25,719 research outputs found
Supermassive Black Holes and Galaxy Formation
The formation of supermassive black holes (SMBH) is intimately related to
galaxy formation, although precisely how remains a mystery. I speculate that
formation of, and feedback from, SMBH may alleviate problems that have arisen
in our understanding of the cores of dark halos of galaxies.Comment: Talk at conference on Matter in the Universe, March 2001, ISSI Ber
Correction of diffraction effects in confocal raman microspectroscopy
A mathematical approach developed to correct depth profiles of
wet-chemically modified polymer films obtained by confocal Raman
microscopy is presented which takes into account scattered contributions originated from a diffraction-limited laser focal volume. It is demonstrated that the problem can be described using a linear Fredholm integral equation of the first kind which correlates apparent and true Raman intensities with the depth resolution curve of the instrument.
The calculations of the corrected depth profiles show that considerable differences between apparent and corrected depth profiles exist at the surface, especially when profiles with strong concentration gradients are dealt with or an instrument with poor depth resolution is used. Degrees of modification at the surface obtained by calculation of the corrected depth profiles are compared with those measured by FTIR-ATR and show an excellent concordance.</p
Locoregional hyperthermia of deep-seated tumours applied with capacitive and radiative systems. A simulation study
Background: Locoregional hyperthermia is applied to deep-seated tumours in the pelvic region. Two very different heating techniques are often applied: capacitive and radiative heating. In this paper, numerical simulations are applied to compare the performance of both techniques in heating of deep-seated tumours. Methods: Phantom simulations were performed for small (30 Ă 20 Ă 50 cm 3 ) and large (45 Ă 30 Ă 50 cm 3 ), homogeneous fatless and inhomogeneous fat-muscle, tissue-equivalent phantoms with a central or eccentric target region. Radiative heating was simulated with the 70 MHz AMC-4 system and capacitive heating was simulated at 13.56 MHz. Simulations were performed for small fatless, small (i.e. fat layer typically 3 cm) patients with cervix, prostate, bladder and rectum cancer. Temperature distributions were simulated using constant hyperthermic-level perfusion values with tissue constraints of 44 °C and compared for both heating techniques. Results: For the small homogeneous phantom, similar target heating was predicted with radiative and capacitive heating. For the large homogeneous phantom, most effective target heating was predicted with capacitive heating. For inhomogeneous phantoms, hot spots in the fat layer limit adequate capacitive heating, and simulated target temperatures with radiative heating were 2â4 °C higher. Patient simulations predicted therapeutic target temperatures with capacitive heating for fatless patients, but radiative heating was more robust for all tumour sites and patient sizes, yielding target temperatures 1â3 °C higher than those predicted for capacitive heating. Conclusion: Generally, radiative locoregional heating yields more favourable simulated temperature distributions for deep-seated pelvic tumours, compared with capacitive heating. Therapeutic temperatures are predicted for capacitive heating in patients with (almost) no fat
Dark-Halo Cusp: Asymptotic Convergence
We propose a model for how the buildup of dark halos by merging satellites
produces a characteristic inner cusp, of a density profile \rho \prop r^-a with
a -> a_as > 1, as seen in cosmological N-body simulations of hierarchical
clustering scenarios. Dekel, Devor & Hetzroni (2003) argue that a flat core of
a<1 exerts tidal compression which prevents local deposit of satellite
material; the satellite sinks intact into the halo center thus causing a rapid
steepening to a>1. Using merger N-body simulations, we learn that this cusp is
stable under a sequence of mergers, and derive a practical tidal mass-transfer
recipe in regions where the local slope of the halo profile is a>1. According
to this recipe, the ratio of mean densities of halo and initial satellite
within the tidal radius equals a given function psi(a), which is significantly
smaller than unity (compared to being 1 according to crude resonance criteria)
and is a decreasing function of a. This decrease makes the tidal mass transfer
relatively more efficient at larger a, which means steepening when a is small
and flattening when a is large, thus causing converges to a stable solution.
Given this mass-transfer recipe, linear perturbation analysis, supported by toy
simulations, shows that a sequence of cosmological mergers with homologous
satellites slowly leads to a fixed-point cusp with an asymptotic slope a_as>1.
The slope depends only weakly on the fluctuation power spectrum, in agreement
with cosmological simulations. During a long interim period the profile has an
NFW-like shape, with a cusp of 1<a<a_as. Thus, a cusp is enforced if enough
compact satellite remnants make it intact into the inner halo. In order to
maintain a flat core, satellites must be disrupted outside the core, possibly
as a result of a modest puffing up due to baryonic feedback.Comment: 37 pages, Latex, aastex.cls, revised, ApJ, 588, in pres
The cosmological origin of the Tully-Fisher relation
We use high-resolution cosmological simulations that include the effects of
gasdynamics and star formation to investigate the origin of the Tully-Fisher
relation in the standard Cold Dark Matter cosmogony. Luminosities are computed
for each model galaxy using their full star formation histories and the latest
spectrophotometric models. We find that at z=0 the stellar mass of model
galaxies is proportional to the total baryonic mass within the virial radius of
their surrounding halos. Circular velocity then correlates tightly with the
total luminosity of the galaxy, reflecting the equivalence between mass and
circular velocity of systems identified in a cosmological context. The slope of
the relation steepens slightly from the red to the blue bandpasses, and is in
fairly good agreement with observations. Its scatter is small, decreasing from
\~0.45 mag in the U-band to ~0.34 mag in the K-band. The particular
cosmological model we explore here seems unable to account for the zero-point
of the correlation. Model galaxies are too faint at z=0 (by about two
magnitudes) if the circular velocity at the edge of the luminous galaxy is used
as an estimator of the rotation speed. The Tully-Fisher relation is brighter in
the past, by about ~0.7 magnitudes in the B-band at z=1, at odds with recent
observations of z~1 galaxies. We conclude that the slope and tightness of the
Tully-Fisher relation can be naturally explained in hierarchical models but
that its normalization and evolution depend strongly on the star formation
algorithm chosen and on the cosmological parameters that determine the
universal baryon fraction and the time of assembly of galaxies of different
mass.Comment: 5 pages, 4 figures included, submitted to ApJ (Letters
Correlations in Nuclear Matter
We analyze the nuclear matter correlation properties in terms of the pair
correlation function. To this aim we systematically compare the results for the
variational method in the Lowest Order Constrained Variational (LOCV)
approximation and for the Bruekner-Hartree-Fock (BHF) scheme. A formal link
between the Jastrow correlation factor of LOCV and the Defect Function (DF) of
BHF is established and it is shown under which conditions and approximations
the two approaches are equivalent. From the numerical comparison it turns out
that the two correlation functions are quite close, which indicates in
particular that the DF is approximately local and momentum independent. The
Equations of State (EOS) of Nuclear Matter in the two approaches are also
compared. It is found that once the three-body forces (TBF) are introduced the
two EOS are fairly close, while the agreement between the correlation functions
holds with or without TBF.Comment: 11 figure
Iron and molybdenum valences in double-perovskite (Sr,Nd)2FeMoO6: electron-doping effect
Double perovskite, (Sr1-xNdx)2FeMoO6, was doped with electrons through
partial substitution of divalent Sr by trivalent Nd (0 < x < 0.2). The Fe
valence and the degree of B-site order were probed by 57Fe Mossbauer
spectroscopy. Replacing Sr by Nd increased the fraction of Fe and Mo atoms
occupying wrong sites, i.e. antisite disorder. It had very little effect on the
Fe valence: a small but visible increase in the isomer shift was seen for the
mixed-valent FeII/III atoms occupying the right site indicating a slight
movement towards divalency of these atoms, which was more than counterbalanced
by the increase in the fraction of antisite Fe atoms with III valence state. It
is therefore argued that the bulk of the electron doping is received by
antisite Mo atoms, which - being surrounded by six MoV/VI atoms - prefer the
lower IV/V valence state. Thus under Nd substitution, the charge-neutrality
requirement inflicts a lattice disorder such that low-valent MoIV/V can exist.Comment: 15 pages, 6 figures, to appear in Solid State Commu
Non-Abelian Chern-Simons-Higgs vortices with a quartic potential
We have constructed numerically non-Abelian vortices in an SU(2)
Chern-Simons-Higgs theory with a quartic Higgs potential. We have analyzed
these solutions in detail by means of improved numerical codes and found some
unexpected features we did not find when a sixth-order Higgs potential was
used. The generic non-Abelian solutions have been generated by using their
corresponding Abelian counterparts as initial guess. Typically, the energy of
the non-Abelian solutions is lower than that of the corresponding Abelian one
(except in certain regions of the parameter space). Regarding the angular
momentum, the Abelian solutions possess the maximal value, although there exist
non-Abelian solutions which reach that maximal value too. In order to classify
the solutions it is useful to consider the non-Abelian solutions with
asymptotically vanishing component of the gauge potential, which may be
labelled by an integer number . For vortex number and above, we have
found uniqueness violation: two different non-Abelian solutions with all the
global charges equal. Finally, we have investigated the limit of infinity Higgs
self-coupling parameter and found a piecewise Regge-like relation between the
energy and the angular momentum.Comment: 9 pages, 13 figure
Polynomial Approximants for the Calculation of Polarization Profiles in the \ion{He}{1} 10830 \AA Multiplet
The \ion{He}{1} multiplet at 10830 \AA is formed in the incomplete
Paschen-Back regime for typical conditions found in solar and stellar
atmospheres. The positions and strengths of the various components that form
the Zeeman structure of this multiplet in the Paschen-Back regime are
approximated here by polynomials. The fitting errors are smaller than
m\AA in the component positions and in the relative
strengths. The approximant polynomials allow for a very fast implementation of
the incomplete Paschen-Back regime in numerical codes for the synthesis and
inversion of polarization profiles in this important multiplet.Comment: ApJ Supplements (in press
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