters in a cold dark matter universe. In any particular cosmology the two scale parameters of the fit, the halo mass and its characteristic density, are strongly correlated. Low-mass halos are significantly denser than more massive systems, a correlation which reflects the higher collapse redshift of small halos. The characteristic density of an equilibrium halo is proportional to the density of the universe at the time it was assembled. A suitable definition of this assembly time allows the same proportionality constant to be used for all the cosmologies that we have tested. We compare our results to previous work on halo density profiles and show that there is good agreement. We also provide a step-by-step analytic procedure, based on the Press-Schechter formalism, which allows accurate equilibrium profiles to be calculated as a function of mass in any hierarchical model.

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arXiv1997

The Formation of Galactic Disks

We study the population of galactic disks expected in current hierarchical clustering models for structure formation. A rotationally supported disk with exponential surface density profile is assumed to form with a mass and angular momentum which are fixed fractions of those of its surrounding dark halo. We assume that haloes respond adiabatically to disk formation, and that only stable disks can correspond to real systems. With these assumptions the predicted population can match both present-day disks and the damped Lyman alpha absorbers in QSO spectra. Good agreement is found provided: (i) the masses of disks are a few percent of those of their haloes; (ii) the specific angular momenta of disks are similar to those of their haloes; (iii) present-day disks were assembled recently (at z<1). In particular, the observed scatter in the size-rotation velocity plane is reproduced, as is the slope and scatter of the Tully-Fisher relation. The zero-point of the TF relation is matched for a stellar mass-to-light ratio of 1 to 2 h in the I-band, consistent with observational values derived from disk dynamics. High redshift disks are predicted to be small and dense, and could plausibly merge together to form the observed population of elliptical galaxies. In many (but not all) currently popular cosmogonies, disks with rotation velocities exceeding 200 km/s can account for a third or more of the observed damped Lyman alpha systems at z=2.5. Half of the lines-of-sight to such systems are predicted to intersect the absorber at r>3kpc/h and about 10% at r>10kpc/h. The cross-section for absorption is strongly weighted towards disks with large angular momentum and so large size for their mass. The galaxy population associated with damped absorbers should thus be biased towards low surface brightness systems.

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arXiv1997

The Structure and Dynamical Evolution of Dark Matter Halos

(Shortened) We use N-body simulations to investigate the structure and dynamical evolution of dark matter halos in galaxy clusters. Our sample consists of nine massive halos from an EdS universe with scale free power spectrum and n = -1. Halos are resolved by ~20000 particles each, with a dynamical resolution of 20-25 kpc. Large scale tidal fields are included up to L=150 Mpc using background particles. The halo formation process can be characterized by the alternation of two dynamical configurations: a merging phase and a relaxation phase, defined by their signature on the evolution of the total mass and rms velocity. Halos spend on average one 1/3 of their evolution in the merging phase and 2/3 in the relaxation phase. Using this definition, we study the density profiles and their change during the halo history. The average density profiles are fitted by the NFW analytical model with an rms residual of 17% between the virial radius Rv and 0.01 Rv. The Hernquist (1990) profiles fits the same halos with an rms residual of 26%. The trend with mass of the scale radius of these fits is marginally consistent with that found by Cole & Lacey (1996): in comparison our halos are more centrally concentrated, and the relation between scale radius and halo mass is slightly steeper. We find a moderately large scatter in this relation, due both to dynamical evolution within halos and to fluctuations in the halo population. We analyze the dynamical equilibrium of our halos using the Jeans' equation, and find that on average they are approximately in equilibrium within their virial radius. Finally, we find that the projected mass profiles of our simulated halos are in very good agreement with the profiles of three rich galaxy clusters derived from strong and weak gravitational lensing observations.

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arXiv1997

A Galaxy-Weighted Measure of the Relative Peculiar Velocity Dispersion

The relative pair dispersion of galaxies has for the past decade been the standard measure of the thermal energy of fluctuations in the observed galaxy distribution. This statistic is known to be unstable, since it is a pair-weighted measure that is very sensitive to rare, rich clusters of galaxies. As a more stable alternative, we here present a single-particle-weighted statistic $σ_1$, which can be considered as an estimate of the one-dimensional rms peculiar velocity dispersion of galaxies relative to their neighbors, and which can be interpreted by means of a filtered version of the Cosmic-Energy equation. We calculate this statistic for the all-sky survey of IRAS galaxies, finding $σ_1=95 \pm 16 $ km/sec. The UGC catalog yields a higher value, $σ_1=130 \pm 15$ km/s. We calibrate our procedure by means of mock catalogs constructed from N-body simulations and find that our method is stable and has modest biases which can easily be corrected. We use the measured values of $σ_1$ in a filtered Layzer-Irvine equation to obtain an estimate of $\tildeΩ\equiv Ω/b^2$. We find that $\tildeΩ\approx 0.14 \pm 0.05$ for both the IRAS and UGC catalogs, which is slightly lower than other recent determinations, but is consistent with a trend of an effective $Ω$ that increases gradually with scale.

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arXiv1997

Estimates for the Luminosity Function of Galaxies and its Evolution

A new method is presented to obtain a non-parametric maximum likelihood estimate of the luminosity function and the selection function of a flux limited redshift survey. The method parameterizes the selection function as a series of step-wise power laws and allows possible evolution of the luminosity function. We also propose a new technique to estimate the rate of evolution of the luminosity function. This is based on a minimization of the observed large-scale power with respect to the evolutionary model. We use an ensemble of mock surveys extracted from a N-body simulation to verify the power of this method. We apply our estimators to the 1.2-Jy survey of IRAS galaxies. We find a far-infrared luminosity function in good agreement with previously published results and evidence for rather strong evolution. If the comoving number density of IRAS galaxies is assumed to scale proportional to (1+z)^P, we estimate P=4.3 +/- 1.4 +/-1.

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arXiv1997

The Physical Origin of Galaxy Scaling Relations

A standard paradigm is now available for the recent evolution (z < 10) of structure on galactic and larger scales. Most of the matter is assumed to be dark and dissipationless and to cluster hierarchically from gaussian initial conditions. Gas moves under the gravitational influence of this dark matter, settling dissipatively at the centres of dark halos to form galaxies. The evolution of the dark matter component has been studied extensively by N-body simulations. The abundances, density profiles, shape and angular momentum distributions and the formation histories of the dark halo population can all be predicted reliably for any hierarchical cosmogony. The systematic variation of these properties with halo mass can produce scaling relations in the galaxy population. Simple hypotheses for how galaxies condense within dark halos lead to characteristic luminosities, sizes, and spins which are close to those of real spiral and elliptical galaxies. Furthermore, correlations similar to the Tully-Fisher, Faber-Jackson and luminosity-metallicity relations arise quite naturally. A quantitative explanation of the fundamental plane of elliptical galaxies appears within reach.

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arXiv1996

Anisotropies in the Distribution of Satellite Galaxies

We find that satellites of isolated disk galaxies at projected radii between 300 and 500 kpc are distributed asymmetrically about the parent galaxy and aligned preferentially with the disk minor axis. The dynamical timescale at these radii is sufficiently long that the shape of this distribution must reflect the formation history of the outer halo rather than its internal evolution. We also find that the orbital angular momenta of satellites at projected major axis distances of $\ltsim$ 200 kpc tend to align with that of the central disk. These results demonstrate that satellites are dynamically related to their primary galaxy. Satellites drawn from current simulations of hierarchical galaxy formation exhibit neither the systematic alignment nor the net rotation with the central disk that we find in the data.

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arXiv1996

More Satellites of Spiral Galaxies

We present a revised and expanded catalog of satellite galaxies of a set of isolated spiral galaxies similar in luminosity to the Milky Way. This sample of 115 satellites, 69 of which were discovered in our multifiber redshift survey, is used to further probe the results obtained from the original sample (Zaritsky et al. 1993). The satellites are, by definition, at projected separations <~ 500 kpc, have absolute recessional velocity differences with respect to the parent spiral of less than 500 km/s and are at least 2.2 mag fainter than their associated primary galaxy. A key characteristic of this survey is the strict isolation of these systems, which simplifies any dynamical analysis. We find no evidence for a decrease in the velocity dispersion of the satellite system as a function of radius out to galactocentric radii of 400 kpc, suggesting that the halo extends well beyond 200 kpc. Furthermore, the new sample affirms our previous conclusions (Zaritsky et al. 1993) that (1) the velocity difference between a satellite and its primary is not strongly correlated with the rotation speed of the primary, (2) the system of satellites has a slight net rotation (34 \pm 14 km\s) in the same sense as the primary's disk, and (3) that the halo mass of an ~ L* spiral galaxy is in excess of 2 x 10^{12} solar masses.

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arXiv1996

Measuring Omega

We were asked to debate the value of the cosmological mass-density parameter Omega. Is Omega_m=1 in accordance with the simplest model? Is Omega_m much smaller as indicated by some observations? There is conflicting evidence. We lay out the various methods for measuring Omega_m, mention new developments and current estimates, and focus on prospects versus the associated difficulties. We try to shed light on the uncertainties that are responsible for the span of estimates for Omega_m. We divide the methods into the following four classes: a. Global measures of the properties of space-time that constrain combinations of Omega_m and the other cosmological parameters (Lambda, Ho, to). b. Virialized Systems. Nonlinear dynamics within galaxies and clusters on comoving scales 1-10 Mpc/h. c. Large-scale structure. Mildly-nonlinear gravitational dynamics of fluctuations on scales 10-100 Mpc/h, in particular cosmic flows. d. Growth rate of fluctuations from the last scattering of the CMB or from high redshift galaxies to the present. Methods (b) typically yield low values of Omega_m=0.2-0.3. Methods (a) and (c) typically indicate higher values of Omega_m=0.4-1.

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arXiv1996

Dark halo mergers and the formation of a universal profile

We argue that a universal density profile for dark matter halos arises as a natural consequence of hierarchical structure formation. It is a fixed point in the process of repeated mergers. We present analytic and numerical arguments for the emergence of a particular form of the profile. At small radii, the density should vary as $r^{-α}$, with $α$ determined by the way the characteristic density of halos scales with their mass. If small halos are dense, then $α$ is large. The mass-density relation can be related to the power spectrum of initial fluctuations, $P(k)$, through `formation time' arguments. Early structure formation leads to steep cusps. For $P(k)\sim k^n$ we find $α\simeq 3(3+n)/(5+n)$. The universal profile is generated by tidal stripping of small halos as they merge with larger objects.

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arXiv1996

Detection of Strong Evolution in the Population of Early-Type Galaxies

The standard picture holds that giant elliptical galaxies formed in a single burst at high redshift. Aging of their stellar populations subsequently caused them to fade and become redder. The Canada-France Redshift Survey provides a sample of about 125 galaxies with the luminosities and colours of passively evolving giant ellipticals and with 0.1 < z < 1. This sample is inconsistent with the standard evolutionary picture with better than 99.9% confidence. The standard Schmidt test gives = 0.398 when restricted to objects with no detected star formation, and = 0.410 when objects with emission lines are also included. A smaller sample of early-type galaxies selected from tha Hawaii Deep Survey gives equally significant results. With increasing redshift a larger and larger fraction of the nearby elliptical and S0 population must drop out of of the sample, either because the galaxies are no longer single units or because star formation alters their colours. If the remaining fraction is modelled as F=(1+z)^{-γ}, the data imply gamma = 1.5 +- 0.3. At z=1 only about one third of bright E and S0 galaxies were already assembled and had the colours of old passively evolving systems. We discuss the sensitivity of these results to the incompleteness corrections and stellar population models we have adopted. We conclude that neither is uncertain enough to reconcile the observations with the standard picture. Hierarchical galaxy formation models suggest that both merging and recent star formation play a role in the strong evolution we have detected.

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arXiv1996

Two--body heating in numerical galaxy formation experiments

We show that discreteness effects related to classical two-body relaxation produce spurious heating of the gaseous component in numerical simulations of galaxy formation. A simple analytic model demonstrates that this artificial heating will dominate radiative cooling in any simulation where the mass of an individual dark matter particle exceeds a certain critical value. This maximum mass depends only on the cooling function of the gas, on the fraction of the material in gaseous form, and (weakly) on typical temperatures in the gas. It is comparable to, or smaller than, the dark matter particle masses employed in most published simulations of cosmological hydrodynamics and galaxy formation. Any simulation which violates this constraint will be unable to follow cooling flows, although catastrophic cooling of gas may still occur in regions with very short cooling times. We use a series of N--body/smoothed particle hydrodynamics simulations to explore this effect. In simulations which neglect radiative cooling, two--body heating causes a gradual expansion of the gas component. When radiative effects are included, we find that gas cooling is almost completely suppressed for dark matter particle masses above our limit. Although our test simulations use smoothed particle hydrodynamics, similar effects, and a similar critical mass, are expected in any simulation where the dark matter is represented by discrete particles.

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arXiv1996

Large-scale structure at high redshift

I discuss and illustrate the development of large-scale structure in the Universe, emphasising in particular the physical processes and cosmological parameters that most influence the observationally accessible aspects of structure at large redshift. Statistical properties of this structure can be measured from the apparent positions of faint galaxies and quasars; the structure can be mapped in three dimensions by obtaining redshifts for large samples of such objects; it can be studied using foreground absorption in the spectra of quasars; finally the mass distribution can be constrained by measuring the gravitationally induced distortion of background galaxy images. The first and last of these techniques require deep imaging of large areas of the sky with the best possible image quality. The second and third will require 8m-class telescopes with efficient multiobject spectrographs. For QSO absorption line spectroscopy high spectral resolution is also important.

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arXiv1996

The Nature and Observability of Protogalaxies

I discuss recent theoretical work on the formation and evolution of galaxies paying particular attention to the ability of current models to make detailed comparisons with observations of the galaxy population both nearby and at high redshift. These models suggest that much (perhaps most) star formation in the universe took place in objects that are already detected in deep galaxy samples. In addition, they predict that systems with large star formation rates are unlikely to be much more abundant in the past than they are at present. Recent data show that the star formation rate in the nearby universe is, in fact, a substantial fraction of that required to make all the stars seen in galaxies, and that the observed abundance of objects forming stars at rates in excess of 10 solar masses per yr is approximately the same at redshifts of 1.25 and 3.25 as it is at z = 0. Both the epoch of galaxy formation and ``typical'' protogalaxies may already have been observed but not recognised. Thermal emission from dust in such protogalaxies could be detected by a large millimeter array, and molecular line observations could explore the dynamical state of the gas in the more massive systems.

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arXiv1996

Hierarchical Clustering and Galaxy Formation

I review the theory of hierarchical clustering, starting with an historical overview and moving on to a discussion of those aspects of dissipationless clustering under gravity which are most relevant to galaxy formation. I conclude with some comments on the additional problems which arise when including all the other physics needed to build a realistic picture for the origin and evolution of the galaxy population.

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arXiv1996

Violent Relaxation in Hierarchical Clustering

The term ``violent relaxation'' was coined by Donald Lynden-Bell as a memorable oxymoron describing how a stellar dynamical system relaxes from a chaotic initial state to a quasi-equilibrium. His analysis showed that this process is rapid, even for systems with many stars, and that it leads to equilibria which may plausibly be related to bounded isothermal spheres. I review how numerical simulations have improved our understanding of violent relaxation over the last thirty years. It is clear that the process leads to equilibria which depend strongly on the initial state, but which nevertheless have certain common features. A particularly interesting case concerns objects formed in an expanding universe through dissipationless hierarchical clustering from gaussian initial conditions; these may correspond to galaxy clusters or to the dark halos of galaxies. While such objects display a wide range of shapes and spins, the distributions of these properties depend only weakly on the cosmological context and on the initial spectrum of density fluctuations. Halo density profiles appear to have a universal form with a singular central structure and a characteristic density which depends only on formation epoch. Low mass halos typically have earlier formation times and thus higher characteristic densities than high mass halos.

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arXiv1995

The Structure of Cold Dark Matter Halos

We use N-body simulations to investigate the structure of dark halos in the standard Cold Dark Matter cosmogony. Halos are excised from simulations of cosmologically representative regions and are resimulated individually at high resolution. We study objects with masses ranging from those of dwarf galaxy halos to those of rich galaxy clusters. The spherically averaged density profiles of all our halos can be fit over two decades in radius by scaling a simple ``universal'' profile. The characteristic overdensity of a halo, or equivalently its concentration, correlates strongly with halo mass in a way which reflects the mass dependence of the epoch of halo formation. Halo profiles are approximately isothermal over a large range in radii, but are significantly shallower than $r^{-2}$ near the center and steeper than $r^{-2}$ near the virial radius. Matching the observed rotation curves of disk galaxies requires disk mass-to-light ratios to increase systematically with luminosity. Further, it suggests that the halos of bright galaxies depend only weakly on galaxy luminosity and have circular velocities significantly lower than the disk rotation speed. This may explain why luminosity and dynamics are uncorrelated in observed samples of binary galaxies and of satellite/spiral systems. For galaxy clusters, our halo models are consistent both with the presence of giant arcs and with the observed structure of the intracluster medium, and they suggest a simple explanation for the disparate estimates of cluster core radii found by previous authors. Our results also highlight two shortcomings of the CDM model. CDM halos are too concentrated to be consistent with the halo parameters inferred for dwarf irregulars, and the predicted abundance of galaxy halos is larger than the observed abundance of galaxies.

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arXiv1994

Formation and Evolution of Galaxies: Les Houches Lectures

This is a set of lecture notes based on the lectures on the formation and evolution of galaxies which were given by S. White at Les Houches in August 1993. This postscript file does not contain the 16 figures. A version containing these figures can be obtained by writing to: Fr. C. Rickl, MPI fuer Astrophysik, Karl-Schwarzschildstr. 1, 85740 Garching, Germany (car@mpa-garching.mpg.de).

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