677 research outputs found
Deconstructing Approximate Offsets
We consider the offset-deconstruction problem: Given a polygonal shape Q with
n vertices, can it be expressed, up to a tolerance \eps in Hausdorff distance,
as the Minkowski sum of another polygonal shape P with a disk of fixed radius?
If it does, we also seek a preferably simple-looking solution P; then, P's
offset constitutes an accurate, vertex-reduced, and smoothened approximation of
Q. We give an O(n log n)-time exact decision algorithm that handles any
polygonal shape, assuming the real-RAM model of computation. A variant of the
algorithm, which we have implemented using CGAL, is based on rational
arithmetic and answers the same deconstruction problem up to an uncertainty
parameter \delta; its running time additionally depends on \delta. If the input
shape is found to be approximable, this algorithm also computes an approximate
solution for the problem. It also allows us to solve parameter-optimization
problems induced by the offset-deconstruction problem. For convex shapes, the
complexity of the exact decision algorithm drops to O(n), which is also the
time required to compute a solution P with at most one more vertex than a
vertex-minimal one.Comment: 18 pages, 11 figures, previous version accepted at SoCG 2011,
submitted to DC
Deconstructing the Antlia cluster core
The present literature does not give a satisfactory answer to the question
about the nature of the "Antlia galaxy cluster". The radial velocities of
galaxies found in the region around the giant ellipticals NGC 3258/3268 range
from about 1000 km/s to 4000 km/s. We characterise this region and its possible
kinematical and population substructure. We have obtained VLT--VIMOS
multi-object spectra of the galaxy population in the inner part of the Antlia
cluster and measure radial velocities for 45 potential members. We supplement
our galaxy sample with literature data, ending up with 105 galaxy velocities.
We find a large radial velocity dispersion for the entire sample as reported in
previous papers. However, we find three groups at about 1900 km/s, 2800 km/s,
and 3700 km/s, which we interpret as differences in the recession velocities
rather than peculiar velocities. The high radial velocity dispersion of
galaxies in the Antlia region reflects a considerable extension along the line
of sight.Comment: 11 pages, 7 figures, accepted in A&
Holographic Electroweak Symmetry Breaking from D-branes
We observe several interesting phenomena in a technicolor-like model of
electroweak symmetry breaking based on the D4-D8-D8bar system of Sakai and
Sugimoto. The benefit of holographic models based on D-brane configurations is
that both sides of the holographic duality are well understood. We find that
the lightest technicolor resonances contribute negatively to the
Peskin-Takeuchi S-parameter, but heavy resonances do not decouple and lead
generically to large, positive values of S, consistent with standard estimates
in QCD-like theories. We study how the S parameter and the masses and decay
constants of the vector and axial-vector techni-resonances vary over a
one-parameter family of D8-brane configurations. We discuss possibilities for
the consistent truncation of the theory to the first few resonances and suggest
some generic predictions of stringy holographic technicolor models.Comment: REVTeX, 25 pages, 8 eps figures, version published in PR
Probing Outflows in z= 1~2 Galaxies through FeII/FeII* Multiplets
We report on a study of the 2300-2600\AA FeII/FeII* multiplets in the rest-UV
spectra of star-forming galaxies at 1.0<z<2.6 as probes of galactic-scale
outflows. We extracted a mass-limited sample of 97 galaxies at z~1.0-2.6 from
ultra-deep spectra obtained during the GMASS spetroscopic survey in the GOODS
South field with the VLT and FORS2. We obtain robust measures of the rest
equivalent width of the FeII absorption lines down to a limit of W_r>1.5 \AA
and of the FeII* emission lines to W_r>0.5 \AA. Whenever we can measure the
systemic redshift of the galaxies from the [OII] emission line, we find that
both the FeII and MgII absorption lines are blueshifted, indicative that both
species trace gaseous outflows. We also find, however, that the FeII gas has
generally lower outflow velocity relative to that of MgII. We investigate the
variation of FeII line profiles as a function of the radiative transfer
properties of the lines, and find that transitions with higher oscillator
strengths are more blueshifted in terms of both line centroids and line wings.
We discuss the possibility that FeII lines are suppressed by stellar
absorptions. The lower velocities of the FeII lines relative to the MgII
doublet, as well as the absence of spatially extended FeII* emission in 2D
stacked spectra, suggest that most clouds responsible for the FeII absorption
lie close (3~4 kpc) to the disks of galaxies. We show that the FeII/FeII*
multiplets offer unique probes of the kinematic structure of galactic outflows.Comment: 53 pages, 22 Figures, accepted for publication in ApJ, revised
according to referee comment
Detailed abundances from integrated-light spectroscopy: Milky Way globular clusters
We test the performance of our analysis technique for integrated-light
spectra by applying it to seven well-studied Galactic GCs that span a wide
range of metallicities. Integrated-light spectra were obtained by scanning the
slit of the UVES spectrograph on the ESO Very Large Telescope across the
half-light diameters of the clusters. We modelled the spectra using resolved
HST colour-magnitude diagrams (CMDs), as well as theoretical isochrones, in
combination with standard stellar atmosphere and spectral synthesis codes. The
abundances of Fe, Na, Mg, Ca, Ti, Cr, and Ba were compared with literature data
for individual stars in the clusters. The typical differences between iron
abundances derived from our integrated-light spectra and those compiled from
the literature are less than 0.1 dex. A larger difference is found for one
cluster (NGC 6752), and is most likely caused primarily by stochastic
fluctuations in the numbers of bright red giants within the scanned area. As
expected, the alpha-elements (Ca, Ti) are enhanced by about 0.3 dex compared to
the Solar-scaled composition, while the [Cr/Fe] ratios are close to Solar. When
using up-to-date line lists, our [Mg/Fe] ratios also agree well with literature
data. Our [Na/Fe] ratios are, on average, 0.08-0.14 dex lower than average
values quoted in the literature, and our [Ba/Fe] ratios may be overestimated by
0.20-0.35 dex at the lowest metallicities. We find that analyses based on
theoretical isochrones give very similar results to those based on resolved
CMDs. Overall, the agreement between our integrated-light abundance
measurements and the literature data is satisfactory. Refinements of the
modelling procedure, such as corrections for stellar evolutionary and non-LTE
effects, might further reduce some of the remaining offsets.Comment: 35 pages, 16 figures, accepted for A&
A 2MASS All-Sky View of the Sagittarius Dwarf Galaxy: Variation of the Metallicity Distribution Function Along the Sagittarius Stream
We present reliable measurements of the metallicity distribution function
(MDF) at different points along the tidal stream of the Sagittarius (Sgr) dwarf
spheroidal (dSph) galaxy, based on high resolution, echelle spectroscopy of
candidate M giant members of the Sgr system. The Sgr MDF is found to evolve
significantly from a median [Fe/H] ~-0.4 in the core to ~-1.1 dex over a Sgr
leading arm length representing ~2.5-3.0 Gyr of dynamical (i.e. tidal
stripping) age. This is direct evidence that there can be significant chemical
differences between current dSph satellites and the bulk of the stars they have
contributed to the halo. Our results suggest that Sgr experienced a significant
change in binding energy over the past several Gyr, which has substantially
decreased its tidal boundary across a radial range over which there must have
been a significant metallicity gradient in the progenitor galaxy. By accounting
for MDF variation along the debris arms, we approximate the MDF Sgr would have
had several Gyr ago. We also analyze the MDF of a moving group of M giants we
previously discovered towards the North Galactic Cap having opposite radial
velocities to the infalling Sgr leading arm stars there and propose that most
of these represent Sgr trailing arm stars overlapping the Sgr leading arm in
this part of the sky. If so, these trailing arm stars further demonstrate the
strong MDF evolution within the Sgr stream.Comment: The new version is a much-expanded version moved from ApJL to ApJ and
will appear in Dec. 1 editio
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