261 research outputs found
A possible dearth of hot gas in galaxy groups at intermediate redshift
We examine the X-ray luminosity of galaxy groups in the CNOC2 survey, at
redshifts 0.1 < z < 0.6. Previous work examining the gravitational lensing
signal of the CNOC2 groups has shown that they are likely to be genuine,
gravitationally bound objects. Of the 21 groups in the field of view of the
EPIC-PN camera on XMM-Newton, not one was visible in over 100 ksec of
observation, even though three of the them have velocity dispersions high
enough that they would easily be visible if their luminosities scaled with
their velocity dispersions in the same way as nearby groups' luminosities
scale. We consider the possibility that this is due to the reported velocity
dispersions being erroneously high, and conclude that this is unlikely. We
therefore find tentative evidence that groups at intermediate redshift are
underluminous relative to their local cousins.Comment: 16 pages, 5 figures, reference added in section 1, typos corrected,
published in Ap
Properties of the cosmological filament between two clusters: A possible detection of a large-scale accretion shock by
We report on the results of a observation of the plasma in the
filament located between the two massive clusters of galaxies Abell 399 and
Abell 401. Abell 399 (=0.0724) and Abell 401 (=0.0737) are expected to be
in the initial phase of a cluster merger. In the region between the two
clusters, we find a clear enhancement in the temperature of the filament plasma
from 4 keV (expected value from a typical cluster temperature profile) to
6.5 keV. Our analysis also shows that filament plasma is present out to
a radial distance of 15' (1.3 Mpc) from a line connecting the two clusters. The
temperature profile is characterized by an almost flat radial shape with
6-7 keV within 10' or 0.8 Mpc. Across =8'~from the axis, the
temperature of the filament plasma shows a drop from 6.3 keV to 5.1 keV,
indicating the presence of a shock front. The Mach number based on the
temperature drop is estimated to be 1.3. We also successfully
determined the abundance profile up to 15' (1.3 Mpc), showing an almost
constant value (=0.3 solar) at the cluster outskirt. We estimated the
Compton -parameter to be 14.5, which is in
agreement with 's results (14-17 on the filament). The
line of sight depth of the filament is 1.1 Mpc, indicating that the
geometry of filament is likely a pancake shape rather than cylindrical. The
total mass of the filamentary structure is 7.7. We discuss a possible interpretation of the drop of X-ray emission
at the rim of the filament, which was pushed out by the merging activity and
formed by the accretion flow induced by the gravitational force of the
filament.Comment: 8 pages, 8 figures, accepted for publication in A&
L_X-T Relation and Related Properties of Galaxy Clusters
An observational approach is presented to constrain the global structure and
evolution of the intracluster medium based on the ROSAT and ASCA distant
cluster sample. From statistical analysis of the gas density profile and the
connection to the LX-T relation under the beta-model, the scaled gas profile is
nearly universal for the outer region and the LX(>0.2r500) is tightly related
to the temperature through T^3 rather than T^2. On the other hand, a large
density scatter exists in the core region and there is clearly a deviation from
the self-similar scaling for clusters with a small core size. A direct link
between the core size and the radiative cooling timescale suggest that t_cool
is a parameter to control the gas structure and the appearance of small cores
in regular clusters may be much connected with the thermal evolution. We derive
the luminosity-ambient temperature (T') relation, assuming the universal
temperature profile to find the dispersion around the relation significantly
decreases: L_1keV is almost constant for a wide range of t_cool. We further
examined the LX-Tbeta and LX-T'beta relations and showed a trend that merging
clusters segregate from the regular clusters on the planes. A good correlation
between t_cool and the X-ray morphology on the L_1keV-t_cool/t_age plane leads
us to define three phases according to the different level of cooling, and draw
a phenomenological picture: after a cluster collapses and t_cool falls below
t_age, the core cools radiatively with quasi-hydrostatic balancing in the
gravitational potential, and the central density gradually becomes higher to
evolve from an outer-core-dominant cluster to inner-core-dominant cluster.Comment: 39 pages, 37 figures. Accepted for publication in ApJ. Version with
high-quality color figures at
http://cosmic.riken.jp/ota/publications/index.htm
Unravelling the origin of large-scale magnetic fields in galaxy clusters and beyond through Faraday Rotation Measures with the SKA
We investigate the possibility for the SKA to detect and study the magnetic
fields in galaxy clusters and in the less dense environments surrounding them
using Faraday Rotation Measures. To this end, we produce 3-dimensional magnetic
field models for galaxy clusters of different masses and in different stages of
their evolution, and derive mock rotation measure observations of background
radiogalaxies. According to our results, already in phase I, we will be able to
infer the magnetic field properties in galaxy clusters as a function of the
cluster mass, down to solar-masses. Moreover, using cosmological
simulations to model the gas density, we have computed the expected rotation
measure through shock-fronts that occur in the intra-cluster medium during
cluster mergers. The enhancement in the rotation measure due to the density
jump will permit to constraint the magnetic field strength and structure after
the shock passage. SKA observations of polarised sources located behind galaxy
clusters will answer several questions about the magnetic field strength and
structure in galaxy clusters, and its evolution with cosmic time.Comment: 9 pages, 4 Figures, to appear as part of 'Cosmic Magnetism' in
Proceedings 'Advancing Astrophysics with the SKA (AASKA14)', PoS(AASKA14
SKA Deep Polarization and Cosmic Magnetism
Deep surveys with the SKA1-MID array offer for the first time the opportunity
to systematically explore the polarization properties of the microJy source
population. Our knowledge of the polarized sky approaching these levels is
still very limited. In total intensity the population will be dominated by
star-forming and normal galaxies to intermediate redshifts (), and
low-luminosity AGN to high redshift. The polarized emission from these objects
is a powerful probe of their intrinsic magnetic fields and of their magnetic
environments. For redshift of order 1 and above the broad bandwidth of the
mid-bands span the Faraday thick and thin regimes allowing study of the
intrinsic polarization properties of these objects as well as depolarization
from embedded and foreground plasmas. The deep field polarization images will
provide Rotation Measures data with very high solid angle density allowing a
sensitive statistical analysis of the angular variation of RM on critical
arc-minute scales from a magnetic component of Large Scale Structure of the
Universe.Comment: 9 pages, 3 figures; to appear as part of 'Cosmic Magnetism' in
Proceedings 'Advancing Astrophysics with the SKA (AASKA14)', PoS(AASKA14)11
Why Do Only Some Galaxy Clusters Have Cool Cores?
Flux-limited X-ray samples indicate that about half of rich galaxy clusters
have cool cores. Why do only some clusters have cool cores while others do not?
In this paper, cosmological N-body + Eulerian hydrodynamic simulations,
including radiative cooling and heating, are used to address this question as
we examine the formation and evolution of cool core (CC) and non-cool core
(NCC) clusters. These adaptive mesh refinement simulations produce both CC and
NCC clusters in the same volume. They have a peak resolution of 15.6 h^{-1} kpc
within a (256 h^{-1} Mpc)^3 box. Our simulations suggest that there are
important evolutionary differences between CC clusters and their NCC
counterparts. Many of the numerical CC clusters accreted mass more slowly over
time and grew enhanced cool cores via hierarchical mergers; when late major
mergers occurred, the CC's survived the collisions. By contrast, NCC clusters
experienced major mergers early in their evolution that destroyed embryonic
cool cores and produced conditions that prevented CC re-formation. As a result,
our simulations predict observationally testable distinctions in the properties
of CC and NCC beyond the core regions in clusters. In particular, we find
differences between CC versus NCC clusters in the shapes of X-ray surface
brightness profiles, between the temperatures and hardness ratios beyond the
cores, between the distribution of masses, and between their supercluster
environs. It also appears that CC clusters are no closer to hydrostatic
equilibrium than NCC clusters, an issue important for precision cosmology
measurements.Comment: 17 emulateapj pages, 17 figures, replaced with version accepted to
Ap
The first measurement of temperature standard deviation along the line-of-sight in galaxy clusters
Clusters of galaxies are mainly formed by merging of smaller structures,
according to the standard cosmological scenario. If the mass of a substructure
is >10% of that of a galaxy cluster, the temperature distribution of the
intracluster medium (ICM) in a merging cluster becomes inhomogeneous. Various
methods have been used to derive the two-dimensional projected temperature
distribution of the ICM. However, methods for studying temperature distribution
along the line-of-sight through the cluster were absent. In this paper, we
present the first measurement of the temperature standard deviation along the
line-of-sight, using as a reference case the multifrequency SZ measurements of
the Bullet Cluster. We find that the value of the temperature standard
deviation is high and equals to (10.6+/-3.8) keV in the Bullet Cluster. This
result shows that the temperature distribution in the Bullet Cluster is
strongly inhomogeneous along the line-of-sight and provides a new method for
studying galaxy clusters in depth.Comment: 5 pages, 1 figure, published in MNRAS Letter
Can electron distribution functions be derived through the Sunyaev-Zel'dovich effect?
Measurements of the Sunyaev-Zel'dovich (hereafter SZ) effect distortion of
the cosmic microwave background provide methods to derive the gas pressure and
temperature of galaxy clusters. Here we study the ability of SZ effect
observations to derive the electron distribution function (DF) in massive
galaxy clusters.
Our calculations of the SZ effect include relativistic corrections considered
within the framework of the Wright formalism and use a decomposition technique
of electron DFs into Fourier series. Using multi-frequency measurements of the
SZ effect, we find the solution of a linear system of equations that is used to
derive the Fourier coefficients; we further analyze different frequency samples
to decrease uncertainties in Fourier coefficient estimations.
We propose a method to derive DFs of electrons using SZ multi-frequency
observations of massive galaxy clusters. We found that the best frequency
sample to derive an electron DF includes high frequencies =375, 600, 700,
857 GHz. We show that it is possible to distinguish a Juttner DF from a
Maxwell-Bolzman DF as well as from a Juttner DF with the second electron
population by means of SZ observations for the best frequency sample if the
precision of SZ intensity measurements is less than 0.1%. We demonstrate by
means of 3D hydrodynamic numerical simulations of a hot merging galaxy cluster
that the morphologies of SZ intensity maps are different for frequencies
=375, 600, 700, 857 GHz. We stress that measurements of SZ intensities at
these frequencies are a promising tool for studying electron distribution
functions in galaxy clusters.Comment: 11 pages, 12 figures, published in Astronomy and Astrophysic
Estimating extragalactic Faraday rotation
(abridged) Observations of Faraday rotation for extragalactic sources probe
magnetic fields both inside and outside the Milky Way. Building on our earlier
estimate of the Galactic contribution, we set out to estimate the extragalactic
contributions. We discuss the problems involved; in particular, we point out
that taking the difference between the observed values and the Galactic
foreground reconstruction is not a good estimate for the extragalactic
contributions. We point out a degeneracy between the contributions to the
observed values due to extragalactic magnetic fields and observational noise
and comment on the dangers of over-interpreting an estimate without taking into
account its uncertainty information. To overcome these difficulties, we develop
an extended reconstruction algorithm based on the assumption that the
observational uncertainties are accurately described for a subset of the data,
which can overcome the degeneracy with the extragalactic contributions. We
present a probabilistic derivation of the algorithm and demonstrate its
performance using a simulation, yielding a high quality reconstruction of the
Galactic Faraday rotation foreground, a precise estimate of the typical
extragalactic contribution, and a well-defined probabilistic description of the
extragalactic contribution for each data point. We then apply this
reconstruction technique to a catalog of Faraday rotation observations. We vary
our assumptions about the data, showing that the dispersion of extragalactic
contributions to observed Faraday depths is most likely lower than 7 rad/m^2,
in agreement with earlier results, and that the extragalactic contribution to
an individual data point is poorly constrained by the data in most cases.Comment: 20 + 6 pages, 19 figures; minor changes after bug-fix; version
accepted for publication by A&A; results are available at
http://www.mpa-garching.mpg.de/ift/faraday
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