1,296 research outputs found
Relativistic jet models for two low-luminosity radio galaxies: evidence for backflow?
We show that asymmetries in total intensity and linear polarization between
the radio jets and counter-jets in two lobed Fanaroff-Riley Class I (FR I)
radio galaxies, B2 0206+35 (UGC 1651) and B2 0755+37 (NGC 2484), can be
accounted for if these jets are intrinsically symmetrical, with decelerating
relativistic outflows surrounded by mildly relativistic backflows. Our
interpretation is motivated by sensitive, well-resolved Very Large Array
imaging which shows that both jets in both sources have a two-component
structure transverse to their axes. Close to the jet axis, a centrally-darkened
counter-jet lies opposite a centrally-brightened jet, but both are surrounded
by broader collimated emission that is brighter on the counter-jet side. We
have adapted our previous models of FR I jets as relativistic outflows to
include an added component of symmetric backflow. We find that the observed
radio emission, after subtracting contributions from the extended lobes, is
well described by models in which decelerating outflows with parameters similar
to those derived for jets in plumed FR I sources are surrounded by backflows
containing predominantly toroidal magnetic fields. These return to within a few
kpc of the galaxies with velocities of roughly 0.25c and radiate with a
synchrotron spectral index close to 0.55. We discuss whether such backflow is
to be expected in lobed FR I sources and suggest ways in which our hypothesis
can be tested by further observations.Comment: 22 pages, 17 figures, accepted for publication in MNRA
Series-produced Helium II Cryostats for the LHC Magnets: Technical Choices, Industrialisation, Costs
Assembled in 8 continuous segments of approximately 2.7 km length each, the He II cryostats for the 1232 cryodipoles and 474 Short Straight Sections (SSS housing the quadrupoles) must fulfil tight technical requirements. They have been produced by industry in large series according to cost-effective industrial production methods to keep expenditure within the financial constraints of the project and assembled under contract at CERN. The specific technical requirements of the generic systems of the cryostat (vacuum, cryogenic, electrical distribution, magnet alignment) are briefly recalled, as well as the basic design choices leading to the definition of their components (vacuum vessels, thermal shielding, supporting systems). Early in the design process emphasis was placed on the feasibility of manufacturing techniques adequate for large series production of components, optimal tooling for time-effective assembly methods, and reliable quality assurance systems. An analytical review of the costs of the cryostats from component procurement to final assembly, tests and interconnection in the machine is presented and compared with initial estimates, together with an appraisal of the results and lessons learned
Structure of the magnetoionic medium around the FR Class I radio galaxy 3C 449
The goal of this work is to constrain the strength and structure of the
magnetic field associated with the environment of the radio source 3C 449,
using observations of Faraday rotation, which we model with a structure
function technique and by comparison with numerical simulations. We assume that
the magnetic field is a Gaussian, isotropic random variable and that it is
embedded in the hot intra-group plasma surrounding the radio source. For this
purpose, we present detailed rotation measure images for the polarized radio
source 3C 449, previously observed with the Very Large Array at seven
frequencies between 1.365 and 8.385 GHz. We quantify the statistics of the
magnetic-field fluctuations by deriving rotation measure structure functions,
which we fit using models derived from theoretical power spectra. We quantify
the errors due to sampling by making multiple two-dimensional realizations of
the best-fitting power spectrum.We also use depolarization measurements to
estimate the minimum scale of the field variations. We then make
three-dimensional models with a gas density distribution derived from X-ray
observations and a random magnetic field with this power spectrum. Under these
assumptions we find that both rotation measure and depolarization data are
consistent with a broken power-law magnetic-field power spectrum, with a break
at about 11 kpc and slopes of 2.98 and 2.07 at smaller and larger scales
respectively. The maximum and minimum scales of the fluctuations are around 65
and 0.2 kpc, respectively. The average magnetic field strength at the cluster
centre is 3.5 +/-1.2 micro-G, decreasing linearly with the gas density within
about 16 kpc of the nucleus.Comment: 19 pages; 14 figures; accepted for publication on A&A. For a high
quality version use ftp://ftp.eso.org/pub/general/guidetti
Multifrequency VLA observations of the FR I radio galaxy 3C 31: morphology, spectrum and magnetic field
We present high-quality VLA images of the FR I radio galaxy 3C 31 in the
frequency range 1365 to 8440 MHz with angular resolutions from 0.25 to 40
arcsec. Our new images reveal complex, well resolved filamentary substructure
in the radio jets and tails. We also use these images to explore the spectral
structure of 3C 31 on large and small scales. We infer the apparent magnetic
field structure by correcting for Faraday rotation. Some of the intensity
substructure in the jets is clearly related to structure in their apparent
magnetic field: there are arcs of emission where the degree of linear
polarization increases, with the apparent magnetic field parallel to the ridges
of the arcs. The spectral indices are significantly steeper (0.62) within 7
arcsec of the nucleus than between 7 and 50 arcsec (0.52 - 0.57). The spectra
of the jet edges are also slightly flatter than the average for their
surroundings. At larger distances, the jets are clearly delimited from
surrounding larger-scale emission both by their flatter radio spectra and by
sharp brightness gradients. The spectral index of 0.62 in the first 7 arcsec of
3C 31's jets is very close to that found in other FR I galaxies where their
jets first brighten in the radio and where X-ray synchrotron emission is most
prominent. Farther from the nucleus, where the spectra flatten, X-ray emission
is fainter relative to the radio. The brightest X-ray emission from FR I jets
is therefore not associated with the flattest radio spectra, but with a
particle-acceleration process whose characteristic energy index is 2.24. The
spectral flattening with distance from the nucleus occurs where our
relativistic jet models require deceleration, and the flatter-spectra at the
jet edges may be associated with transverse velocity shear. (Slightly abridged)Comment: 17 pages, 13 figures, accepted for publication in MNRA
The ATESP 5 GHz radio survey. II. Physical properties of the faint radio population
One of the most debated issues about sub-mJy radio sources, which are
responsible for the steepening of the 1.4 GHz source counts, is the origin of
their radio emission. Particularly interesting is the possibility of combining
radio spectral index information with other observational properties to assess
whether the sources are triggered by star formation or nuclear activity. The
aim of this work is to study the optical and near infrared properties of a
complete sample of 131 radio sources with S>0.4 mJy, observed at both 1.4 and 5
GHz as part of the ATESP radio survey. We use deep multi-colour (UBVRIJK)
images, mostly taken in the framework of the ESO Deep Public Survey, to
optically identify and derive photometric redshifts for the ATESP radio
sources. Deep optical coverage and extensive colour information are available
for 3/4 of the region covered by the radio sample. Typical depths of the images
are U~25, B~26, V~25.4, R~25.5, I~24.3, 19.5<K_s<20.2, J<22.2. Optical/near
infrared counterparts are found for ~78% (66/85) of the radio sources in the
region covered by the deep multi-colour imaging, and for 56 of these reliable
estimates of the redshift and type are derived. We find that many of the
sources with flat radio spectra are characterised by high radio-to-optical
ratios (R>1000), typical of classical powerful radio galaxies and quasars.
Flat-spectrum sources with low R values are preferentially identified with
early type galaxies, where the radio emission is most probably triggered by
low-luminosity active galactic nuclei. Considering both early type galaxies and
quasars as sources with an active nucleus, such sources largely dominate our
sample (78%). Flat-spectrum sources associated with early type galaxies are
quite compact (d<10-30 kpc), suggesting core-dominated radio emission.Comment: 15 pages, 13 figures, accepted for pubblication in A&
The ATESP Radio Survey II. The Source Catalogue
This paper is part of a series reporting the results of the Australia
Telescope ESO Slice Project (ATESP) radio survey obtained at 1400 MHz with the
Australia Telescope Compact Array (ATCA) over the region covered by the ESO
Slice Project (ESP) galaxy redshift survey. The survey consists of 16 radio
mosaics with ~8"x14" resolution and uniform sensitivity (1sigma noise level ~79
microJy) over the whole area of the ESP redshift survey (~26 sq. degrees at
decl. -40 degr). Here we present the catalogue derived from the ATESP survey.
We detected 2960 distinct radio sources down to a flux density limit of ~0.5
mJy (6sigma), 1402 being sub-mJy sources. We describe in detail the procedure
followed for the source extraction and parameterization. The internal accuracy
of the source parameters was tested with Monte Carlo simulations and possible
systematic effects (e.g. bandwidth smearing) have been quantified.Comment: 14 pages, 14 Postscript figures, Accepted for publication in A&A
Suppl. Corrected typos and added Journal Referenc
Are radio galaxies and quiescent galaxies different? Results from the analysis of HST brightness profiles
We present a study of the optical brightness profiles of early type galaxies,
using a number of samples of radio galaxies and optically selected elliptical
galaxies. For the radio galaxy samples--B2 of Fanaroff-Riley type I and 3C of
Fanaroff-Riley type II-- we determined a number of parameters that describe a
"Nuker-law" profile, which were compared with those already known for the
optically selected objects. We find that radio active galaxies are always of
the "core" type (i.e. an inner Nuker law slope gamma < 0.3). However, there are
core-type galaxies which harbor no significant radio source and which are
indistinguishable from the radio active galaxies. We do not find any radio
detected galaxy with a power law profile (gamma > 0.5). This difference is not
due to any effect with absolute magnitude, since in a region of overlap in
magnitude the dichotomy between radio active and radio quiescent galaxies
remains. We speculate that core-type objects represent the galaxies that have
been, are, or may become, radio active at some stage in their lives; active and
non-active core-type galaxies are therefore identical in all respects except
their eventual radio-activity: on HST scales we do not find any relationship
between boxiness and radio-activity. There is a fundamental plane, defined by
the parameters of the core (break radius r_b and break brightness mu_b), which
is seen in the strong correlation between r_b and mu_b. The break radius is
also linearly proportional to the optical Luminosity in the band. Moreover,
for the few galaxies with an independently measured black hole mass, the break
radius turns out to be tightly correlated with M_{BH}. The black hole mass
correlates even better with the combination of fundamental plane parameters r_b
and mu_b, which represents the central velocity dispersion.Comment: Accepted for publication in A&A, 20 Pages, 9 figure
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