arXiv subjects

Joel R. Primack

Publications and source records attributed to Joel R. Primack.

At least 163 records · Page 9Linked to original sources

arXiv1998

The nature of Lyman-break galaxies

Using semi-analytic models of galaxy formation, we investigate the properties of $z\sim3$ galaxies and compare them with the observed population of Lyman-break galaxies (LBGs). In addition to the usual quiescent mode of star formation, we introduce a physical model for starbursts triggered by galaxy-galaxy interactions. We find that with the merger rate that arises naturally in the CDM-based merging hierarchy, a significant fraction of bright galaxies identified at high redshift ($z \ga 2$) are likely to be low-mass, bursting satellite galaxies. The abundance of LBGs as a function of redshift and the luminosity function of LBGs both appear to be in better agreement with the data when the starburst mode is included, especially when effects of dust extinction are considered. The objects that we identify as LBGs have observable properties including low velocity dispersions that are in good agreement with the available data. In this ``Bursting Satellite'' scenario, quiescent star formation at $z\ga2$ is relatively inefficient and most of the observed LBGs are starbursts triggered by satellite mergers within massive halos. In high-resolution N-body simulations, we find that the most massive dark matter halos cluster at redshift $z\sim 3$ much as the LBGs are observed to do. This is true for both the $Ω=1$ CHDM model and low-$Ω$ \LCDM and OCDM models, all of which have fluctuation power spectra $P(k)$ consistent with the distribution of low-redshift galaxies. The Bursting Satellite scenario can resolve the apparent paradox of LBGs that cluster like massive dark matter halos but have narrow linewidths and small stellar masses.

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arXiv1998

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

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)

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arXiv1998

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

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.

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arXiv1998

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

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.

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arXiv1997

Clusters in Various Cosmological Models: Abundance and Evolution

The combination of measurements of the local abundance of rich clusters of galaxies and its evolution to higher redshift offers the possibility of a direct measurement of $Ω_0$ with little contribution from other cosmological parameters. We investigate the significance of recent claims that this evolution indicates that $Ω_0$ must be small. The most recent cluster velocity dispersion function from a compilation including the ESO Northern Abell Cluster Survey (ENACS) results in a significantly higher normalization for models, corresponding to $σ_8\approx 0.6$ for $Ω_0=1$, compared to the Eke, Cole, & Frenk result of $σ_8=0.52\pm 0.04$. Using the ENACS data for a $z=0$ calibration results in strong evolution in the abundance of clusters, and we find that the velocity dispersion function is consistent with $Ω_0=1$. The results are dependent upon the choice and analysis of low-redshift and high-redshift data, so at present, the data is not good enough to determine $Ω_0$ unambiguously.

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arXiv1997

Dark Matter and Structure Formation in the Universe

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 $H_0 \equiv 100 h \kmsmpc$, $Ω_0$, $Ω_Λ$ and $Ω_b$) 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 ($Λ$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 $Ω=1$ 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.

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arXiv1996

Favored Variants of Cold Dark Matter Cosmologies

We discuss variants of Cold Dark Matter (CDM) dominated cosmological models that give good agreement with a range of observations. We consider models with hot dark matter, tilt, $Ω< 1$, or a cosmological constant. We also discuss the sensitivity of the results to other parameters, such as the Hubble parameter and the baryon fraction. We obtain constraints by combining the COBE data, cluster abundances, abundance of damped Lyman-$α$ systems at $z\sim3$, the small-angle Cosmic Microwave Background anisotropy, and the small-scale non-linear power spectrum. We present non-linear power spectra from a new suite of N-body simulations for the ``best-bet'' models from each category.

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arXiv1996

Non-Gaussian Fluctuations and Primordial Black Holes from Inflation

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's, and 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 cut-offs in the probability of large positive fluctuations, meaning that Gaussian distributions would vastly over-produce PBH's. The standard results that link inflation-produced power spectra and PBH number densities must then be reconsidered, since they rely quite heavily on the Gaussian assumption. We point out that since the probability distributions depend 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.

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arXiv1996

Status of Cosmological Parameters: $Ω_0\approx 0.3$ vs. $Ω=1$

The cosmological parameters that I discuss are the Hubble parameter $H_0 \equiv 100 h$ km s$^{-1}$ Mpc$^{-1}$, the age of the universe $t_0$, the average density $Ω_0$, and the cosmological constant $Λ$. To focus the discussion, I concentrate on the the value of $Ω_0$ in currently popular models in which most of the dark matter is cold, especially Cold + Hot Dark Matter (CHDM) and flat ($Ω_0 + Ω_Λ=1$) low-$Ω$ CDM with a Cosmological Constant ($Λ$CDM). The evidence would favor small $Ω_0 \approx 0.3$ if (1) the Hubble parameter actually has the high value $h \approx 0.75$ favored by many observers, and $t_0 \geq 13$ Gy; or (2) the baryonic/total mass ratio in clusters of galaxies is actually $\sim 15$\%, about 3 times larger than expected for standard BBN in an $Ω=1$ universe, $Ω_b \approx 0.0125 h^{-2}$, despite the recent measurement by Tytler of $D/H=2.4\times 10^{-5}$ in two high-redshift Lyman limit systems, implying $Ω_b\approx 0.024 h^{-2}$. The evidence would favor $Ω=1$ if (1) the POTENT analysis of galaxy peculiar velocity data is right, in particular regarding outflows from voids or the inability to obtain the present-epoch non- Gaussian density distribution from Gaussian initial fluctuations in a low- $Ω$ universe; or (2) the preliminary LSND report indicating neutrino mass $\gsim 2.4$ eV is right, since that would be too much hot dark matter to allow significant structure formation in a low-$Ω_0$ $Λ$CDM model. Statistics on gravitational lensing of quasars provide an upper limit on $Λ$, and the preliminary results on the deceleration parameter $q_0=Ω_0/2-Ω_Λ$ on very large scales from high-redshift Type Ia supernovae suggest that $Ω_0 \sim 1$ and $Ω_Λ$ is small.

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arXiv1996

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

The velocity dispersion of galaxies on small scales ($r\sim1h^{-1}$ Mpc), $σ_{12}(r)$, can be estimated from the anisotropy of the galaxy-galaxy correlation function in redshift space. We apply this technique to ``mock-catalogs'' extracted from N-body simulations of several different variants of Cold Dark Matter dominated cosmological models to obtain results which may be consistently compared to similar results from observations. We find a large variation in the value of $σ_{12}(1 h^{-1} Mpc)$ in different regions of the same simulation. We conclude that this statistic should not be considered to conclusively rule out any of the cosmological models we have studied. We attempt to make the statistic more robust by removing clusters from the simulations using an automated cluster-removing routine, but this appears to reduce the discriminatory power of the statistic. However, studying $σ_{12}$ as clusters with different internal velocity dispersions are removed leads to interesting information about the amount of power on cluster and subcluster scales. We also compute the pairwise velocity dispersion directly and compare this to the values obtained using the Davis-Peebles method, and find that the agreement is fairly good. We evaluate the models used for the mean streaming velocity and the pairwise peculiar velocity distribution in the original Davis-Peebles method by comparing the models with the results from the simulations.

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arXiv1996

A Reanalysis of Small Scale Velocity Dispersion in the CfA1 Survey

The velocity dispersion of galaxies on scales of $r\sim1h^{-1}$ Mpc, $σ_{12}(r)$, may be estimated from the anisotropy of the galaxy-galaxy correlation function in redshift space. We present a reanalysis of the CfA1 survey, correct an error in the original analysis of Davis and Peebles (1983), and find that $σ_{12}(r)$ is extremely sensitive to the details of how corrections for infall into the Virgo cluster are applied. We conclude that a robust value of $σ_{12}$ cannot be obtained from this survey. We also discuss results from other redshift surveys, including the effect of removing clusters.

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arXiv1995

Cluster Cores, Gravitational Lensing, and Cosmology

Many multiply--imaged quasars have been found over the years, but none so far with image separation in excess of $8\arcsec$. The absence of such large splittings has been used as a test of cosmological models: the standard Cold Dark Matter model has been excluded on the basis that it predicts far too many large--separation double images. These studies assume that the lensing structure has the mass profile of a singular isothermal sphere. However, such large splittings would be produced by very massive systems such as clusters of galaxies, for which other gravitational lensing data suggest less singular mass profiles. Here we analyze two cases of mass profiles for lenses: an isothermal sphere with a finite core radius (density $ρ\propto (r^2+r_{core}^2)^{-1})$, and a Hernquist profile ($ρ\propto r^{-1}(r+a)^{-3}$). We find that small core radii $r_{core} \sim 30 h^{-1}$ kpc, as suggested by the cluster data, or large $a \gsim 300 h^{-1}$ kpc, as needed for compatibility with gravitational distortion data, would reduce the number of large--angle splittings by an order of magnitude or more. Thus, it appears that these tests are sensitive both to the cosmological model (number density of lenses) and to the inner lens structure, which is unlikely to depend sensitively on the cosmology, making it difficult to test the cosmological models by large--separation quasar lensing until we reliably know the structure of the lenses themselves.

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arXiv1995

Improved Ways to Compare Simulations to Data

Theoretical models for structure formation with Gaussian initial fluctuations have been worked out in considerable detail and compared with observations on various scales. It is on nonlinear scales $\lsim 10 \ h^{-1}\ {\rm Mpc}$ that the greatest differences exist between $Ω=1$ models that have been normalized to agree on the largest scales with the COBE data; here especially there is a need for better statistical tests which are simultaneously {\it robust}, {\it discriminatory}, and {\it interpretable}. The era at which galaxy and cluster formation occurs is also a critical test of some models. Needs for the future include faster and cleverer codes, better control of cosmic variance in simulations, better understanding of processes leading to galaxy formation, better ways of comparing observational data with models, and better access to observational and simulation data.

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arXiv1995

Status of Cosmological Parameters: Can $Ω=1$?

I discuss the Hubble parameter $H_0$, the age of the universe $t_0$, the density $Ω_0$, and the cosmological constant $Λ$. To focus the discussion, I concentrate on the issue of the value of the density $Ω_0$ in currently popular cosmological models in which most of the dark matter is cold, especially Cold + Hot Dark Matter (CHDM) and flat low- $Ω$ CDM with a Cosmological Constant ($Λ$CDM). The evidence would favor a small $Ω_0 \approx 0.3$ if (1) the Hubble parameter actually has the high value $h \approx 0.8$ favored by many observers, and the age of the universe $t_0 \geq 13$ Gy; or (2) the baryonic/total mass ratio in clusters is actually $\sim 20$\%, about 3-4 times larger than expected for standard Big Bang Nucleosynthesis in an $Ω=1$ universe, and standard BBN is actually right in predicting that the density of ordinary matter $Ω_b$ lies in the range $0.009 \leq Ω_b h^2 \leq 0.02$. The evidence would favor $Ω=1$ if (1) the POTENT analysis of galaxy peculiar velocity data is right, in particular regarding outflows from voids or the inability to obtain the present-epoch non-Gaussian density distribution from Gaussian initial fluctuations in a low-$Ω$ universe; or (2) the preliminary report from LSND indicating a neutrino mass $\geq 2.4$ eV is right, since that would be too much hot dark matter to allow significant structure formation in a low-$Ω$ $Λ$CDM model. Statistics on gravitational lensing of quasars provide a strong upper limit on $Λ$. Reliable data on all of these issues is becoming available so rapidly today that a clear decision between these alternatives may be possible within the next few years.

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arXiv1995

Do experiments and astrophysical considerations suggest an inverted neutrino mass hierarchy?

The recent results from the Los Alamos neutrino oscillation experiment, together with assumptions of neutrino oscillation solutions for the solar and atmospheric neutrino deficit problems, may place powerful constraints on any putative scheme for neutrino masses and mixings. Assuming the validity of these experiments and assumptions, we argue that a nearly unique spectrum of neutrino masses emerges as a fit, if two additional astrophysical arguments are adopted: (1) the sum of the light neutrino masses is $\sim 5\ {\rm eV}$, as large scale structure simulations with mixed cold plus hot dark matter seem to suggest; and (2) $r$-process nucleosynthesis originates in neutrino-heated ejecta from Type II supernovae. In this fit, the masses of the neutrinos must satisfy $m_{ν_e} \approx m_{ν_s} \approx 2.7\ {\rm eV}$ (where $ν_e$ is split from a sterile species, $ν_s$, by $\sim {10}^{-6} \ {\rm eV}$) and $m_{ν_τ} \approx m_{ν_μ} \approx 1.1\ {\rm eV}$ (where these species are split by $\sim {10}^{-2} \ {\rm eV}$). We discuss alternative neutrino mass spectra that are allowed if we decline to adopt certain experiments or astrophysical models.

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arXiv1994

Probing the Era of Galaxy Formation via TeV Gamma Ray Absorption by the Near Infrared Extragalactic Background

We present models of the extragalactic background light (EBL) based on several scenarios of galaxy formation and evolution. We have treated galaxy formation with the Press-Schecter approximation for both cold dark matter (CDM) and cold+hot dark matter (CHDM) models. Galaxy evolution has been treated by considering a variety of stellar types, different initial mass functions and star formation histories, and with an accounting of dust absorption and emission. We find that the dominant factor influencing the EBL is the epoch of galaxy formation. A recently proposed method for observing the EBL utilizing the absorption of $\sim 0.1$ to 10 TeV gamma-rays from active galactic nuclei (AGN) is shown to be capable of discriminating between different galaxy formation epochs. Observations of TeV gamma-ray bursts (GRB) would constrain or eliminate models in which the GRB sources lie at cosmological distances.

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arXiv1994

Cold + Hot Dark Matter Cosmology with $m(ν_μ) \approx m(ν_τ) \approx 2.4$ ev

Cold + Hot Dark Matter (CHDM) $Ω=1$ cosmological models require a total neutrino mass $\sim 5$ eV. Because recent data support the $ν_μ\to ν_τ$ oscillation explanation of the cosmic ray $ν_μ$ deficit, which requires that $m(ν_μ) \approx m(ν_τ)$, this suggests that $m(ν_μ) \approx m(ν_τ) \approx 2.4$ eV. The linear calculations and N-body simulation reported here indicate that an $Ω=1$ CHDM model with two 2.4 eV neutrinos (designated C$ν^2$DM) agrees remarkably well with all available observations, but only if the Hubble parameter $h \approx 0.5$. We also show that even one 2.4 eV neutrino raises serious difficulties for low-$Ω$ flat CDM models.

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arXiv1994

Observational and Theoretical Constraints on Singular Dark Matter Halos

The distribution of dark matter around galactic or cluster halos has usually been assumed to be approximately isothermal with a non-zero core radius, which is expected to be of the order of the size of the visible matter distribution. Recently, the possibility has been raised that dark matter halos might be singular in the sense that the dark matter density $ρ$ could increase monotonically with radius $r$ down to a very small distance from the center of galaxies or clusters. Such central cusps in the dark matter density could lead to a high flux of gamma rays from WIMP dark matter annihilation. Here we analyze two possibilities that have been discussed in the literature, $ρ\propto r^{-n}$ with $n \approx 1\ {\rm or}\ 2$, and point out that such density profiles are excluded by gravitional lensing analyses on cluster scales and by the rotation curves of gas-rich, halo-dominated dwarf spirals on small scales. We also point out that if spiral galaxies form by gas infall inside dark matter halos, as they are expected to do in any hierarchical clustering model, such profiles almost always lead to falling rotation curves after infall, contrary to observations.

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