5,184 research outputs found
Extended emission around GPS radio sources
Extended radio emission detected around a sample of GHz Peaked Spectrum (GPS)
radio sources is discussed. Evidence for extended emission which is related to
the GPS source is found in 6 objects out of 33. Three objects are associated
with quasars with core-jet pc-scale morphology, and three are identified with
galaxies with symmetric (CSO) radio morphology. We conclude that the core-jet
GPS quasars are likely to be beamed objects with a continuous supply of energy
from the core to the kpc scale. It is also possible that low surface brightness
extended radio emission is present in other GPS quasars but the emission is
below our detection limit due to the high redshifts of the objects. On the
other hand, the CSO/galaxies with extended large scale emission may be
rejuvenated sources where the extended emission is the relic of previous
activity. In general, the presence of large scale emission associated with GPS
galaxies is uncommon, suggesting that in the context of the recurrent activity
model, the time scale between subsequent bursts is in general longer than the
radiative lifetime of the radio emission from the earlier activity.Comment: 18 paged, 18 figures, accepted for publication on A&
Interference of Conversion and Bremsstrahlung Amplitudes in the Decay K_L -> mu^+ mu^- gamma
In the region of large mu^+ mu^- invariant mass, the decay spectrum of K_L ->
mu^+ mu^- gamma deviates from the Dalitz pair spectrum, as a result of
interference between conversion (K_L -> gamma^* gamma -> mu^+ mu^- gamma) and
bremsstrahlung amplitudes. The latter is proportional to the K_L -> mu^+ mu^-
matrix element, whose 2 gamma-absorptive part appears to dominate the observed
K_L -> mu^+ mu^- decay rate. We examine the extent to which a scrutiny of the
K_L -> mu^+ mu^- gamma spectrum in the end-point region could provide evidence
on the real part of the K_L -> mu^+ mu^- amplitude. As a by-product, we obtain
the absorptive part of the K_L -> gamma^* gamma form factor, using data on the
K_L -> pi^+ pi^- gamma spectrum.Comment: 7 pages, 4 figure
The lives of FR I radio galaxies
After a brief introduction to the morphological properties of FRI radio
sources, we discuss the possibility that FRI jets are relativistic at their
bases and decelerate quickly to non-relativistic velocities. From two-frequency
data we determine spectral index distributions and consequently the ages of FRI
sources. We show that in the large majority of cases synchrotron theory
provides unambiguous and plausible answers; in a few objects re-acceleration of
electrons may be needed. The derived ages are of the order 10^7-10^8 years, 2-4
times larger than the ages inferred from dynamical arguments and a factor 5-10
larger than the ages of FRII sources. The linear sizes of FRI and FRII sources
make it unlikely that many FRII's evolve into FRI's. A brief discussion is
given of the possibility that radio sources go through different cycles of
activity.Comment: 19 pages, including 13 figures, to appear in `Life Cycles of Radio
Galaxies', ed. J. Biretta et al., New Astronomy Review
Crystallization in a model glass: influence of the boundary conditions
Using molecular dynamics calculations and the Voronoi tessellation, we study
the evolution of the local structure of a soft-sphere glass versus temperature
starting from the liquid phase at different quenching rates. This study is done
for different sizes and for two different boundary conditions namely the usual
cubic periodic boundary conditions and the isotropic hyperspherical boundary
conditions for which the particles evolve on the surface of a hypersphere in
four dimensions. Our results show that for small system sizes, crystallization
can indeed be induced by the cubic boundary conditions. On the other hand we
show that finite size effects are more pronounced on the hypersphere and that
crystallization is artificially inhibited even for large system sizes.Comment: 11 pages, 2 figure
HST\/NICMOS observations of the host galaxies of powerful radio sources: Does size matter?
We present near-infrared J and K band imaging of a sample of powerful radio source host galaxies with the Hubble Space Telescope NICMOS2 camera. These sources have been selected on their double lobed radio structure, and include a wide range of projected radio source sizes. The largest projected linear sizes range from the compact Gigahertz Peaked Spectrum (GPS, 20 kpc) classical doubles (FR II radio sources). We investigate the dependence of host galaxy properties (including near-IR surface brightness profiles) on radio source size, using both our own and published data. The absolute magnitudes and surface brightness profiles are consistent with the host galaxies being regular giant elliptical galaxies rather than Brightest Cluster Galaxies (BCGs). We find that the GPS, CSS, and FR II host galaxies are a uniform class of objects, consistent with a scenario in which a powerful radio source evolves along this size sequence
Induced Compton Scattering in Gigahertz Peak Spectrum Sources
We revisit the shocked shell model for the class of Active Galactic Nuclei
known as Gigahertz Peak Spectrum sources, incorporating new observational data
on the radiation brightness temperatures. We argue that in addition to
free-free absorption, induced Compton scattering will also have an important
effect in forming the ~GHz peak and in shaping the radio spectra that
characterize these sources. Indeed, our arguments suggest that GPS sources may
provide the first real evidence for the role of induced Compton scattering in
extragalactic radio sources.Comment: 12 pages, 1 figure, AAS LaTeX style with epsf, to appear in ApJ
Letter
A complete sample of GHz-Peaked-Spectrum radio sources and its radio properties
We define a complete sample of thirty-three GHz-Peaked-Spectrum (GPS) radio
sources based on their spectral properties. We present measurements of the
radio spectra and polarization of the complete sample and a list of additional
GPS sources which fail one or more criteria to be included in the complete
sample. The majority of the data have been obtained from quasi-simultaneous
multi-frequency observations at the Very Large Array (VLA) during 3 observing
sessions. Low frequency data from the Westerbork Synthesis Radio Telescope
(WSRT) and from the literature have been combined with the VLA data in order to
better define the spectral shape.
The objects presented here show a rather wide range of spectral indices at
high and low frequencies, including a few cases where the spectral index below
the turnover is close to the theoretical value of 2.5 typical of self-absorbed
incoherent synchrotron emission. Faint and diffuse extended emission is found
in about 10% of the sources. In the majority of the GPS sources, the fractional
polarization is found to be very low, consistent with the residual instrumental
polarization of 0.3%.Comment: 23 pages, 16 figures, uses psfig.sty, A&AS in pres
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