3,634 research outputs found
Completely dark galaxies: their existence, properties, and strategies for finding them
There are a number of theoretical and observational hints that large numbers
of low-mass galaxies composed entirely of dark matter exist in the field. The
theoretical considerations follow from the prediction of cold dark matter
theory that there exist many low-mass galaxies for every massive one. The
observational considerations follow from the observed paucity of these low-mass
galaxies in the field but not in dense clusters of galaxies; this suggests that
the lack of small galaxies in the field is due to the inhibition of star
formation in the galaxies as opposed to the fact that their small dark matter
halos do not exist. In this work we outline the likely properties of low-mass
dark galaxies, and describe observational strategies for finding them, and
where in the sky to search. The results are presented as a function of the
global properties of dark matter, in particular the presence or absence of a
substantial baryonic dark matter component. If the dark matter is purely cold
and has a Navarro, Frenk and White density profile, directly detecting dark
galaxies will only be feasible with present technology if the galaxy has a
maximum velocity dispersion in excess of 70 km/s, in which case the dark
galaxies could strongly lens background objects. This is much higher than the
maximum velocity dispersions in most dwarf galaxies. If the dark matter in
galaxy halos has a baryonic component close to the cosmic ratio, the
possibility of directly detecting dark galaxies is much more realistic; the
optimal method of detection will depend on the nature of the dark matter. A
number of more indirect methods are also discussed.Comment: 12 pages, 4 figures, MNRAS in pres
A study of CTD cables and lowering systems
This study first reviews both the electrical and mechanical modes
and causes of failure of electromechanical (E/M) cables used to lower
deep sea sensors, such as CTD instruments, from oceanographic ships.
It then outlines measures or steps that could be taken to correct
some of the deficiencies observed and improve the systems presently used.
These measures include quality control, tests, operational limits,
improved handling and maintenance, improved machinery.
The study then surveys alternative cables for lowering the sensors
and convey the information from the sensors back to the ship. These
alternatives include strength members other than steel (Kevlar)and signal
carriers other than conventional copper conductors (fiber optics).
The final section - Conclusions - summarizes the recommendations,
based on this study, for improving the reliability of present and future
CTD lowering systems.Prepared for the Office of Naval Research under Contract
N00014-7l6-C-0197; NR 083-400
Seasonal variability and coastal upwelling near Cape Santa Marta (Brazil)
The Santa Marta Grande Cape (CSM) area, a relatively steep and narrow continental shelf off southeast Brazil, is under the influence of the Brazil Current and intense seasonal variations of the wind field that affect the composition of water masses and their circulation. Hydrographic data, satellite-derived wind stress, and numerical simulations are used to characterize the seasonal and shorter period variability of the oceanographic conditions off CSM, in the area between 27S and 30S. The study is focused on the upwelling episodes and the dynamical mechanisms associated with these events. Field data and numerical results indicate significant seasonal changes in the oceanographic structure, the water mass composition, and the dynamical balance. During fall and winter, when southwesterly winds prevail and the frequency of meteorological frontal systems increases, the region is characterized by waters of continental origin, the Plata Plume Water (PPW). During spring and summer, deeper waters reach the surface layer due to the action of northeasterly winds. Coastal upwelling events are revealed by surface temperature minima observed south of CSM. Synoptic data show that these events occur in pulses associated with changes in the wind direction. Analysis of the numerical results shows that this particular location of the upwelling band results from the synergy between shelf-break upwelling and the regional shelf circulation induced by the northeasterly winds, both of which are modulated by changes in the coastline orientation and shelf width.Fil: Campos, Paula C.. Universidade Federal do Rio Grande do Sul; BrasilFil: Moller Jr., Osmar O.. Universidade Federal do Rio Grande do Sul; BrasilFil: Piola, Alberto Ricardo. Universidad de Buenos Aires; Argentina. Ministerio de Defensa. Armada Argentina. Servicio de Hidrografia Naval; Argentina. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas; ArgentinaFil: Palma, Elbio Daniel. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas. Centro CientĂfico TecnolĂłgico BahĂa Blanca. Instituto Argentino de OceanografĂa (i); Argentina. Universidad Nacional del Sur; Argentin
Finding active galactic nuclei through Fink
We present the Active Galactic Nuclei (AGN) classifier as currently
implemented within the Fink broker. Features were built upon summary statistics
of available photometric points, as well as color estimation enabled by
symbolic regression. The learning stage includes an active learning loop, used
to build an optimized training sample from labels reported in astronomical
catalogs. Using this method to classify real alerts from the Zwicky Transient
Facility (ZTF), we achieved 98.0% accuracy, 93.8% precision and 88.5% recall.
We also describe the modifications necessary to enable processing data from the
upcoming Vera C. Rubin Observatory Large Survey of Space and Time (LSST), and
apply them to the training sample of the Extended LSST Astronomical Time-series
Classification Challenge (ELAsTiCC). Results show that our designed feature
space enables high performances of traditional machine learning algorithms in
this binary classification task.Comment: Accepted for the Machine learning and the Physical Sciences workshop
of NeurIPS 202
Towards a Notion of Distributed Time for Petri Nets
We set the ground for research on a timed extension of Petri nets where time parameters are associated with tokens and arcs carry constraints that qualify the age of tokens required for enabling. The novelty is that, rather than a single global clock, we use a set of unrelated clocks --- possibly one per place --- allowing a local timing as well as distributed time synchronisation. We give a formal definition of the model and investigate properties of local versus global timing, including decidability issues and notions of processes of the respective models
Statistics of Magnification Perturbations by Substructure in the Cold Dark Matter Cosmological Model
We study the statistical properties of magnification perturbations by
substructures in strong lensed systems using linear perturbation theory and an
analytical substructure model including tidal truncation and a continuous
substructure mass spectrum. We demonstrate that magnification perturbations are
dominated by perturbers found within a tidal radius of an image, and that
sizable magnification perturbations may arise from small, coherent
contributions from several substructures within the lens halo. We find that the
root-mean-square (rms) fluctuation of the magnification perturbation is 10% to
20% and both the average and rms perturbations are sensitive to the mass
spectrum and density profile of the perturbers. Interestingly, we find that
relative to a smooth model of the same mass, the average magnification in
clumpy models is lower (higher) than that in smooth models for positive
(negative) parity images. This is opposite from what is observed if one assumes
that the image magnification predicted by the best-fit smooth model of a lens
is a good proxy for what the observed magnification would have been if
substructures were absent. While it is possible for this discrepancy to be
resolved via nonlinear perturbers, we argue that a more likely explanation is
that the assumption that the best-fit lens model is a good proxy for the
magnification in the absence of substructure is not correct. We conclude that a
better theoretical understanding of the predicted statistical properties of
magnification perturbations by CDM substructure is needed in order to affirm
that CDM substructures have been unambiguously detected.Comment: ApJ accepted, minor change
Effects of Ellipticity and Shear on Gravitational Lens Statistics
We study the effects of ellipticity in lens galaxies and external tidal shear
from neighboring objects on the statistics of strong gravitational lenses. For
isothermal lens galaxies normalized so that the Einstein radius is independent
of ellipticity and shear, ellipticity {\it reduces} the lensing cross section
slightly, and shear leaves it unchanged. Ellipticity and shear can
significantly enhance the magnification bias, but only if the luminosity
function of background sources is steep. Realistic distributions of ellipticity
and shear {\it lower} the total optical depth by a few percent for most source
luminosity functions, and increase the optical depth only for steep luminosity
functions. The boost in the optical depth is noticeable (>5%) only for surveys
limited to the brightest quasars (L/L_* > 10). Ellipticity and shear broaden
the distribution of lens image separations but do not affect the mean.
Ellipticity and shear naturally increase the abundance of quadruple lenses
relative to double lenses, especially for steep source luminosity functions,
but the effect is not enough (by itself) to explain the observed
quadruple-to-double ratio. With such small changes to the optical depth and
image separation distribution, ellipticity and shear have a small effect on
cosmological constraints from lens statistics: neglecting the two leads to
biases of just Delta Omega_M = 0.00 \pm 0.01 and Delta Omega_Lambda = -0.02 \pm
0.01 (where the errorbars represent statistical uncertainties in our
calculations).Comment: Optical depth normalization discussed. Matches the published versio
Simulation of gauge transformations on systems of ultracold atoms
We show that gauge transformations can be simulated on systems of ultracold
atoms. We discuss observables that are invariant under these gauge
transformations and compute them using a tensor network ansatz that escapes the
phase problem. We determine that the Mott-insulator-to-superfluid critical
point is monotonically shifted as the induced magnetic flux increases. This
result is stable against the inclusion of a small amount of entanglement in the
variational ansatz.Comment: 14 pages, 6 figure
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