Cosmology with redshift surveys of radio sources

We use the K-z relation for radio galaxies to illustrate why it has proved difficult to obtain definitive cosmological results from studies based entirely on catalogues of the brightest radio sources, e.g. 3C. To improve on this situation we have been undertaking redshift surveys of complete samples drawn from the fainter 6C and 7C radio catalogues. We describe these surveys, and illustrate the new studies they are allowing. We also discuss our `filtered' 6C redshift surveys: these have led to the discovery of a radio galaxy at z=4.4, and are sensitive to similar objects at higher redshift provided the space density of these objects is not declining too rapidly with z. There is currently no direct evidence for a sharp decline in the space density of radio galaxies for z > 4, a result only barely consistent with the observed decline of flat-spectrum radio quasars.Comment: 8 pages Latex, To appear in the "Cosmology with the New Radio Surveys" Conference - Tenerife 13-15 January 199

A Combined EIS-NVSS Survey Of Radio Sources (CENSORS) III: Spectroscopic observations

The Combined EIS-NVSS Survey Of Radio Sources (CENSORS) is a 1.4GHz radio survey selected from the NRAO VLA Sky Survey (NVSS) and complete to a flux-density of 7.2mJy. It targets the ESO Imaging Survey (EIS) Patch D, which is a 3 by 2 square degree field centred on 09 51 36.0, -21 00 00 (J2000). This paper presents the results of spectroscopic observations of 143 of the 150 CENSORS sources. The primary motivation for these observations is to achieve sufficient spectroscopic completeness so that the sample may be used to investigate the evolution of radio sources. The observations result in secure spectroscopic redshifts for 63% of the sample and likely redshifts (based on a single emission line, for example) for a further 8%. Following the identification of the quasars and star-forming galaxies in the CENSORS sample, estimated redshifts are calculated for the remainder of the sample via the K-z relation for radio galaxies. Comparison of the redshift distribution of the CENSORS radio sources to distributions predicted by the various radio luminosity function evolution models of Dunlop & Peacock 1990, results in no good match. This demonstrates that this sample can be used to expand upon previous work in that field.Comment: Accepted for publication in MNRAS. This version has some reduced resolution figures and 13 associated gif files. A version with all figures incorporated (at full resolution) is available at http://www.roe.ac.uk/~pnb/papers/censors_spectro.pd

Direct imaging of a digital-micromirror device for configurable microscopic optical potentials

Programable spatial light modulators (SLMs) have significantly advanced the configurable optical trapping of particles. Typically, these devices are utilized in the Fourier plane of an optical system, but direct imaging of an amplitude pattern can potentially result in increased simplicity and computational speed. Here we demonstrate high-resolution direct imaging of a digital micromirror device (DMD) at high numerical apertures (NA), which we apply to the optical trapping of a Bose-Einstein condensate (BEC). We utilise a (1200 x 1920) pixel DMD and commercially available 0.45 NA microscope objectives, finding that atoms confined in a hybrid optical/magnetic or all-optical potential can be patterned using repulsive blue-detuned (532 nm) light with 630(10) nm full-width at half-maximum (FWHM) resolution, within 5% of the diffraction limit. The result is near arbitrary control of the density the BEC without the need for expensive custom optics. We also introduce the technique of time-averaged DMD potentials, demonstrating the ability to produce multiple grayscale levels with minimal heating of the atomic cloud, by utilising the high switching speed (20 kHz maximum) of the DMD. These techniques will enable the realization and control of diverse optical potentials for superfluid dynamics and atomtronics applications with quantum gases. The performance of this system in a direct imaging configuration has wider application for optical trapping at non-trivial NAs.Comment: 9 page

Measuring the Virial Temperature of Galactic Halos Through Electron Scattering of Quasar Emission Lines

Semi-analytic models of galaxy formation postulate the existence of virialized gaseous halos around galaxies at high redshifts. A small fraction of the light emitted by any high-redshift quasar is therefore expected to scatter off the free electrons in the halo of its host galaxy. The broadening of the scattered emission lines of the quasar can be used to measure the temperature of these electrons. For gas in virial equilibrium, the velocity width of the scattered line profile is larger by a factor of (m_p/m_e)^{1/2}=43 than the velocity dispersion of the host galaxy and reaches >10,000 km/s for the massive galaxies and groups in which bright quasars reside. In these systems, the scattered width exceeds the intrinsic width of the quasar lines and hence can be used to measure the virial temperature of the quasar host. The high degree of polarization of the scattered radiation can help filter out the extended scattered light from the central emission by the quasar and its host galaxy. The signal-to-noise ratio of the spectral broadening can be enhanced by matching the full spectrum of the scattered radiation to a template of the unscattered quasar spectrum. Although the central fuzz around low-redshift quasars is dominated by starlight, the fuzz around high-redshift quasars might be dominated by scattering before galaxies have converted most of their gas reservoirs into stars.Comment: 10 pages, 1 figures, submitted to ApJ Letter

The Global Star Formation Rate from the 1.4 GHz Luminosity Function

The decimetric luminosity of many galaxies appears to be dominated by synchrotron emission excited by supernova explosions. Simple models suggest that the luminosity is directly proportional to the rate of supernova explosions of massive stars averaged over the past 30 Myr. The proportionality may be used together with models of the evolving 1.4 GHz luminosity function to estimate the global star formation rate density in the era z < 1. The local value is estimated to be 0.026 solar masses per year per cubic megaparsec, some 50% larger than the value inferred from the Halpha luminosity density. The value at z ~ 1 is found to be 0.30 solar masses per year per cubic megaparsec. The 10-fold increase in star formation rate density is consistent with the increase inferred from mm-wave, far-infrared, ultra-violet and Halpha observations.Comment: 10 pages, 2 figures, Astrophysical Journal Letters (in press); new PS version has improved figure placemen

Multiplexed communication over a high-speed quantum channel

In quantum information systems it is of particular interest to consider the best way in which to use the non-classical resources consumed by that system. Quantum communication protocols are integral to quantum information systems and are amongst the most promising near-term applications of quantum information science. Here we show that a multiplexed, digital quantum communications system supported by comb of vacuum squeezing has a greater channel capacity per photon than a source of broadband squeezing with the same analogue bandwidth. We report on the time-resolved, simultaneous observation of the first dozen teeth in a 2.4 GHz comb of vacuum squeezing produced by a sub-threshold OPO, as required for such a quantum communications channel. We also demonstrate multiplexed communication on that channel

Avoiding spurious breaks in binned luminosity functions

We show that using either the method of Page & Carrera or the well-known $1/V_a$ method to construct the binned luminosity function (LF) of a flux limited sample of Active Galactic Nuclei (AGN) can produce an artificial flattening (or steepening in the case of negative evolution) of the binned LF for bins intersected by the flux cutoff of the sample. This effect is more pronounced for samples with steep and strongly evolving parent LFs but is still present even for non-evolving LFs. As a result of this distortion of the true LF, fitting a model LF to binned data may lead to errors in the estimation of the parameters and may even prompt the erroneous use of broken power law functions. We compute the expected positions of apparent breaks in the binned LF. We show that these spurious breaks in the binned LFs can be avoided if the binning is done in the flux--redshift plane instead of the typically used luminosity--redshift plane. Binning in the flux--redshift plane can be used in conjunction with the binning in the luminosity--redshift plane to test for real breaks in the binned LFs and to identify the features that are the result of binning biases. We illustrate this effect for most typical forms of luminosity dependence and redshift evolution and show how the proposed method helps address this problem. We also apply this method to the MOJAVE AGN sample and show that it eliminates an apparent break in the binned LF.Comment: 7 pages and 7 figures. Accepted for publication in Ap

Many-body theory of gamma spectra from positron-atom annihilation

A many-body theory approach to the calculation of gamma spectra of positron annihilation on many-electron atoms is developed. We evaluate the first-order correlation correction to the annihilation vertex and perform numerical calculations for the noble gas atoms. Extrapolation with respect to the maximal orbital momentum of the intermediate electron and positron states is used to achieve convergence. The inclusion of correlation corrections improves agreement with experimental gamma spectra.Comment: 25 pages, 9 figures, submitted to J. Phys.