6,684 research outputs found
The hot gas content of fossil galaxy clusters
We investigate the properties of the hot gas in four fossil galaxy systems
detected at high significance in the Planck Sunyaev-Zeldovich (SZ) survey.
XMM-Newton observations reveal overall temperatures of kT ~ 5-6 keV and yield
hydrostatic masses M500,HE > 3.5 x 10e14 Msun, confirming their nature as bona
fide massive clusters. We measure the thermodynamic properties of the hot gas
in X-rays (out to beyond R500 in three cases) and derive their individual
pressure profiles out to R ~ 2.5 R500 with the SZ data. We combine the X-ray
and SZ data to measure hydrostatic mass profiles and to examine the hot gas
content and its radial distribution. The average Navarro-Frenk-White (NFW)
concentration parameter, c500 = 3.2 +/- 0.4, is the same as that of relaxed
`normal' clusters. The gas mass fraction profiles exhibit striking variation in
the inner regions, but converge to approximately the cosmic baryon fraction
(corrected for depletion) at R500. Beyond R500 the gas mass fraction profiles
again diverge, which we interpret as being due to a difference in gas clumping
and/or a breakdown of hydrostatic equilibrium in the external regions. Overall
our observations point to considerable radial variation in the hot gas content
and in the gas clumping and/or hydrostatic equilibrium properties in these
fossil clusters, at odds with the interpretation of their being old, evolved
and undisturbed. At least some fossil objects appear to be dynamically young.Comment: 4 pages, 2 figures. Accepted for publication in A&
CMB Constraints on WIMP Annihilation: Energy Absorption During the Recombination Epoch
We compute in detail the rate at which energy injected by dark matter
annihilation heats and ionizes the photon-baryon plasma at z ~ 1000, and
provide accurate fitting functions over the relevant redshift range for a broad
array of annihilation channels and DM masses. The resulting perturbations to
the ionization history can be constrained by measurements of the CMB
temperature and polarization angular power spectra. We show that models which
fit recently measured excesses in 10-1000 GeV electron and positron cosmic rays
are already close to the 95% confidence limits from WMAP. The recently launched
Planck satellite will be capable of ruling out a wide range of DM explanations
for these excesses. In models of dark matter with Sommerfeld-enhanced
annihilation, where sigma v rises with decreasing WIMP velocity until some
saturation point, the WMAP5 constraints imply that the enhancement must be
close to saturation in the neighborhood of the Earth.Comment: 17 pages, 6 figures, v2 extends discussion of constraints on
Sommerfeld-enhanced model
An Ab Initio Approach to the Solar Coronal Heating Problem
We present an ab initio approach to the solar coronal heating problem by
modelling a small part of the solar corona in a computational box using a 3D
MHD code including realistic physics. The observed solar granular velocity
pattern and its amplitude and vorticity power spectra, as reproduced by a
weighted Voronoi tessellation method, are used as a boundary condition that
generates a Poynting flux in the presence of a magnetic field. The initial
magnetic field is a potential extrapolation of a SOHO/MDI high resolution
magnetogram, and a standard stratified atmosphere is used as a thermal initial
condition. Except for the chromospheric temperature structure, which is kept
fixed, the initial conditions are quickly forgotten because the included
Spitzer conductivity and radiative cooling function have typical timescales
much shorter than the time span of the simulation. After a short initial start
up period, the magnetic field is able to dissipate 3-4 10^6 ergs cm^{-2} s^{-1}
in a highly intermittent corona, maintaining an average temperature of K, at coronal density values for which emulated images of the Transition
Region And Coronal Explorer(TRACE) 171 and 195 pass bands reproduce observed
photon count rates.Comment: 12 pages, 14 figures. Submitted to Ap
The stellar mass function of galaxies in Planck-selected clusters at 0.5 < z < 0.7: new constraints on the timescale and location of satellite quenching
We study the abundance of star-forming and quiescent galaxies in a sample of
21 massive clusters at 0.5<z<0.7, detected with the Planck satellite. We
measure the cluster galaxy stellar mass function (SMF), which is a fundamental
observable to study and constrain the formation and evolution of galaxies. Our
measurements are based on homogeneous and deep multi-band photometry spanning
u- to the Ks-band for each cluster and are supported by spectroscopic data from
different programs. The galaxy population is separated between quiescent and
star-forming galaxies based on their rest-frame U-V and V-J colours. The SMF is
compared to that of field galaxies at the same redshifts, using data from the
COSMOS/UltraVISTA survey. We find that the shape of the SMF of star-forming
galaxies does not depend on environment, while the SMF of quiescent galaxies
has a significantly steeper low-mass slope in the clusters compared to the
field. We estimate the environmental quenching efficiency (f_EQ), i.e. the
probability for a galaxy that would normally be star forming in the field, to
be quenched due to its environment. The f_EQ shows no stellar-mass dependence
in any environment, but it increases from 40% in the cluster outskirts to ~90%
in the cluster centres. The radial signature of f_EQ provides constraints on
where the dominant quenching mechanism operates in these clusters and on what
timescale. Exploring these using a simple model based on galaxy orbits obtained
from an N-body simulation, we find a clear degeneracy between both parameters.
For example, the quenching process may either be triggered on a long (~3 Gyr)
time scale at large radii (r~8R_500), or happen well within 1 Gyr at r<R_500.
The radius where quenching is triggered is at least r_quench> 0.67R_500
(95%CL). The ICM density at this location suggests that ram-pressure stripping
of the cold gas is a likely cause of quenching. [Abridged]Comment: 16 pages, 12 figures, accepted for publication in A&
Kinematics of Galaxies in the Hubble Deep Field South: Discovery of a Very Massive Spiral at z=0.6
We report the first results from a study of the internal kinematics, based on
spatially resolved H_alpha velocity profiles, of three galaxies at redshift
z~0.6 and one at redshift z~0.8, detected by ISOCAM in the Hubble Deep Field
South. The kinematics are derived from high resolution near-infrared VLT
spectroscopy. One of the galaxies is a massive spiral which possesses a very
large rotational velocity of 460 km/s and contains a mass of 10^12 M_solar
(within 20 kpc), significantly higher than the dynamical masses measured in
most other local and high redshift spirals. Two of the galaxies comprise a
counter-rotating interacting system, while the fourth is also a large spiral.
The observed galaxies are representative examples of the morphologies
encountered among ISOCAM galaxies. The mass-to-light (M /L_bol) ratios of
ISOCAM galaxies lie between those of local luminous IR galaxies and massive
spirals. We measure an offset of 1.6+/-0.3 mag in the rest frame B-band and of
0.7+/-0.3 mag in the rest frame I-band when we compare the four ISOCAM galaxies
to the local Tully-Fisher B and I-band relations. We conclude that the large IR
luminosity of the ISOCAM population results from a combination of large mass
and efficient triggering of star formation. Since ISOCAM galaxies contribute
significantly to the Cosmic Infrared Background our results imply that a
relatively small number of very massive and IR luminous objects contribute
significantly to the IR background and star formation activity near z~0.7.Comment: accepted for publication in the Astrophysical Journal (Part 1
Three‐dimensional study of Mars upper thermosphere/ionosphere and hot oxygen corona: 1. General description and results at equinox for solar low conditions
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/94871/1/jgre2685.pd
The Extreme Spin of the Black Hole in Cygnus X-1
The compact primary in the X-ray binary Cygnus X-1 was the first black hole
to be established via dynamical observations. We have recently determined
accurate values for its mass and distance, and for the orbital inclination
angle of the binary. Building on these results, which are based on our favored
(asynchronous) dynamical model, we have measured the radius of the inner edge
of the black hole's accretion disk by fitting its thermal continuum spectrum to
a fully relativistic model of a thin accretion disk. Assuming that the spin
axis of the black hole is aligned with the orbital angular momentum vector, we
have determined that Cygnus X-1 contains a near-extreme Kerr black hole with a
spin parameter a/M>0.95 (3\sigma). For a less probable (synchronous) dynamical
model, we find a/M>0.92 (3\sigma). In our analysis, we include the
uncertainties in black hole mass, orbital inclination angle and distance, and
we also include the uncertainty in the calibration of the absolute flux via the
Crab. These four sources of uncertainty totally dominate the error budget. The
uncertainties introduced by the thin-disk model we employ are particularly
small in this case given the extreme spin of the black hole and the disk's low
luminosity.Comment: Paper III of three papers on Cygnus X-1; 21 pages including 5 figures
and 12 tables, ApJ in press. The paper is significantly restructured; two
further tests of the robustness of our spin measurement are presented, and
our error analysis has been substantially improved; the conclusions are
unchange
Self-assembly in solution of a reversible comb-shaped supramolecular polymer
We report a single step synthesis of a polyisobutene with a bis-urea moiety
in the middle of the chain. In low polarity solvents, this polymer
self-assembles by hydrogen bonding to form a combshaped polymer with a central
hydrogen bonded backbone and polyisobutene arms. The comb backbone can be
reversibly broken, and consequently, its length can be tuned by changing the
solvent, the concentration or the temperature. Moreover, we have proved that
the bulkiness of the side-chains have a strong influence on both the
self-assembly pattern and the length of the backbone. Finally, the density of
arms can be reduced, by simply mixing with a low molar mass bis-urea
Testing the performance of a blind burst statistic
In this work we estimate the performance of a method for the detection of
burst events in the data produced by interferometric gravitational wave
detectors. We compute the receiver operating characteristics in the specific
case of a simulated noise having the spectral density expected for Virgo, using
test signals taken from a library of possible waveforms emitted during the
collapse of the core of Type II Supernovae.Comment: 8 pages, 6 figures, Talk given at the GWDAW2002 worksho
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