181 research outputs found
Cosmological Implications of Number Counts of Clusters of Galaxies: logN-logS in X-Ray and Submm Bands
We compute the number counts of clusters of galaxies, the logN-logS relation,
in several X-ray and submm bands on the basis of the Press-Schechter theory. We
pay particular attention to a set of theoretical models which well reproduce
the ROSAT 0.5-2 keV band logN-logS, and explore possibilities to further
constrain the models from future observations with ASCA and/or at submm bands.
The latter is closely related to the European PLANCK mission and the Japanese
Large Millimeter and Submillimeter Array (LMSA) project. We exhibit that one
can break the degeneracy in an acceptable parameter region on the plane by combining the ROSAT logN-logS and the submm number counts.
Models which reproduce the ROSAT band logN-logS will have str at in the ASCA 2-10 keV band, and at in the submm (0.85mm) band. The
amplitude of the logN-logS is very sensitive to the model parameters in the
submm band. We also compute the redshift evolution of the cluster number counts
and compare with that of the X-ray brightest Abell-type clusters. The results,
although still preliminary, point to low density ()
universes. The contribution of clusters to the X-ray and submm background
radiations is shown to be insignificant in any model compatible with the ROSAT
logN-logS.Comment: 18 pages, 6 figures and 1 table. Figures revised. Accepted for
publication in PAS
Monte-Carlo Modeling of Non-Gravitational Heating Processes in Galaxy Clusters
We consider non-gravitational heating effects on galaxy clusters on the basis
of the Monte-Carlo modeling of merging trees of dark matter halos combined with
the thermal evolution of gas inside each halo. Under the assumption of
hydrostatic equilibrium and the isothermal gas profiles, our model takes
account of the metallicity evolution, metallicity-dependent cooling of gas,
supernova energy feedback, and heating due to jets of radio galaxies in a
consistent manner. The observed properties of galaxy clusters can be explained
in models with higher non-gravitational heating efficiency than that in the
conventional model. Possibilities include jet heating by the Fanaroff-Riley
Type II radio galaxies, and the enhanced star formation efficiency and/or
supernova energy feedback, especially at high redshifts.Comment: 29 pages, 12 figures. To appear in PASJ, February 25, 200
Reliability of merger tree realizations of dark halos in the Monte-Carlo modeling of galaxy formation
We examine the reliability of the merger trees generated for the Monte-Carlo
modeling of galaxy formation. In particular we focus on the cold gas fraction
predicted from the merger trees with different assumptions on the progenitor
distribution function, the timestep, and the mass resolution. We show that the
cold gas fraction is sensitive to the accuracy of the merger trees at
small-mass scales of progenitors at high redshifts. One can reproduce the
Press-Schechter prediction to a reasonable degree by adopting a fairly large
number of redshift bins, N_{step} ~ 1000 in generating merger trees, which is a
factor of ten larger than the canonical value used in previous literature.Comment: 9 pages, 10 figures. To appear in PASJ, October 25, 200
Systematic bias in the estimate of cluster mass and the fluctuation amplitude from cluster abundance statistics
We revisit the estimate of the mass fluctuation amplitude, sigma_8, from the
observational X-ray cluster abundance. In particular, we examine the effect of
the systematic difference between the cluster virial mass estimated from the
X-ray spectroscopy, M_{vir, spec}, and the true virial mass of the
corresponding halo, M_{vir}. Mazzotta et al. (2004) recently pointed out the
possibility that alpha_M = M_{vir, spec}/M_{vir} is systematically lower than
unity. We perform the statistical analysis combining the latest X-ray cluster
sample and the improved theoretical models and find that sigma_8 \sim 0.76 +/-
0.01 + 0.50 (1-alpha_M) for 0.5 \le alpha_M \le 1, where the quoted errors are
statistical only. Thus if alpha_M \sim 0.7, the value of sigma_8 from cluster
abundance alone is now in better agreement with other cosmological data
including the cosmic microwave background, the galaxy power spectrum and the
weak lensing data. The current study also illustrates the importance of
possible systematic effects in mapping real clusters to underlying dark halos
which changes the interpretation of cluster abundance statistics.Comment: 13 pages, 4 figures. To appear in PASJ, April 25, 200
Systematic Errors in the Hubble Constant Measurement from the Sunyaev-Zel'dovich effect
The Hubble constant estimated from the combined analysis of the
Sunyaev-Zel'dovich effect and X-ray observations of galaxy clusters is
systematically lower than those from other methods by 10-15 percent. We examine
the origin of the systematic underestimate using an analytic model of the
intracluster medium (ICM), and compare the prediction with idealistic triaxial
models and with clusters extracted from cosmological hydrodynamical
simulations. We identify three important sources for the systematic errors;
density and temperature inhomogeneities in the ICM, departures from
isothermality, and asphericity. In particular, the combination of the first two
leads to the systematic underestimate of the ICM spectroscopic temperature
relative to its emission-weighed one. We find that these three systematics well
reproduce both the observed bias and the intrinsic dispersions of the Hubble
constant estimated from the Sunyaev-Zel'dovich effect.Comment: 26 pages, 7 figures, accepted for publication in ApJ, Minor change
On the Design Rationale of SIMON Block Cipher: Integral Attacks and Impossible Differential Attacks against SIMON Variants
SIMON is a lightweight block cipher designed by NSA in 2013. NSA presented the specification and the implementation efficiency, but they did not provide detailed security analysis nor the design rationale. The original SIMON has rotation constants of , and Kölbl {\it et al}.~regarded the constants as a parameter , and analyzed the security of SIMON block cipher variants against differential and linear attacks for all the choices of .
This paper complements the result of Kölbl {\it et al}.~by considering integral and impossible differential attacks. First, we search the number of rounds of integral distinguishers by using a supercomputer. Our search algorithm follows the previous approach by Wang {\it et al}., however, we introduce a new choice of the set of plaintexts satisfying the integral property. We show that the new choice indeed extends the number of rounds for several parameters. We also search the number of rounds of impossible differential characteristics based on the miss-in-the-middle approach.
Finally, we make a comparison of all parameters from our results and the observations by Kölbl {\it et al}. Interesting observations are obtained, for instance we find that the optimal parameters with respect to the resistance against differential attacks are not stronger than the original parameter with respect to integral and impossible differential attacks. We also obtain a parameter that is better than the original parameter with respect to security against these four attacks
Extracting Galaxy Cluster Gas Inhomogeneity from X-ray Surface Brightness: A Statistical Approach and Application to Abell 3667
Our previous analysis indicates that small-scale fluctuations in the
intracluster medium (ICM) from cosmological hydrodynamic simulations follow the
lognormal distribution. In order to test the lognormal nature of the ICM
directly against X-ray observations of galaxy clusters, we develop a method of
extracting statistical information about the three-dimensional properties of
the fluctuations from the two-dimensional X-ray surface brightness.
We first create a set of synthetic clusters with lognormal fluctuations.
Performing mock observations of these synthetic clusters, we find that the
resulting X-ray surface brightness fluctuations also follow the lognormal
distribution fairly well. Systematic analysis of the synthetic clusters
provides an empirical relation between the density fluctuations and the X-ray
surface brightness. We analyze \chandra observations of the galaxy cluster
Abell 3667, and find that its X-ray surface brightness fluctuations follow the
lognormal distribution. While the lognormal model was originally motivated by
cosmological hydrodynamic simulations, this is the first observational
confirmation of the lognormal signature in a real cluster. Finally we check the
synthetic cluster results against clusters from cosmological hydrodynamic
simulations. As a result of the complex structure exhibited by simulated
clusters, the empirical relation shows large scatter. Nevertheless we are able
to reproduce the true value of the fluctuation amplitude of simulated clusters
within a factor of two from their X-ray surface brightness alone.
Our current methodology combined with existing observational data is useful
in describing and inferring the statistical properties of the three dimensional
inhomogeneity in galaxy clusters.Comment: 34 pages, 17 figures, accepted for publication in Ap
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