1,781 research outputs found
Faraday rotation variations along radio jets: the magnetic field in galaxy and group halos
Our modelling of FR I radio jets as decelerating, relativistic flows allows
us to derive their orientations accurately. We present images of Faraday
rotation for two of these these objects (3C 31 and NGC 315) and show that the
fluctuations of rotation measure (RM) are larger in the fainter (receding)
jets, as expected if the rotation occurs in the hot galaxy/group halos. The gas
density is much lower in NGC 315 and the RM fluctuations are only just
detectable.Comment: 3 pages, 2 figures, requires an.cls. To appear in the Proceedings of
the International Conference: "The Origin and Evolution of Cosmic Magnetism";
Bologna 29 August - 2 September 2005; eds R. Beck, G. Brunetti, L. Feretti,
and B. Gaensler (Astronomische Nachrichten, 2006
Simulations of the interaction of cold gas with radio jets
A new scenario for the interaction of a jet with a background medium with
cold clouds is investigated by means of hydrodynamic turbulence simulations
with cooling. The idea is that the cold clouds are overtaken by a radio cocoon
and stirred up by turbulence in this cocoon. The 2D multiphase turbulence
simulations contain all the three gas phases and have a number of interesting
properties. The produced power spectrum is proportional to the inverse square
of the wavevector. The Mach number - density relation may explain the observed
velocities in emission line gas associated with radio galaxies. The model also
explains the increased optical visibility within the radio structures, the
correlation between emission line and radio luminosity, and the evolution of
the alignment effect with source size. I also propose this to be a useful model
for the recently discovered neutral outflows in nearby radio sources.Comment: Contribution to the proceedings of "The Fate of Gas in Galaxies",
Dwingeloo, July 2006, 8 pages 6 figures. To appear in New Astronomy Reviews,
Vol. 51 (2007), eds. Morganti, Oosterloo, Villar-Martin & van Gorko
The Discovery of Extended Thermal X-ray Emission from PKS 2152-699: Evidence for a `Jet-cloud' Interaction
A Chandra ACIS-S observation of PKS 2152-699 reveals thermal emission from a
diffuse region around the core and a hotspot located 10" northeast from the
core. This is the first detection of thermal X-ray radiation on kiloparsec
scales from an extragalactic radio source. Two other hotspots located 47"
north-northeast and 26" southwest from the core were also detected. Using a
Raymond-Smith model, the first hotspot can be characterized with a thermal
plasma temperature of 2.6 K and an electron number density of 0.17
cm. These values correspond to a cooling time of about 1.6
yr. In addition, an emission line from the hotspot, possibly Fe xxv, was
detected at rest wavelength 10.04\AA.
The thermal X-ray emission from the first hotspot is offset from the radio
emission but is coincident with optical filaments detected with broadband
filters of HST/WFPC2. The best explanation for the X-ray, radio, and optical
emission is that of a `jet-cloud' interaction.
The diffuse emission around the nucleus of PKS 2152-699 can be modeled as a
thermal plasma with a temperature of 1.2 K and a luminosity of
1.8 erg s. This emission appears to be asymmetric with a
small extension toward Hotspot A, similar to a jet. An optical hotspot (EELR)
is seen less than an arcsecond away from this extension in the direction of the
core. This indicates that the extension may be caused by the jet interacting
with an inner ISM cloud, but entrainment of hot gas is unavoidable. Future
observations are discussed.Comment: To appear in the Astrophysical Journal 21 pages, 5 Postscript
figures, 1 table, AASTeX v. 5.
More evidence for extinction of activity in galaxies
This Research Note amends an article in which we showed that radio-loud
quasars can become radio-quiet. Exploring the analogy between galactic nuclei
and X-ray binaries (XRB), we pointed out there that this transition in quasars
could be identified with a switch from low/hard to high/soft state in
microquasars. Here, we present the evidence that traces of past occurrences of
this kind of phenomena can be found in normal but once active galaxies. Based
on the properties of a few such "post-active" galaxies that are representative
for a much wider group, it has been argued that they have reached the
evolutionary stages when their nuclei, which were radio-loud in the past, now,
mimicking the behaviour of XRBs, remain in the intermediate state on their way
towards quiescence or even have already entered the quiescent state. It follows
that the full evolutionary track of XRBs can be mapped onto the evolution of
galaxies. The above findings are in line with those reported recently for IC
2497, a galaxy that 70,000 years ago or less hosted a quasar but now appears as
a normal one. This scenario stems from the presence of Hanny's Voorwerp, a
nebulous object in its vicinity excited by that QSO in the epoch when IC 2497
was active. The post-active galaxies we deal with here are accompanied by
extremely weak and diffuse relic radio lobes that were inflated during their
former active period. These relics can be regarded as radio analogues of
Hanny's Voorwerp.Comment: 10 pages, 6 figures, A&A in pres
Abell 2111: An Optical and Radio Study of the Richest Butcher-Oemler Cluster
We present an in-depth analysis of the Butcher-Oemler cluster A2111,
including new optical spectroscopy plus a deep Very Large Array (VLA) radio
continuum observation. These are combined with optical imaging from the Sloan
Digital Sky Survey (SDSS) to assess the activity and properties of member
galaxies. Prior X-ray studies have suggested A2111 is a head-on cluster merger,
a dynamical state which might be connected to the high level of activity
inferred from its blue fraction. We are able to directly assess this claim,
using our spectroscopic data to identify 95 cluster members among 196 total
galaxy spectra. These galaxy velocities do not themselves provide significant
evidence for the merger interpretation, however they are consistent with it
provided the system is viewed near the time of core passage and at a viewing
angle >~30 degrees different from the merger axis. The SDSS data allow us to
confirm the high blue fraction for A2111, f_b = 0.15 +/- 0.03 based on
photometry alone and f_b = 0.23 +/- 0.03 using spectroscopic data to remove
background galaxies. We are able to detect 175 optical sources from the SDSS in
our VLA radio data, of which 35 have redshift information. We use the SDSS
photometry to determine photometric redshifts for the remaining 140
radio-optical sources. In total we identify up to 26 cluster radio galaxies, 14
of which have spectroscopic redshifts. The optical spectroscopy and radio data
reveal a substantial population of dusty starbursts within the cluster. The
high blue fraction and prevalence of star formation is consistent with the
hypothesis that dynamically-active clusters are associated with more active
member galaxies than relaxed clusters.Comment: To appear in AJ; 53 pages including 10 figures and several long
table
The first bent double lobe radio source in a known cluster filament: Constraints on the intra-filament medium
We announce the first discovery of a bent double lobe radio source (DLRS) in
a known cluster filament. The bent DLRS is found at a distance of 3.4 Mpc from
the center of the rich galaxy cluster, Abell~1763. We derive a bend angle
alpha=25deg, and infer that the source is most likely seen at a viewing angle
of Phi=10deg. From measuring the flux in the jet between the core and further
lobe and assuming a spectral index of 1, we calculate the minimum pressure in
the jet, (8.0+-3.2)x10^-13 dyn/cm^2, and derive constraints on the
intra-filament medium (IFM) assuming the bend of the jet is due to ram
pressure. We constrain the IFM to be between (1-20)x10^-29 gm/cm^3. This is
consistent with recent direct probes of the IFM and theoretical models. These
observations justify future searches for bent double lobe radio sources located
several Mpc from cluster cores, as they may be good markers of super cluster
filaments.Comment: 13 pages, 4 figures, accepted in ApJ Letter
The Cygnus X region XXIII. Is 18P87 galactic or extragalactic?
The radio source 18P87, previously thought to be a point source, has been
serendipitously found to be resolved into a core-jet geometry in VLA maps. HI
absorption of continuum emission (in data from the Canadian Galactic Plane
Survey) appears in gas with radial velocities > +2 km/s but not in brightly
emitting gas at lower radial velocity. Examination of further archival
observations at radio, infrared and optical wavelengths suggests that the
"obvious" interpretation as a radio galaxy requires a rather unusual object of
this kind and a highly unusual local line of sight. We argue that 18P87 may be
a Galactic object, a local astrophysical jet. If this is correct it could have
arisen from outbursts of a microquasar.Comment: Accepted for publication in Astronomische Nachrichte
A Deep Chandra Observation of the X-shaped Radio Galaxy 4C +00.58: A Candidate for Merger-induced Reorientation?
Although rapid reorientation of a black hole spin axis (lasting less than a
few Myr) has been suggested as a mechanism for the formation of wings in
X-shaped radio galaxies (XRGs), to date no convincing case of reorientation has
been found in any XRG. Alternative wing formation models such as the
hydrodynamic backflow models are supported by observed trends indicating that
XRGs form preferentially with jets aligned along the major axis of the
surrounding medium and wings along the minor axis. In this Letter, we present a
deep Chandra observation of 4C +00.58, an oddball XRG with its jet oriented
along the minor axis. By using the X-ray data in tandem with available radio
and optical data, we estimate relevant timescales with which to evaluate wing
formation models. The hydrodynamic models have difficulty explaining the long
wings, whereas the presence of X-ray cavities (suggesting jet activity along a
prior axis) and a potential stellar shell (indicating a recent merger) favor a
merger-induced reorientation model.Comment: 14 pages, 4 figures, accepted by ApJ
A Semi-automatic Search for Giant Radio Galaxy Candidates and their Radio-Optical Follow-up
We present results of a search for giant radio galaxies (GRGs) with a
projected largest linear size in excess of 1 Mpc. We designed a computational
algorithm to identify contiguous emission regions, large and elongated enough
to serve as GRG candidates, and applied it to the entire 1.4-GHz NRAO VLA Sky
survey (NVSS). In a subsequent visual inspection of 1000 such regions we
discovered 15 new GRGs, as well as many other candidate GRGs, some of them
previously reported, for which no redshift was known. Our follow-up
spectroscopy of 25 of the brighter hosts using two 2.1-m telescopes in Mexico,
and four fainter hosts with the 10.4-m Gran Telescopio Canarias (GTC), yielded
another 24 GRGs. We also obtained higher-resolution radio images with the Karl
G. Jansky Very Large Array for GRG candidates with inconclusive radio
structures in NVSS.Comment: 4 pages, 1 figure, to appear in the proceedings of The Universe of
Digital Sky Surveys, Naples, Italy, Nov 25-28, 2014; Astrophysics and Space
Science, eds. N.R. Napolitano et a
Star Formation and AGN in the Core of the Shapley Supercluster: A VLA Survey of A3556, A3558, SC1327-312, SC1329-313, and A3562
The core of the Shapley supercluster (A3556, A3558, SC1327-312, SC1329-313,
and A3562) is an ideal region in which to study the effects of cluster mergers
on the activity of individual galaxies. This paper presents the most
comprehensive radio continuum investigation of the region, relying on a
63-pointing mosaic obtained with the Very Large Array yielding an areal
coverage of nearly 7 square degrees. The mosaic provides a typical sensitivity
of about 80 uJy at a resolution of 16", enabling detection of galaxies with
star formation rates as low as 1 solar mass per year. The radio data are
complemented by optical imaging in B and R, producing a catalog of 210
radio-detected galaxies with m_R <= 17.36 (M_R <= -19). At least 104 of these
radio-detected galaxies are members of the supercluster on the basis of public
velocity measurements. Across the entire core of the supercluster, there
appears to be a significant deficit of radio galaxies at intermediate optical
magnitudes (M_R between -21 and -22). This deficit is offset somewhat by an
increase in the frequency with which brighter galaxies (M_R less than -22) host
radio sources. More dramatic is the highly significant increase in the
probability for fainter galaxies (M_R between -20 and -21) in the vicinity of
A3562 and SC1329-313 to be associated with radio emission. The radio and
optical data for these sources strongly suggest that these active galaxies are
powered by star formation. In conjunction with recent X-ray analysis, this is
interpreted as young starbursts related to the recent merger of SC1329-313 with
A3562 and the rest of the supercluster.Comment: Accepted by AJ; 50 pages, including 16 figures (for full resolution
PDF, see http://mywebpages.comcast.net/nealamiller2/Shapley_pp.pdf
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