1,721,117 research outputs found
Spectral imaging of galaxy clusters with Planck
Get PDFThe Sunyaev–Zeldovich (SZ) effect is a promising tool for detecting the presence of hot gas out to the galaxy cluster peripheries. We developed a spectral imaging algorithm dedicated to the SZ observations of nearby galaxy clusters with Planck, with the aim of revealing gas density anisotropies related to the filamentary accretion of materials, or pressure discontinuities induced by the propagation of shock fronts. To optimize an unavoidable trade-off between angular resolution and precision of the SZ flux measurements, the algorithm performs a multi-scale analysis of the SZ maps as well as of other extended components, such as the cosmic microwave background (CMB) anisotropies and the Galactic thermal dust. The demixing of the SZ signal is tackled through kernel-weighted likelihood maximizations. The CMB anisotropies are further analyzed through a wavelet analysis, while the Galactic foregrounds and SZ maps are analyzed via a curvelet analysis that best preserves their anisotropic details. The algorithm performance has been tested against mock observations of galaxy clusters obtained by simulating the Planck High Frequency Instrument and by pointing at a few characteristic positions in the sky. These tests suggest that Planck should easily allow us to detect filaments in the cluster peripheries and detect large-scale shocks in colliding galaxy clusters that feature favorable geometry
Temperature structure of the intergalactic medium within seven nearby and bright clusters of galaxies observed with XMM-Newton
No full textAims.Using a newly developed algorithm, we map, to the
highest angular resolution allowed by the data, the temperature
structure of the intra-cluster medium (ICM) within a nearly complete
X-ray flux limited sample of galaxy clusters in the redshift range
between and . Our sample contains seven bright
clusters of galaxies observed with XMM-Newton: Abell 399, Abell 401,
Abell 478, Abell 1795, Abell 2029, Abell 2065, Abell 2256.
Methods.We use a multi-scale spectral mapping algorithm especially designed
to map spectroscopic observables from X-ray extended emission of the
ICM. By means of a wavelet analysis, this algorithm couples
spatially resolved spectroscopy with a structure detection approach.
Derived from a former algorithm using Haar wavelets, our algorithm
is now implemented with B-spline wavelets in order to perform a more
regular analysis of the signal. Compared to other adaptive
algorithms, our method has the advantage of analysing spatially the
gas temperature structure itself, instead of being primarily driven
by the geometry of gas brightness.
Results.For the four clusters in our
sample that are major mergers, we find a rather complex thermal
structure with strong thermal variations consistent with their
dynamics. For two of them, A2065 and A2256, we perform a 3-d
analysis of cold front-like features evidenced from the gas
temperature and brightness maps. Furthermore, we detect a
significant non-radial thermal structure outside the cool core
region of the other 3 more “regular” clusters, with relative
amplitudes of about about 10% and typical sizes ranging between 2 and 3 arcmin. We investigate possible
implications of this thermal
structure on the mass estimates, by extracting the surface
brightness and temperature profiles from complementary sectors in
the “regular” clusters A1795 and A2029, corresponding to hottest
and coldest regions in the maps. For A2029, the temperature and
surface brightness gradients seem to compensate each other, leading
to a consistent mass profile. For A1795, however, the temperature
structure leads to a significant mass discrepancy in the innermost
cluster region. The third “regular” cluster, A478, is located in a
particular sky region characterised by strong variations of neutral
hydrogen column density, Nh, even on angular scales smaller than the
cluster itself. For this cluster, we derive a spectroscopic Nh map
and investigate the origin of Nh structure by discussing its
correlation with galactic emission of dust in the infrared
Temperature structure of the intra-cluster medium within relaxed clusters of galaxies
No full textUsing a wavelet algorithm, we have mapped the temperature structure of
the three relaxed clusters of galaxies Abell 478, Abell 1795 and Abell
2029. The findings of significant non-radial thermal structures
outside the core region of two of these clusters question the validity
limits of the elliptical symmetry hypothesis required for deriving
cluster mass profiles from gas brightness and temperature profiles
measurements
A performance comparison between graph and hypergraph : Topologies for passive star WDM lightwave networks
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Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Shock Heating of the Merging Galaxy Cluster A521
Get PDFA521 is an interacting galaxy cluster located at z = 0.247, hosting a low-frequency radio halo connected to an eastern radio relic. Previous Chandra observations hinted at the presence of an X-ray brightness edge at the position of the relic, which may be a shock front. We analyze a deep observation of A521 recently performed with XMM-Newton in order to probe the cluster structure up to the outermost regions covered by the radio emission. The cluster atmosphere exhibits various brightness and temperature anisotropies. In particular, two cluster cores appear to be separated by two cold fronts. We find two shock fronts, one that was suggested by Chandra and that is propagating to the east, and another to the southwestern cluster outskirt. The two main interacting clusters appear to be separated by a shock-heated region, which exhibits a spatial correlation with the radio halo. The outer edge of the radio relic coincides spatially with a shock front, suggesting that this shock is responsible for the generation of cosmic-ray electrons in the relic. The propagation direction and Mach number of the shock front derived from the gas density jump, M = 2.4 +/- 0.2, are consistent with expectations from the radio spectral index, under the assumption of Fermi I acceleration mechanism
MAPPING THE PARTICLE ACCELERATION IN THE COOL CORE OF THE GALAXY CLUSTER RX J1720.1+2638
No full textWe present new deep, high-resolution radio images of the diffuse minihalo in the cool core of the galaxy cluster RX J1720.1+2638. The images have been obtained with the Giant Metrewave Radio Telescope at 317, 617, and 1280 MHz and with the Very Large Array at 1.5, 4.9, and 8.4 GHz, with angular resolutions ranging from 1 '' to 10 ''. This represents the best radio spectral and imaging data set for any minihalo. Most of the radio flux of the minihalo arises from a bright central component with a maximum radius of similar to 80 kpc. A fainter tail of emission extends out from the central component to form a spiral-shaped structure with a length of similar to 230 kpc, seen at frequencies 1.5 GHz and below. We find indication of a possible steepening of the total radio spectrum of the minihalo at high frequencies. Furthermore, a spectral index image shows that the spectrum of the diffuse emission steepens with increasing distance along the tail. A striking spatial correlation is observed between the minihalo emission and two cold fronts visible in the Chandra X-ray image of this cool core. These cold fronts confine the minihalo, as also seen in numerical simulations of minihalo formation by sloshing-induced turbulence. All these observations favor the hypothesis that the radio-emitting electrons in cluster cool cores are produced by turbulent re-acceleration
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