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Search Results: 1 - 10 of 461495 matches for " A. Peck "
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A Day in the Life of Fish Larvae: Modeling Foraging and Growth Using Quirks
Klaus B. Huebert, Myron A. Peck
PLOS ONE , 2014, DOI: 10.1371/journal.pone.0098205
Abstract: This article introduces “Quirks,” a generic, individual-based model synthesizing over 40 years of empirical and theoretical insights into the foraging behavior and growth physiology of marine fish larvae. In Quirks, different types of larvae are defined by a short list of their biological traits, and all foraging and growth processes (including the effects of key environmental factors) are modeled following one unified set of mechanistic rules. This approach facilitates ecologically meaningful comparisons between different species and environments. We applied Quirks to model young exogenously feeding larvae of four species: 5.5-mm European anchovy (Engraulis encrasicolus), 7-mm Atlantic cod (Gadus morhua), 13-mm Atlantic herring (Clupea harengus), and 7-mm European sprat (Sprattus sprattus). Modeled growth estimates explained the majority of variability among 53 published empirical growth estimates, and displayed very little bias: 0.65%±1.2% d?1 (mean ± standard error). Prey organisms of ~67% the maximum ingestible prey length were optimal for all larval types, in terms of the expected ingestion per encounter. Nevertheless, the foraging rate integrated over all favorable prey sizes was highest when smaller organisms made up >95% of the prey biomass under the assumption of constant normalized size spectrum slopes. The overall effect of turbulence was consistently negative, because its detrimental influence on prey pursuit success exceeded its beneficial influence on prey encounter rate. Model sensitivity to endogenous traits and exogenous environmental factors was measured and is discussed in depth. Quirks is free software and open source code is provided.
Evidence for a Circumnuclear Disk in 1946+708
A. B. Peck,G. B. Taylor
Physics , 2001, DOI: 10.1086/321699
Abstract: A number of extragalactic radio sources which exhibit symmetric jets on parsec scales have now been found to have neutral hydrogen absorption at or near the systemic velocities of their host galaxies. Understanding the spatial distribution and kinematics of the HI detected toward the central parsecs of these sources provides an important test of unified schemes for AGN. We present results of Global VLBI Network observations of the redshifted 21 cm HI line toward the Compact Symmetric Object 1946+708 (z=0.101). We find significant structure in the gas on parsec scales. The peak column density of the HI (N_HI~3x10^23 cm^-2(Ts/8000K)) occurs near the center of activity of the source, as does the highest velocity dispersion (FWHM 350 to 400 km/s). There is also good evidence for a torus of ionized gas with column density 7x10^22 cm^-2}. The jets in 1946+708 exhibit bi-directional motion measurable on timescales of a few years. The resulting unique information about the geometry of the continuum source greatly assists in the interpretation of the gas distribution, which is strongly suggestive of a circumnuclear torus of neutral atomic and ionized material with one or more additional compact clumps of gas along the line of sight to the approaching jet.
Identifying Compact Symmetric Objects in the Southern Sky
G. B. Taylor,A. B. Peck
Physics , 2003, DOI: 10.1086/378267
Abstract: We present results of multifrequency polarimetric VLBA observations of 20 compact radio sources. The observations represent the northern and southern extensions of a large survey undertaken to identify Compact Symmetric Objects (CSOs) Observed in the Northern Sky (COINS). CSOs are young radio galaxies whose jet axes lie close to the plane of the sky, and whose appearance is therefore not dominated by relativistic beaming effects. The small linear sizes of CSOs make them valuable for studies of both the evolution of radio galaxies and testing unified schemes for active galactic nuclei (AGN). In this paper we report on observations made of 20 new CSO candidates discovered in the northern and southern extremities of the VLBA Calibrator Survey. We identify 4 new CSOs, and discard 12 core-jet sources. The remaining 4 sources remain candidates pending further investigation. We present continuum images at 5 GHz and 15 GHz and, where relevant, images of the polarized flux density and spectral index distributions for the 8 new CSOs and CSO candidates.
The COINS Sample - VLBA Identifications of Compact Symmetric Objects
A. B. Peck,G. B. Taylor
Physics , 1999, DOI: 10.1086/308746
Abstract: We present results of multifrequency polarimetric VLBA observations of 34 compact radio sources. The observations are part of a large survey undertaken to identify CSOs Observed in the Northern Sky (COINS). Compact Symmetric Objects (CSOs) are of particular interest in the study of the physics and evolution of active galaxies. Based on VLBI continuum surveys of ~2000 compact radio sources, we have defined a sample of 52 CSOs and CSO candidates. In this paper, we identify 18 previously known CSOs, and introduce 33 new CSO candidates. We present continuum images at several frequencies and, where possible, images of the polarized flux density and spectral index distributions for the 33 new candidates and one previously known but unconfirmed source. We find evidence to support the inclusion of 10 of these condidates into the class of CSOs. Thirteen candidates, including the previously unconfirmed source, have been ruled out. Eleven sources require further investigation. The addition of the 10 new confirmed CSOs increases the size of this class of objects by 50%.
Polarization Limits in Compact Symmetric Objects
A. B. Peck,G. B. Taylor
Physics , 2000,
Abstract: We present results of multifrequency polarimetric VLBA observations of 34 compact radio sources. The observations are part of a large survey undertaken to identify CSOs Observed in the Northern Sky (COINS). Based on VLBI continuum surveys of ~2000 compact radio sources, we have defined a sample of 52 CSOs and CSO candidates. Positive identification of CSOs is contingent upon acquiring multi-frequency observations in order to correctly identify the core of the source, which is expected to have a strongly inverted spectrum. We also expect CSOs to exhibit very little polarized flux. Despite the fact that synchrotron emission is intrinsically highly polarized, less than 0.5% fractional polarization is seen in low resolution studies of CSOs at frequencies up to 5 GHz. One possible explanation for the low observed linear polarization from CSOs is that their radiation is depolarized as it passes through a magnetized plasma associated with a circumnuclear torus. This interpretation is consistent with the unified scheme of AGN, and also with the recent detections of free-free and HI absorption in CSOs. Here we present limits on the polarized flux density at 8.4 GHz and ~1 mas resolution for 21 CSOs and candidates in the COINS sample.
Global VLBI Observations of HI Absorption toward NGC 3894
A. B. Peck,G. B. Taylor
Physics , 2000,
Abstract: One of the most important problems in the study of AGN is understanding the detailed geometry, physics, and evolution of the central engines and their environments. The leading models involve an accretion disk and torus around a central black hole. Much of this torus should be comprised of atomic gas, detectable in absorption toward the bright inner radio jets. In the last few years, a number of compact symmetric radio sources have been found to exhibit HI absorption, at or near the systemic velocity, toward the central parsecs. Understanding the kinematics of the HI detected toward the central parsecs of these sources will provide an important test of this model and of unified schemes for AGN. We present results of Global VLBI Network observations at 1.4 GHz toward the active nucleus of the nearby elliptical galaxy NGC 3894 (a.k.a. 1146+596, z=0.01068). The center of activity in this source and the orientation of the jets with respect to our line of sight have been determined using VLBI studies of the proper motions of jet components. The 21 cm atomic hydrogen line is seen in absorption slightly redshifted with respect to the systemic velocity toward the core, jet, and counterjet of this source. The structure of the HI in this source is complicated. We find several distinct components present along the lines of sight to the approaching and receding jets, making interpretation challenging.
New VLBA Identifications of Compact Symmetric Objects
A. B. Peck,G. B. Taylor
Physics , 1999, DOI: 10.1016/S1387-6473(01)00197-X
Abstract: The class of radio sources known as Compact Symmetric Objects (CSOs) is of particular interest in the study of the evolution of radio galaxies. CSOs are thought to be young (probably ~10^4 years), and a very high fraction of them exhibit HI absorption toward the central parsecs. The HI, which is thought to be part of a circumnuclear torus of accreting gas, can be observed using the VLBA with high enough angular resolution to map the velocity field of the gas. This velocity field provides new information on the accretion process in the central engines of these young sources. We have identified 9 new CSOs from radio continuum observations for the VLBA Calibrator Survey, increasing the number of known CSOs by almost 50%.
Imaging HI Absorption toward Symmetric Radio Galaxies - Evidence for a Circumnuclear Torus
A. B. Peck,G. B. Taylor
Physics , 1999, DOI: 10.1016/S1387-6473(01)00192-0
Abstract: Recent VLBI observations have identified several compact radio sources which have symmetric structures on parsec scales, and exhibit HI absorption which appears to be associated with the active nucleus. These sources are uniquely well suited to investigations into the physics of the central engines, in particular to studies of the kinematics of the gas within 100 pc of the core. In these compact sources, it is reasonable to assume that this circumnuclear material is accreting onto, and ``feeding'', the central engine. We present results of HI imaging studies of 3 symmetric radio galaxies which show evidence of a circumnuclear torus.
Another Look at Becker’s Irrational Consumer  [PDF]
Richard M. Peck
Theoretical Economics Letters (TEL) , 2012, DOI: 10.4236/tel.2012.23047
Abstract: Becker’s paper \"Irrational Consumers and Economic Theory\" ([1]) is a classic. This paper shows how to parameterize the process of selecting points randomly on a budget set. This parameterization also simplifies the proof that average demand curves are downward sloping and satisfy the weak axiom of revealed preference. In addition, we show that the probability distribution of random choices does not need to be restricted to a uniform distribution which Becker assumes. In fact, the distribution can be arbitrary.
The Tin-Man and the TAM – A Journey Into M-Learning in the Land of Aus
Blake Peck a *, Cecil Deans a , Lynette Stockhausen a
World Journal on Educational Technology , 2010,
Abstract: Within the past few years the virtues and pitfalls of Podcasting specifically within the higher education environment have been extolled. However, there is little, if any, discussion of how academic staff have undergone this period of transformation in an era of technological adoption and sweeping pedagogic change. Using the Technology Adoption Model (Davis 1989) as a theoretical platform and the characters from the classic film the Wizard of Oz as signposts, this paper explores and describes a staff development journey that introduced M-Learning pedagogies. More specifically the way in which Podcast technology was introduced in an undergraduate Bachelor of Nursing program conducted in a regional University in Victoria, Australia. The journey’s resultant destination reveals that courage; passion and an openness to try something new are essential for the successful introduction of new electronic pedagogies by academics.
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