2,906 research outputs found
Dust in Hot Plasma of Nearby Dusty Elliptical Galaxies Observed with the Spitzer Space Telescope
We report on mid- and far-IR Spitzer observations of 7 nearby dusty
elliptical galaxies by using the Multiband Imaging Photometer (MIPS) and
Infrared Spectrograph (IRS). Our sample galaxies are known to contain an
excessive amount of interstellar dust against sputtering destruction in hot
plasma filling the interstellar space of elliptical galaxies. In order to study
the origin and the properties of the excess dust in the hot plasma, we selected
galaxies with a wide range of X-ray luminosities but similar optical luminos
ities for our Spitzer Guest Observers (GO1) program. The 7 galaxies are
detected at the MIPS 24 um, 70 um, and 160 um bands; the far- to mid-IR flux
ratios of relatively X-ray-bright elliptical galaxies are lower than those of
X-ray-faint galaxies. From the IRS spectra, polycyclic aromatic hydrocarbon
(PAH) emission features are detected significantly from 5 of the 7 galaxies; t
he emission intensities are weaker as the X-ray luminosity of the galaxy is
larger. We have found a correlation between the far- to mid-IR flux ratio and
the equivalent width of the PAH emission feature. We have obtained apparent
spatial correspondence between mid-IR and X-ray distributions in the outer
regions for the three X-ray-brightest galaxies in our sample. Possible
interpretations for our observational results are discussed.Comment: 25 pages, 7 figures, accepted for publication in Publications of the
Astronomical Society of Japa
Investigating the hard X-ray emission from the hottest Abell cluster A2163 with Suzaku
We present the results from Suzaku observations of the hottest Abell galaxy
cluster A2163 at . To study the physics of gas heating in cluster
mergers, we investigated hard X-ray emission from the merging cluster A2163,
which hosts the brightest synchrotron radio halo. We analyzed hard X-ray
spectra accumulated from two-pointed Suzaku observations. Non-thermal hard
X-ray emission should result from the inverse Compton (IC) scattering of
relativistic electrons by the CMB photons. To measure this emission, the
dominant thermal emission in the hard X-ray band must be modeled in detail. To
this end, we analyzed the combined broad-band X-ray data of A2163 collected by
Suzaku and XMM-Newton, assuming single- and multi-temperature models for
thermal emission and the power-law model for non-thermal emission. From the
Suzaku data, we detected significant hard X-ray emission from A2163 in the
12-60 keV band at the level (or at the level if a
systematic error is considered). The Suzaku HXD spectrum alone is consistent
with the single-T thermal model of gas temperature keV. From the XMM
data, we constructed a multi-T model including a very hot ( keV)
component in the NE region. Incorporating the multi-T and the power-law models
into a two-component model with a radio-band photon index, the 12-60 keV energy
flux of non-thermal emission is constrained within . The 90% upper limit of detected IC
emission is marginal ( in the
12-60 keV). The estimated magnetic field in A2163 is .
While the present results represent a three-fold increase in the accuracy of
the broad band spectral model of A2163, more sensitive hard X-ray observations
are needed to decisively test for the presence of hard X-ray emission due to IC
emission.Comment: 7 pages, 7 figures, A&A accepted. Minor correctio
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
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
Forming Clusters of Galaxies as the Origin of Unidentified GeV Gamma-Ray Sources
Over half of GeV gamma-ray sources observed by the EGRET experiment have not
yet been identified as known astronomical objects. There is an isotropic
component of such unidentified sources, whose number is about 60 in the whole
sky. Here we calculate the expected number of dynamically forming clusters of
galaxies emitting gamma-rays by high energy electrons accelerated in the shock
wave when they form, in the framework of the standard theory of structure
formation. We find that a few tens of such forming clusters should be
detectable by EGRET and hence a considerable fraction of the isotropic
unidentified sources can be accounted for, if about 5% of the shock energy is
going into electron acceleration. We argue that these clusters are very
difficult to detect in x-ray or optical surveys compared with the conventional
clusters, because of their extended angular size of about 1 degree. Hence they
define a new population of ``gamma-ray clusters''. If this hypothesis is true,
the next generation gamma-ray telescopes such as GLAST will detect more than a
few thousands of gamma-ray clusters. It would provide a new tracer of
dynamically evolving structures in the universe, in contrast to the x-ray
clusters as a tracer of hydrodynamically stabilized systems. We also derive the
strength of magnetic field required for the extragalactic gamma-ray background
by structure formation to extend up to 100 GeV as observed, that is about
10^{-5} of the shock-heated baryon energy density.Comment: Accepted by ApJ after minor revisions. Received May 9, Accepted
August 3. 8 pages including 2 figure
Biases on cosmological parameters by general relativity effects
General relativistic corrections to the galaxy power spectrum appearing at
the horizon scale, if neglected, may induce biases on the measured values of
the cosmological parameters. In this paper, we study the impact of general
relativistic effects on non standard cosmologies such as scenarios with a time
dependent dark energy equation of state, with a coupling between the dark
energy and the dark matter fluids or with non-Gaussianities. We then explore
whether general relativistic corrections affect future constraints on
cosmological parameters in the case of a constant dark energy equation of state
and of non-Gaussianities. We find that relativistic corrections on the power
spectrum are not expected to affect the foreseen errors on the cosmological
parameters nor to induce large biases on them.Comment: 17 pages, 5 figures, one added figure, results of Tab. I revised,
version accepted for publication in PR
Measuring the Cosmic Equation of State with Counts of Galaxies
The classical dN/dz test allows the determination of fundamental cosmological
parameters from the evolution of the cosmic volume element. This test is
applied by measuring the redshift distribution of a tracer whose evolution in
number density is known. In the past, ordinary galaxies have been used as such
a tracer; however, in the absence of a complete theory of galaxy formation,
that method is fraught with difficulties. In this paper, we propose studying
instead the evolution of the apparent abundance of dark matter halos as a
function of their circular velocity, observable via the linewidths or rotation
speeds of visible galaxies. Upcoming redshift surveys will allow the linewidth
distribution of galaxies to be determined at both z~1 and the present day. In
the course of studying this test, we have devised a rapid, improved
semi-analytic method for calculating the circular velocity distribution of dark
halos based upon the analytic mass function of Sheth et al. (1999) and the
formation time distribution of Lacey & Cole (1993). We find that if selection
effects are well-controlled and minimal external constraints are applied, the
planned DEEP Redshift Survey should allow the measurement of the cosmic
equation-of-state parameter w to 10% (as little as 3% if Omega_m has been
well-determined from other observations). This type of test has the potential
also to provide a constraint on any evolution of w such as that predicted by
``tracker'' models.Comment: 4 pages plus 3 embedded figures; version approved by Ap. J. Letters.
A greatly improved error analysis has been added, along with a figure showing
complementarity to other cosmological test
Imaging Simulations of the Sunyaev-Zel'dovich Effect for ALMA
We present imaging simulations of the Sunyaev-Zel'dovich effect of galaxy
clusters for the Atacama Large Millimeter/submillimeter Array (ALMA) including
the Atacama Compact Array (ACA). In its most compact configuration at 90GHz,
ALMA will resolve the intracluster medium with an effective angular resolution
of 5 arcsec. It will provide a unique probe of shock fronts and relativistic
electrons produced during cluster mergers at high redshifts, that are hard to
spatially resolve by current and near-future X-ray detectors. Quality of image
reconstruction is poor with the 12m array alone but improved significantly by
adding ACA; expected sensitivity of the 12m array based on the thermal noise is
not valid for the Sunyaev-Zel'dovich effect mapping unless accompanied by an
ACA observation of at least equal duration. The observations above 100 GHz will
become excessively time-consuming owing to the narrower beam size and the
higher system temperature. On the other hand, significant improvement of the
observing efficiency is expected once Band 1 is implemented in the future.Comment: 16 pages, 12 figures. Accepted for publication in PASJ. Note added in
proof is include
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