5,164 research outputs found
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
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.
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