The radio properties of radio-quiet quasars     

Marek J. Kukula  James S. Dunlop  David H. Hughes  & Steve Rawlings 《Monthly notices of the Royal Astronomical Society》1998,297(2):366-382

Although radio-quiet quasars (RQQs) constitute ≳ 90 per cent of optically identified quasar samples, their radio properties are only poorly understood. In this paper we present the results of a multi-frequency VLA study of 27 low-redshift RQQs. We detect radio emission from 20 objects, half of which are unresolved (≤ 0.24 arcsec). In cases where significant structure can be resolved, double, triple and linear radio sources on scales of a few kpc are found. The radio emission (typically) has a steep spectrum (α ∼ 0.7, where S  ∝ ν^−α), and high brightness temperatures ( T _B ≥ 10⁵ K) are measured in some of the radio components. The RQQs form a natural extension to the radio luminosity–absolute magnitude distribution of nearby Seyfert 1s. We conclude that a significant fraction of the radio emission in RQQs originates in a compact nuclear source directly associated with the quasar. There are no significant differences between the radio properties of RQQs with elliptical hosts and those in disc galaxies within the current sample.  相似文献   

Collocation,dissipation and [overshoot] for time integration schemes in structural dynamics     

Hans M. Hilber  Thomas J. R. Hughes 《地震工程与结构动力学》1978,6(1):99-117

The concept of collocation, originally used by Wilson in the development of dissipative algorithms for structural dynamics, is systematically generalized and analysed. Optimal schemes within this class are developed and compared with a recently proposed family of dissipative algorithms, called a methods. The α methods are found to be superior on the basis of standard measures of dissipation and dispersion. It is pointed out that the tendency to overshoot is an important and independent factor which should be considered in an evaluation of an implicit scheme. The basis for studying overshoot is discussed and the optimal collocation and α methods are compared. It is found that pathological overshooting is an inherent property of collocation schemes, whereas the overshooting characteristics of the α methods are good.  相似文献   

Distribution surfaces in solvent extraction systems     

M.A. Hughes  S. Andersson  C. Forrest 《International Journal of Mineral Processing》1975,2(3):267-276

In the solvent extraction of metals, the distribution of the metal is usually dependent on the concentrations of several different species in the two phases. Despite this fact, distribution isotherms are usually presented as plots of C_o as a function of C_aq and these are used for a McCabe-Thiele type evaluation of a number of stages and flow rates, etc., each individual isotherm being valid only for one set of initial conditions; this treatment limits the possibilities of optimizing the system. In the present paper an extension of three-dimensional plots is suggested with special reference to the Cu/LIX64N extraction system. The distribution surface is defined and the general properties of the plots are outlined.A procedure is outlined whereby the surface can be represented on a two dimensional plot and a theoretical model for the surface can be superimposed.The paper also shows how distribution ratios (D values) are related to the surface.  相似文献   

On a Physically Realistic Fast Dynamo     

Kim  Eun-Jin  Hughes  D. W.  Soward  A. M. 《Studia Geophysica et Geodaetica》1998,42(3):335-342

As a step towards a physically realistic model of a fast dynamo, we study numerically a kinematic dynamo driven by convection in a rapidly rotating cylindrical annulus. Convection maintains the quasi-geostrophic balance whilst developing more complicated time-dependence as the Rayleigh number is increased. We incorporate the effects of Ekman suction and investigate dynamo action resulting from a chaotic flow obtained in this manner. We examine the growth rate as a function of magnetic Prandtl number Pm, which is proportional to the magnetic Reynolds number. Even for the largest value of Pm considered, a clearly identifiable asymptotic behaviour is not established. Nevertheless the available evidence strongly suggests a fast dynamo process.  相似文献   

Generalisation of a Nonlinear Dynamo Model     

Tanner  Sarah E. M.  Hughes  D. W. 《Studia Geophysica et Geodaetica》1998,42(3):343-349

In order to gain a better understanding of the physical processes underlying fast dynamo action it is instructive to investigate the structure of both the magnetic field and the velocity field after the dynamo saturates. Previously, computational results have been presented (Cattaneo, Hughes and Kim, 1996) that indicate, in particular, that Lagrangian chaos is suppressed in the dynamical regime of the dynamo. Here we extend their model by removing the assumption of neglecting the inertial term. This allows for an investigation into the effect of this term on the evolution of the dynamo via a comparison of the two models. Our results indicate that this term plays a crucial role in the physics of the dynamo.  相似文献   

Improved numerical dissipation for time integration algorithms in structural dynamics     

Hans M. Hilber  Thomas J. R. Hughes  Robert L. Taylor 《地震工程与结构动力学》1977,5(3):283-292

A new family of unconditionally stable one-step methods for the direct integration of the equations of structural dynamics is introduced and is shown to possess improved algorithmic damping properties which can be continuously controlled. The new methods are compared with members of the Newmark family, and the Houbolt and Wilson methods.  相似文献   

The use of ²¹⁰Pb as a heavy metal tracer in the Susquehanna River system     

Dale M Lewis 《Geochimica et cosmochimica acta》1977,41(11):1557-1564

Analysis of soil profiles and shallow ground water in the Susquehanna River basin, northeastern U.S.A., indicates that the atmospheric flux of ²¹⁰Pb is efficiently scavenged by the organic-rich horizons of the soils. This atmospherically supplied ²¹⁰Pb in soil profiles can only be lost from the system by soil erosion. Based on the annual sediment yield of the Susquehanna River and the excess ²¹⁰Pb concentration in particulate matter, a mean residence time of 2000 yr is calculated for metals similar to Pb in soil profiles.The West Branch of the Susquehanna River (WBSR) is strongly affected by acid mine drainage and is low in pH and high in dissolved ( <0.4 μm) ²¹⁰Pb, Fe and Mn. Along its course iron hydroxide is precipitating at a pH of between 4 and 4.5 and the ²¹⁰Pb supplied by the acid mine water is diminished by about 25% as a result of dilution. As the WBSR enters the Valley and Ridge Province of the Appalachians it has a ²¹⁰Pb concentration of ~ 0.2 dpm/l. At this juncture it receives a considerable influx of alkalinity from tributaries draining carbonate terranes, resulting in neutralization of the sulfuric acid and increase of the river pH to around 6.5–7. This pH adjustment is accompanied by the precipitation of Fe and Mn. Due to the slow rate of Mn removal from solution, the Mn precipitation extends a considerable distance down river from the point of acid neutralization. Analyses for ²¹⁰Pb in the river at points in or below the region of Mn precipitation show that ²¹⁰Pb is rapidly scavenged from solution onto suspended particles. From the data it is possible to calculate the removal rate of Pb from water in the presence of Fe and Mn hydroxides and other particles. At a pH of 4–4.5 Pb removal is nonexistent relative to the river flow rate, but at a pH of 6.5–7 the ²¹⁰Pb data indicate a residence time of <0.7 day for dissolved Pb.  相似文献   

Shark nets trap new bounty     

David Hughes 《Marine pollution bulletin》1974,5(9):133

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A broad-band spectroscopic search for CO line emission in HDF850.1: the brightest submillimetre object in the Hubble Deep Field-North     

J. Wagg  D. H. Hughes  I. Aretxaga  E. L. Chapin  J. S. Dunlop  E. Gaztañaga  M. Devlin 《Monthly notices of the Royal Astronomical Society》2007,375(2):745-752

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Variations in 24-μm morphologies among galaxies in the Spitzer Infrared Nearby Galaxies Survey: new insights into the Hubble sequence     

G. J. Bendo  D. Calzetti  C. W. Engelbracht  R. C. Kennicutt Jr    M. J. Meyer  M. D. Thornley  F. Walter  D. A. Dale  A. Li  E. J. Murphy 《Monthly notices of the Royal Astronomical Society》2007,380(4):1313-1334

To study the distribution of star formation and dust emission within nearby galaxies, we measured five morphological parameters in the 3.6- and 24-μm wavebands for 65 galaxies in the Spitzer Infrared Nearby Galaxies Survey (SINGS) and eight galaxies that were serendipitously observed by SINGS. The morphological parameters demonstrate strong variations along the Hubble sequence, including statistically significant differences between S0/a-Sab and Sc-Sd galaxies. Early-type galaxies are generally found to be compact, centralized, symmetric sources in the 24-μm band, while late-type galaxies are generally found to be extended, asymmetric sources. These results suggest that the processes that increase the real or apparent sizes of galaxies' bulges also lead to more centralized 24-μm dust emission. Several phenomena, such as strong nuclear star formation, Seyfert activity, or outer ring structures, may cause galaxies to deviate from the general morphological trends observed at 24 μm. We also note that the 24-μm morphologies of Sdm-Im galaxies are quite varied, with some objects appearing very compact and symmetric but others appearing diffuse and asymmetric. These variations reflect the wide variation in star formation in irregular galaxies as observed at other wavelengths. The variations in the 24-μm morphological parameters across the Hubble sequence mirror many of the morphological trends seen in other tracers of the ISM and in stellar emission. However, the 24-μm morphological parameters for the galaxies in this sample do not match the morphological parameters measured in the stellar wavebands. This implies that the distribution of dust emission is related to but not equivalent to the distribution of stellar emission.  相似文献