9,615 research outputs found
3C 295, a cluster and its cooling flow at z=0.46
We present ROSAT HRI data of the distant and X-ray luminous (L_x(bol)=2.6^
{+0.4}_{-0.2} 10^{45}erg/sec) cluster of galaxies 3C 295. We fit both a
one-dimensional and a two-dimensional isothermal beta-model to the data, the
latter one taking into account the effects of the point spread function (PSF).
For the error analysis of the parameters of the two-dimensional model we
introduce a Monte-Carlo technique. Applying a substructure analysis, by
subtracting a cluster model from the data, we find no evidence for a merger,
but we see a decrement in emission South-East of the center of the cluster,
which might be due to absorption. We confirm previous results by Henry &
Henriksen(1986) that 3C 295 hosts a cooling flow. The equations for the simple
and idealized cooling flow analysis presented here are solely based on the
isothermal beta-model, which fits the data very well, including the center of
the cluster. We determine a cooling flow radius of 60-120kpc and mass accretion
rates of dot{M}=400-900 Msun/y, depending on the applied model and temperature
profile. We also investigate the effects of the ROSAT PSF on our estimate of
dot{M}, which tends to lead to a small overestimate of this quantity if not
taken into account. This increase of dot{M} (10-25%) can be explained by a
shallower gravitational potential inferred by the broader overall profile
caused by the PSF, which diminishes the efficiency of mass accretion. We also
determine the total mass of the cluster using the hydrostatic approach. At a
radius of 2.1 Mpc, we estimate the total mass of the cluster (M{tot}) to be
(9.2 +/- 2.7) 10^{14}Msun. For the gas to total mass ratio we get M{gas}/M{tot}
=0.17-0.31, in very good agreement with the results for other clusters of
galaxies, giving strong evidence for a low density universe.Comment: 26 pages, 7 figures, accepted for publication in Ap
Size matters: cardinality-constrained clustering and outlier detection via conic optimization
Plain vanilla K-means clustering has proven to be successful in practice, yet it suffers from outlier sensitivity and may produce highly unbalanced clusters. To mitigate both shortcomings, we formulate a joint outlier detection and clustering problem, which assigns a prescribed number of datapoints to an auxiliary outlier cluster and performs cardinality-constrainedK-means clustering on the residual dataset, treating the cluster cardinalities as a given input. We cast this problem as a mixed-integer linear program (MILP) that admits tractable semidefinite and linear programming relaxations. We propose deterministic rounding schemes thattransform the relaxed solutions to feasible solutions for the MILP. We also prove that these solutions areoptimal in the MILP if a cluster separation condition holds
Measuring cosmological bulk flows via the kinematic Sunyaev-Zeldovich effect in the upcoming cosmic microwave background maps
We propose a new method to measure the possible large-scale bulk flows in the
Universe from the cosmic microwave background (CMB) maps from the upcoming
missions, MAP and Planck. This can be done by studying the statistical
properties of the CMB temperature field at many X-ray cluster positions. At
each cluster position, the CMB temperature fluctuation will be a combination of
the Sunyaev-Zeldovich (SZ) kinematic and thermal components, the cosmological
fluctuations and the instrument noise term. When averaged over many such
clusters the last three will integrate down, whereas the first one will be
dominated by a possible bulk flow component. In particular, we propose to use
all-sky X-ray cluster catalogs that should (or could) be available soon from
X-ray satellites, and then to evaluate the dipole component of the CMB field at
the cluster positions. We show that for the MAP and Planck mission parameters
the dominant contributions to the dipole will be from the terms due to the SZ
kinematic effect produced by the bulk flow (the signal we seek) and the
instrument noise (the noise in our signal). Computing then the expected
signal-to-noise ratio for such measurement, we get that at the 95 % confidence
level the bulk flows on scales >100h^{-1} Mpc can be probed down to the
amplitude of km/sec with the MAP data and down to only 30 km/sec with
the Planck mission.Comment: Astrophysical Journal Letters, in pres
Discovery of X-ray emission rom the distant lensing cluster of galaxies CL2236-04 at z = 0.552
X-ray emission from the distant lensing cluster CL2236-04 at = 0.552 was
discovered by ASCA and ROSAT/HRI observations. If the spherical symmetric mass
distribution model of the cluster is assumed, the lensing estimate of the
cluster mass is a factor of two higher than that obtained from X-ray
observations as reported for many distant clusters. However, the elliptical and
clumpy lens model proposed by Kneib et al.(1993) is surprisingly consistent
with the X-ray observations assuming that the X-ray emitting hot gas is
isothermal and in a hydrostatic equilibrium state. The existence of the cooling
flow in the central region of the cluster is indicated by the short central
cooling time and the excess flux detected by ROSAT/HRI compared to the ASCA
flux. However, it is shown that even if the AXJ2239-0429 has a cooling flow in
the central region, the temperature measured by ASCA which is the mean
emission-weighted cluster temperature in this case, should not be cooler than
and different from the virial temperature of the cluster. Therefore, we
conclude that the effect of the clumpiness and non-zero ellipticity in the mass
distribution of the cluster are essential to explain the observed feature of
the giant luminous arc, and there is no discrepancy between strong lensing and
X-ray estimation of the mass of the cluster in this cluster.Comment: 18 pages, including 4 postscripts figs, LaTex. To appear in Part 1 of
The Astrophysical Journa
Cluster Mass Estimate and a Cusp of the Mass Density Distribution in Clusters of Galaxies
We study density cusps in the center of clusters of galaxies to reconcile
X-ray mass estimates with gravitational lensing masses. For various mass
density models with cusps we compute X-ray surface brightness distribution, and
fit them to observations to measure the range of parameters in the density
models. The Einstein radii estimated from these density models are compared
with Einstein radii derived from the observed arcs for Abell 2163, Abell 2218,
and RX J1347.5-1145. The X-ray masses and lensing masses corresponding to these
Einstein radii are also compared. While steeper cusps give smaller ratios of
lensing mass to X-ray mass, the X-ray surface brightnesses estimated from
flatter cusps are better fits to the observations. For Abell 2163 and Abell
2218, although the isothermal sphere with a finite core cannot produce giant
arc images, a density model with a central cusp can produce a finite Einstein
radius, which is smaller than the observed radii. We find that a total mass
density profile which declines as produces the largest radius
in models which are consistent with the X-ray surface brightness profile. As
the result, the extremely large ratio of the lensing mass to the X-ray mass is
improved from 2.2 to 1.4 for Abell 2163, and from 3 to 2.4 for Abell 2218. For
RX J1347.5-1145, which is a cooling flow cluster, we cannot reduce the mass
discrepancy.Comment: 23 pages, 10 figures, Latex, uses aasms4.sty, accepted for
publication in ApJ, Part
The cluster M-T relation from temperature profiles observed with ASCA and ROSAT
We calibrate the galaxy cluster mass - temperature relation using the
temperature profiles of intracluster gas observed with ASCA (for hot clusters)
and ROSAT (for cool groups). Our sample consists of apparently relaxed clusters
for which the total masses are derived assuming hydrostatic equilibrium. The
sample provides data on cluster X-ray emission-weighted cooling flow-corrected
temperatures and total masses up to r_1000. The resulting M-T scaling in the
1-10 keV temperature range is M_1000 = (1.23 +- 0.20)/h_50 10^15 Msun (T/10
keV)^{1.79 +- 0.14} with 90% confidence errors, or significantly (99.99%
confidence) steeper than the self-similar relation M propto T^{3/2}. For any
given temperature, our measured mass values are significantly smaller compared
to the simulation results of Evrard et al. (1996) that are frequently used for
mass-temperature scaling. The higher-temperature subsample (kT > 4 keV) is
consistent with M propto T^{3/2}, allowing the possibility that the
self-similar scaling breaks down at low temperatures, perhaps due to heating by
supernovae that is more important for low-temperature groups and galaxies as
suggested by earlier works.Comment: 8 pages, 2 figures, accepted by Ap
Security of practical private randomness generation
Measurements on entangled quantum systems necessarily yield outcomes that are
intrinsically unpredictable if they violate a Bell inequality. This property
can be used to generate certified randomness in a device-independent way, i.e.,
without making detailed assumptions about the internal working of the quantum
devices used to generate the random numbers. Furthermore these numbers are also
private, i.e., they appear random not only to the user, but also to any
adversary that might possess a perfect description of the devices. Since this
process requires a small initial random seed, one usually speaks of
device-independent randomness expansion.
The purpose of this paper is twofold. First, we point out that in most real,
practical situations, where the concept of device-independence is used as a
protection against unintentional flaws or failures of the quantum apparatuses,
it is sufficient to show that the generated string is random with respect to an
adversary that holds only classical-side information, i.e., proving randomness
against quantum-side information is not necessary. Furthermore, the initial
random seed does not need to be private with respect to the adversary, provided
that it is generated in a way that is independent from the measured systems.
The devices, though, will generate cryptographically-secure randomness that
cannot be predicted by the adversary and thus one can, given access to free
public randomness, talk about private randomness generation.
The theoretical tools to quantify the generated randomness according to these
criteria were already introduced in [S. Pironio et al, Nature 464, 1021
(2010)], but the final results were improperly formulated. The second aim of
this paper is to correct this inaccurate formulation and therefore lay out a
precise theoretical framework for practical device-independent randomness
expansion.Comment: 18 pages. v3: important changes: the present version focuses on
security against classical side-information and a discussion about the
significance of these results has been added. v4: minor changes. v5: small
typos correcte
Dark Matter and Baryon Fraction at the Virial Radius in Abell 2256
We combine ASCA and ROSAT X-ray data to constrain the radial dark matter
distribution in the primary cluster of A2256, free from the isothermality
assumption. Both instruments indicate that the temperature declines with
radius. The region including the central galaxy has a multicomponent spectrum,
which results in a wide range of allowed central temperatures. We find that the
secondary subcluster has a temperature and luminosity typical of a rich
cluster; however, the ASCA temperature map shows no signs of an advanced
merger. It is therefore assumed that the primary cluster is in hydrostatic
equilibrium. The data then require dark matter density profiles steeper than
rho ~ r^-2.5 in its outer part. Acceptable models have a total mass within
r=1.5 Mpc (the virial radius) of 6.0+-1.5 10^14 Msun at the 90% confidence,
about 1.6 times smaller than the mass derived assuming isothermality. Near the
center, dark matter profiles with and without central cusps are consistent with
the data. Total mass inside the X-ray core (r=0.26 Mpc) is 1.28+-0.08 10^14
Msun, which exceeds the isothermal value by a factor of 1.4. Although the
confidence intervals above may be underestimates since they do not include
possible asymmetry and departures from hydrostatic equilibrium, the behavior of
the mass distribution, if applicable to other clusters, can bring into better
agreement X-ray and lensing mass estimates, but aggravate the ``baryon
catastrophe''. The observed considerable increase in the gas content with
radius, not anticipated by simulations, may imply that a significant fraction
of thermal gas energy comes from sources other than gravity and merger shocks.Comment: Added dynamic argument against advanced merger. Latex, 10 pages, 3
figures; uses emulateapj.sty. ApJ in pres
Simulations of galactic winds and starbursts in galaxy clusters
We present an investigation of the metal enrichment of the intra-cluster
medium (ICM) by galactic winds and merger-driven starbursts. We use combined
N-body/hydrodynamic simulations with a semi-numerical galaxy formation model.
The mass loss by galactic winds is obtained by calculating transonic solutions
of steady state outflows, driven by thermal, cosmic ray and MHD wave pressure.
The inhomogeneities in the metal distribution caused by these processes are an
ideal tool to reveal the dynamical state of a galaxy cluster. We present
surface brightness, X-ray emission weighted temperature and metal maps of our
model clusters as they would be observed by X-ray telescopes like XMM-Newton.
We show that X-ray weighted metal maps distinguish between pre- or post-merger
galaxy clusters by comparing the metallicity distribution with the
galaxy-density distribution: pre-mergers have a metallicity gap between the
subclusters, post-mergers a high metallicity between subclusters. We apply our
approach to two observed galaxy clusters, Abell 3528 and Abell 3921, to show
whether they are pre- or post-merging systems. The survival time of the
inhomogeneities in the metallicity distribution found in our simulations is up
to several Gyr. We show that galactic winds and merger-driven starbursts enrich
the ICM very efficiently after z=1 in the central (~ 3 Mpc radius) region of a
galaxy cluster.Comment: 18 pages, 25 figures, 2 tables, accepted for publication in A&A, more
technical details added - results are unaffected, high resolution PDF version
is available at http://astro.uibk.ac.at/Kapferer.pd
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