5,917 research outputs found
Synchrotron Spectra and Ages of Compact Steep Spectrum Radio Sources
The high-frequency integrated spectra of Compact Steep Spectrum (CSS) sources
show breaks with a moderate spectral steepening well fitted by continuous
injection synchrotron spectra. In lobe-dominated CSS sources the radiative ages
deduced by the synchrotron theory are in the range of up to 0.1 Myears, if
equipartition magnetic fields are assumed. These radiative ages are well
correlated with the source size indicating that the CSS sources are young. In
order to maintain the frustration scenario, in which the sources' lifetimes are
about 10 Myears, their equipartition magnetic field would be systematically
decreased by a factor of more than 20. To complete the sample used in this
work, we conducted observations at 230 GHz with the IRAM 30-m telescope of
those sources which did not have such high-frequency observations up to now.Comment: 10 pages, 6 figures, 2 tables, accepted for publication in Astron. &
Astrophys.; typos corrected; gzipped postscript version also available at:
http://multivac.jb.man.ac.uk:8000/ceres/papers/papers.html
http://gladia.astro.rug.nl:8000/ceres/papers/papers.htm
The B3-Vla CSS sample. III: Evn & Merlin images at 18 cm
EVN and MERLIN observations at 18 cm are presented for 18 Compact
Steep--spectrum radio Sources (CSSs) from the B3-VLA CSS sample. These sources
were marginally resolved in previous VLA A-configuration observations at 4.9
and 8.4 GHz or had peculiar morphologies, two of them looking like core-jets.
The MERLIN images basically confirm the VLA structures at 8.4 GHz while the EVN
and/or the combined images reveal several additional details.Comment: 17 pages, many low resoltion figures, A&A accepted. A higher
resolution gzipped postscript file can be found at
http://www.ira.cnr.it/~ddallaca/h3443.ps.g
Measurements of the Higgs boson properties at the ATLAS experiment
The ATLAS and CMS Collaborations observed a new resonance
decaying in two photons, in four leptons via Z-boson pair production, and in two Wbosons decaying leptonically. With the total amount of LHC run-1 data, amounting to about 25 fb
â1 collected at âs = 7TeV and 8TeV, it is now possible to probe the fundamental properties of this particle: mass, spin, couplings, showing that it is compatible with a Standard Model Higgs boson. This document contains a review
of the most recent results from the ATLAS Collaboration
Parsec-scale morphology and spectral index distribution in faint high frequency peakers
We investigate the parsec-scale structure of 17 high frequency peaking radio
sources from the faint HFP sample. VLBA observations were carried out at two
adjacent frequencies, 8.4 and 15.3 GHz, both in the optically-thin part of the
spectrum, to obtain the spectral index information. We found that 64% of the
sources are resolved into subcomponents, while 36% are unresolved even at the
highest frequency. Among the resolved sources, 7 have a morphology and a
spectral index distribution typical of young radio sources, while in other 4
sources, all optically associated with quasars, the radio properties resemble
those of the blazar population. The equipartition magnetic field of the single
components are a few tens milliGauss, similar to the values found in the
hotspots of young sources with larger sizes. Such high magnetic fields cause
severe radiative losses, precluding the formation of extended lobe structures
emitting at centimeter wavelengths. The magnetic fields derived in the various
components of individual source are usually very different, indicating a non
self-similar source evolution, at least during the very first stages of the
source growth.Comment: 14 pages, 5 figures, accepted for publication in MNRA
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
Low Power Compact Radio Galaxies at High Angular Resolution
We present sub-arcsecond resolution multi-frequency (8 and 22 GHz)
VLA images of five low power compact (LPC) radio sources, and phase
referenced VLBA images at 1.6 GHz of their nuclear regions. At the VLA
resolution we resolve the structure and identify component positions and flux
densities. The phase referenced VLBA data at 1.6 GHz reveals flat-spectrum,
compact cores (down to a few milliJansky) in four of the five sources. The
absolute astrometry provided by the phase referencing allows us to identify the
center of activity on the VLA images. Moreover, these data reveal rich
structures, including two-sided jets and secondary components. On the basis of
the arcsecond scale structures and of the nuclear properties, we rule out the
presence of strong relativistic effects in our LPCs, which must be
intrinsically small (deprojected linear sizes <~ 10 kpc). Fits of continuous
injection models reveal break frequencies in the GHz domain, and ages in the
range 10^5-10^7 yrs. In LPCs, the outermost edge may be advancing more slowly
than in more powerful sources or could even be stationary; some LPCs might also
have ceased their activity. In general, the properties of LPCs can be related
to a number of reasons, including, but not limited to: youth, frustration, low
kinematic power jets, and short-lived activity in the radio.Comment: 15 pages, 9 .eps figures, accepted by A&
VLBI images at 327 MHz of compact steep spectrum and GHz-peaked spectrum sources from the 3C and PW samples
We present results on global very long baseline interferometry (VLBI) observations at 327 MHz of 18 compact steep-spectrum (CSS) and GHz-peaked spectrum (GPS) radio sources from the 3C and the Peacock & Wall catalogues. About 80 per cent of the sources have a 'double/triple' structure. The radio emission at 327 MHz is dominated by steep-spectrum extended structures, while compact regions become predominant at higher frequencies. As a consequence, we could unambiguously detect the core region only in three sources, likely due to self-absorption affecting its emission at this low frequency. Despite their low surface brightness, lobes store the majority of the source energy budget, whose correct estimate is a key ingredient in tackling the radio source evolution. Low-frequency VLBI observations able to disentangle the lobe emission from that of other regions are therefore the best way to infer the energetics of these objects. Dynamical ages estimated from energy budget arguments provide values between 2 Ă 103 and 5 Ă 104 yr, in agreement with the radiative ages estimated from the fit of the integrated synchrotron spectrum, further supporting the youth of these objects. A discrepancy between radiative and dynamical ages is observed in a few sources where the integrated spectrum is dominated by hotspots. In this case the radiative age likely represents the time spent by the particles in these regions, rather than the source age
The Radio Luminosity Function of the NEP Distant Cluster Radio Galaxies
A complete sample of 18 X-ray selected clusters of galaxies belonging to the
ROSAT North Ecliptic Pole (NEP) survey has been observed with the Very Large
Array at 1.4 GHz. These are the most distant clusters in the X-ray survey with
redshift in the range 0.3 < z < 0.8.Seventy-nine radio sources are detected
within half an Abell radius with an observed peak brightness >=0.17 mJy/beam,
except for three sources, belonging to the same cluster, which have a higher
peak brightness limit of 0.26 mJy/beam. The NEP field source counts are in good
agreement with the source counts of a comparison survey, the VLA-VIRMOS deep
field survey, indicating that the NEP sample is statistically complete.
Thirty-two out of the 79 sources are within 0.2 Abell radii, twenty-two of them
are considered cluster members based on spectroscopic redshifts or their
optical magnitude and morphological classification. The cluster radio galaxies
are used to construct the Radio Luminosity Function (RLF) of distant X-ray
selected clusters. A comparison with two nearby cluster RLFs shows that the NEP
RLF lies above the local ones, has a steeper slope at low radio powers (<=
10^(24) W/Hz) and shows no evidence for a break at about 6 X 10^(24) W/Hz which
is observed in the nearby cluster RLFs. We discuss briefly the origin and
possible explanations of the differences observed in the radio properties of
nearby and distant clusters of galaxies. The main result of this study is that
the RLF of the distant X-ray clusters is very different from that of the local
rich Abell clusters.Comment: 32 pages, 14 figures, Latex file with use of bib.tex. To appear in
Astronomy and Astrophysics, Main Journal. To appear in Astronomy and
Astrophysics Main Journal. To appear in Astronomy & Astrophysics, Main
Journa
Physical parameters in the hot spots and jets of Compact Symmetric Objects
We present a model to determine the physical parameters of jets and hot spots
of a sample of CSOs under very basic assumptions like synchrotron emission and
minimum energy conditions. Based on this model we propose a simple evolutionary
scenario for these sources assuming that they evolve in ram pressure
equilibrium with the external medium and constant jet power. The parameters of
our model are constrained from fits of observational data (radio luminosity,
hot spot radius and hot spot advance speed) versus projected linear size. From
these plots we conclude that CSOs evolve self-similarly and that their radio
luminosity increases with linear size along the first kiloparsec. Assuming that
the jets feeding CSOs are relativistic from both kinematical and
thermodynamical points of view, we use the values of the pressure and particle
number density within the hot spots to estimate the fluxes of momentum
(thrust), energy, and particles of these relativistic jets. The mean jet power
obtained in this way is within an order of magnitude that inferred for FRII
sources, which is consistent with CSOs being the possible precursors of large
doubles. The inferred flux of particles corresponds to, for a barionic jet,
about a 10% of the mass accreted by a black hole of at
the Eddington limit, pointing towards a very efficient conversion of accretion
flow into ejection, or to a leptonic composition of jets.Comment: 11 pages, 2 figures. Accepted for publication in Astrophysical
Journa
A Parsec Scale Accelerating Radio Jet in the Giant Radio Galaxy NGC315
Observations of the core of the giant radio galaxy NGC315 made with VLBI
interferometers are discussed in the context of a relativistic jet. The
sidedness asymmetry suggests Doppler favoritism from a relativistic jet. The
presence of moving features in the jet as well as jet counter--jet brightness
ratios hint at an accelerating, relativistic jet. An increasing jet velocity is
also supported by a comparison of the jet's observed properties with the
predictions of an adiabatic expansion model. On the parsec scale, the jet is
unpolarized at a wavelength of 6 cm to a very high degree in clear distinction
to the high polarization seen on the kiloparsec scale.Comment: 24 pages with 8 figures. ApJ in pres
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