11,374 research outputs found
Shape Smoothing using Double Offsets
It has been observed for a long time that the operation consisting of offseting a solid by a quantity and then offseting its complement by $
The origin of peak-offsets in weak-lensing maps
Centroid positions of peaks identified in weak lensing mass maps often show
offsets with respect to other means of identifying halo centres, like position
of the brightest cluster galaxy or X-ray emission centroid. Here we study the
effect of projected large-scale structure (LSS), smoothing of mass maps, and
shape noise on the weak lensing peak positions. Additionally we compare the
offsets in mass maps to those found in parametric model fits. Using ray-tracing
simulations through the Millennium Run -body simulation, we find that
projected LSS does not alter the weak-lensing peak position within the limits
of our simulations' spatial resolution, which exceeds the typical resolution of
weak lensing maps. We conclude that projected LSS, although a major contaminant
for weak-lensing mass estimates, is not a source of confusion for identifying
halo centres. The typically reported offsets in the literature are caused by a
combination of shape noise and smoothing alone. This is true for centroid
positions derived both from mass maps and model fits.Comment: 6 pages, 4 figures, accepted for publication in MNRAS, significant
additions to v
Cartographic Algorithms: Problems of Implementation and Evaluation and the Impact of Digitising Errors
Cartographic generalisation remains one of the outstanding challenges in digital cartography and Geographical Information Systems (GIS). It is generally assumed that computerisation will lead to the removal of spurious variability introduced by the subjective decisions of individual cartographers. This paper demonstrates through an inâdepth study of a line simplification algorithm that computerisation introduces its own sources of variability. The algorithm, referred to as the DouglasâPeucker algorithm in cartographic literature, has been widely used in image processing, pattern recognition and GIS for some 20 years. An analysis of this algorithm and study of some implementations in wide use identify the presence of variability resulting from the subjective decisions of software implementors. Spurious variability in software complicates the processes of evaluation and comparison of alternative algorithms for cartographic tasks. No doubt, variability in implementation could be removed by rigorous study and specification of algorithms. Such future work must address the presence of digitising error in cartographic data. Our analysis suggests that it would be difficult to adapt the DouglasâPeucker algorithm to cope with digitising error without altering the method. Copyright © 1991, Wiley Blackwell. All rights reserve
Re-growth of stellar disks in mature galaxies: The two component nature of NGC 7217 revisited with VIRUS-W
Previous studies have reported the existence of two counter-rotating stellar
disks in the early-type spiral galaxy NGC7217. We have obtained high-resolution
optical spectroscopic data (R ~ 9000) with the new fiber-based Integral Field
Unit instrument VIRUS-W at the 2.7m telescope of the McDonald Observatory in
Texas. Our analysis confirms the existence of two components. However, we find
them to be co-rotating. The first component is the more luminous (~ 77% of the
total light), has the higher velocity dispersion (~ 170 km/s) and rotates
relatively slowly (projected = 50 km/s). The lower luminosity second
component, (~ 23% of the total light), has a low velocity dispersion (~ 20
km/s) and rotates quickly (projected = 150 km/s). The difference in
the kinematics of the two stellar components allows us to perform a kinematic
decomposition and to measure the strengths of their Mg and Fe Lick indices
separately. The rotational velocities and dispersions of the less luminous and
faster component are very similar to those of the interstellar gas as measured
from the [OIII] emission. Morphological evidence of active star formation in
this component further suggests that NGC7217 may be in the process of
(re)growing a disk inside a more massive and higher dispersion stellar halo.
The kinematically cold and regular structure of the gas disk in combination
with the central almost dust-free morphology allows us to compare the dynamical
mass inside of the central 500pc with predictions from a stellar population
analysis. We find agreement between the two if a Kroupa stellar initial mass
function is assumed.Comment: accepted for publication by MNRA
HST/Acs Weak-Lensing and Chandra X-Ray Studies of the High-Redshift Cluster MS 1054-0321
We present Hubble Space Telescope/Advanced Camera for Surveys (ACS)
weak-lensing and Chandra X-ray analyses of MS 1054-0321 at z=0.83, the most
distant and X-ray luminous cluster in the Einstein Extended Medium-Sensitivity
Survey (EMSS). The high-resolution mass reconstruction through ACS weak-lensing
reveals the complicated dark matter substructure in unprecedented detail,
characterized by the three dominant mass clumps with the four or more minor
satellite groups within the current ACS field. The direct comparison of the
mass map with the Chandra X-ray image shows that the eastern weak-lensing
substructure is not present in the X-ray image and, more interestingly, the two
X-ray peaks are displaced away from the hypothesized merging direction with
respect to the corresponding central and western mass clumps, possibly because
of ram pressure. In addition, as observed in our previous weak-lensing study of
another high-redshift cluster CL 0152-1357 at z=0.84, the two dark matter
clumps of MS 1054-0321 seem to be offset from the galaxy counterparts. We
examine the significance of these offsets and discuss a possible scenario,
wherein the dark matter clumps might be moving ahead of the cluster galaxies.
The non-parametric weak-lensing mass modeling gives a projected mass of M(r<1
Mpc)=(1.02+-0.15)x 10^{15} solar mass, where the uncertainty reflects both the
statistical error and the cosmic shear effects. Our temperature measurement of
T=8.9_{-0.8}^{+1.0} keV utilizing the newest available low-energy quantum
efficiency degradation prescription for the Chandra instrument, together with
the isothermal beta description of the cluster (r_c=16"+-15" and
beta=0.78+-0.08), yields a projected mass of M(r<1 Mpc)=(1.2+-0.2) x 10^{15}
solar mass, consistent with the weak-lensing result.Comment: Accepted for publication in apj. Full-resolution version can be
downloaded from http://acs.pha.jhu.edu/~mkjee/ms1054.pd
Frequency-Dependent Template Profiles for High Precision Pulsar Timing
Pulsar timing experiments require high fidelity template profiles in order to
minimize the biases in pulse time-of-arrival (TOA) measurements and their
uncertainties. Efforts to acquire more precise TOAs given fixed effective area
of telescopes, finite receiver noise, and limited integration time have led
pulsar astronomers to the solution of implementing ultra-wideband receivers.
This solution, however, has run up against the problem that pulse profile
shapes evolve with frequency, which raises the question of how to properly
measure and analyze TOAs obtained using template-matching methods. This paper
proposes a new method for one facet of this problem, that of template profile
generation, and demonstrates it on the well-timed millisecond pulsar
J1713+0747. Specifically, we decompose pulse profile evolution into a linear
combination of basis eigenvectors, the coefficients of which change slowly with
frequency such that their evolution is modeled simply by a sum of low degree
piecewise polynomial spline functions. These noise-free, high fidelity,
frequency-dependent templates can be used to make measurements of so-called
"wideband TOAs" simultaneously with an estimate of the instantaneous dispersion
measure. The use of wideband TOAs is becoming important for pulsar timing array
experiments, as the volume of datasets comprised of conventional, subbanded
TOAs are quickly becoming unwieldly for the Bayesian analyses needed to uncover
latent gravitational wave signals. Although motivated by high precision timing
experiments, our technique is applicable in more general pulsar observations.Comment: 16 pages, 6 figures, accepted to Ap
A Digital Archive of HI 21 cm Line Spectra of Optically-targeted Galaxies
We present a homogeneous compilation of HI spectral parameters extracted from
global 21 cm line spectra for some 9000 galaxies in the local universe
(heliocentric velocity -200 < V_Sun < 28,000 km/s) obtained with a variety of
large single dish radio telescopes but reanalyzed using a single set of
parameter extraction algorithms. Corrections to the observed HI line flux for
source extent and pointing offsets and to the HI line widths for instrumental
broadening and smoothing are applied according to model estimates to produce a
homogenous catalog of derived properties with quantitative error estimates.
Where the redshift is available from optical studies, we also provide flux
measurements for an additional 156 galaxies classified as marginal HI
detections and rms noise limits for 494 galaxies classified as nondetections.
Given the diverse nature of the observing programs contributing to it, the
characteristics of the combined dataset are heterogeneous, and as such, the
compilation is neither integrated HI line flux nor peak flux limited. However,
because of the large statistical base and homogenous reprocessing, the spectra
and spectral parameters of galaxies in this optically targeted sample can be
used to complement data obtained at other wavelengths to characterize the
properties of galaxies in the local universe and to explore the large scale
structures in which they reside.Comment: 13 pages, 9 figures, 3 external online tables, accepted for
publication in ApJ
A Structural and Dynamical Study of Late-Type, Edge-On Galaxies: I. Sample Selection and Imaging Data
We present optical (B & R) and infrared (K_s) images and photometry for a
sample of 49 extremely late-type, edge-on disk galaxies selected from the Flat
Galaxy Catalog of Karenchentsev et al. (1993). Our sample was selected to
include galaxies with particularly large axial ratios, increading the
likelihood that the galaxies in the sample are truly edge-on. We have also
concentrated the sample on galaxies with low apparent surface brightness, in
order to increase the representation of intrinisically low surface brightness
galaxies. Finally, the sample was chosen to have no apprarent bulges or optical
warps so that the galaxies represent undisturbed, ``pure disk'' systems. The
resulting sample forms the basis for a much larger spectroscopic study designed
to place constraints on the physical quantities and processes which shape disk
galaxies. The imaging data presented in this paper has been painstakingly
reduced and calibrated to allow accurate surface photometry of features as
faint as 30 mag/sqr-arcsec in B and 29 mag/sqr-arcsec in R on scales larger
than 10 arcsec. Due to limitations in sky subtraction and flat fielding, the
infrared data can reach only to 22.5 mag/sqr-arcsec in K_s on comparable
scales. As part of this work, we have developed a new method for quantifying
the reliability of surface photometry, which provides useful diagnostics for
the presence of scattered light, optical emission from infrared cirrus, and
other sources of non-uniform sky backgrounds.Comment: scheduled to appear in the Astronomical Journal, LaTeX, 36 pages
including 7 pages of figures (fig 1-2,4). A low resolution version of Figure
3 is included in JPEG format; contours are seriously degraded. A full
resolution Postscript version of Figure 3 (10.6Mb,gzipped) is available
through anonymous ftp at
ftp://ftp.astro.washington.edu/pub/users/jd/FGC/dalcanton.f3.ps.g
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