Deceleration of Relativistic Radio Components and the morphologies of Gigahertz Peaked Spectrum Sources

A relativistic radio component, which moves in a direction close to the sky plane, will increase in flux density when it decelerates. This effect is the basis for the qualitative model for GPS galaxies we present in this paper, which can explain their low-variability convex spectrum, their compact double or compact symmetric morphology, and the lack of GPS quasars at similar redshifts. Components are expelled from the nucleus at relativistic speeds at a large angle to the line of sight, and are decelerated (eg. by ram-pressure or entrainment of the external gas) before contributing to a mini-lobe. The young components are Doppler boosted in the direction of motion but appear fainter for the observer. The non-relativistic mini-lobes dominate the structure and are responsible for the low variability in flux density and the convex radio spectrum as well as the compact double angular morphology. Had the same source been orientated at a small angle to the line of sight, the young components would be boosted in the observer's direction resulting in a flat and variable radio spectrum at high frequencies. Hence the characteristic convex spectrum of a GPS source would not be seen. These sources at small angles to the line of sight are probably identified with quasars, and are not recognized as GPS sources, but are embedded in the large population of flat spectrum variable quasars and BL Lac objects. This leads to a deficiency in GPS/CSOs identified with quasars.Comment: 11 pages, LaTeX, accepted by A&A 26/Jan/199

Statistical Properties of Radio Emission from the Palomar Seyfert Galaxies

We have carried out an analysis of the radio and optical properties of a statistical sample of 45 Seyfert galaxies from the Palomar spectroscopic survey of nearby galaxies. We find that the space density of bright galaxies (-22 mag <= M_{B_T} <= -18 mag) showing Seyfert activity is (1.25 +/- 0.38) X 10^{-3} Mpc^{-3}, considerably higher than found in other Seyfert samples. Host galaxy types, radio spectra, and radio source sizes are uncorrelated with Seyfert type, as predicted by the unified schemes for active galaxies. Approximately half of the detected galaxies have flat or inverted radio spectra, more than expected based on previous samples. Surprisingly, Seyfert 1 galaxies are found to have somewhat stronger radio sources than Seyfert 2 galaxies at 6 and 20 cm, particularly among the galaxies with the weakest nuclear activity. We suggest that this difference can be accommodated in the unified schemes if a minimum level of Seyfert activity is required for a radio source to emerge from the vicinity of the active nucleus. Below this level, Seyfert radio sources might be suppressed by free-free absorption associated with the nuclear torus or a compact narrow-line region, thus accounting for both the weakness of the radio emission and the preponderance of flat spectra. Alternatively, the flat spectra and weak radio sources might indicate that the weak active nuclei are fed by advection-dominated accretion disks.Comment: 18 pages using emulateapj5, 13 embedded figures, accepted by Ap

A new sample of faint Gigahertz Peaked Spectrum radio sources

The Westerbork Northern Sky Survey (WENSS) has been used to select a sample of Gigahertz Peaked Spectrum (GPS) radio sources at flux densities one to two orders of magnitude lower than bright GPS sources investigated in earlier studies. Sources with inverted spectra at frequencies above 325 MHz have been observed with the WSRT at 1.4 and 5 GHz and with the VLA at 8.6 and 15 GHz to select genuine GPS sources. This has resulted in a sample of 47 GPS sources with peak frequencies ranging from ~500 MHz to >15 GHz, and peak flux densities ranging from ~40 to ~900 mJy. Counts of GPS sources in our sample as a function of flux density have been compared with counts of large scale sources from WENSS scaled to 2 GHz, the typical peak frequency of our GPS sources. The counts can be made similar if the number of large scale sources at 2 GHz is divided by 250, and their flux densities increase by a factor of 10. On the scenario that all GPS sources evolve into large scale radio sources, these results show that the lifetime of a typical GPS source is ~250 times shorter than a typical large scale radio source, and that the source luminosity must decrease by a factor of ~10 in evolving from GPS to large scale radio source. However, we note that the redshift distributions of GPS and large scale radio sources are different and that this hampers a direct and straightforward interpretation of the source counts. Further modeling of radio source evolution combined with cosmological evolution of the radio luminosity function for large sources is required.Comment: LaTeX, 15 pages, 8 figs. To be published in A&AS. For more info see http://www.ast.cam.ac.uk/~snelle

An outline of polar spaces: basics and advances

This paper is an extended version of a series of lectures on polar spaces given during the workshop and conference 'Groups and Geometries', held at the Indian Statistical Institute in Bangalore in December 2012. The aim of this paper is to give an overview of the theory of polar spaces focusing on some research topics related to polar spaces. We survey the fundamental results about polar spaces starting from classical polar spaces. Then we introduce and report on the state of the art on the following research topics: polar spaces of infinite rank, embedding polar spaces in groups and projective embeddings of dual polar spaces

Income polarisation, expenditure and the Australian urban middle class

Hervorming Sociale Regelgevin

Measuring Cosmological Parameters with the JVAS and CLASS Gravitational Lens Surveys

The JVAS (Jodrell Bank-VLA Astrometric Survey) and CLASS (Cosmic Lens All-Sky Survey) are well-defined surveys containing about ten thousand flat-spectrum radio sources. For many reasons, flat-spectrum radio sources are particularly well-suited as a population from which one can obtain unbiased samples of gravitational lenses. These are by far the largest gravitational (macro)lens surveys, and particular attention was paid to constructing a cleanly-defined sample for the survey itself and for the underlying luminosity function. Here we present the constraints on cosmological parameters, particularly the cosmological constant, derived from JVAS and combine them with constraints from optical gravitational lens surveys, `direct' measurements of $\Omega_{0}$, $H_{0}$ and the age of the universe, and constraints derived from CMB anisotropies, before putting this final result into the context of the latest results from other, independent cosmological tests.Comment: LaTeX, 9 pages, 6 PostScript figures, uses texas.sty. To appear in the Proceedings of the 19th Texas Symposium on Relativistic Astrophysics and Cosmology (CD-ROM). Paper version available on request. Actual poster (A0 and A4 versions) available from http://multivac.jb.man.ac.uk:8000/helbig/research/publications/info/ texas98.htm

The new two-image gravitational lens system CLASS B2319+051

We report the discovery of a new two-image gravitational lens system from the Cosmic Lens All-Sky Survey, CLASS B2319+051. Radio imaging with the Very Large Array (VLA) and Multi-Element Radio-Linked Interferometer Network (MERLIN) shows two compact components with a flux density ratio of 5:1, separated by 1.36 arcsec. Observations with the Very Long Baseline Array (VLBA) resolve each of the radio components into a pair of parity-reversed subcomponents. Hubble Space Telescope (HST) observations with the Near-Infrared Camera and Multi-Object Spectrometer (NICMOS) show a bright elliptical galaxy (G1) coincident with the radio position, and a second irregular galaxy (G2) 3.4 arcsec to the northwest. Previous spectroscopic studies have indicated that these galaxies are at different redshifts: z(G1) = 0.624, z(G2) = 0.588. Infrared counterparts to the lensed radio components are not detected in the NICMOS image, and the source redshift has not yet been determined. Preliminary mass modeling based on the VLBA subcomponent data indicates that the lensing potential includes a strong external shear contribution. A VLA monitoring program is currently being undertaken to measure the differential time delay.Comment: 16 pages, 7 figs, several typos corrected, AJ in press (August 2001