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Articles 451 - 476 of 476
Full-Text Articles in Astrophysics and Astronomy
A Rigorous Reanalysis Of The Iras Variable Population: Scale Lengths, Asymmetries, And Microlensing, S Nikolaev, Md Weinberg
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 …
The Effect Of The Galactic Spheroid On Globular Cluster Evolution, C Murali, Md Weinberg
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⊙ …
Globular Cluster Evolution In M87 And Fundamental Plane Ellipticals, C Murali, Md Weinberg
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 …
Intergalactic Helium Absorption In Cold Dark Matter Models, Rac Croft, Dh Weinberg, N Katz, L Hernquist
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 …
Metal Lines In Cosmological Models Of Lyman-Alpha Absorbers, U Hellsten, L Hernquist, N Katz, D Weinberg
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 …
Hubble Flow Broadening Of The Lyman-Alpha Forest And Its Implications, Dh Weinberg, L Hernquist, N Katz, R Croft, J Miralda-Escude
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 …
Characterization Of Lyman Alpha Spectra And Predictions Of Structure Formation Models: A Flux Statistics Approach, R Ac Croft, Dh Weinberg, L Hernquist, N Katz
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 …
Evolution Of The Galactic Globular Cluster System, C Murali, Md Weinberg
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 …
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
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 …
Simulating Cosmic Structure Formation, Dh Weinberg, N Katz, L Hernquist
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 …
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
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.
Testing Cosmological Models Against The Abundance Of Damped Lyman-Alpha Absorbers, Jp Gardner, N Katz, Dh Weinberg, L Hernquist
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, …
The Fate Of Cannibalized Fundamental-Plane Ellipticals, Martin D. Weinberg
The Fate Of Cannibalized Fundamental-Plane Ellipticals, Martin D. Weinberg
Astronomy Department Faculty Publication Series
Evolution and disruption of galaxies orbiting in the gravitational field of a larger cluster galaxy are driven by three coupled mechanisms: 1) the heating due to its time dependent motion in the primary; 2) mass loss due to the tidal strain field; and 3) orbital decay. Previous work demonstrated that tidal heating is effective well inside the impulse approximation limit. Not only does the overall energy increase over previous predictions, but the work is done deep inside the secondary galaxy, e.g. at or inside the half mass radius in most cases. Here, these ideas applied to cannibalization of elliptical galaxies …
Ricci Tensor Of Diagonal Metric, K. Z. Win
Ricci Tensor Of Diagonal Metric, K. Z. Win
Astronomy Department Faculty Publication Series
Efficient formulae of Ricci tensor for an arbitrary diagonal metric are presented.
The Dynamics Of The Galatic Bar, Martin D. Weinberg
The Dynamics Of The Galatic Bar, Martin D. Weinberg
Astronomy Department Faculty Publication Series
No abstract provided.
The Global Rate And Efficiency Of Star Formation In Spiral Galaxies As A Function Of Morphology And Environment, Judith S. Young, Lori Allen, Jeffrey D.P. Kenney, Amy Lesser, Brooks Rownd
The Global Rate And Efficiency Of Star Formation In Spiral Galaxies As A Function Of Morphology And Environment, Judith S. Young, Lori Allen, Jeffrey D.P. Kenney, Amy Lesser, Brooks Rownd
Astronomy Department Faculty Publication Series
CCD images of Hα and R-band emission in 120 spiral galaxies were obtained using the now-retired No. 1-0.9 m telescope of Kitt Peak National Observatory. These images were used to derive the distribution and total flux of continuum-subtracted Hα line emission, and therefore the Hα surface brightnesses and high mass star formation rates in these galaxies. We find a small but significant variation in the mean Hα surface brightness for spiral galaxies along the Hubble sequence; the Sd-Ir galaxies exhibit a mean Hα surface brightness 1.4 times higher than the Sbc-Scd galaxies, and 2-3 times higher than the Sa-Sb galaxies. …
Production Of Milky-Way Structure By The Magellanic-Clouds, Md Weinberg
Production Of Milky-Way Structure By The Magellanic-Clouds, Md Weinberg
Astronomy Department Faculty Publication Series
Previous attempts at disturbing the galactic disk by the Magellanic Clouds relied on direct tidal forcing. However, by allowing the halo to actively respond rather than remain a rigid contributor to the rotation curve, the Clouds may produce a wake in the halo which then distorts the disk. Recent work reported here suggests that the Magellanic Clouds use this mechanism to produce disk distortions sufficient to allow for both the radial location, position angle and sign of the HI warp and observed anomalies in stellar kinematics toward the galactic anticenter and LSR motion.
Adiabatic Invariants In Stellar Dynamics .2. Gravitational Shocking, Md Weinberg
Adiabatic Invariants In Stellar Dynamics .2. Gravitational Shocking, Md Weinberg
Astronomy Department Faculty Publication Series
A new theory of gravitational shocking based on time-dependent perturbation theory shows that the changes in energy and angular momentum due to a slowly varying disturbance are not exponentially small for stellar dynamical systems in general. It predicts significant shock heating by slowly varying perturbations previously thought to be negligible according to the adiabatic criterion. The theory extends the scenarios traditionally computed only with the impulse approximation and is applicable to a wide class of disturbances. The approach is applied specifically to the problem of the disk shocking of star clusters.
Adiabatic Invariants In Stellar Dynamics .3. Application To Globular-Cluster Evolution, Md Weinberg
Adiabatic Invariants In Stellar Dynamics .3. Application To Globular-Cluster Evolution, Md Weinberg
Astronomy Department Faculty Publication Series
The previous two companion papers demonstrate that slowly varying perturbations do not result in adiabatic cutoffs and provide a formalism for computing the long term effects of time dependent perturbations on stellar systems. Here, the theory is implemented in Fokker Planck code and a suite of runs illustrating the effects of shock heating on globular cluster evolution are described. Shock heating alone results in a considerable mass loss for clusters with Rg < S kpc: a concentration c = 1.5 cluster with Rg = 8kpc loses up to 95% of the initial mass in 15 Gyr. Only those with concentration c < 1.3 survive disk shock inside of this radius. Other effects, such as mass loss by stellar evolution, will increase this survival bound. Loss of the initial halo together with mass segregation leads to mass spectral induces, x, which may be considerably large than their initial values.
Kinematic Signature Of A Rotating Bar Near A Resonance, Md Weinberg
Kinematic Signature Of A Rotating Bar Near A Resonance, Md Weinberg
Astronomy Department Faculty Publication Series
There have been several recent suggestions that the Milky Way has rotating bar-like features based on HI and star count data. In this paper, I show that such features cause distinctive stellar kinematic signatures near OLR and ILR. The effects of these resonances may be observable far from the peak density of the pattern and relatively nearby the solar position. The details of the kinematic signatures depend on the evolutionary history of the ‘bar’ and therefore velocity data, both systemic and velocity dispersion, may be used to probe the evolutionary history as well as the present state of the Galaxy. …
Adiabatic Invariants In Stellar Dynamics .1. Basic Concepts, Md Weinberg
Adiabatic Invariants In Stellar Dynamics .1. Basic Concepts, Md Weinberg
Astronomy Department Faculty Publication Series
The adiabatic criterion, widely used in astronomical dynamics, is based on the harmonic oscillator. It asserts that the change in action under a slow varying perturbation is exponentially small. Recent mathematical results precisely define the conditions for invariance show that this model does not apply in general. In particular, a slowly varying perturbation may cause significant evolution stellar dynamical systems even if its time scale is longer than any internal orbital time scale. This additional 'heating' may have serious implications for the evolution of star clusters and dwarf galaxies which are subject to long-term environmental forces. the mathematical developments leading …
Weakly Damped Modes In Star-Clusters And Galaxies, Md Weinberg
Weakly Damped Modes In Star-Clusters And Galaxies, Md Weinberg
Astronomy Department Faculty Publication Series
A perturber may excite a coherent mode in a star cluster or galaxy. If the stellar system is stable, it is commonly assumed that such a mode will be strongly damped and therefore of little practical consequence other than redistributing momentum and energy deposited by the perturber. This paper demonstrates that this assumption is false; weakly damped modes exist and may persist long enough to have observable consequences. To do this, a method for investigating the dispersion relation for spherical stellar systems and for locating weakly damped modes in particular is developed and applied to King models of varying concentration. …
(Co)-C-12 (3-2) And (1-0) Emission-Line Observations Of Nearby Starburst Starburst Galaxy Nuclei, Nicholas Devereux, Yoshiaki Taniguchi, D. B. Sanders, N. Nakai, J. S. Young
(Co)-C-12 (3-2) And (1-0) Emission-Line Observations Of Nearby Starburst Starburst Galaxy Nuclei, Nicholas Devereux, Yoshiaki Taniguchi, D. B. Sanders, N. Nakai, J. S. Young
Astronomy Department Faculty Publication Series
New measurements of the ^12^CO (1-0) and ^12^CO (3-2) line emission are presented for the nuclei of seven nearby starburst galaxies selected from a complete sample of 21 nearby starburst galaxies for which the nuclear star formation rates are measured to be comparable to the archetype starburst galaxies M82 and NGC 253. The new observations capitalize on the coincidence between the beam size of the 45 m Nobeyama telescope at 115 GHz and that of the 15 m James Clerk Maxwell Telescope at 345 GHz to measure the value of the ^12^CO (3-2)/(1-0) emission line ratio in a 15" ( …
4 Co Outflows And Dense Gas In The Dark Cloud Complex Near Ic5146, Kazuhito Dobashi, Toshikazu Onishi, Takahiro Iwata, Tomoo Nagahama, Nimesh Patel, Ronald L. Snell, Yasuo Fukui
4 Co Outflows And Dense Gas In The Dark Cloud Complex Near Ic5146, Kazuhito Dobashi, Toshikazu Onishi, Takahiro Iwata, Tomoo Nagahama, Nimesh Patel, Ronald L. Snell, Yasuo Fukui
Astronomy Department Faculty Publication Series
No abstract provided.
Bar-Spheroid Interaction In Galaxies, Lars Hernquist, Martin D. Weinberg
Bar-Spheroid Interaction In Galaxies, Lars Hernquist, Martin D. Weinberg
Astronomy Department Faculty Publication Series
The secular evolution of barred galaxies is explored using N-body simulation and linear analysis. In particular, we examine the interaction between bars and spheroidal components of galaxies. According to our calculations, this interaction drives secular transfer of angular momentum from bars to spheroids, primarily through resonant coupling. A moderately strong bar, having mass within corotation ~0.3 times the enclosed spheroid mass, is predicted to shed all its angular momentum typically in less than ~10^9^ yr, which is quite short compared to the ages of galactic disks. Even shorter depletion time scales are found for relatively more massive bars. Given that …
1300 Micron Continuum And C18o Line Mapping Of The Giant Molecular Cloud Cores In Orion, W49, And W51, F. Peter Schloerb, Ronald L. Snell, P. R. Schwartz
1300 Micron Continuum And C18o Line Mapping Of The Giant Molecular Cloud Cores In Orion, W49, And W51, F. Peter Schloerb, Ronald L. Snell, P. R. Schwartz
Astronomy Department Faculty Publication Series
Observations of the 1300-micron continuum emission and the C(O-18) spectral-line emission from three well-studied giant molecular cloud cores: Orion, W49, and W51 are presented. The observations were obtained at the Five College Radio Astronomy Observatory, and they provide a means to examine the consistency of these two methods to trace the column density structure of molecular clouds. A good general correlation is found between the 1300-micron continuum, which traces the column density of dust, and the C(O-18) J = 2-1 line emission, which traces the column density of molecular gas, when the effects of source temperature are taken into consideration. …