Showing papers by "Joel R. Primack published in 1997"

The Cores of Dark Matter Dominated Galaxies: theory vs. observations

Abstract: We use the rotation curves of a sample of dark matter dominated dwarf and low-surface brightness (LSB) late-type galaxies to study their radial mass distributions. We find that the shape of the rotation curves is remarkably similar for all (both dwarf and LSB) galaxies in the sample, suggesting a self-similar density distribution of their dark matter (DM) halos. This shape can be reproduced well by a density profile with a shallow central cusp (rho(r)\propto 1/r^gamma, gamma\approx 0.2-0.4), corresponding to a steeply rising velocity curve (v(r)\propto r^g, g\approx 0.9-0.8). We further show that the observed shape of the rotation curves is well matched by the density profiles of DM halos formed in very high resolution simulations of the CDM, LCDM, and CHDM models of structure formation. This is surprising in light of several previous studies which suggested that the structure of simulated DM halos is inconsistent with the dynamics of dwarf galaxies. We discuss possible explanations for this discrepancy. We show that although the mass distribution in the hierarchically formed halos is on average consistent with the shape of rotation curves of dark matter dominated galaxies, the scatter of the individual profiles around the average is substantial and should not be neglected in comparisons with the data. Finally, we show that the DM halos in our hierarchical simulations and the real galaxies in our sample exhibit a similar decrease in their characteristic densities with increasing characteristic radial scales, and increase in their maximum rotation velocities with increasing radii at which their maximum velocity occurs. (Abridged)

Non-Gaussian fluctuations and primordial black holes from inflation

Abstract: We explore the role of non-Gaussian fluctuations in primordial black hole (PBH) formation and show that the standard Gaussian assumption, used in all PBH formation papers to date, is not justified. Since large spikes in power are usually associated with flat regions of the inflaton potential, quantum fluctuations become more important in the field dynamics, leading to mode-mode coupling and non-Gaussian statistics. Moreover, PBH production requires several {sigma} (rare) fluctuations in order to prevent premature matter dominance of the universe, so we are necessarily concerned with distribution tails, where any intrinsic skewness will be especially important. We quantify this argument by using the stochastic slow-roll equation and a relatively simple analytic method to obtain the final distribution of fluctuations. We work out several examples with toy models that produce PBH{close_quote}s, and test the results with numerical simulations. Our examples show that the naive Gaussian assumption can result in errors of many orders of magnitude. For models with spikes in power, our calculations give sharp cutoffs in the probability of large positive fluctuations, meaning that Gaussian distributions would vastly overproduce PBH{close_quote}s. The standard results that link inflation-produced power spectra and PBH number densities must then be reconsidered, since they rely quitemore » heavily on the Gaussian assumption. We point out that since the probability distributions depend strongly on the nature of the potential, it is impossible to obtain results for general models. However, calculating the distribution of fluctuations for any specific model seems to be relatively straightforward, at least in the single inflaton case. {copyright} {ital 1997} {ital The American Physical Society}« less

CDM-Variant Cosmological Models - I: Simulations and Preliminary Comparisons

Abstract: We present two matched sets of five simulations each, covering five presently favored simple modifications to the standard cold dark matter (CDM) scenario. One simulation suite, with a linear box size of 75 Mpc/h, is designed for high resolution and good statistics on the group/poor cluster scale, and the other, with a box size of 300 Mpc/h, is designed for good rich cluster statistics. All runs had 57 million cold particles, and models with massive neutrinos had an additional 113 million hot particles. We consider separately models with massive neutrinos, tilt, curvature, and a nonzero cosmological constant in addition to the standard CDM model. We find that our tilted Omega+Omega_Lambda=1 (TLCDM) model produces too much small-scale power by a factor of ~3, and our open Lambda=0 (OCDM) model also exceeds observed small-scale power by a factor of 2. In addition, we take advantage of the large dynamic range in detectable halo masses our simulations allow to check the shape of the Press-Schechter approximation. We find good fits at cluster masses for delta_c=1.27--1.35 for a Gaussian filter and delta_c=1.57--1.73 for a tophat filter. However, Press-Schechter overpredicts the number density of halos compared to the simulations in the high resolution suite by a weakly cosmology-dependent factor of 1.5--2 at galaxy and group masses, which cannot be fixed by adjusting delta_c within reasonable bounds. An appendix generalizes the spherical collapse model to any isotropic cosmology.

Implications of Spikes in the Redshift Distribution of $z\sim3$ Galaxies

Abstract: We address the high peaks found by Steidel et al (1997) in the redshift distribution of ``Lyman-break'' objects (LBOs) at redshift z~3. The highest spike represents a relative overdensity of 2.6 in the distribution of LBOs in pixels of comoving size ~10Mpc/h. We examine the likelihood of such a spike in the redshift distribution within a suite of models for the evolution of structure in the Universe, including models with Omega=1 (SCDM and CHDM) and with Omega=0.4-0.5 (LCDM and OCDM). Using high-resolution dissipationless N-body simulations, we analyze deep pencil-beam surveys from these models in the same way that they are actually observed, identifying LBOs with the most massive dark matter halos. We find that all the models (with SCDM as a marginal exception) have a substantial probability of producing spikes similar to those observed, because the massive halos are much more clumped than the underlying matter -- i.e., they are biased. Therefore, the likelihood of such a spike is not a good discriminator among these models. We find in these models that the mean biasing parameter b of LBOs with respect to dark matter varies within a range b ~2-5 on a scale of ~10Mpc/h. We also compute the two-body correlation functions of LBOs predicted in these models. The LBO correlation functions are less steep than galaxies today (gamma ~1.4), but show similar or slightly longer correlation lengths.

Dark Matter and Structure Formation

Abstract: This chapter aims to present an introduction to current research on the nature of the cosmological dark matter and the origin of galaxies and large scale structure within the standard theoretical framework: gravitational collapse of fluctuations as the origin of structure in the expanding universe. General relativistic cosmology is summarized, and the data on the basic cosmological parameters (to and H0 ≡ 100hkms −1 Mpc −1 , 0, � and b) are reviewed. Various particle physics candidates for hot, warm, and cold dark matter are briefly reviewed, together with current constraints and experiments that could detect or eliminate them. Also included is a very brief summary of the theory of cosmic defects, and a somewhat more extended exposition of the idea of cosmological inflation with a summary of some current models of inflation. The remainder is a discussion of observational constraints on cosmological model building, emphasizing models in which most of the dark matter is cold and the primordial fluctuations are the sort predicted by inflation. It is argued that the simplest models that have a hope of working are Cold Dark Matter with a cosmological constant (�CDM) if the Hubble parameter is high (h > ∼ 0.7), and Cold + Hot Dark Matter (CHDM) if the Hubble parameter and age permit an = 1 cosmology, as seems plausible in light of the data from the Hipparcos astrometric satellite. The most attractive variants of these models and the critical tests for each are discussed. To be published as Chapter 1 of Formation of Structure in the Universe, Proceedings of the Jerusalem Winter School 1996, edited by A. Dekel and J.P. Ostriker (Cambridge University Press).

Inclination Effects in Spiral Galaxy Gravitational Lensing

Abstract: Spheroidal components of spiral galaxies have been considered to be the only dynamically important component in gravitational lensing studies to date. Here we point out that including the disk component can have a significant effect, which depends on the disk inclination, on a variety of lensing properties that are relevant to present studies and future surveys. As an example, we look at the multiple image system B1600+434, which was recently identified as being lensed by a spiral galaxy. We find that by including the disk component, one can understand the fairly large image separation as being caused by the inclination of a typical spiral rather than the presence of a very massive halo. The fairly low magnification ratio can also be readily understood if the disk is included. We also discuss how such lensed systems might allow one to constrain parameters of spiral galaxies such as a disk-to-halo mass ratio and disk mass scale length. Another example we consider is the quasar multiple-lensing cross section, which we find can increase many-fold at high inclination for a typical spiral. Finally, we discuss the changes in the gravitational lensing effects on damped Lyα systems when disk lensing is included.

Cold + Hot and Cold Dark Matter Cosmologies: Analysis of Numerical Simulations

Abstract: We present a series of four simulations of cold dark matter (CDM) and cold + hot dark matter (CHDM) cosmologies, which we analyze together in this and subsequent papers. These dissipationless simulations were done using the particle mesh method with a 5123 mesh, corresponding to a resolution of approximately 200 kpc for an assumed Hubble parameter of H0 = 50 km s-1 Mpc-1, and with approximately 17 × 106 cold and (for CHDM) an additional 34 × 106 hot particles. In this paper we discuss the power spectrum and correlation functions in real and redshift space, with comparisons to the CfA2 and IRAS redshift data, the pairwise velocity of galaxies in real space, and the distribution of hot and cold particles in CHDM simulations. We confirm that CHDM with cold/hot/baryon density ratios Ωc/Ων/Ωb = 0.6/0.3/0.1 is a good fit to a wide variety of present-epoch data, much better than CDM. In particular, with reasonable assumptions about identification of galaxies and biasing, we find that the power spectrum from our CHDM simulations agrees rather well with both the CfA2 and IRAS power spectra in both the nonlinear and linear regimes. New variants of the CHDM scenario (e.g., with 20% of the mass in hot particles or with two massive neutrinos) predict a significantly larger rate of formation of galaxies at high redshift, which may be needed to explain some observational data. At the same time, the difference between the variants is rather small at z = 0. The results presented in this paper are interesting for two purposes: (i) For a rough comparison with other classes of models (like CDM or models with cosmological constant ΛCDM) at z = 0—indeed, we have used the simulations described here as a test bed for developing a number of new statistics for quantifying large-scale structure and comparing it to observations; (ii) As a reference point for comparison between different variants of the CHDM model. In addition, we explain here how we modify the usual Zeldovich approximation used to set up the initial conditions for both cold and hot particles in our simulations, taking into account that the growth rates of both kinds of fluctuations are different from the usual CDM case. We have also added an Appendix on issues of resolution in particle mesh simulations.

Galaxy Groups, CDM/CHDM Cosmologies, and the Value of Ω0

Abstract: We present techniques for identifying and analyzing galaxy groups and apply them to large-scale particle-mesh (PM) N-body simulations of structure formation in three Ω0 = 1 cosmological models: cold plus hot dark matter (CHDM), with Ωcold = 0.6, Ων = 0.3, and Ωbaryon = 0.1 at bias b ≡ σ−18=1.5; and two cold dark matter (CDM) models, at bias b = 1.5 and b = 1.0. Groups are identified with the adaptive friends-of-friends algorithm of Nolthenius. Our most important conclusions follow. The standard group M/L method gives Ω0 0.08 for the CfA1 survey (for redshift link parameter V5 = 350), and, applied to our Ω0 = 1 simulations, it gives Ω0 0.12 for CHDM (V5 = 350) and Ω0 0.35 for CDM (V5 = 600). This Ω bias appears to be even stronger at higher resolution. We show quantitatively how three different effects conspire to produce this large discrepancy, and we conclude that low observed Ω values need not argue for a low-Ω universe. Our preferred statistics of groups show promise in becoming powerful discriminators between Gaussian cosmological models, whose Ων differ and are robust against several methods for assigning luminosity to dark matter halos, and for merging CfA1 data. However, our latest results at higher resolution show such strong sensitivity to how massive overmergers are broken up that more reliable ways of identifying luminous galaxies within large-scale simulations will be necessary before these statistics can provide reliable discrimination. When overmergers are broken up, the median virial-to-DM mass Mvir/MDM of three-dimensional-selected groups is ~1 for all simulations. Groups with MDM > 1014 M☉ appear virialized in all simulations. We measure global (not pairwise) velocity biases bv, similar to previous studies. Within three-dimensional-selected groups, CHDM and CDM with b = 1.5 show a stronger bias of bv = 0.7-0.8, while CDM with b = 1.0 shows groups of bv 1.

Dark Matter and Structure Formation in the Universe

Abstract: This chapter aims to present an introduction to current research on the nature of the cosmological dark matter and the origin of galaxies and large scale structure within the standard theoretical framework: gravitational collapse of fluctuations as the origin of structure in the expanding universe. General relativistic cosmology is summarized, and the data on the basic cosmological parameters ($t_o$ and $H_0 \equiv 100 h \kmsmpc$, $\Omega_0$, $\Omega_\Lambda$ and $\Omega_b$) are reviewed. Various particle physics candidates for hot, warm, and cold dark matter are briefly reviewed, together with current constraints and experiments that could detect or eliminate them. Also included is a very brief summary of the theory of cosmic defects, and a somewhat more extended exposition of the idea of cosmological inflation with a summary of some current models of inflation. The remainder is a discussion of observational constraints on cosmological model building, emphasizing models in which most of the dark matter is cold and the primordial fluctuations are the sort predicted by inflation. It is argued that the simplest models that have a hope of working are Cold Dark Matter with a cosmological constant ($\Lambda$CDM) if the Hubble parameter is high ($h \gsim 0.7$), and Cold + Hot Dark Matter (CHDM) if the Hubble parameter and age permit an $\Omega=1$ cosmology, as seems plausible in light of the data from the Hipparcos astrometric satellite. The most attractive variants of these models and the critical tests for each are discussed.

A Reanalysis of Small-Scale Velocity Dispersion in the CfA1 Survey

Abstract: The velocity dispersion of galaxies on scales of r D 1 h~1 Mpc, may be estimated from the p 12 (r), anisotropy of the galaxy-galaxy correlation function in redshift space. We present a reanalysis of the CfA1 survey in which we correct an error in the original analysis of Davis & Peebles. We give a detailed breakdown of how the value of depends on various aspects of the way in which corrections for p 12 (r) infall into the Virgo Cluster are applied. We -nd that is extremely sensitive to the details of these p 12 (r) corrections. We conclude that a robust value of cannot be obtained from this survey. p 12 Subject headings: cosmology: observations E galaxies: clusters: general E galaxies: distances and redshifts E large-scale structure of universe

Statistical Tests for CHDM and ΛCDM Cosmologies

Abstract: We apply several statistical estimators to high-resolution N-body simulations of two currently viable cosmological models: a mixed dark matter model, having ?? = 02 contributed by two massive neutrinos (C + 2?DM), and a cold dark matter model with cosmological constant (?CDM) with ?0 = 03 and h = 07 Our aim is to compare simulated galaxy samples with the Perseus-Pisces redshift survey (PPS) We consider the n-point correlation functions (n = 2-4); the N-count probability functions PN, including the void probability function P0; and the underdensity probability function U (where fixes the underdensity threshold in percentage of the average) We find that P0 (for which PPS and CfA2 data agree) and P1 distinguish efficiently between the models, while U is only marginally discriminatory On the contrary, the reduced skewness and kurtosis are, respectively, S3 22 and S4 6-7 in all cases, quite independent of the scale, in agreement with hierarchical scaling predictions and estimates based on redshift surveys Among our results, we emphasize the remarkable agreement between PPS data and C + 2?DM in all the tests performed In contrast, the above ?CDM model has serious difficulties in reproducing observational data if galaxies and matter overdensities are related in a simple way

The Small Scale Velocity Dispersion of Galaxies: A Comparison of Cosmological Simulations

Abstract: The velocity dispersion of galaxies on small scales (r D 1 h~1 Mpc), can be estimated from the p 12 (r), anisotropy of the galaxy-galaxy correlation function in redshift space (Davis & Peebles). We apply this technique to "" mock catalogs II extracted from N-body simulations of several di†erent variants of cold dark matterEdominated cosmological models, including models with cold ) hot dark matter, to obtain results that may be compared consistently with similar results from observations. We -nd a large varia- tion in the value of h~1 Mpc) in di†erent regions of the same simulation. We investigate the e†ects p 12 (1 of removing clusters from the simulations, using an automated cluster-removing routine, and -nd that this reduces the sky variance but also reduces the discrimination between models. However, studying p 12 as clusters with di†erent internal velocity dispersions are removed leads to interesting information about the amount of power on cluster and subcluster scales. We compute the pairwise velocity dispersion directly, in order to check the Davis-Peebles method, and -nd agreement of better than 20% in all the models studied. We also calculate the mean streaming velocity and the pairwise peculiar velocity dis- tribution in the simulations, and compare these with the models used in the Davis-Peebles method. We -nd that the model for the mean streaming velocity may be a substantial source of error in the calcu- lation of p 12 . Subject headings: cosmology: theory E galaxies: clusters: general E galaxies: distances and redshifts E large-scale structure of universe

Linearizing the Observed Power Spectrum

Abstract: Reconstruction of the linear power spectrum from observational data provides a way to compare cosmological models to a large amount of data, as Peacock & Dodds (1994, 1996) have shown. By applying the appropriate corrections to the observational power spectrum it is possible to recover the underlying linear power spectrum for any cosmological model. Using extensive N-body simulations we demonstrate that the method is applicable to a wide range of cosmological models. However, we find that the recovery of the linear power spectrum from observations following PD94 is misleading because the corrections are model- dependent. When we apply the proper corrections for a given model to the observational power spectrum, we find that no model in our test group recovers the linear power spectrum well for the bias suggested by PD94 between Abell, Radio, Optical, and IRAS catalogs 4.5:1.9:1.3:1, with b_IRAS=1. When we allow b_IRAS to vary we find that: (i)CHDM models give very good fits to observations if optically-selected galaxies are slightly biased b_Opt=1.1 (ii) Most LCDM models give worse but acceptable fits if blue galaxies are considerably antibiased: 0.6

Inclination Effects in Spiral Galaxy Gravitational Lensing

Abstract: Spheroidal components of spiral galaxies have been considered the only dynamically important component in gravitational lensing studies thus far. Here we point out that including the disk component can have a significant effect, depending on the disk inclination, on a variety of lensing properties that are relevant to present studies and future surveys. As an example, we look at the multiple image system B1600+434, recently identified as being lensed by a spiral galaxy. We find that including the disk component one can understand the fairly large image separation as being due to the inclination of a typical spiral, rather than the presence of a very massive halo. The fairly low magnification ratio can also be readily understood if the disk is included. We also discuss how such lensed systems might allow one to constrain parameters of spiral galaxies such as a disk-to-halo mass ratio, and disk mass scale length. Another example we consider is the quasar multiple-lensing cross section, which we find can increase many-fold at high inclination for a typical spiral. Finally, we discuss the changes in the gravitational lensing effects on damped Lyman alpha systems (DLAS) when disk lensing is included.