143 research outputs found
A Complete Sample of Megaparsec Size Double Radio Sources from SUMSS
We present a complete sample of megaparsec-size double radio sources compiled
from the Sydney University Molonglo Sky Survey (SUMSS). Almost complete
redshift information has been obtained for the sample. The sample has the
following defining criteria: Galactic latitude |b| > 12.5 deg, declination <
-50 deg and angular size > 5 arcmin. All the sources have projected linear size
larger than 0.7 Mpc (assuming H_o = 71 km/s/Mpc). The sample is chosen from a
region of the sky covering 2100 square degrees. In this paper, we present
843-MHz radio images of the extended radio morphologies made using the Molonglo
Observatory Synthesis Telescope (MOST), higher resolution radio observations of
any compact radio structures using the Australia Telescope Compact Array
(ATCA), and low resolution optical spectra of the host galaxies from the 2.3-m
Australian National University (ANU) telescope at Siding Spring Observatory.
The sample presented here is the first in the southern hemisphere and
significantly enhances the database of known giant radio sources. The giant
radio sources with linear size exceeding 0.7 Mpc have an abundance of (215
Mpc)^(-3) at the sensitivity of the survey. In the low redshift universe, the
survey may be suggesting the possibility that giant radio sources with relict
lobes are more numerous than giant sources in which beams from the centre
currently energize the lobes.Comment: 67 pages, 29 figures, for full resolution figures see
http://www.astrop.physics.usyd.edu.au/SUMSS/PAPERS/Submit-May11-ms.pd
The Abundance of X-Shaped Radio Sources I. VLA Survey of 52 Sources With Off-Axis Distortions
Cheung identified a sample of 100 candidate X-shaped radio galaxies using the
NRAO FIRST survey; these are small-axial-ratio extended radio sources with
off-axis emission. Here we present radio images of 52 of these sources that
have been made from archival Very Large Array data with resolution of about 1
arcsec. Fifty-one of the 52 were observed at 1.4 GHz, seven were observed at
1.4 GHz and 5 GHz, and one was observed only at 5 GHz. We also present overlays
of the SDSS red images for 48 of the sources, and DSS II overlays for the
remainder. Optical counterparts have been identified for most sources, but
there remain a few empty fields. Our higher resolution VLA images along with
FIRST survey images of the sources in the sample reveal that extended
extragalactic radio sources with small axial ratios are largely (60%) cases of
double radio sources with twin lobes that have off-axis extensions, usually
with inversion-symmetric structure. The available radio images indicate that at
most 20% sources might be genuine X-shaped radio sources that could have formed
by a restarting of beams in a new direction following an interruption and axis
flip. The remaining 20% are in neither of these categories. The implications of
this result for the gravitational wave background are discussed in Roberts,
Saripalli, and Subrahmanyan.Comment: ApJ Supplements (accepted
Probing the pc- and kpc-scale environment of the powerful radio galaxy Hercules A
We present the kpc-scale behaviour of the powerful extragalactic radio source
Hercules A and the behaviour of the intracluster gas in which the radio source
is situated. We have found that Hercules A exhibits a strong Laing-Garrington
effect. The X-ray observations have revealed an extended X-ray emission
elongated along the radio galaxy axis. The estimated temperature of the cluster
is kT = 2.45 keV and the central electron density is no~7.8 x 10^(-3) cm^(-3)
which reveals a hot, dense environment in which Hercules A is situated. From
the combined study of the radio and X-ray data we have estimated a central
value of 3<Bo (muG)<9. We also present the most recent results from the
analysis of the radio data on the pc-scale structure of the radio galaxy,
observed at 18 cm by the EVN-MERLIN array. A faint but compact radio source,
coincident with the optical centre of Hercules A was detected by the EVN at 18
mas resolution. The total flux density of the EVN core is 14.6 mJy. Its angular
size is 18 x 7 mas with a position angle of ~139 degrees. There is also
evidence for extended emission in the NW-SE direction, most probably from the
eastern pc-scale jet. If this is true then there is a misalignment between the
direction of the pc-eastern and the aligned kpc-scale jets of ~35 degrees.Comment: 7 pages, 2 figures, Submitted to PAS
Hard X-ray selected giant radio galaxies - I. The X-ray properties and radio connection
We present the first broad-band X-ray study of the nuclei of 14 hard X-ray
selected giant radio galaxies, based both on the literature and on the analysis
of archival X-ray data from NuSTAR, XMM-Newton, Swift and INTEGRAL. The X-ray
properties of the sources are consistent with an accretion-related X-ray
emission, likely originating from an X-ray corona coupled to a radiatively
efficient accretion flow. We find a correlation between the X-ray luminosity
and the radio core luminosity, consistent with that expected for AGNs powered
by efficient accretion. In most sources, the luminosity of the radio lobes and
the estimated jet power are relatively low compared with the nuclear X-ray
emission. This indicates that either the nucleus is more powerful than in the
past, consistent with a restarting of the central engine, or that the giant
lobes are dimmer due to expansion losses.Comment: 11 pages, 3 figures. Accepted for publication in MNRA
A Case for Renewed Activity in the Giant Radio Galaxy J0116-473
We present ATCA radio observations of the giant radio galaxy J0116-473 at 12
and 22 cm wavelengths in total intensity and polarization. The images clearly
reveal a bright inner-double structure within more extended edge-brightened
lobe emission. The lack of hotspots at the ends of the outer lobes, the strong
core and the inner-double structure with its edge-brightened morphology lead us
to suggest that this giant radio galaxy is undergoing a renewed nuclear
activity: J0116-473 appears to be a striking example of a radio galaxy where a
young double source is evolving within older lobe material. We also report the
detection of a Mpc-long linear feature which is oriented perpendicular to the
radio axis and has a high fractional polarization.Comment: 25 pages, 10 figures, appeared in 2002 ApJ, 565, 25
PKS B1545-321: Bow shocks of a relativistic jet?
Sensitive, high resolution images of the double-double radio galaxy PKS
B1545-321 reveal detailed structure, which we interpret in the light of
previous work on the interaction of restarted jets with pre-existing relict
cocoons. We have also examined the spectral and polarization properties of the
source, the color distribution in the optical host and the environment of this
galaxy in order to understand its physical evolution. We propose that the
restarted jets generate narrow bow shocks and that the inner lobes are a
mixture of cocoon plasma reaccelerated at the bow shock and new jet material
reaccelerated at the termination shock. The dynamics of the restarted jets
implies that their hot spots advance at mildly relativistic speeds with
external Mach numbers of at least 5. The existence of supersonic hot spot Mach
numbers and bright inner lobes is the result of entrainment causing a reduction
in the sound speed of the pre-existing cocoon. The interruption to jet activity
in PKS B1545-321 has been brief - lasting less than a few percent of the
lifetime of the giant radio source. The host
galaxy is located at the boundary of a large scale filamentary structure, and
shows blue patches in color distribution indicative of a recent merger, which
may have triggered the Mpc-scale radio galaxy.Comment: 26 pages including 1 table and 16 figures. To appear in MNRA
Multiwavelength Observations of the Second Largest Known FR II Radio Galaxy, NVSS 2146+82
We present multi-frequency VLA, multicolor CCD imaging, optical spectroscopy,
and ROSAT HRI observations of the giant FR II radio galaxy NVSS 2146+82. This
galaxy, which was discovered by the NRAO VLA Sky Survey (NVSS), has an angular
extent of nearly 20' from lobe to lobe. The radio structure is normal for an FR
II source except for its large size and regions in the lobes with unusually
flat radio spectra. Our spectroscopy indicates that the optical counterpart of
the radio core is at a redshift of z=0.145, so the linear size of the radio
structure is ~4 h_50^-1 Mpc. This object is therefore the second largest FR II
known (3C 236 is ~6 h_50^-1 Mpc). Optical imaging of the field surrounding the
host galaxy reveals an excess number of candidate galaxy cluster members above
the number typically found in the field surrounding a giant radio galaxy. WIYN
HYDRA spectra of a sample of the candidate cluster members reveal that six
share the same redshift as NVSS 2146+82, indicating the presence of at least a
``rich group'' containing the FR II host galaxy. ROSAT HRI observations of NVSS
2146+82 place upper limits on the X-ray flux of 1.33 x 10^-13 ergs cm^-2 s^-1
for any hot IGM and 3.52 x 10^-14 ergs cm^-2 s^-1 for an X-ray AGN, thereby
limiting any X-ray emission at the distance of the radio galaxy to that typical
of a poor group or weak AGN. Several other giant radio galaxies have been found
in regions with overdensities of nearby galaxies, and a separate study has
shown that groups containing FR IIs are underluminous in X-rays compared to
groups without radio sources. We speculate that the presence of the host galaxy
in an optically rich group of galaxies that is underluminous in X-rays may be
related to the giant radio galaxy phenomenon.Comment: 46 pages, 15 figures, AASTeX aaspp4 style, accepted for publication
in A
On the relationship between a giant radio galaxy MSH 05-22 and the ambient large-scale galaxy structure
We present a comparison of the properties of a giant radio galaxy and the
ambient intergalactic medium, whose properties are inferred from the
large-scale distribution in galaxies. The double lobes of the radio galaxy MSH
05-22 are giant--1.8 Mpc projected linear size--and interacting with the
environment outside the interstellar medium and coronal halo associated with
the host galaxy. The radio lobes appear to be relicts and the double structure
is asymmetric. We have examined the large-scale structure in the galaxy
distribution surrounding the radio source. The host galaxy of MSH 05-22 is
associated with a small group that lies close to the boundary of sheet-like and
filamentary density enhancements, and adjacent to a void. Assuming that the
galaxies trace gas, the asymmetries in the radio morphology in this case study
appear related to the anisotropy in the medium. However, the observed
overdensities and structure formation models for the heating of the
intergalactic medium (IGM) suggest a density-temperature product for the IGM
environment that is an order of magnitude below that expected from the
properties of the radio source. The discordance suggests that even sources like
MSH 05-22, which are observed in the relatively low-density IGM environment
associated with the filamentary large-scale structure and have multiple
signatures of being relicts, may be overpressured and evolving towards an
equilibrium relaxed state with the ambient IGM. Alternately, it is speculated
that astrophysical feedback originating in galaxy overdensities observed 1-2
Mpc to the N and NE of MSH 05-22 might be the mechanism for the heating of the
ambient IGM gas.Comment: 27 pages including 1 table and 13 figures. To appear in Ap
Renewed activity in the radio galaxy PKS B1545-321: twin edge-brightened beams within diffuse radio lobes
Australia Telescope Compact Array (ATCA) images of the giant radio galaxy PKS
B1545-321 show a pair of oppositely directed beams emerging from a radio core
and ending in bright components that are symmetrically located on either side.
These inner beams are embedded within edge-brightened outer lobes of lower
surface brightness and the bright ends of the inner beams are well recessed
from the ends of the outer lobes. The inner beams and diffuse surrounding lobes
share a common central core and radio axis. We propose that the observed inner
beams are double lobes which have been created within relic outer lobes as a
consequence of a restarting of the central activity; therefore, PKS B1545-321
is a rare opportunity for examining the development of restarted beams within a
relic synchrotron plasma cocoon. The inner double representing the new episode
has among the highest axial ratios found in typical edge-brightened radio
galaxies. The low radio luminosity of the inner double, the narrow and constant
transverse extent of its cocoon and the relatively low brightness of the
hotspots at its ends are consistent with the almost ballistic propagation
expected for a beam that has a low density contrast and is advancing within a
relatively light ambient medium.Comment: 30 pages, 12 figures, to appear in ApJ June, 200
- …