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Astrophysics and Astronomy

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University of Massachusetts Amherst

Astronomy Department Faculty Publication Series

1998

Articles 1 - 14 of 14

Full-Text Articles in Physical Sciences and Mathematics

Effect Of The Magellanic Clouds On The Milky Way Disk And Vice Versa, Martin D. Weinberg Nov 1998

Effect Of The Magellanic Clouds On The Milky Way Disk And Vice Versa, Martin D. Weinberg

Astronomy Department Faculty Publication Series

The satellite-disk interaction provides limits on halo properties in two ways: (1) physical arguments motivate the excitation of observable Galactic disk structure in the presence of a massive halo, although precise limits on halo parameters are scenario-dependent; (2) conversely, the Milky Way as a whole has significant dynamical effect on LMC structure and this interaction also leads to halo limits. Together, these scenarios give strong corroboration of our current gravitational mass estimates and suggests a rapidly evolving LMC.

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Fluctuations In Finite-N Equilibrium Stellar Systems, Md Weinberg Jan 1998

Fluctuations In Finite-N Equilibrium Stellar Systems, Md 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 6 and the quadrupole noise by 50 per cent for a King model profile. The dipole noise is amplified by a factor of 15 for the core-free Hernquist model. The predictions are computed by summing over the wakes caused by each star in the system — the dressed-particle formalism of Rostoker & Rosenbluth — and are demonstrated by N-body simulation. This result implies that a collisionless N-body simulation is impossible; the fluctuation noise which …

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Dynamics Of An Interacting Luminous Disc, Dark Halo And Satellite Companion, Md Weinberg Jan 1998

Dynamics Of An Interacting Luminous Disc, Dark Halo And Satellite Companion, Md Weinberg

Astronomy Department Faculty Publication Series

This paper describes a method, based on linear perturbation theory, to determine 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 disc. 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 …

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New Insight On Galaxy Structure From Galphat I. Motivation, Methodology, And Benchmarks For Sersic Models, Ilsang Yoon, Martin D. Weinberg Jan 1998

New Insight On Galaxy Structure From Galphat I. Motivation, Methodology, And Benchmarks For Sersic Models, Ilsang Yoon, Martin D. Weinberg

Astronomy Department Faculty Publication Series

We introduce a new galaxy image decomposition tool, GALPHAT (GALaxy PHotometric ATtributes), to provide full posterior probability distributions and reliable confidence intervals for all model parameters. GALPHAT is designed to yield a high speed and accurate likelihood computation, using grid interpolation and Fourier rotation. We benchmark this approach using an ensemble of simulated Sersic model galaxies over a wide range of observational conditions: the signal-to-noise ratio S/N, the ratio of galaxy size to the PSF and the image size, and errors in the assumed PSF; and a range of structural parameters: the half-light radius $r_e$ and the Sersic index $n$. …

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Morphological Transformation From Galaxy Harassment, B Moore, G Lake, N Katz Jan 1998

Morphological Transformation From Galaxy Harassment, B Moore, G Lake, N Katz

Astronomy Department Faculty Publication Series

Galaxy morphologies in clusters have undergone a remarkable transition over the past several billion yr. Distant clusters at z ~ 0.4 are filled with small spiral galaxies, many of which are disturbed and show evidence of multiple bursts of star formation. This population is absent from nearby clusters, where spheroidals comprise the faint end of the luminosity function. Our numerical simulations follow the evolution of disk galaxies in a rich cluster resulting from encounters with brighter galaxies and the cluster's tidal field, or "galaxy harassment." After a bursting transient phase, they undergo a complete morphological transformation from "disks" to "spheroidals." …

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Recovery Of The Power Spectrum Of Mass Fluctuations From Observations Of The Ly Alpha Forest, Rac Croft, Dh Weinberg, N Katz, L Hernquist Jan 1998

Recovery Of The Power Spectrum Of Mass Fluctuations From Observations Of The Ly Alpha Forest, Rac Croft, Dh Weinberg, N Katz, L Hernquist

Astronomy Department Faculty Publication Series

We present a method to recover the shape and amplitude of the power spectrum of mass fluctuations, P(k), from observations of the high-redshift Lyα forest. The method is motivated by the physical picture that has emerged from hydrodynamic cosmological simulations and related semianalytic models, in which typical Lyα forest lines arise in a diffuse, continuous, fluctuating intergalactic medium. The thermal state of this low-density gas (δρ/ρ 10) is governed by simple physical processes, which lead to a tight correlation between the Lyα optical depth and the underlying matter density. To recover the mass power spectrum, we (1) …

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Origin And Evolution Of The Cepheus Bubble, Na Patel, Pf Goldsmith, Mh Heyer, Rl Snell, P Pratap Jan 1998

Origin And Evolution Of The Cepheus Bubble, Na Patel, Pf Goldsmith, Mh Heyer, Rl Snell, P Pratap

Astronomy Department Faculty Publication Series

We have imaged a 10° × 10° region of the Cepheus bubble in the J = 1-0 line of CO and the 21 cm line of atomic hydrogen. The CO emission defines a giant expanding shell 120 pc in diameter, which is similar to that seen in the IRAS sky maps. We estimate the total gas mass in the region to be ~4 × 105M☉. The total kinetic energy from the observed spread of velocities of the molecular clouds is ~1051ergs. We suggest that the members of earlier generations of massive stars in NGC 7160 are responsible for …

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Constraining The Metallicity Of The Low-Density Ly Alpha Forest Using O Vi Absorption, R Dave, U Hellsten, L Hernquist, N Katz, Dh Weinberg Jan 1998

Constraining The Metallicity Of The Low-Density Ly Alpha Forest Using O Vi Absorption, R Dave, U Hellsten, L Hernquist, N Katz, Dh Weinberg

Astronomy Department Faculty Publication Series

We present a systematic search for O VI (1032, 1037 Å) absorption in a Keck High-Resolution Echelle Spectrometer spectrum of the z = 3.62 quasar Q1422+231, with the goal of constraining the metallicity and ionization state of the low-density intergalactic medium (IGM). Comparison of C IV absorption measurements to models of the Lyα forest based on cosmological simulations shows that absorbers with NH I 1014.5 cm-2 have a mean carbon abundance [C/H] -2.5, assuming a metagalactic photoionizing background with the spectral shape predicted by Haardt and Madau in 1996. In these models, lower column density absorption arises …

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The Observability Of Metal Lines Associated With The Ly Alpha Forest, U Hellsten, L Hernquist, N Katz, Dh Weinberg Jan 1998

The Observability Of Metal Lines Associated With The Ly Alpha Forest, U Hellsten, L Hernquist, N Katz, Dh Weinberg

Astronomy Department Faculty Publication Series

We develop a prescription for characterizing the strengths of metal lines associated with Lyα forest absorbers (LYFAs) of a given neutral hydrogen column density NH I and metallicity [O/H]. This line observability index (LOX) is line specific and translates, for weak lines, into a measure of the equivalent width. It can be evaluated quickly for thousands of transitions within the framework of a given model of the Lyα forest, providing a ranking of the absorption lines in terms of their strengths and enabling model builders to select the lines that deserve more detailed consideration, i.e., those that should be …

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Cosmological Simulations With Scale-Free Initial Conditions. I. Adiabatic Hydrodynamics, Jm Owen, Dh Weinberg, Ae Evrard, L Hernquist, N Katz Jan 1998

Cosmological Simulations With Scale-Free Initial Conditions. I. Adiabatic Hydrodynamics, Jm Owen, Dh Weinberg, Ae Evrard, L Hernquist, N Katz

Astronomy Department Faculty Publication Series

We analyze hierarchical structure formation based on scale-free initial conditions in an Einstein-de Sitter universe, including a baryonic component with Ωbary = 0.05. We present three independent, smoothed particle hydrodynamics (SPH) simulations, performed at two resolutions (323 and 643 dark matter and baryonic particles) and with two different SPH codes (TreeSPH and P3MSPH). Each simulation is based on identical initial conditions, which consist of Gaussian-distributed initial density fluctuations that have a power spectrum P(k) k-1. The baryonic material is modeled as an ideal gas subject only to shock heating and adiabatic heating …

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The Formation Of Quasars In Low-Luminosity Hosts Via Galaxy Harassment, G Lake, N Katz, B Moore Jan 1998

The Formation Of Quasars In Low-Luminosity Hosts Via Galaxy Harassment, G Lake, N Katz, B Moore

Astronomy Department Faculty Publication Series

We have simulated disk galaxies undergoing continual bombardment by other galaxies in a rich cluster. "Galaxy harassment" leads to dramatic evolution of smaller disk galaxies and provides an extremely effective mechanism for fueling a central quasar. Within a few billion years after a small disk galaxy enters the cluster environment, up to 90% of its gas can be driven into the inner 500 pc. Up to half of the mass can be transferred in a burst lasting just 100-200 Myr. This transport of gas to the center of galaxy is far more efficient than any mechanism proposed before. Galaxy harassment …

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Cosmology With The Lyman-Alpha Forest, Dh Weinberg, S Burles, Rac Croft, R Dave, G Gomez, L Hernquist, N Katz, D Kirkman, S Liu, J Miralda-Escude, M Penttini, J Phillips, D Tytler, J Wright Jan 1998

Cosmology With The Lyman-Alpha Forest, Dh Weinberg, S Burles, Rac Croft, R Dave, G Gomez, L Hernquist, N Katz, D Kirkman, S Liu, J Miralda-Escude, M Penttini, J Phillips, D Tytler, J Wright

Astronomy Department Faculty Publication Series

We outline the physical picture of the high-redshift Ly-alpha forest that has emerged from cosmological simulations, describe statistical characteristics of the forest that can be used to test theories of structure formation, present a preliminary comparison between simulation results and measurements from Keck HIRES spectra, and discuss a recent determination of the slope and amplitude of the linear mass power spectrum P(k) at z=2.5 from moderate resolution spectra. The physical picture is simple if each QSO spectrum is viewed as a continuous non-linear map of the line-of-sight density field rather than a collection of discrete absorption lines. The distribution of …

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Cosmology From The Structure Of The Lyman-Alpha Forest, R Croft, D Weinberg, N Katz, L Hernquist Jan 1998

Cosmology From The Structure Of The Lyman-Alpha Forest, R Croft, D Weinberg, N Katz, L Hernquist

Astronomy Department Faculty Publication Series

A convincing physical picture for the Lyman-alpha forest has emerged from simulations and related semi-analytic studies of structure formation models. Observations can be be used in the context of this picture to study cosmology using the structure of the forest. With the availability of well motivated predictions, not only has it become possible to test models directly, but the physical processes involved appear to be simple enough that we can attempt to reconstruct aspects of the underlying cosmology from observations. We briefly summarise the method of Croft et al (1997) (astro-ph/9708018) for recovering the primordial mass power spectrum …

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Simulating Gas At High Redshift, N Katz, L Hernquist, Dh Weinberg Jan 1998

Simulating Gas At High Redshift, N Katz, L Hernquist, Dh Weinberg

Astronomy Department Faculty Publication Series

We discuss simulations of gas at high redshift. We briefly review the methods used and the results for quasar absorption lines. We present gas mass functions and galaxy correlation functions for 5 different cosmological models. Galaxies should be detectable at redshifts greater than 2 by SKAI, and measurements of the gas maWe discuss simulations of gas at high redshift. We briefly review the methods used and the results for quasar absorption lines. We present gas mass functions and galaxy correlation functions for 5 different cosmological models. Galaxies should be detectable at redshifts greater than 2 by SKAI, and measurements of …

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