28,629 research outputs found
Phase-space shapes of clusters and rich groups of galaxies
Clusters and groups of galaxies are highly aspherical, with shapes
approximated by nearly prolate ellipsoids of revolution. An equally fundamental
property is the shape of these objects in velocity space which is the
anisotropy of the global velocity dispersion tensor. Here we make use of
kinematical data comprising around 600 nearby clusters and rich groups of
galaxies from the SDSS to place constraints on the phase-space shapes of these
objects, i.e. their shapes in both position and velocity space. We show that
the line of sight velocity dispersion normalised by a mass dependent velocity
scale correlates with the apparent elongation, with circular (elongated)
clusters exhibiting an excessive (decremental) normalised velocity dispersion.
This correlation holds for dynamically young or old clusters and, therefore, it
originates from projecting their intrinsic phase-space shapes rather than from
dynamical evolution. It signifies that clusters are preferentially prolate not
only in position space, but also in velocity space. The distribution of the
axial ratios in position space is found to be well approximated by a Gaussian
with a mean 0.66+/-0.01 and a dispersion 0.07+/-0.008. The velocity ellipsoids
representing the shapes in velocity space are more spherical, with a mean axial
ratio of 0.78+/-0.03. This finding has important implications for mass
measurements based on the line of sight velocity dispersion profiles in
individual clusters. For typical axial ratios of the velocity ellipsoids in the
analysed cluster sample, systematic errors on the mass estimates inferred from
the line of sight velocity dispersions become comparable to statistical
uncertainties for galaxy clusters with as few as 40 spectroscopic redshifts.Comment: 9 pages, 7 figures; published in A&A; typo in eq. 5 correcte
OGLE-2005-BLG-071Lb, the Most Massive M-Dwarf Planetary Companion?
We combine all available information to constrain the nature of
OGLE-2005-BLG-071Lb, the second planet discovered by microlensing and the first
in a high-magnification event. These include photometric and astrometric
measurements from Hubble Space Telescope, as well as constraints from higher
order effects extracted from the ground-based light curve, such as microlens
parallax, planetary orbital motion and finite-source effects. Our primary
analysis leads to the conclusion that the host of Jovian planet
OGLE-2005-BLG-071Lb is an M dwarf in the foreground disk with mass M= 0.46 +/-
0.04 Msun, distance D_l = 3.3 +/- 0.4 kpc, and thick-disk kinematics v_LSR ~
103 km/s. From the best-fit model, the planet has mass M_p = 3.8 +/- 0.4 M_Jup,
lies at a projected separation r_perp = 3.6 +/- 0.2 AU from its host and so has
an equilibrium temperature of T ~ 55 K, i.e., similar to Neptune. A degenerate
model less favored by \Delta\chi^2 = 2.1 (or 2.2, depending on the sign of the
impact parameter) gives similar planetary mass M_p = 3.4 +/- 0.4 M_Jup with a
smaller projected separation, r_\perp = 2.1 +/- 0.1 AU, and higher equilibrium
temperature T ~ 71 K. These results from the primary analysis suggest that
OGLE-2005-BLG-071Lb is likely to be the most massive planet yet discovered that
is hosted by an M dwarf. However, the formation of such high-mass planetary
companions in the outer regions of M-dwarf planetary systems is predicted to be
unlikely within the core-accretion scenario. There are a number of caveats to
this primary analysis, which assumes (based on real but limited evidence) that
the unlensed light coincident with the source is actually due to the lens, that
is, the planetary host. However, these caveats could mostly be resolved by a
single astrometric measurement a few years after the event.Comment: 51 pages, 12 figures, 3 tables, Published in Ap
Ag-coverage-dependent symmetry of the electronic states of the Pt(111)-Ag-Bi interface: The ARPES view of a structural transition
We studied by angle-resolved photoelectron spectroscopy the strain-related
structural transition from a pseudomorphic monolayer (ML) to a striped
incommensurate phase in an Ag thin film grown on Pt(111). We exploited the
surfactant properties of Bi to grow ordered Pt(111)-xMLAg-Bi trilayers with 0 <
x < 5 ML, and monitored the dispersion of the Bi-derived interface states to
probe the structure of the underlying Ag film. We find that their symmetry
changes from threefold to sixfold and back to threefold in the Ag coverage
range studied. Together with previous scanning tunneling microscopy and
photoelectron diffraction data, these results provide a consistent microscopic
description of the coverage-dependent structural transition.Comment: 10 pages, 9 figure
Reconstructing the Local Twist of Coronal Magnetic Fields and the Three-Dimensional Shape of the Field Lines from Coronal Loops in EUV and X-Ray Images
Non-linear force-free fields are the most general case of force-free fields,
but the hardest to model as well. There are numerous methods of computing such
fields by extrapolating vector magnetograms from the photosphere, but very few
attempts have so far made quantitative use of coronal morphology. We present a
method to make such quantitative use of X-Ray and EUV images of coronal loops.
Each individual loop is fit to a field line of a linear force-free field,
allowing the estimation of the field line's twist, three-dimensional geometry
and the field strength along it.
We assess the validity of such a reconstruction since the actual corona is
probably not a linear force-free field and that the superposition of linear
force-free fields is generally not itself a force-free field. To do so, we
perform a series of tests on non-linear force-free fields, described in Low &
Lou (1990). For model loops we project field lines onto the photosphere. We
compare several results of the method with the original field, in particular
the three-dimensional loop shapes, local twist (coronal alpha), distribution of
twist in the model photosphere and strength of the magnetic field. We find
that, (i) for these trial fields, the method reconstructs twist with mean
absolute deviation of at most 15% of the range of photospheric twist, (ii) that
heights of the loops are reconstructed with mean absolute deviation of at most
5% of the range of trial heights and (iii) that the magnitude of non-potential
contribution to photospheric field is reconstructed with mean absolute
deviation of at most 10% of the maximal value.Comment: submitted to Ap
The inner Galactic bulge: evidence for a nuclear bar?
Recent data from the VVV survey have strengthened evidence for a structural
change in the Galactic bulge inwards of |l|<=4 deg. Here we show with an N-body
barred galaxy simulation that a boxy bulge formed through the bar and buckling
instabilities effortlessly matches measured bulge longitude profiles for red
clump stars. The same simulation snapshot was earlier used to clarify the
apparent boxy bulge - long bar dichotomy, for the same orientation and scaling.
The change in the slope of the model longitude profiles in the inner few
degrees is caused by a transition from highly elongated to more nearly
axisymmetric isodensity contours in the inner boxy bulge. This transition is
confined to a few degrees from the Galactic plane, thus the change of slope is
predicted to disappear at higher Galactic latitudes. We also show that the
nuclear star count map derived from this simulation snapshot displays a
longitudinal asymmetry similar to that observed in the 2MASS data, but is less
flattened to the Galactic plane than the 2MASS map. These results support the
interpretation that the Galactic bulge originated from disk evolution, and
question the evidence advanced from star count data for the existence of a
secondary nuclear bar in the Milky Way.Comment: ApJL in press, 4 figure
Towards multiple 3D bone surface identification and reconstruction using few 2D X-ray images for intraoperative applications
This article discusses a possible method to use a small number, e.g. 5, of conventional 2D X-ray images to reconstruct multiple 3D bone surfaces intraoperatively. Each boneâs edge contours in X-ray images are automatically identified. Sparse 3D landmark points of each bone are automatically reconstructed by pairing the 2D X-ray images. The reconstructed landmark point distribution on a surface is approximately optimal covering main characteristics of the surface. A statistical shape model, dense point distribution model (DPDM), is then used to fit the reconstructed optimal landmarks vertices to reconstruct a full surface of each bone separately. The reconstructed surfaces can then be visualised and manipulated by surgeons or used by surgical robotic systems
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