779 research outputs found
Radio jet refraction in galactic atmospheres with static pressure gradients
A theory of double radio sources which have a 'Z' or 'S' morphology is proposed, based on the refraction of radio jets in the extended atmosphere of an elliptical galaxy. The model describes a collimated jet of supersonic material bending self-consistently under the influence of external static pressure gradients. Gravity and magnetic fields are neglected in the simplest case except insofar as they determine the static pressure distribution. The calculation is a straightforward extension of a method used to calculate a ram-pressure model for twin radio trails ('C' morphology). It may also be described as a continuous-jet version of a buoyancy model proposed in 1973. The model has the added virtue of invoking a galactic atmosphere similar to those already indicated by X-ray measurements of some other radio galaxies and by models for the collimation of other radio jets
Asymmetry of jets, lobe size and spectral index in radio galaxies and quasars
We investigate the correlations between spectral index, jet side and extent
of the radio lobes for a sample of nearby FRII radio galaxies. In
Dennett-Thorpe et al. (1997) we studied a sample of quasars and found that the
high surface brightness regions had flatter spectra on the jet side (explicable
as a result of Doppler beaming) whilst the extended regions had spectral
asymmetries dependent on lobe length. Unified schemes predict that asymmetries
due to beaming will be much smaller in narrow-line radio galaxies than in
quasars: we therefore investigate in a similar manner, a sample of radio
galaxies with detected jets. We find that spectral asymmetries in these objects
are uncorrelated with jet sidedness at all brightness levels, but depend on
relative lobe volume. Our results are not in conflict with unified schemes, but
suggest that the differences between the two samples are due primarily to power
or redshift, rather than to orientation. We also show directly that hotspot
spectra steepen as a function of radio power or redshift. Whilst a shift in
observed frequency due to the redshift may account for some of the steepening,
it cannot account for all of it, and a dependence on radio power is required.Comment: accepted for publication in MNRAS, 10 pages; typos/minor correctio
Ecological Niche Models and Coalescent Analysis of Gene Flow Support Recent Allopatric Isolation of Parasitoid Wasp Populations in the Mediterranean
Background: The integration of multiple complementary approaches is a powerful way to understand the processes of diversification and speciation. The parasitoid wasp Aphidius transcaspicus Telenga (Hymenoptera: Braconidae) is a parasitoid of Hyalopterus aphids across a wide geographic range. This species shows a remarkable degree of genetic structure among western, central, and eastern Mediterranean population clusters. In this paper we attempt to better characterize this genetic structure. Methodology/Principal Findings: We use a Bayesian coalescent analysis of gene flow under the Isolation with Migration model using mitochondrial and microsatellite markers together with climate-based ecological niche models to better understand the genetic structure of A. transcaspicus in the Mediterranean. The coalescent analysis revealed low levels of migration among western and eastern Mediterranean populations (Nm,1) that were not statistically distinguishable from zero. Niche models showed that localities within population clusters each occupy areas of continuously high environmental suitability, but are separated from each other by large regions of completely unsuitable habitat that could limit dispersal. Overall, environmental characteristics were similar among the population clusters, though significant differences did emerge. Conclusions/Significance: These results support contemporary allopatric isolation of Mediterranean populations of A
A relativistic model of the radio jets in NGC 315
We apply our intrinsically symmetrical, decelerating relativistic jet model
to deep VLA imaging of the inner 140 arcsec of the giant low-luminosity radio
galaxy NGC 315. An optimized model accurately fits the data in both total
intensity and linear polarization. We infer that the velocity, emissivity and
field structure in NGC 315 are very similar to those of the other
low-luminosity sources we have modelled, but that all of the physical scales
are larger by a factor of about 5. We derive an inclination to the line of
sight of 38 degrees for the jets. Where they first brighten, their on-axis
velocity is approximately v/c = 0.9. They decelerate to v/c = 0.4 between 8 and
18 kpc from the nucleus and the velocity thereafter remains constant. The speed
at the edge of the jet is roughly 0.6 of the on-axis value where it is best
constrained, but the transverse velocity profile may deviate systematically
from the Gaussian form we assume. The proper emissivity profile is split into
three power-law regions separated by shorter transition zones. In the first of
these, at 3 kpc (the flaring point) the jets expand rapidly at constant
emissivity, leading to a large increase in the observed brightness on the
approaching side. At 10 kpc, the emissivity drops abruptly by a factor of 2.
Where the jets are well resolved their rest-frame emission is
centre-brightened. The magnetic field is modelled as random on small scales but
anisotropic and we rule out a globally ordered helical configuration. To a
first approximation, the field evolves from a mixture of longitudinal and
toroidal components to predominantly toroidal, but it also shows variations in
structure along and across the jets, with a significant radial component in
places. Simple adiabatic models fail to fit the emissivity variations.Comment: 20 pages, 17 figures, MNRAS (in press
The discovery of diffuse steep spectrum sources in Abell 2256
Context: Hierarchical galaxy formation models indicate that during their
lifetime galaxy clusters undergo several mergers. Here we report on the
discovery of three diffuse radio sources in the periphery of Abell 2256, using
the GMRT.
Aims: The aim of the observations was to search for diffuse ultra-steep
spectrum radio sources within the galaxy cluster Abell 2256.
Methods: We have carried out GMRT 325 MHz radio continuum observations of
Abell 2256. V, R and I band images of the cluster were taken with the 4.2m WHT.
Results: We have discovered three diffuse elongated radio sources located
about 1 Mpc from the cluster center. Two are located to the west of the cluster
center, and one to the southeast. The sources have a measured physical extent
of 170, 140 and 240 kpc, respectively. The two western sources are also visible
in deep low-resolution 115-165 MHz WSRT images, although they are blended into
a single source. For the combined emission of the blended source we find an
extreme spectral index of -2.05\pm 0.14 between 140 and 351 MHz. The extremely
steep spectral index suggests these two sources are most likely the result of
adiabatic compression of fossil radio plasma due to merger shocks.
Conclusions: The discovery of the steep spectrum sources implies the
existence of a population of faint diffuse radio sources in (merging) clusters
with such steep spectra that they have gone unnoticed in higher frequency
(\gtrsim 1 GHz) observations. An exciting possibility therefore is that such
sources will determine the general appearance of clusters in low-frequency high
resolution radio maps as will be produced by for example LOFAR or LWA.Comment: 5 pages, 2 figures, accepted for publication in A&A on October 16,
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Radio Wavelength Constraints on the Sources of the Far Infrared Background
The cosmic far infrared background detected recently by the COBE-DIRBE team
is presumably due, in large part, to the far infrared (FIR) emission from all
galaxies. We take the well-established correlation between FIR and radio
luminosity for individual galaxies and apply it to the FIR background. We find
that these sources make up about half of the extragalactic radio background,
the other half being due to AGN. This is in agreement with other radio
observations, which leads us to conclude that the FIR-radio correlation holds
well for the very faint sources making up the FIR background, and that the FIR
background is indeed due to star-formation activity (not AGN or other possible
sources). If these star-forming galaxies have a radio spectral index between
0.4 and 0.8, and make up 40 to 60% of the extragalactic radio background, we
find that they have redshifts between roughly 1 and 2, in agreement with recent
estimates by Madau et al. of the redshift of peak star-formation activity. We
compare the observed extragalactic radio background to the integral over the
logN-logS curve for star-forming radio sources, and find that the slope of the
curve must change significantly below about 1 microjansky. At 1 microjansky,
the faint radio source counts predict about 25 sources per square arcminute,
and these will cause SIRTF to be confusion limited at 160micron.Comment: 10 pages including 1 figure, AASTeX, accepted by Ap
Multifrequency observations of the jets in the radio galaxy NGC 315
We present images of the jets in the nearby radio galaxy NGC 315 made with
the VLA at five frequencies between 1.365 and 5 GHz with resolutions between
1.5 and 45 arcsec FWHM. Within 15 arcsec of the nucleus, the spectral index of
the jets is 0.61. Further from the nucleus, the spectrum is flatter, with
significant transverse structure. Between 15 and 70 arcsec from the nucleus,
the spectral index varies from 0.55 on-axis to 0.44 at the edge. This spectral
structure suggests a change of dominant particle acceleration mechanism with
distance from the nucleus and the transverse gradient may be associated with
shear in the jet velocity field. Further from the nucleus, the spectral index
has a constant value of 0.47. We derive the distribution of Faraday rotation
over the inner +/-400 arcsec of the radio source and show that it has three
components: a constant term, a linear gradient (both probably due to our
Galaxy) and residual fluctuations at the level of 1 - 2 rad/m^2. These residual
fluctuations are smaller in the brighter (approaching) jet, consistent with the
idea that they are produced by magnetic fields in a halo of hot plasma that
surrounds the radio source. We model this halo, deriving a core radius of
approximately 225 arcsec and constraining its central density and
magnetic-field strength. We also image the apparent magnetic-field structure
over the first +/-200 arcsec from the nucleus.Comment: 20 pages, 17 figures, accepted for publication in MNRAS. Minor
corrections to match published version, including a short note on emission
from the background galaxy FGC011
Current-Controlled Negative Differential Resistance due to Joule Heating in TiO2
We show that Joule heating causes current-controlled negative differential
resistance (CC-NDR) in TiO2 by constructing an analytical model of the
voltage-current V(I) characteristic based on polaronic transport for Ohm's Law
and Newton's Law of Cooling, and fitting this model to experimental data. This
threshold switching is the 'soft breakdown' observed during electroforming of
TiO2 and other transition-metal-oxide based memristors, as well as a precursor
to 'ON' or 'SET' switching of unipolar memristors from their high to their low
resistance states. The shape of the V(I) curve is a sensitive indicator of the
nature of the polaronic conduction.Comment: 13 pages, 2 figure
Measuring the Reduced Shear
Neglecting the second order corrections in weak lensing measurements can lead
to a few percent uncertainties on cosmic shears, and becomes more important for
cluster lensing mass reconstructions. Existing methods which claim to measure
the reduced shears are not necessarily accurate to the second order when a
point spread function (PSF) is present. We show that the method of Zhang (2008)
exactly measures the reduced shears at the second order level in the presence
of PSF. A simple theorem is provided for further confirming our calculation,
and for judging the accuracy of any shear measurement method at the second
order based on its properties at the first order. The method of Zhang (2008) is
well defined mathematically. It does not require assumptions on the
morphologies of galaxies and the PSF. To reach a sub-percent level accuracy,
the CCD pixel size is required to be not larger than 1/3 of the Full Width at
Half Maximum (FWHM) of the PSF. Using a large ensemble (> 10^7) of mock
galaxies of unrestricted morphologies, we find that contaminations to the shear
signals from the noise of background photons can be removed in a well defined
way because they are not correlated with the source shapes. The residual shear
measurement errors due to background noise are consistent with zero at the
sub-percent level even when the amplitude of such noise reaches about 1/10 of
the source flux within the half-light radius of the source. This limit can in
principle be extended further with a larger galaxy ensemble in our simulations.
On the other hand, the source Poisson noise remains to be a cause of systematic
errors. For a sub-percent level accuracy, our method requires the amplitude of
the source Poisson noise to be less than 1/80 ~ 1/100 of the source flux within
the half-light radius of the source, corresponding to collecting roughly 10^4
source photons.Comment: 18 pages, 3 figures, 4 tables, minor changes from the previous
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