674 research outputs found
Structures of the magnetoionic media around the FR I radio galaxies 3C 31 and Hydra A
We use high-quality VLA images of the Fanaroff & Riley Class I radio galaxy
3C 31 at six frequencies in the range 1365 to 8440MHz to explore the spatial
scale and origin of the rotation measure (RM) fluctuations on the line of sight
to the radio source. We analyse the distribution of the degree of polarization
to show that the large depolarization asymmetry between the North and South
sides of the source seen in earlier work largely disappears as the resolution
is increased. We show that the depolarization seen at low resolution results
primarily from unresolved gradients in a Faraday screen in front of the
synchrotron-emitting plasma. We establish that the residual degree of
polarization in the short-wavelength limit should follow a Burn law and we fit
such a law to our data to estimate the residual depolarization at high
resolution. We show that the observed RM variations over selected areas of 3C
31 are consistent with a power spectrum of magnetic fluctuations in front of 3C
31 whose power-law slope changes significantly on the scales sampled by our
data. The power spectrum can only have the form expected for Kolmogorov
turbulence on scales <5 kpc. On larger scales we find a flatter slope. We also
compare the global variations of RM across 3C 31 with the results of
three-dimensional simulations of the magnetic-field fluctuations in the
surrounding magnetoionic medium. We show that our data are consistent with a
field distribution that favours the plane perpendicular to the jet axis -
probably because the radio source has evacuated a large cavity in the
surrounding medium. We also apply our analysis techniques to the case of Hydra
A, where the shape and the size of the cavities produced by the source in the
surrounding medium are known from X-ray data. (Abridged)Comment: 33 pages, 25 figures, accepted for publication in MNRA
The Entrainment-Limited Evolution of FR II Sources: Maximum Sizes and A Possible Connection to FR Is
We construct a simple theoretical model to investigate how entrainment
gradually erodes high-speed FR II jets. This process is described by embedding
a mixing-layer model developed originally to describe FR I objects in a
self-similar model for the lobe structure of classical FR II sources. Following
the classical FR II models, we assume that the lobe is dominated by the
particles injected from the central jet. The entrainment produces a boundary
shear layer which acts at the interface between the dense central jet and the
less denser surrounding lobe, and the associated erosion of the jet places
interesting limits on the maximum size of FR II sources. The model shows that
this limit depends mainly on the initial bulk velocity of the relativistic jet
triggered. The bulk velocities of FR IIs suggested by our model are in good
agreement with that obtained from direct pc-scale observations on ordinary
radio galaxies and quasars. Finally, we discuss how FR IIs may evolve into FR
Is upon reaching their maximum, entrainment-limited sizes.Comment: 9 pages, 5 figures, accepted for publication in MNRA
The magnetized medium around the radio galaxy B2 0755+37: an interaction with the intra-group gas
We explore the magneto-ionic environment of the isolated radio galaxy B2
0755+37 using detailed imaging of the distributions of Faraday rotation and
depolarization over the radio source from Very Large Array observations at
1385,1465 and 4860 MHz and new X-ray data from XMM-Newton. The Rotation Measure
(RM) distribution is complex, with evidence for anisotropic fluctuations in two
regions. The approaching lobe shows low and uniform RM in an unusual `stripe'
along an extension of the jet axis and a linear gradient transverse to this
axis over its Northern half. The leading edge of the receding lobe shows
arc-like RM structures with sign reversals. Elsewhere, the RM structures are
reasonably isotropic. The RM power spectra are well described by cut-off power
laws with slopes ranging from 2.1 to 3.2 in different sub-regions. The
corresponding magnetic-field autocorrelation lengths, where well-determined,
range from 0.25 to 1.4 kpc. It is likely that the fluctuations are mostly
produced by compressed gas and field around the leading edges of the lobes. We
identify areas of high depolarization around the jets and inner lobes. These
could be produced by dense gas immediately surrounding the radio emission
containing a magnetic field which is tangled on small scales. We also identify
four ways in which the well known depolarization (Faraday depth) asymmetry
between jetted and counter-jetted lobes of extended radio sources can be
modified by interactions with the surrounding medium.Comment: 16 pages, 13 figures, accepted for publication in MNRAS. Full
resolution paper available at: ftp://ftp.ira.inaf.it/pub/outgoing/guidetti/
Subjects: Cosmology and Extragalactic Astrophysics (astro-ph.CO
The TOOT00 Redshift Survey of Radio Sources
We present first results from the study of the TOOT00 region consisting of 47
radio sources brighter than 100 mJy at 151 MHz. We have 81% spectroscopic
redshift completeness. From the K-z diagram we deduce that the host galaxies
are similar to ~3 L* passively evolved elliptical galaxies and thus estimate
the redshifts of the 9 sources without a secure spectroscopic redshift yielding
a median redshift of 1.287. Above the RLF break we have a quasar fraction f ~
0.3 although the quasars appear reddened; below the RLF break f -> 0 if we
exclude flat-spectrum radio sources. We present a histogram of the number of
TOOT00 radio sources versus their redshift which looks broadly like the Willott
et al. (2001) prediction for TOOT, although the observed ratio of high to low
redshift objects is somewhat lower than the prediction.Comment: 4 pages, 5 figures, to be published in AN (proceedings of the Granada
Workshop on High Redshift Radio Galaxies
Bumps and rings in a two-dimensional neural field: splitting and rotational instabilities
In this paper we consider instabilities of localised solutions in planar neural field firing rate models of Wilson-Cowan or Amari type. Importantly we show that angular perturbations can destabilise spatially localised solutions. For a scalar model with Heaviside firing rate function we calculate symmetric one-bump and ring solutions explicitly and use an Evans function approach to predict the point of instability and the shapes of the dominant growing modes. Our predictions are shown to be in excellent agreement with direct numerical simulations. Moreover, beyond the instability our simulations demonstrate the emergence of multi-bump and labyrinthine patterns.
With the addition of spike-frequency adaptation, numerical simulations of the resulting vector model show that it is possible for structures without rotational symmetry, and in particular multi-bumps, to undergo an instability to a rotating wave. We use a general argument, valid for smooth firing rate functions, to establish the conditions necessary to generate such a rotational instability. Numerical continuation of the rotating wave is used to quantify the emergent angular velocity as a bifurcation parameter is varied. Wave stability is found via the numerical evaluation of an associated eigenvalue problem
On the Populations of Radio Galaxies with Extended Morphology at z<0.3
Extended extragalactic radio sources have traditionally been classified into
FR I and II types, based on the ratio r of the separation S between the
brightest regions on either sides of the host galaxy and the total size T of
the radio source. Here we examine the distribution of various physical
properties as a function of r of 1040 luminous radio galaxies (RGs) at z<0.3
from the SDSS, NVSS, and FIRST. About 2/3 of the RGs are lobe-dominated (LD),
and 1/3 have prominent jets. If we follow the original definition of the FR
types (a division based solely on r), FR I and FR II RGs overlap in their host
galaxy properties. However, the rare, LD sources with r>0.8 AND OIII5007 line
luminosity >10^6 Lsun are markedly different on average from the rest of the
RGs, in the sense that they are hosted in lower mass galaxies, live in
relatively sparse environments, and have higher accretion rates onto the
central SMBH. Thus these objects and the rest of RGs form a well-defined
dichotomy. Motivated by the stark differences in the nuclear emission line
properties of the RG subsamples, we suggest that the accretion rate onto the
SMBH may play the primary role in creating the different morphologies. At
relatively high accretion rates, the accretion system may produce powerful jets
that create the "classical double" morphology (roughly corresponding to the LD
sources with r>0.8 and emission lines); at lower accretion rates the jets from
a radiatively inefficient accretion flow generate radio lobes without apparent
"hot spots" at the edge (corresponding to the majority of LD sources). At
slightly lower accretion rates AND in galaxies with dense galactic structure,
sources with prominent jets result. It is possible that while the high
accretion rate systems could affect sub-Mpc scale environments, the jets from
lower accretion rate systems may efficiently suppress activity within the host
galaxies.Comment: ApJ, accepted. 20 pages, 16 figures, 4 tables. Image quality of
Figures 1 & 2 degraded due to size limits. Table 4 is available and kept
up-to-date at http://member.ipmu.jp/yen-ting.lin/RG/index.html. Higher
resolution version available at
http://member.ipmu.jp/yen-ting.lin/Data/ytlin.pd
On Dynamical Models for Radio Galaxies
The tailed radio galaxies that have been called ``Type I'' are not a uniform
set. To study their dynamics, we have used the Ledlow-Owen data set, which
provides a new sample of 250 radio galaxies in nearby Abell clusters. These
sources divide into two clear categories based on their radio morphology. Type
A sources (``straight'') contain nearly straight jets which are embedded in
outer radio lobe. Type B sources (``tailed'') have a well-collimated jet flow
which undergoes a sudden transition, at an inner hot spot, to a less collimated
flow which continues on and forms a radio tail. We have not found any
separation of these classes in terms of radio power, radio flux size, galaxy
power or external gas density. We propose the difference is due to the
development, or not, of a disruptive flow instability, such as
Kelvin-Helmholtz, and the saturation of the instability when it develops.Comment: 10 pages, 4 figures, to appear in 'Life Cycles of Radio Galaxies',
ed. J. Biretta etal, New Astronomy Review
The inner jet of radio galaxy NGC 315 as observed with Chandra and the VLA
We present Chandra X-ray results for the jet, nucleus, and gaseous atmosphere
of NGC 315, a nearby radio galaxy whose jet kinematics are known through deep
radio mapping. Diffuse X-ray synchrotron emission is detected from the jet out
to 30 arcsec from the nucleus, through regions both of fast bulk flow and
deceleration. The X-ray to radio flux ratio drops considerably where the flow
decelerates, but the X-ray and radio emissions show similar transverse extents
throughout, requiring distributed particle acceleration to maintain the supply
of X-ray-emitting electrons. A remarkable knotty filament within the jet is
seen in both the radio and X-ray, contributing roughly 10 per cent of the
diffuse emission along its extent at both wavelengths. No completely
satisfactory explanation for the filament is found, though its oscillatory
appearance, roughly aligned magnetic field, and requirements for particle
acceleration, suggest that it is a magnetic strand within a shear layer between
fast inner and slower outer flow.Comment: Accepted for publication in the MNRAS. 13 pages,14 figures (some in
colour
Distant FR I radio galaxies in the Hubble Deep Field: implications for the cosmological evolution of radio-loud AGN
Deep and high resolution radio observations of the Hubble Deep Field and
flanking fields have shown the presence of two distant edge-darkened FR I radio
galaxies, allowing for the first time an estimate of their high redshift space
density. If it is assumed that the space density of FR I radio galaxies at z>1
is similar to that found in the local universe, then the chance of finding two
FR I radio galaxies at these high radio powers in such a small area of sky is
1
than at present, effectively ruling out the possibility that FR I radio sources
undergo no cosmological evolution. We suggest that FR I and FR II radio
galaxies should not be treated as intrinsically distinct classes of objects,
but that the cosmological evolution is simply a function of radio power with FR
I and FR II radio galaxies of similar radio powers undergoing similar
cosmological evolutions. Since low power radio galaxies have mainly FR I
morphologies and high power radio galaxies have mainly FR II morphologies, this
results in a generally stronger cosmological evolution for the FR IIs than the
FR Is. We believe that additional support from the V/Vmax test for evolving and
non-evolving populations of FR IIs and FR Is respectively is irrelevant, since
this test is sensitive over very different redshift ranges for the two classes.Comment: LaTeX, 6 pages, 3 figs. To appear in MNRA
An X-ray and Optical Investigation of the Environments Around Nearby Radio Galaxies
Investigations of the cluster environment of radio sources have not shown a
correlation between radio power and degree of clustering. However, it has been
demonstrated that extended X-ray luminosity and galaxy clustering do exhibit a
positive correlation. This study investigates a complete sample of 25 nearby (z
less than 0.06) radio galaxies which are not cataloged members of Abell
clusters. The environment of these radio galaxies is studied in both the X-ray
and the optical by means of the ROSAT All-Sky Survey (RASS), ROSAT pointed
observations, and the Palomar optical Digitized Sky Survey (DSS). X-ray
luminosities and extents are determined from the RASS, and the DSS is used to
quantify the degree of clustering via the spatial two-point correlation
coefficient, Bgg. Of the 25 sources, 20 are greater than sigma detections in
the X-ray and 11 possessed Bgg's significantly in excess of that expected for
an isolated galaxy. Adding the criterion that the X-ray emission be resolved,
10 of the radio galaxies do appear to reside in poor clusters with extended
X-ray emission suggestive of the presence of an intracluster medium. Eight of
these galaxies also possess high spatial correlation coefficients. Taken
together, these data suggest that the radio galaxies reside in a low richness
extension of the Abell clusters. The unresolved X-ray emission from the other
galaxies is most likely associated with AGN phenomena. Furthermore, although
the sample size is small, it appears that the environments of FR I and FR II
sources differ. FR I's tend to be more frequently associated with extended
X-ray emission (10 of 18), whereas FR II's are typically point sources or
non-detections in the X-ray (none of the 7 sources exhibit extended X-ray
emission).Comment: 28 page postscript file including figures and tables, plus one
landscape table and 5 GIF figure
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