Showing papers by "Sloan Fellows published in 1999"

Power Spectra for Cold Dark Matter and its Variants

Abstract: The bulk of recent cosmological research has focused on the adiabatic cold dark matter model and its simple extensions. Here we present an accurate —tting formula that describes the matter transfer func- tions of all common variants, including mixed dark matter models. The result is a function of wavenum- ber, time, and six cosmological parameters: the massive neutrino density, number of neutrino species degenerate in mass, baryon density, Hubble constant, cosmological constant, and spatial curvature. We show how observational constraintse.g., the shape of the power spectrum, the abundance of clusters and damped Lya systems, and the properties of the Lya forestcan be extended to a wide range of cosmologies, which includes variations in the neutrino and baryon fractions in both high-density and low-density universes. Subject headings: cosmology: theorydark matterlarge-scale structure of universe

Cosmic Complementarity: Joint Parameter Estimation from Cosmic Microwave Background Experiments and Redshift Surveys

Abstract: We study the ability of future cosmic microwave background anisotropy experiments and redshift surveys to constrain a 13-dimensional parameterization of the adiabatic cold dark matter model. Each alone is unable to determine all parameters to high accuracy. However, considered together, one data set resolves the difficulties of the other, allowing certain degenerate parameters to be determined with far greater precision. We treat in detail the degeneracies involving the classical cosmological parameters, massive neutrinos, tensor-scalar ratio, bias, and reionization optical depth as well as how redshift surveys can resolve them. We discuss the opportunities for internal and external consistency checks on these measurements. Previous papers on parameter estimation have generally treated smaller parameter spaces; in direct comparisons to these works, we tend to find weaker constraints and suggest numerical explanations for the discrepancies.

The Cosmological Origin of the Tully-Fisher Relation

Abstract: We use high-resolution cosmological simulations that include the effects of gasdynamics and star formation to investigate the origin of the Tully-Fisher relation in the standard cold dark matter cosmogony. Stars are assumed to form in collapsing, Jeans-unstable gas clumps at a rate set by the local gas density and the dynamical/cooling timescale. The energetic feedback from stellar evolution is assumed to heat the gas-surrounding regions of ongoing star formation, where it is radiated away very rapidly. The star formation algorithm thus has little effect on the rate at which gas cools and collapses, and, as a result, most galaxies form their stars very early. Luminosities are computed for each model galaxy using their full star formation histories and the latest spectrophotometric models. We find that the stellar mass of model galaxies is proportional to the total baryonic mass within the virial radius of their surrounding halos. Circular velocity then correlates tightly with the total luminosity of the galaxy, which reflects the equivalence between mass and circular velocity of systems identified in a cosmological context. The slope of the relation steepens slightly from the blue to the red bandpasses and is in fairly good agreement with observations. Its scatter is small, decreasing from ~0.38 mag in the U band to ~0.24 mag in the K band. The particular cosmological model we explore here seems unable to account for the zero point of the correlation. Model galaxies are too faint at z=0 (by about 2 mag) if the circular velocity at the edge of the luminous galaxy is used as an estimator of the rotation speed. The model Tully-Fisher relation is brighter in the past by ~0.7 mag in the B band at z=1, which is at odds with recent observations of z~1 galaxies. We conclude that the slope and tightness of the Tully-Fisher relation can be naturally explained in hierarchical models, but that its normalization and evolution depend strongly on the star formation algorithm chosen and on the cosmological parameters that determine the universal baryon fraction and the time of assembly of galaxies of different mass.

Maximal Disks and the Tully-Fisher Relation

Abstract: We show that for luminous, nonbarred, high surface brightness (HSB) spirals the Tully-Fisher relation (TFR) residuals can be used to estimate the relative mass contributions of the stellar disk and the dark halo at the peak of the disk rotation, near 22 exponential scale lengths For maximal disks, a large fraction (085?01) of the total rotational support, V22, at such radii should arise from their stellar mass Therefore, the disk size or surface-brightness should be a significant additional parameter in the TFR At a given absolute luminosity, Mr, more compact disks (as measured by the disk scale length Rexp) should have higher rotation speeds, V22 Using a well-defined sample of late-type spirals, deviations, ?log V22, and ?log Rexp, from the mean relations, V22(Mr) and Rexp(Mr), are not significantly correlated The case of ?log V22/?log Rexp=-05 expected for a maximal disk is ruled out for the majority of these HSB galaxies We model adiabatic infall of varying amounts of luminous matter into dark matter halos to explore the range of possible values for ?log V22/?log Rexp From this, we find that the TFR residuals require a mean value of Vdisk~06Vtotal, fairly insensitive to the details of the initial dark matter halo and to the presence of a bulge This translates to Mhalo~06Mtotal within 22Rexp or roughly twice more dark matter in the inner parts of late-type spirals than previously accounted for by maximum disk fits We show that any stellar population differences between disks of different scale lengths lead to lower values of Vdisk/Vtotal Our result is independent of the shape of the luminosity profile and relies only on the assumption of adiabatic contraction and that the dark matter halo rotation rises in the central parts of the galaxy Submaximal disks establish a natural continuity between HSB and low surface brightness galaxies, which appear to be completely dark matter dominated even in their inner regions

How Universal Are the Density Profiles of Dark Halos

Abstract: We use high-resolution N-body simulations to investigate the formation of virialized halos due to the gravitational collapse of collisionless matter. A variety of formation scenarios are studied, ranging from hierarchical clustering to monolithic radial collapse. The goal of these experiments was to study departures from the universal density profiles recently found to arise in cosmological settings. However, we found that even for models that exhibit quite a different formation history, the density and velocity dispersion profiles of the virialized halos are strikingly similar. Power-law density profiles do not result even in models with initial power-law profiles and without initial substructure or nonradial motions. Such initial conditions give rise to a radial orbit instability that leads to curved velocity dispersion and density profiles. The shapes of the density profiles in all our models are well parameterized by the profiles of halos formed in a generic cosmological setting. Our results show that the universality of dark halo density profiles does not depend crucially on hierarchical merging as has been suggested recently in the literature. Rather it arises because apparently different collapse histories produce a near universal angular momentum distribution among the halo particles. We conclude that the general form of the density and velocity dispersion profiles of virialized halos in an expanding universe are robust outcomes of gravitational collapse, nearly independent of the initial conditions and the formation history.

Axisymmetric Three-Integral Models for Galaxies

Abstract: We describe an improved, practical method for constructing galaxy models that match an arbitrary set of observational constraints, without prior assumptions about the phase-space distribution function (DF). Our method is an extension of Schwarzschild's orbit superposition technique. As in Schwarzschild's original implementation, we compute a representative library of orbits in a given potential. We then project each orbit onto the space of observables, consisting of position on the sky and line-of-sight velocity, while properly taking into account seeing convolution and pixel binning. We find the combination of orbits that produces a dynamical model that best fits the observed photometry and kinematics of the galaxy. A new element of this work is the ability to predict and match to the data the full line-of-sight velocity profile shapes. A dark component (such as a black hole and/or a dark halo) can easily be included in the models. In an earlier paper (Rix et al.) we described the basic principles and implemented them for the simplest case of spherical geometry. Here we focus on the axisymmetric case. We first show how to build galaxy models from individual orbits. This provides a method to build models with fully general DFs, without the need for analytic integrals of motion. We then discuss a set of alternative building blocks, the two-integral and the isotropic components, for which the observable properties can be computed analytically. Models built entirely from the two-integral components yield DFs of the form f(E, Lz), which depend only on the energy E and angular momentum Lz. This provides a new method to construct such models. The smoothness of the two-integral and isotropic components also makes them convenient to use in conjunction with the regular orbits. We have tested our method by using it to reconstruct the properties of a two-integral model built with independent software. The test model is reproduced satisfactorily, either with the regular orbits, or with the two-integral components. This paper mainly deals with the technical aspects of the method, while applications to the galaxies M32 and NGC 4342 are described elsewhere (van der Marel et al.; Cretton & van den Bosch).

Angular Momentum Redistribution by Waves in the Sun

Abstract: We calculate the angular momentum transport by gravito-inertial-Alfven waves and show that, so long as prograde and retrograde gravity waves are excited to roughly the same amplitude, the sign of angular momentum deposit in the radiative interior of the Sun is such as to lead to an exponential growth of any existing small radial gradient of rotation velocity just below the convection zone. This leads to formation of a strong thin shear layer (of thickness about 0.3% R☉) near the top of the radiative zone of the Sun on a timescale of order 20 yr. When the magnitude of differential rotation across this layer reaches about 0.1 μHz, the layer becomes unstable to shear instability and undergoes mixing, and the excess angular momentum deposited in the layer is returned to the convection zone. The strong shear in this layer generates a toroidal magnetic field which is also deposited in the convection zone when the layer becomes unstable. This could possibly start a new magnetic activity cycle seen at the surface.

Angular momentum redistribution by waves in the Sun

Abstract: We calculate the angular momentum transport by gravito-inertial-Alfv\'en waves and show that, so long as prograde and retrograde gravity waves are excited to roughly the same amplitude, the sign of angular momentum deposit in the radiative interior of the Sun is such as to lead to an exponential growth of any existing small radial gradient of rotation velocity just below the convection zone. This leads to formation of a strong thin shear layer (of thickness about 0.3% R_\odot) near the top of the radiative zone of the Sun on a time-scale of order 20 years. When the magnitude of differential rotation across this layer reaches about 0.1 \mu Hz, the layer becomes unstable to shear instability and undergoes mixing, and the excess angular momentum deposited in the layer is returned to the convection zone. The strong shear in this layer generates toroidal magnetic field which is also deposited in the convection zone when the layer becomes unstable. This could possibly start a new magnetic activity cycle seen at the surface.

A Tully-Fisher Relation for S0 Galaxies

Abstract: We present an I-band Tully-Fisher relation (TFR) for 18 nearby S0 galaxies using kinematics derived from long-slit spectroscopy of stellar absorption lines. Our estimates of the circular velocity, Vc, at 2–3 exponential disk scale lengths account for line-of-sight projection and for the stellar random motions through an asymmetric drift correction. Uniform and accurate distance calibration is available from recent surface brightness fluctuation measurements. Despite the care taken in estimating both Vc and MI, the TFR shows an intrinsic scatter of about 0.7 mag in MI or 0.15 in log Vc. This result is surprising, because S0 galaxies appear to have both the simple kinematics of disk galaxies and the simple stellar populations of early-type galaxies. Remarkably, in this sample of overall rotation-dominated galaxies, the central stellar velocity dispersion is a better predictor of the total I-band luminosity (through the fundamental plane relations) than the circular speed at several exponential scale lengths. Furthermore, the TFR zero point, or the mean stellar I-band luminosity at a given Vc, differs by only about 0.5 mag between our sample of S0s and a comparison sample of late-type spirals, once both data sets are brought onto a consistent distance scale. This offset is less than expected if S0s are former spiral galaxies with prematurely truncated star formation (4 Gyrs ago).

Galaxy Formation and the Kinematics of Damped Lyα Systems

Abstract: A model of damped Lyα systems is presented based on randomly moving clouds in spherical halos. We use the Press-Schechter model for the abundance of halos and assume that each halo has a similar population of clouds, with total mass and spatial distribution constrained to fit observations of the column density distribution. We show that the kinematics of the multiple absorbing components revealed in absorption profiles of the low-ionization lines, presented by Prochaska & Wolfe, are consistent with our spherical halo model. The presence of multiple absorbing components with a large covering factor, combined with the small impact parameters of the systems predicted in our analytical model and in numerical simulations, implies a high rate of energy dissipation in cloud collisions. We calculate the rate of energy dissipation in our model and show that it is far greater than the rate at which energy can be supplied by gravitational mergers of halos. This poses a possible problem for the model of merging protogalactic clumps of Haehnelt et al., based on numerical simulations. We also present new constraints on the amplitude of the power spectrum in hierarchical theories required to account for the observed velocity dispersion in the absorbers. We find that the linearly extrapolated rms fluctuation at redshift z = 4 on spheres of radius R = 100 km s-1 H-1(z) [where H(z) is the Hubble constant at redshift z] must be greater than 0.75. Although this limit is obtained only for our specific model of the absorbing components, it should not be highly model-dependent because the velocity dispersion of the absorbers is essentially determined by the velocity dispersion of the halos where the gas is moving.

Measuring the Cosmological Geometry from the Lyα Forest along Parallel Lines of Sight

Abstract: The possibility of measuring the cosmological geometry using the redshift space correlation function of the Lyα forest in multiple lines of sight as a function of angular and velocity separation is discussed. The geometric parameter to be measured is f(z)≡c-1H(z)DA(z), where H(z) is the Hubble constant and DA(z) the angular diameter distance at redshift z. The correlation function is computed in linear theory, assuming that the Lyα forest is a result of gravitational instability in a photoionized intergalactic medium. We describe a method to measure the correlation from observations with the Gaussianization procedure of Croft et al. to map the observed Lyα forest transmitted flux to an approximation of the linear density field. The effect of peculiar velocities on the shape of the recovered power spectrum is pointed out. We estimate the error in recovering the f(z) factor from observations due to the variance in the Lyα absorbers. We show that at least ~25 pairs of quasars (separations <3') are needed to distinguish a flat Ω0=1 universe from a universe with Ω0=0.2, ΩΛ=0.8. A second parameter that is obtained from the correlation function of the Lyα forest is βΩ(z)0.6/b (affecting the magnitude of the peculiar velocities), where b is a linear theory bias of the Lyα forest. In the theory of the Lyα forest assumed here, the parameter β can be predicted from numerical simulations; once β is known, the number of quasar pairs needed to constrain f is reduced to about six. On small scales, where the correlation function is higher, f(z) should be measurable with fewer quasars, but nonlinear effects must then be taken into account. The anisotropy of the nonlinear redshift space correlation function as a function of scale should also provide a precise quantitative test of the gravitational instability theory of the Lyα forest.

The core structure of galaxy clusters from gravitational lensing

Abstract: We examine gravitational lensing constraints on the structure of galaxy clusters and compare them with the results of cosmological N-body simulations of cluster formation in cold dark matter -dominated universes. We find that cluster core masses, as measured by the observed location of giant tangential arcs, generally exceed those of dark matter halos of similar velocity dispersion. The magnitude of the discrepancy is a strong function of cluster mass. Arc properties in the most massive clusters in the sample (i.e., those with velocity dispersion ? ~ 1500-2000 km s-1) are essentially consistent with the N-body predictions. On the other hand, giant arcs in ? ~ 1000 km s-1 clusters can be reconciled with cold dark matter cluster halos only if their lensing power, i.e., central surface mass density, has been increased substantially by the presence of a massive (~3 ? 1012 h-1 M?) central galaxy and of significant substructure. Best agreement is found if the mass of the central galaxy and the effects of substructure are approximately independent of cluster mass. Massive central galaxies with steep inner density profiles are also needed to explain a clear trend, observed in our data set, between the radial thickness of giant tangential arcs and the velocity dispersion of the lensing cluster. The position and redshift of radial arcs may be used as independent tests of these results, but at present the data set available is too limited to have a significant impact on these conclusions. Our results depend only weakly on the cosmological model adopted and suggest that structural parameters of clusters derived from strong lensing studies cannot usefully constrain the values of cosmological parameters.

Large Stellar Disks in Small Elliptical Galaxies

Abstract: We present stellar kinematics along the principal axes of seven elliptical galaxies less luminous than MB=-19.5, which extend beyond the half-light radii for all systems in this photometrically selected sample. At large radii, the kinematics not only confirm that rotation and "diskiness" are important in faint elliptical galaxies, as was previously known, but show that rotation dominates: the stars at large galactocentric distances have (V/σ)max~2, similar to the disks in bona fide S0 galaxies. A comparison with published simulations of dissipationless mergers is not straightforward. Yet, within Re, the observed galaxies seem to rotate somewhat faster than 3:1 merger remnants, arguing against major mergers as the dominant mechanism in the final shaping of low-luminosity elliptical galaxies and favoring instead the dissipative formation of a disk.

The Structure of the Central Disk of NGC 1068: A Clumpy Disk Model

Abstract: NGC 1068 is one of the best-studied Seyfert II galaxies, for which the black hole mass has been determined from the Doppler velocities of water maser. We show that the standard α-disk model of NGC 1068 gives disk mass between the radii of 0.65 and 1.1 pc (the region from which water maser emission is detected) to be about 7 × 107 M☉ (for α = 0.1), more than 4 times the black hole mass, and a Toomre Q-parameter for the disk is ~0.001. This disk is therefore highly self-gravitating and is subject to large-amplitude density fluctuations. We conclude that the standard α-viscosity description for the structure of the accretion disk is invalid for NGC 1068. In this paper, we develop a new model for the accretion disk. The disk is considered to be composed of gravitationally bound clumps; accretion in this clumped disk model arises because of gravitational interaction of clumps with each other and the dynamical frictional drag exerted on clumps from the stars in the central region of the galaxy. The clumped disk model provides a self-consistent description of the observations of NGC 1068. The computed temperature and density are within the allowed parameter range for water maser emission, and the rotational velocity in the disk falls off as r-0.35.

The structure of the central disk of NGC 1068: a clumpy disk model

Abstract: NGC 1068 is one of the best studied Seyfert II galaxies, for which the blackhole mass has been determined from the Doppler velocities of water maser. We show that the standard $\alpha$-disk model of NGC 1068 gives disk mass between the radii of 0.65 pc and 1.1 pc (the region from which water maser emission is detected) to be about 7x10$^7$ M$_\odot$ (for $\alpha=0.1$), more than four times the blackhole mass, and a Toomre Q-parameter for the disk is $\sim$0.001. This disk is therefore highly self-gravitating and is subject to large-amplitude density fluctuations. We conclude that the standard $\alpha$-viscosity description for the structure of the accretion disk is invalid for NGC 1068. In this paper we develop a new model for the accretion disk. The disk is considered to be composed of gravitationally bound clumps; accretion in this clumped disk model arises because of gravitational interaction of clumps with each other and the dynamical frictional drag exerted on clumps from the stars in the central region of the galaxy. The clumped disk model provides a self-consistent description of the observations of NGC 1068. The computed temperature and density are within the allowed parameter range for water maser emission, and the rotational velocity in the disk falls off as $r^{-0.35}$.

Microlensing in the galactic bulge: Effects of the disk behind the bulge

Abstract: A large number of microlensing events have been observed in the direction of the Galactic bulge, with a measured optical depth in the range 2-3×10−6. It has been shown that most of these events are due to bulge stars being lensed by other bulge stars or by foreground disk stars. Among the stars observed in the bulge fields there should also be disk stars located behind the bulge; here we consider their effect on the microlensing rates. The optical depth of background disk stars is much higher than that of typical bulge stars, reaching 10-5 at 6 kpc behind the bulge. Thus, although background disk stars are a very small fraction of the stars in Baade's window, we find that ~5%-10% of the optical depth should be due to disk stars more than 3 kpc behind the bulge. This fraction is sensitive to the luminosity function of disk stars at large scale height, to the magnitude cutoff of the survey, and to the amplification bias effect causing large numbers of "blended" events. We consider also the effect of a warp and flare in the disk at large distances behind the bulge; this could increase the optical depth from the background disk to ~20% of the total. Events on background disk stars should on average be longer than other events and could be distinguished as well by measuring the proper motion or distance of the stars that have been microlensed. The number of these events could be an interesting probe to the structure and stellar population of the far side of the Galactic disk.

Foregrounds and Forecasts for the Cosmic Microwave Background

Abstract: One of the main challenges facing upcoming CMB experiments will be to distinguish the cosmological signal from foreground contamination. We present a comprehensive treatment of this problem and study how foregrounds degrade the accuracy with which the Boomerang, MAP and Planck experiments can measure cosmological parameters. Our foreground model includes not only the normalization, frequency dependence and scale dependence for each physical component, but also variations in frequency dependence across the sky. When estimating how accurately cosmological parameter can be measured, we include the important complication that foreground model parameters (we use about 500) must be simultaneously measured from the data as well. Our results are quite encouraging: despite all these complications, precision measurements of most cosmological parameters are degraded by less than a factor of 2 for our main foreground model and by less than a factor of 5 in our most pessimistic scenario. Parameters measured though large-angle polarization signals suffer more degradation: up to 5 in the main model and 25 in the pessimistic case. The foregrounds that are potentially most damaging and therefore most in need of further study are vibrating dust emission and point sources, especially those in the radio frequencies. It is well-known that E and B polarization contain valuable information about reionization and gravity waves, respectively. However, the cross-correlation between polarized and unpolarized foregrounds also deserves further study, as we find that it carries the bulk of the polarization information about most other cosmological parameters.