350 research outputs found
Extracting few representative reconciliations with Host-Switches (Extended Abstract)
Phylogenetic tree reconciliation is the approach commonly used to in- vestigate the coevolution of sets of organisms such as hosts and symbionts. Given a phylogenetic tree for each such set, respectively denoted by H and S, together with a mapping Ï of the leaves of S to the leaves of H, a reconciliation is a mapping Ï of the internal vertices of S to the vertices of H which extends Ï with some constraints.
Given a cost for each reconciliation, a huge number of most parsimonious ones are possible, even exponential in the dimension of the trees. Without further information, any biological interpretation of the underlying coevolution would require that all optimal solutions are enumerated and examined. The latter is however impossible without pro- viding some sort of high level view of the situation. One approach would be to extract a small number of representatives, based on some notion of similarity or of equivalence between the reconciliations.
In this paper, we define two equivalence relations that allow one to identify many reconciliations with a single one, thereby reducing their number. Extensive experiments indicate that the number of output solutions greatly decreases in general. By how much clearly depends on the constraints that are given as input
Mass profiles and concentration-dark matter relation in X-ray luminous galaxy clusters
(Abriged) Assuming that the hydrostatic equilibrium holds between the
intracluster medium and the gravitational potential, we constrain the NFW
profiles in a sample of 44 X-ray luminous galaxy clusters observed with
XMM-Newton in the redshift range 0.1-0.3. We evaluate several systematic
uncertainties that affect our reconstruction of the X-ray masses. We measure
the concentration c200, the dark mass M200 and the gas mass fraction within
R500 in all the objects of our sample, providing the largest dataset of mass
parameters for galaxy clusters in this redshift range. We confirm that a tight
correlation between c200 and M200 is present and in good agreement with the
predictions from numerical simulations and previous observations. When we
consider a subsample of relaxed clusters that host a Low-Entropy-Core (LEC), we
measure a flatter c-M relation with a total scatter that is lower by 40 per
cent. From the distribution of the estimates of c200 and M200, with associated
statistical (15-25%) and systematic (5-15%) errors, we use the predicted values
from semi-analytic prescriptions calibrated through N-body numerical runs and
measure sigma_8*Omega_m^(0.60+-0.03)= 0.45+-0.01 (at 2 sigma level, statistical
only) for the subsample of the clusters where the mass reconstruction has been
obtained more robustly, and sigma_8*Omega_m^(0.56+-0.04) = 0.39+-0.02 for the
subsample of the 11 more relaxed LEC objects. With the further constraint from
the fgas distribution in our sample, we break the degeneracy in the
sigma_8-Omega_m plane and obtain the best-fit values sigma_8~1.0+-0.2
(0.75+-0.18 when the subsample of the more relaxed objects is considered) and
Omega_m = 0.26+-0.01.Comment: 21 pages. A&A in press. Minor revisions to match accepted version.
Corrected 2nd and 3rd column in Table 3, and equation (A.4
Gas clumping in galaxy clusters
The reconstruction of galaxy cluster's gas density profiles is usually
performed by assuming spherical symmetry and averaging the observed X-ray
emission in circular annuli. In the case of a very inhomogeneous and asymmetric
gas distribution, this method has been shown to return biased results in
numerical simulations because of the dependence of the X-ray emissivity.
We propose a method to recover the true density profiles in the presence of
inhomogeneities, based on the derivation of the azimuthal median of the surface
brightness in concentric annuli. We demonstrate the performance of this method
with numerical simulations, and apply it to a sample of 31 galaxy clusters in
the redshift range 0.04-0.2 observed with ROSAT/PSPC. The clumping factors
recovered by comparing the mean and the median are mild and show a slight trend
of increasing bias with radius. For , we measure a clumping factor
, which indicates that the thermodynamic properties and
hydrostatic masses measured in this radial range are only mildly affected by
this effect. Comparing our results with three sets of hydrodynamical numerical
simulations, we found that non-radiative simulations significantly overestimate
the level of inhomogeneities in the ICM, while the runs including cooling, star
formation, and AGN feedback reproduce the observed trends closely. Our results
indicate that most of the accretion of X-ray emitting gas is taking place in
the diffuse, large-scale accretion patterns rather than in compact structures.Comment: 12 pages, 11 figures, accepted for publication in MNRAS.
Largely-improved version compared to v1, method and comparison with
simulations update
The cool core state of Planck SZ-selected clusters versus X-ray selected samples: evidence for cool core bias
We characterized the population of galaxy clusters detected with the SZ
effect with Planck, by measuring the cool core state of the objects in a
well-defined subsample of the Planck catalogue. We used as indicator the
concentration parameter Santos et al. (2008). The fraction of cool core
clusters is and does not show significant indications of
evolution in the redshift range covered by our sample. We compare the
distribution of the concentration parameter in the Planck sample with the one
of the X-ray selected sample MACS (Mann & Ebeling, 2011): the distributions are
significantly different and the cool core fraction in MACS is much higher (). Since X-ray selected samples are known to be biased towards cool
cores due to the presence of their prominent surface brightness peak, we
simulated the impact of the "cool core bias" following Eckert et al. (2011). We
found that it plays a large role in the difference between the fractions of
cool cores in the two samples. We examined other selection effects that could
in principle bias SZ-surveys against cool cores but we found that their impact
is not sufficient to explain the difference between Planck and MACS. The
population of X-ray under-luminous objects, which are found in SZ-surveys but
missing in X-ray samples (Planck Collaboration 2016), could possibly contribute
to the difference, as we found most of them to be non cool cores, but this
hypothesis deserves further investigation.Comment: Accepted for publication in MNRA
Where does the gas fueling star formation in BCGs originate?
We investigate the relationship between X-ray cooling and star formation in
brightest cluster galaxies (BCGs). We present an X-ray spectral analysis of the
inner regions, 10-40 kpc, of six nearby cool core clusters (z<0.35) observed
with Chandra ACIS. This sample is selected on the basis of the high star
formation rate (SFR) observed in the BCGs. We restrict our search for cooling
gas to regions that are roughly cospatial with the starburst. We fit single-
and multi-temperature mkcflow models to constrain the amount of isobarically
cooling intracluster medium (ICM). We find that in all clusters, below a
threshold temperature ranging between 0.9 and 3 keV, only upper limits can be
obtained. In four out of six objects, the upper limits are significantly below
the SFR and in two, namely A1835 and A1068, they are less than a tenth of the
SFR. Our results suggests that a number of mechanisms conspire to hide the
cooling signature in our spectra. In a few systems the lack of a cooling
signature may be attributed to a relatively long delay time between the X-ray
cooling and the star burst. However, for A1835 and A1068, where the X-ray
cooling time is shorter than the timescale of the starburst, a possible
explanation is that the region where gas cools out of the X-ray phase extends
to very large radii, likely beyond the core of these systems.Comment: to appear in A&
Ni abundance in the core of the Perseus Cluster: an answer to the significance of resonant scattering
Using an XMM-Newton observation of the Perseus cluster we show that the
excess in the flux of the 7-8 keV line complex previously detected by ASCA and
BeppoSAX is due to an overabundance of Nickel rather than to an anomalously
high Fe He/Fe He ratio. This observational fact leads to the
main result that resonant scattering, which was assumed to be responsible for
the supposed anomalous Fe He/Fe He ratio, is no longer required.
The absence of resonant scattering points towards the presence of significant
gas motions (either turbulent or laminar) in the core of the Perseus cluster.Comment: 29 pages, 10 bw figures, accepted for publication in the
Astrophysical Journa
The stripping of a galaxy group diving into the massive cluster A2142
Structure formation in the current Universe operates through the accretion of
group-scale systems onto massive clusters. The detection and study of such
accreting systems is crucial to understand the build-up of the most massive
virialized structures we see today. We report the discovery with XMM-Newton of
an irregular X-ray substructure in the outskirts of the massive galaxy cluster
Abell 2142. The tip of the X-ray emission coincides with a concentration of
galaxies. The bulk of the X-ray emission of this substructure appears to be
lagging behind the galaxies and extends over a projected scale of at least 800
kpc. The temperature of the gas in this region is 1.4 keV, which is a factor of
~4 lower than the surrounding medium and is typical of the virialized plasma of
a galaxy group with a mass of a few 10^13M_sun. For this reason, we interpret
this structure as a galaxy group in the process of being accreted onto the main
dark-matter halo. The X-ray structure trailing behind the group is due to gas
stripped from its original dark-matter halo as it moves through the
intracluster medium (ICM). This is the longest X-ray trail reported to date.
For an infall velocity of ~1,200 km s-1 we estimate that the stripped gas has
been surviving in the presence of the hot ICM for at least 600 Myr, which
exceeds the Spitzer conduction timescale in the medium by a factor of >~400.
Such a strong suppression of conductivity is likely related to a tangled
magnetic field with small coherence length and to plasma microinstabilities.
The long survival time of the low-entropy intragroup medium suggests that the
infalling material can eventually settle within the core of the main cluster.Comment: 11 pages, 7 figures, accepted for publication in A&
A textbook example of ram-pressure stripping in the Hydra A/A780 cluster
In the current epoch, one of the main mechanisms driving the growth of galaxy clusters is the continuous accretion of group-scale halos. In this process, the ram pressure applied by the hot intracluster medium on the gas content of the infalling group is responsible for stripping the gas from its dark-matter halo, which gradually leads to the virialization of the infalling gas in the potential well of the main cluster. Using deep wide-field observations of the poor cluster Hydra A/A780 with XMM-Newton and Suzaku, we report the discovery of an infalling galaxy group 1.1 Mpc south of the cluster core. The presence of a substructure is confirmed by a dynamical study of the galaxies in this region. A wake of stripped gas is trailing behind the group over a projected scale of 760 kpc. The temperature of the gas along the wake is constant at kT ~ 1.3 keV, which is about a factor of two less than the temperature of the surrounding plasma. We observe a cold front pointing westwards compared to the peak of the group, which indicates that the group is currently not moving in the direction of the main cluster, but is moving along an almost circular orbit. The overall morphology of the group bears remarkable similarities with high-resolution numerical simulations of such structures, which greatly strengthens our understanding of the ram-pressure stripping process
On the connection between turbulent motions and particle acceleration in galaxy clusters
Giant radio halos are Mpc-scale diffuse radio sources associated with the
central regions of galaxy clusters. The most promising scenario to explain the
origin of these sources is that of turbulent re-acceleration, in which MeV
electrons injected throughout the formation history of galaxy clusters are
accelerated to higher energies by turbulent motions mostly induced by cluster
mergers. In this Letter, we use the amplitude of density fluctuations in the
intracluster medium as a proxy for the turbulent velocity and apply this
technique to a sample of 51 clusters with available radio data. Our results
indicate a segregation in the turbulent velocity of radio halo and radio quiet
clusters, with the turbulent velocity of the former being on average higher by
about a factor of two. The velocity dispersion recovered with this technique
correlates with the measured radio power through the relation , which implies that the radio power is
nearly proportional to the turbulent energy rate. Our results provide an
observational confirmation of a key prediction of the turbulent re-acceleration
model and possibly shed light on the origin of radio halos.Comment: Submitted to ApJ Letter
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