6,252 research outputs found
Buoyancy-driven inflow to a relic cold core: the gas belt in radio galaxy 3C 386
We report measurements from an XMM-Newton observation of the low-excitation
radio galaxy 3C 386. The study focusses on an X-ray-emitting gas belt, which
lies between and orthogonal to the radio lobes of 3C 386 and has a mean
temperature of keV, cooler than the extended group atmosphere.
The gas in the belt shows temperature structure with material closer to the
surrounding medium being hotter than gas closer to the host galaxy. We suggest
that this gas belt involves a `buoyancy-driven inflow' of part of the group-gas
atmosphere where the buoyant rise of the radio lobes through the ambient medium
has directed an inflow towards the relic cold core of the group.
Inverse-Compton emission from the radio lobes is detected at a level consistent
with a slight suppression of the magnetic field below the equipartition value.Comment: 11 pages, 10 figures, accepted for publication in MNRA
Kelvin-Helmholtz instabilities at the sloshing cold fronts in the Virgo cluster as a measure for the effective ICM viscosity
Sloshing cold fronts (CFs) arise from minor merger triggered gas sloshing.
Their detailed structure depends on the properties of the intra-cluster medium
(ICM): hydrodynamical simulations predict the CFs to be distorted by
Kelvin-Helmholtz instabilities (KHIs), but aligned magnetic fields, viscosity,
or thermal conduction can suppress the KHIs. Thus, observing the detailed
structure of sloshing CFs can be used to constrain these ICM properties. Both
smooth and distorted sloshing CFs have been observed, indicating that the KHI
is suppressed in some clusters, but not in all. Consequently, we need to
address at least some sloshing clusters individually before drawing general
conclusions about the ICM properties. We present the first detailed attempt to
constrain the ICM properties in a specific cluster from the structure of its
sloshing CF. Proximity and brightness make the Virgo cluster an ideal target.
We combine observations and Virgo-specific hydrodynamical sloshing simulations.
Here we focus on a Spitzer-like temperature dependent viscosity as a mechanism
to suppress the KHI, but discuss the alternative mechanisms in detail. We
identify the CF at 90 kpc north and north-east of the Virgo center as the best
location in the cluster to observe a possible KHI suppression. For viscosities
10% of the Spitzer value KHIs at this CF are suppressed. We describe
in detail the observable signatures at low and high viscosities, i.e. in the
presence or absence of KHIs. We find indications for a low ICM viscosity in
archival XMM-Newton data and demonstrate the detectability of the predicted
features in deep Chandra observations.Comment: Accepted for ApJ; 15 pages, 11 figures. A movie can be found here:
http://www.hs.uni-hamburg.de/DE/Ins/Per/Roediger/research.html#Virgo-viscou
Focusing on the extended X-ray emission in 3C 459 with a Chandra follow-up observation
6 pages, 4 figures. Reproduced with permission from Astronomy & Astrophysics. © 2019 ESO.Aims. We investigated the X-ray emission properties of the powerful radio galaxy 3C 459 revealed by a recent Chandra follow-up observation carried out in October 2014 with a 62 ks exposure. Methods. We performed an X-ray spectral analysis from a few selected regions on an image obtained from this observation and also compared the X-ray image with a 4.9 GHz VLA radio map available in the literature. Results. The dominant contribution comes from the radio core but significant X-ray emission is detected at larger angular separations from it, surrounding both radio jets and lobes. According to a scenario in which the extended X-ray emission is due to a plasma collisionally heated by jet-driven shocks and not magnetically dominated, we estimated its temperature to be ∼0.8 keV. This hot gas cocoon could be responsible for the radio depolarization observed in 3C 459, as recently proposed also for 3C 171 and 3C 305. On the other hand, our spectral analysis and the presence of an oxygen K edge, blueshifted at 1.23 keV, cannot exclude the possibility that the X-ray radiation originating from the inner regions of the radio galaxy could be intercepted by some outflow of absorbing material intervening along the line of sight, as already found in some BAL quasars.Peer reviewe
Viscous Kelvin-Helmholtz instabilities in highly ionised plasmas
Transport coefficients in highly ionised plasmas like the intra-cluster
medium (ICM) are still ill-constrained. They influence various processes, among
them the mixing at shear flow interfaces due to the Kelvin-Helmholtz
instability (KHI). The observed structure of potential mixing layers can be
used to infer the transport coefficients, but the data interpretation requires
a detailed knowledge of the long-term evolution of the KHI under different
conditions. Here we present the first systematic numerical study of the effect
of constant and temperature-dependent isotropic viscosity over the full range
of possible values. We show that moderate viscosities slow down the growth of
the KHI and reduce the height of the KHI rolls and their rolling-up.
Viscosities above a critical value suppress the KHI. The effect can be
quantified in terms of the Reynolds number Re = U{\lambda}/{\nu}, where U is
the shear velocity, {\lambda} the perturbation length, and {\nu} the kinematic
viscosity. We derive the critical Re for constant and temperature dependent,
Spitzer-like viscosities, an empirical relation for the viscous KHI growth time
as a function of Re and density contrast, and describe special behaviours for
Spitzer-like viscosities and high density contrasts. Finally, we briefly
discuss several astrophysical situations where the viscous KHI could play a
role, i.e., sloshing cold fronts, gas stripping from galaxies, buoyant
cavities, ICM turbulence, and high velocity clouds.Comment: Accepted by MNRAS. 22 pages, 21 figure
The Disturbed 17 keV Cluster Associated with the Radio Galaxy 3C 438
We present results from a {\em Chandra} observation of the cluster gas
associated with the FR II radio galaxy 3C 438. This radio galaxy is embedded
within a massive cluster with gas temperature 17 keV and bolometric
luminosity of 6 ergs s. It is unclear if this high
temperature represents the gravitational mass of the cluster, or if this is an
already high ( 11 keV) temperature cluster that has been heated
transiently. We detect a surface brightness discontinuity in the gas that
extends 600 kpc through the cluster. The radio galaxy 3C 438 is too small
(110 kpc across) and too weak to have created this large disturbance in
the gas. The discontinuity must be the result of either an extremely powerful
nuclear outburst or the major merger of two massive clusters. If the observed
features are the result of a nuclear outburst, it must be from an earlier epoch
of unusually energetic nuclear activity. However, the energy required
( ergs) to move the gas on the observed spatial scales strongly
supports the merger hypothesis. In either scenario, this is one of the most
extreme events in the local Universe.Comment: 13 pages, 4 figures, 1 table - accepted for publication in the
Astrophysical Journal Letter
WATCAT: a tale of wide-angle tailed radio galaxies
We present a catalog of 47 wide-angle tailed radio galaxies (WATs), the
WATCAT; these galaxies were selected by combining observations from the
National Radio Astronomy Observatory/Very Large Array Sky Survey (NVSS), the
Faint Images of the Radio Sky at Twenty-Centimeters (FIRST), and the Sloan
Digital Sky Survey (SDSS), and mainly built including a radio morphological
classification. We included in the catalog only radio sources showing two-sided
jets with two clear "warmspots" (i.e., jet knots as bright as 20% of the
nucleus) lying on the opposite side of the radio core, and having classical
extended emission resembling a plume beyond them. The catalog is limited to
redshifts z 0.15, and lists only sources with radio emission extended
beyond 30 kpc from the host galaxy. We found that host galaxies of WATCAT
sources are all luminous (-20.5 Mr -23.7), red early-type
galaxies with black hole masses in the range M M. The spectroscopic classification indicates that they
are all low-excitation galaxies (LEGs). Comparing WAT multifrequency properties
with those of FRI and FRII radio galaxies at the same redshifts, we conclude
that WATs show multifrequency properties remarkably similar to FRI radio
galaxies, having radio power of typical FRIIs
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