4,198 research outputs found
The Unwantables: An Exploration of Visual Narrative
The Unwantables is a creative project that has evolved from my lifelong interest in and relationship with visual stories. To me the most compelling aspect of visual storytelling is imagination . The author possesses the ability to create new worlds, interesting characters, and situations that can have the potential to communicate about any topic. My project explores visual narrative as a communication tool to explain the role of imagination in my life
ENSO dynamics in current climate models: an investigation using nonlinear dimensionality reduction
International audienceLinear dimensionality reduction techniques, notably principal component analysis, are widely used in climate data analysis as a means to aid in the interpretation of datasets of high dimensionality. These linear methods may not be appropriate for the analysis of data arising from nonlinear processes occurring in the climate system. Numerous techniques for nonlinear dimensionality reduction have been developed recently that may provide a potentially useful tool for the identification of low-dimensional manifolds in climate data sets arising from nonlinear dynamics. Here, we apply Isomap, one such technique, to the study of El Niño/Southern Oscillation variability in tropical Pacific sea surface temperatures, comparing observational data with simulations from a number of current coupled atmosphere-ocean general circulation models. We use Isomap to examine El Niño variability in the different datasets and assess the suitability of the Isomap approach for climate data analysis. We conclude that, for the application presented here, analysis using Isomap does not provide additional information beyond that already provided by principal component analysis
Photometric Properties of 47 Clusters of Galaxies: I. The Butcher-Oemler Effect
We present gri CCD photometry of 44 Abell clusters and 4 cluster candidates.
Twenty one clusters in our sample have spectroscopic redshifts. Fitting a
relation between mean g, r and i magnitudes, and redshift for this subsample,
we have calculated photometric redshifts for the remainder with an estimated
accuracy of 0.03. The resulting redshift range for the sample is 0.03<z<0.38.
Color-magnitude diagrams are presented for the complete sample and used to
study evolution of the galaxy population in the cluster environment. Our
observations show a strong Butcher-Oemler effect (Butcher & Oemler 1978, 1984),
with an increase in the fraction of blue galaxies (f_B) with redshift that
seems more consistent with the steeper relation estimated by Rakos and
Schombert (1995) than with the original one by Butcher & Oemler (1984).
However, in the redshift range between ~ 0.08 and 0.2, where most of our
clusters lie, there is a wide range of f_B values, consistent with no redshift
evolution of the cluster galaxy population. A large range of f_B values is also
seen between ~ 0.2 and 0.3, when Smail at al. (1998) x-ray clusters are added
to our sample. The discrepancies between samples underscore the need for an
unbiased sample to understand how much of the Butcher-Oemler effect is due to
evolution, and how much to selection effects. We also tested the idea proposed
by Garilli et al. (1996) that there is a population of unusually red galaxies
which could be associated either with the field or clusters, but we find that
these objects are all near the limiting magnitude of the images (20.5<r<22) and
have colors that are consistent with those expected for stars or field galaxies
at z ~ 0.7.Comment: 35 pages including 8 figures, submitted to A
Absolute Proper Motion of the Fornax Dwarf Spheroidal Galaxy from Photographic and HST WFPC2 Data
We have measured the absolute proper motion of the Fornax dwarf spheroidal
galaxy from a combination of photographic plate material and HST WFPC2 data
that provide a time baseline of up to 50 years. The extragalactic reference
frame consists of 8 QSO images and 48 galaxies. The absolute proper motion is
mu_alpha cos(delta) = 0.59 +-0.16 mas/yr and mu_delta = -0.15 +- 0.16 mas/yr.
The corresponding orbit of Fornax is polar, with an eccentricity of 0.27, and a
radial period of 4.5 Gyr. Fornax's current location is near pericenter. The
direction of the motion of Fornax supports the notion that Fornax belongs to
the Fornax-LeoI-LeoII-Sculptor-Sextans stream as hypothesized by Lynden-Bell
(1976, 1982) and Majewski (1994).
According to our orbit determination, Fornax crossed the Magellanic plane
\~190 Myr ago, a time that coincides with the termination of the star-formation
process in Fornax. We propose that ram-pressure stripping due to the passage of
Fornax through a gaseous medium denser than the typical intragalactic medium
left behind from the LMC may have caused the end of star formation in Fornax.
The excess, anomalous clouds within the South Galactic pole region of the
Magellanic Stream whose origin has long been debated in the literature as
constituents of either the Magellanic Stream or of the extragalactic Sculptor
group, are found to lie along the orbit of Fornax. We speculate that these
clouds are stripped material from Fornax as the dwarf crossed the Magellanic
Clouds' orbit.Comment: Accepted for publication in Astronomical Journal. The version with
high resolution figures can be found at
ftp://pegasus.astro.yale.edu/pub/dana/paper
Double percolation effects and fractal behavior in magnetic/superconducting hybrids
Perpendicular magnetic anisotropy ferromagnetic/ superconducting (FM/SC)
bilayers with a labyrinth domain structure are used to study nucleation of
superconductivity on a fractal network, tunable through magnetic history. As
clusters of reversed domains appear in the FM layer, the SC film shows a
percolative behavior that depends on two independent processes: the arrangement
of initial reversed domains and the fractal geometry of expanding clusters. For
a full labyrinth structure, the behavior of the upper critical field is typical
of confined superconductivity on a fractal network.Comment: 15 pages, 5 figure
Recommended from our members
Analysis Of Sequential Barycenter Random Probability Measures via Discrete Constructions
Hill and Monticino (1998) introduced a constructive method for generating random probability measures with a prescribed mean or distribution on the mean. The method involves sequentially generating an array of barycenters that uniquely defines a probability measure. This work analyzes statistical properties of the measures generated by sequential barycenter array constructions. Specifically, this work addresses how changing the base measures of the construction affects the statististics of measures generated by the SBA construction. A relationship between statistics associated with a finite level version of the SBA construction and the full construction is developed. Monte Carlo statistical experiments are used to simulate the effect changing base measures has on the statistics associated with the finite level construction
Shapes and Shears, Stars and Smears: Optimal Measurements for Weak Lensing
We present the theoretical and analytical bases of optimal techniques to
measure weak gravitational shear from images of galaxies. We first characterize
the geometric space of shears and ellipticity, then use this geometric
interpretation to analyse images. The steps of this analysis include:
measurement of object shapes on images, combining measurements of a given
galaxy on different images, estimating the underlying shear from an ensemble of
galaxy shapes, and compensating for the systematic effects of image distortion,
bias from PSF asymmetries, and `"dilution" of the signal by the seeing. These
methods minimize the ellipticity measurement noise, provide calculable shear
uncertainty estimates, and allow removal of systematic contamination by PSF
effects to arbitrary precision. Galaxy images and PSFs are decomposed into a
family of orthogonal 2d Gaussian-based functions, making the PSF correction and
shape measurement relatively straightforward and computationally efficient. We
also discuss sources of noise-induced bias in weak lensing measurements and
provide a solution for these and previously identified biases.Comment: Version accepted to AJ. Minor fixes, plus a simpler method of shape
weighting. Version with full vector figures available via
http://www.astro.lsa.umich.edu/users/garyb/PUBLICATIONS
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