68,282 research outputs found
Extragalactic radio source evolution under the dual-population unification scheme
We show that a dual-population unification scheme provides a successful
paradigm with which to describe the evolution and beaming of all bright
extragalactic radio sources. The paradigm consists of two intrinsic
radio-source populations, based on the two distinct radio-galaxy morphologies
of Fanaroff-Riley classes I and II. These represent the `unbeamed' or `side-on'
parent populations of steep radio spectra; the `beamed' source types including
flat-spectrum quasars and BL Lac objects, arise through the random alignment of
their radio-axis to our line-of-sight where Doppler-beaming of the relativistic
radio jets produces highly anisotropic radio emission.Comment: 18 pages & 18 postscript figures, accepted for publication in MNRA
Self-assembled ErAs islands in GaAs for optical-heterodyne THz generation
We report photomixer devices fabricated on a material consisting of self-assembled ErAs islands in GaAs, which is grown by molecular beam epitaxy. The devices perform comparably and provide an alternative to those made from low-temperature-grown GaAs. The photomixer's frequency response demonstrates that the material is a photoconductor with subpicosecond response time, in agreement with time-resolved differential reflectance measurements. The material also provides the other needed properties such as high photocarrier mobility and high breakdown field, which exceeds 2×10^5 V/cm. The maximum output power before device failure at frequencies of 1 THz was of order 0.1 µW. This material has the potential to allow engineering of key photomixer properties such as the response time and dark resistance
The spectral characteristics of the 2dFGRS-NVSS galaxies
We have analysed the 2dF spectra of a sample of galaxies common to the 2dF
galaxy redshift survey (2dFGRS, Colless 1999) and the NRAO VLA sky survey
(NVSS, Condon et al. 1998). Our sample comprises 88 galaxies selected by Sadler
et al. (1999) from 30 2dFGRS fields observed in 1998. In this paper we discuss
how this and future, much larger, samples of 2dFGRS-NVSS galaxies can be
interpreted via analysis of those galaxies with strong narrow emission lines.
Using diagnostic line ratio measurements we confirm the majority of the eyeball
classifications of Sadler et al. (1999), although many galaxies show evidence
of being `composite' galaxies - a mixture of AGN plus starburst components.Comment: 11 pages, 5 figures, accepted for publication in PAS
Equivalence of two mathematical forms for the bound angular momentum of the electromagnetic field
It is shown that the mathematical form, obtained in a recent paper, for the
angular momentum of the electromagnetic field in the vicinity of electric
charge is equivalent to another form obtained previously by Cohen-Tannoudji,
Dupont-Roc and Gilbert. In this version of the paper an improved derivation is
given.Comment: 4 pages pdf, simpler derivatio
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