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1997

Astronomy Department Faculty Publication Series

Articles 1 - 14 of 14

Full-Text Articles in Physical Sciences and Mathematics

Fluctuations In Finite N Equilibrium Stellar Systems, Martin D. Weinberg Jul 1997

Fluctuations In Finite N Equilibrium Stellar Systems, Martin D. Weinberg

Astronomy Department Faculty Publication Series

Gravitational amplification of Poisson noise in stellar systems is important on large scales. For example, it increases the dipole noise power by roughly a factor of six and the quadrupole noise by 50% for a King model profile. The dipole noise is amplified by a factor of fifteen for the core-free Hernquist model. The predictions are computed using the dressed-particle formalism of Rostoker & Rosenbluth (1960) and are demonstrated by n-body simulation. This result implies that a collisionless n-body simulation is impossible; The fluctuation noise which causes relaxation is an intrinic part of self gravity. In other words, eliminating two-body …

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Dynamics Of An Interacting Luminous Disk, Dark Halo, And Satellite Companion, Martin D. Weinberg Jul 1997

Dynamics Of An Interacting Luminous Disk, Dark Halo, And Satellite Companion, Martin D. Weinberg

Astronomy Department Faculty Publication Series

This paper describes a method for determining the dynamical interaction between extended halo and spheroid components and an environmental disturbance. One finds that resonant interaction between a galaxy and passing interlopers or satellite companions can carry the disturbance inward, deep inside the halo, where it can perturb the disk. Applied to the Milky Way for example, the LMC and SMC appear to be sufficient to cause the observed Galactic warp and possibly seed other asymmetries. This is a multi-scale interaction in which the halo wake has a feature at roughly half the satellite orbital radius due to a 2:1 orbital …

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A Rigorous Reanalysis Of The Iras Variable Population: Scale Lengths, Asymmetries, And Microlensing, S Nikolaev, Md Weinberg Jan 1997

A Rigorous Reanalysis Of The Iras Variable Population: Scale Lengths, Asymmetries, And Microlensing, S Nikolaev, Md Weinberg

Astronomy Department Faculty Publication Series

Previous work reported a bar signature in color-selected IRAS variable stars. Here, we estimate the source density of these variables while consistently accounting for spatial incompleteness in the data using a likelihood approach. The existence of the bar is confirmed with a shoulder at a approx 3 kpc, an axis ratio of a:b = 2.2-2.7, and a position angle of 19° ± 1°. The ratio of nonaxisymmetric to axisymmetric components gives a similar estimate for the bar size of a = 3.3 ± 0.1 kpc and a position angle of phi0 = 24° ± 2°. We estimate a scale length …

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The Effect Of The Galactic Spheroid On Globular Cluster Evolution, C Murali, Md Weinberg Jan 1997

The Effect Of The Galactic Spheroid On Globular Cluster Evolution, C Murali, Md Weinberg

Astronomy Department Faculty Publication Series

We study the combined effects of relaxation, tidal heating and binary heating on globular cluster evolution, exploring the physical consequences of external effects and examining evolutionary trends in the MilkyWay population. Our analysis demonstrates that heating on circular and low-eccentricity orbits can dominate cluster evolution. The results also predict rapid evolution on eccentric orbits either due to strong relaxation caused by the high densities needed for tidal limitation or due to efficient bulge shocking of low density clusters. The combination of effects leads to strong evolution of the population as a whole. For example, within the solar circle, tidally-limited 105M⊙ …

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Globular Cluster Evolution In M87 And Fundamental Plane Ellipticals, C Murali, Md Weinberg Jan 1997

Globular Cluster Evolution In M87 And Fundamental Plane Ellipticals, C Murali, Md Weinberg

Astronomy Department Faculty Publication Series

The globular cluster population in M87 has decreased measurably through dynamical evolution caused by relaxation, binary heating and time-dependent tidal perturbation. For fundamental plane ellipticals in general, cluster populations evolve more rapidly in smaller galaxies because of the higher mass density. A simple evolutionary model reproduces the observed trend in specific frequency with luminosity for an initially constant relationship. Fits of theoretically evolved populations to M87 cluster data from McLaughlin et al. (1994) show the following: 1) dynamical effects drive evolution in the initial mass and space distributions and can account for the large core in the spatial profile as …

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Intergalactic Helium Absorption In Cold Dark Matter Models, Rac Croft, Dh Weinberg, N Katz, L Hernquist Jan 1997

Intergalactic Helium Absorption In Cold Dark Matter Models, Rac Croft, Dh Weinberg, N Katz, L Hernquist

Astronomy Department Faculty Publication Series

Observations from the Hopkins Ultraviolet Telescope and the Hubble Space Telescope have recently detected He II absorption along the lines of sight to two high-redshift quasars. We use cosmological simulations with gas dynamics to investigate He II absorption in the cold dark matter (CDM) theory of structure formation. We consider two Ω = 1 CDM models with different normalizations and one open universe (Ω0 = 0.4) CDM model. The simulations incorporate the photoionizing UV background spectrum computed by Haardt & Madau, which is based on the output of observed quasars and reprocessing by the Lyα forest. The simulated …

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Metal Lines In Cosmological Models Of Lyman-Alpha Absorbers, U Hellsten, L Hernquist, N Katz, D Weinberg Jan 1997

Metal Lines In Cosmological Models Of Lyman-Alpha Absorbers, U Hellsten, L Hernquist, N Katz, D Weinberg

Astronomy Department Faculty Publication Series

The metal absorption lines found in association with \lya absorbers of moderate to high HI column density contain valuable information about the metallicity and ionization conditions within the absorbers and offer a stronger test of models of the intergalactic medium at \sim 3$ than HI absorption lines alone.
We have developed a method to predict the strengths of metal absorption lines within the framework of cosmological models for the \lya forest. The method consists of evaluating a quantity, the Line Observability Index, for a database of hundreds of candidate metal lines, allowing a comprehensive identification of the lines the model …

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Hubble Flow Broadening Of The Lyman-Alpha Forest And Its Implications, Dh Weinberg, L Hernquist, N Katz, R Croft, J Miralda-Escude Jan 1997

Hubble Flow Broadening Of The Lyman-Alpha Forest And Its Implications, Dh Weinberg, L Hernquist, N Katz, R Croft, J Miralda-Escude

Astronomy Department Faculty Publication Series

Lyman-alpha forest lines in QSO spectra have typical widths of 20-50 km/s. Low column density absorbers in cosmological simulations are large, diffuse structures, and the Hubble flow across the spatially extended absorber is usually the dominant contribution to the width of its associated absorption line. Thermal broadening is unimportant over most of the spectrum, and peculiar velocities tend to make absorption features narrower rather than broader. As a consequence of Hubble flow broadening, there is a close relation between local Lyman-alpha optical depth and local neutral hydrogen density, which is well approximated by the Gunn-Peterson formula. The physics that governs …

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Characterization Of Lyman Alpha Spectra And Predictions Of Structure Formation Models: A Flux Statistics Approach, R Ac Croft, Dh Weinberg, L Hernquist, N Katz Jan 1997

Characterization Of Lyman Alpha Spectra And Predictions Of Structure Formation Models: A Flux Statistics Approach, R Ac Croft, Dh Weinberg, L Hernquist, N Katz

Astronomy Department Faculty Publication Series

In gravitational instability models, \lya absorption arises from a continuous fluctuating medium, so that spectra provide a non-linear one-dimensional ``map'' of the underlying density field. We characterise this continuous absorption using statistical measures applied to the distribution of absorbed flux. We describe two simple members of a family of statistics which we apply to simulated spectra in order to show their sensitivity as probes of cosmological parameters (H$_{0}$, $\Omega$, the initial power spectrum of matter fluctuations) and the physical state of the IGM. We make use of SPH simulation results to test the flux statistics, as well as presenting a …

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Evolution Of The Galactic Globular Cluster System, C Murali, Md Weinberg Jan 1997

Evolution Of The Galactic Globular Cluster System, C Murali, Md Weinberg

Astronomy Department Faculty Publication Series

We study the dynamical evolution of disk and halo globular clusters in the Milky Way using a series of Fokker-Planck calculations combined with parametric statistical models. Our sample of 113 clusters with velocity data is predicted to descend from an initial population of 250 clusters, implying more than a factor of two decrease in population size due to evolution. Approximately 200 of these clusters are in a halo component and 50 in a disk component. The estimated initial halo population follows a coreless R−3.38 density profile in good agreement with current estimates for the distribution of halo field stars. The …

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The Opacity Of The Ly Alpha Forest And Implications For Omega(B) And The Ionizing Background, M Rauch, J Miraldaescude, Wl Sargent, Ta Barlow, Dh Weinberg, L Hernquist, N Katz, R Cen, Jp Ostriker Jan 1997

The Opacity Of The Ly Alpha Forest And Implications For Omega(B) And The Ionizing Background, M Rauch, J Miraldaescude, Wl Sargent, Ta Barlow, Dh Weinberg, L Hernquist, N Katz, R Cen, Jp Ostriker

Astronomy Department Faculty Publication Series

We have measured the distribution function of the flux decrement D = 1 - e-τ caused by Lyα forest absorption from intervening gas in the lines of sight to high-redshift QSOs from a sample of seven high-resolution QSO spectra obtained with the Keck telescope. The observed flux decrement distribution function (FDDF) is compared with the FDDF from two simulations of the Lyα forest: a ΛCDM model (with Ω = 0.4, Λ = 0.6), computed with the Eulerian code of Cen & Ostriker, and a standard cold dark matter (SCDM) model (with Ω = 1), computed with the smoothed particle …

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Simulating Cosmic Structure Formation, Dh Weinberg, N Katz, L Hernquist Jan 1997

Simulating Cosmic Structure Formation, Dh Weinberg, N Katz, L Hernquist

Astronomy Department Faculty Publication Series

We describe cosmological simulation techniques and their application to studies of cosmic structure formation, with particular attention to recent hydrodynamic simulations of structure in the high redshift universe. Collisionless N-body simulations with Gaussian initial conditions produce a pattern of sheets, filaments, tunnels, and voids that resembles the observed large scale galaxy distribution. Simulations that incorporate gas dynamics and dissipation form dense clumps of cold gas with sizes and masses similar to the luminous parts of galaxies. Models based on inflation and cold dark matter predict a healthy population of high redshift galaxies, including systems with star formation rates of 20 …

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New Statistical Measures Of The Lya Forest Spectra For Accurate Comparison To Theoretical Models, J Miralda-Escude, M Rauch, Wlw Sargent, Ta Barlow, Dh Weinberg, L Hernquist, N Katz, R Cen, Jp Ostriker Jan 1997

New Statistical Measures Of The Lya Forest Spectra For Accurate Comparison To Theoretical Models, J Miralda-Escude, M Rauch, Wlw Sargent, Ta Barlow, Dh Weinberg, L Hernquist, N Katz, R Cen, Jp Ostriker

Astronomy Department Faculty Publication Series

We propose a new method of analysis for the \lya forest, namely to measure the 1-point and 2-point joint probability distribution of the transmitted flux. The results for a sample of seven observed quasars and from two simulations of structure formation are shown and compared. Statistically significant differences in the 2-point function between the results of the numerical simulations and the observations are easily found. The analysis we suggest is very simple to apply to observed data sets, and we discuss its superiority over the traditional Voigt-profile fitting algorithms for accurate comparison to the predictions of theoretical models.

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Testing Cosmological Models Against The Abundance Of Damped Lyman-Alpha Absorbers, Jp Gardner, N Katz, Dh Weinberg, L Hernquist Jan 1997

Testing Cosmological Models Against The Abundance Of Damped Lyman-Alpha Absorbers, Jp Gardner, N Katz, Dh Weinberg, L Hernquist

Astronomy Department Faculty Publication Series

We calculate the number of damped Lyα absorbers expected in various popular cosmological models as a function of redshift and compare our predictions with observed abundances. The Press-Schechter formalism is used to obtain the distribution of halos with circular velocity in different cosmologies, and we calibrate the relation between circular velocity and absorption cross section using detailed gasdynamical simulations of a standard cold dark matter (CDM) model. Because of this calibration, our approach makes more realistic assumptions about the absorption properties of collapsed objects than previous, analytic calculations of the damped Lyα abundance. CDM models with Ω0 = 1, …

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