670 research outputs found
Mapping the Gas Turbulence in the Coma Cluster: Predictions for Astro-H
Astro-H will be able for the first time to map gas velocities and detect
turbulence in galaxy clusters. One of the best targets for turbulence studies
is the Coma cluster, due to its proximity, absence of a cool core, and lack of
a central active galactic nucleus. To determine what constraints Astro-H will
be able to place on the Coma velocity field, we construct simulated maps of the
projected gas velocity and compute the second-order structure function, an
analog of the velocity power spectrum. We vary the injection scale, dissipation
scale, slope, and normalization of the turbulent power spectrum, and apply
measurement errors and finite sampling to the velocity field. We find that even
with sparse coverage of the cluster, Astro-H will be able to measure the Mach
number and the injection scale of the turbulent power spectrum--the quantities
determining the energy flux down the turbulent cascade and the diffusion rate
for everything that is advected by the gas (metals, cosmic rays, etc.). Astro-H
will not be sensitive to the dissipation scale or the slope of the power
spectrum in its inertial range, unless they are outside physically motivated
intervals. We give the expected confidence intervals for the injection scale
and the normalization of the power spectrum for a number of possible pointing
configurations, combining the structure function and velocity dispersion data.
Importantly, we also determine that measurement errors on the line shift will
bias the velocity structure function upward, and show how to correct this bias.Comment: 18 pages, 13 figures. Matches published ApJ version, except that it
fixes an error in the left panel of Figure 5 that is being addressed in an
ApJ erratu
American Corn in Russia: Lessons of the People-to-People Diplomacy and Capitalism
This paper is devoted to the “corn diplomacy” in the context of Russian-American relations from the end of the 19th century to the Cold war period. The author focuses her attention on three cases dealt with the American attempts to export their corn and secrets of corn production to the Russian Empire and the Soviet Union. Thеsе thematic priorities give her the brilliant opportunity to analyze two dimensions of American messianic feelings deter- mined the stable long-term perception trends of Russia in the American society. The eco- nomic one arose from the attractive prospects of exports of goods, capital, and technologies into Russian markets (Russia was supposed to learn “the lessons of American capitalism”). The humanitarian one turned a famished and backward Russia into the object of aid from the rich and prosperous America and the Americans—into “international philanthropists”. At the same time one of the main author’s conclusions is that the “corn diplomacy” played an important role in promoting better understanding between Russian and Americans be- came the equivalent of the people-to-people diplomacy.
Stellar kinematics of X-ray bright massive elliptical galaxies
We discuss a simple and fast method for estimating masses of early-type
galaxies from optical data and compare the results with X-ray derived masses.
The optical method relies only on the most basic observables such as the
surface brightness and the line-of-sight velocity dispersion
profiles and provides an anisotropy-independent estimate of the
galaxy circular speed . The mass-anisotropy degeneracy is effectively
overcome by evaluating at a characteristic radius defined
from {\it local} properties of observed profiles. The sweet radius is expected to lie close to , where , and not
far from the effective radius . We apply the method to a sample of
five X-ray bright elliptical galaxies observed with the 6-m telescope BTA-6 in
Russia. We then compare the optical -estimate with the X-ray derived
value, and discuss possible constraints on the non-thermal pressure in the hot
gas and configuration of stellar orbits. We find that the average ratio of the
optical -estimate to the X-ray one is equal to with
scatter, i.e. there is no evidence for the large non-thermal pressure
contribution in the gas at . From analysis of the Lick
indices H, Mgb, Fe5270 and Fe5335, we calculate the mass of the stellar
component within the sweet radius. We conclude that a typical dark matter
fraction inside in the sample galaxies is for the
Salpeter IMF and for the Kroupa IMF.Comment: accepted for publication in MNRA
On the Nature of X-ray Surface Brightness Fluctuations in M87
X-ray images of galaxy clusters and gas-rich elliptical galaxies show a
wealth of small-scale features which reflect fluctuations in density and/or
temperature of the intra-cluster medium. In this paper we study these
fluctuations in M87/Virgo, to establish whether sound waves/shocks, bubbles or
uplifted cold gas dominate the structure. We exploit the strong dependence of
the emissivity on density and temperature in different energy bands to
distinguish between these processes. Using simulations we demonstrate that our
analysis recovers the leading type of fluctuation even in the presence of
projection effects and temperature gradients. We confirm the isobaric nature of
cool filaments of gas entrained by buoyantly rising bubbles, extending to 7' to
the east and south-west, and the adiabatic nature of the weak shocks at 40" and
3' from the center. For features of 5--10 kpc, we show that the central 4'x 4'
region is dominated by cool structures in pressure equilibrium with the ambient
hotter gas while up to 30 percent of the variance in this region can be
ascribed to adiabatic fluctuations. The remaining part of the central 14'x14'
region, excluding the arms and shocks described above, is dominated by
apparently isothermal fluctuations (bubbles) with a possible admixture (at the
level of about 30 percent) of adiabatic (sound waves) and by isobaric
structures. Larger features, of about 30 kpc, show a stronger contribution from
isobaric fluctuations. The results broadly agree with an AGN feedback model
mediated by bubbles of relativistic plasma.Comment: 16 pages, submitted to Ap
The relation between gas density and velocity power spectra in galaxy clusters: high-resolution hydrodynamic simulations and the role of conduction
Exploring the ICM power spectrum can help us to probe the physics of galaxy
clusters. Using high-resolution 3D plasma simulations, we study the statistics
of the velocity field and its relation with the thermodynamic perturbations.
The normalization of the ICM spectrum (density, entropy, or pressure) is
linearly tied to the level of large-scale motions, which excite both gravity
and sound waves due to stratification. For low 3D Mach number M~0.25, gravity
waves mainly drive entropy perturbations, traced by preferentially tangential
turbulence. For M>0.5, sound waves start to significantly contribute, passing
the leading role to compressive pressure fluctuations, associated with
isotropic (or slightly radial) turbulence. Density and temperature fluctuations
are then characterized by the dominant process: isobaric (low M), adiabatic
(high M), or isothermal (strong conduction). Most clusters reside in the
intermediate regime, showing a mixture of gravity and sound waves, hence
drifting towards isotropic velocities. Remarkably, regardless of the regime,
the variance of density perturbations is comparable to the 1D Mach number. This
linear relation allows to easily convert between gas motions and ICM
perturbations, which can be exploited by Chandra, XMM data and by the
forthcoming Astro-H. At intermediate and small scales (10-100 kpc), the
turbulent velocities develop a Kolmogorov cascade. The thermodynamic
perturbations act as effective tracers of the velocity field, broadly
consistent with the Kolmogorov-Obukhov-Corrsin advection theory. Thermal
conduction acts to damp the gas fluctuations, washing out the filamentary
structures and steepening the spectrum, while leaving unaltered the velocity
cascade. The ratio of the velocity and density spectrum thus inverts the
downtrend shown by the non-diffusive models, allowing to probe the presence of
significant conductivity in the ICM.Comment: Accepted by A&A; 15 pages, 10 figures; added insights and references
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A Uniform Contribution of Core-Collapse and Type Ia Supernovae to the Chemical Enrichment Pattern in the Outskirts of the Virgo Cluster
We present the first measurements of the abundances of -elements (Mg,
Si, and S) extending out to beyond the virial radius of a cluster of galaxies.
Our results, based on Suzaku Key Project observations of the Virgo Cluster,
show that the chemical composition of the intra-cluster medium is consistent
with being constant on large scales, with a flat distribution of the Si/Fe,
S/Fe, and Mg/Fe ratios as a function of radius and azimuth out to 1.4 Mpc (1.3
). Chemical enrichment of the intergalactic medium due solely to core
collapse supernovae (SNcc) is excluded with very high significance; instead,
the measured metal abundance ratios are generally consistent with the Solar
value. The uniform metal abundance ratios observed today are likely the result
of an early phase of enrichment and mixing, with both SNcc and type Ia
supernovae (SNIa) contributing to the metal budget during the period of peak
star formation activity at redshifts of 2-3. We estimate the ratio between the
number of SNIa and the total number of supernovae enriching the intergalactic
medium to be between 12-37%, broadly consistent with the metal abundance
patterns in our own Galaxy or with the SNIa contribution estimated for the
cluster cores.Comment: accepted for publication in ApJ
Quantifying properties of ICM inhomogeneities
We present a new method to identify and characterize the structure of the
intracluster medium (ICM) in simulated galaxy clusters. The method uses the
median of gas properties, such as density and pressure, which we show to be
very robust to the presence of gas inhomogeneities. In particular, we show that
the radial profiles of median gas properties are smooth and do not exhibit
fluctuations at locations of massive clumps in contrast to mean and mode
properties. It is shown that distribution of gas properties in a given radial
shell can be well described by a log-normal PDF and a tail. The former
corresponds to a nearly hydrostatic bulk component, accounting for ~99% of the
volume, while the tail corresponds to high density inhomogeneities. We show
that this results in a simple and robust separation of the diffuse and clumpy
components of the ICM. The FWHM of the density distribution grows with radius
and varies from ~0.15 dex in cluster centre to ~0.5 dex at 2r_500 in relaxed
clusters. The small scatter in the width between relaxed clusters suggests that
the degree of inhomogeneity is a robust characteristic of the ICM. It broadly
agrees with the amplitude of density perturbations in the Coma cluster. We
discuss the origin of ICM density variations in spherical shells and show that
less than 20% of the width can be attributed to the triaxiality of the cluster
gravitational potential. As a link to X-ray observations of real clusters we
evaluated the ICM clumping factor with and without high density
inhomogeneities. We argue that these two cases represent upper and lower limits
on the departure of the observed X-ray emissivity from the median value. We
find that the typical value of the clumping factor in the bulk component of
relaxed clusters varies from ~1.1-1.2 at r_500 up to ~1.3-1.4 at r_200, in
broad agreement with recent observations.Comment: 16 pages, 12 figure, accepted to MNRA
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