2,608 research outputs found
Thunderstorm/environment interactions that affect subsequent convection
Mesoscale kinematics and thermodynamics of severe thunderstorm-baroclinic zone interactions, and the development and evolution of mesoscale pressure systems associated with strong convective storms, are being studied in an ongoing research project
Interactions Between Convective Storms and Their Environment
The ways in which intense convective storms interact with their environment are considered for a number of specific severe storm situations. A physical model of subcloud wind fields and vertical wind profiles was developed to explain the often observed intensification of convective storms that move along or across thermal boundaries. A number of special, unusually dense, data sets were used to substantiate features of the model. GOES imagery was used in conjunction with objectively analyzed surface wind data to develop a nowcast technique that might be used to identify specific storm cells likely to become tornadic. It was shown that circulations associated with organized meso-alpha and meso-beta scale storm complexes may, on occasion, strongly modify tropospheric thermodynamic patterns and flow fields
Faint blue objects on the Hubble Deep Field North & South as possible nearby old halo white dwarfs
Using data derived from the deepest and finest angular resolution images of
the universe yet acquired by astronomers at optical wavelengths using the
Hubble Space Telescope (HST) in two postage-stamp sections of the sky (Williams
et al. 1996a,b), plus simple geometrical and scaling arguments, we demonstrate
that the faint blue population of point-source objects detected on those two
fields (M\'endez et al. 1996) could actually be ancient halo white dwarfs at
distances closer than about 2 kpc from the Sun. This finding has profound
implications, as the mass density of the detected objects would account for
about half of the missing dark matter in the Milky-Way (Bahcall and Soneira
1980), thus solving one of the most controversial issues of modern astrophysics
(Trimble 1987, Ashman 1992). The existence of these faint blue objects points
to a very large mass locked into ancient halo white dwarfs. Our estimate
indicates that they could account for as much as half of the dark matter in our
Galaxy, confirming the suggestions of the MACHO microlensing experiment (Alcock
et al. 1997). Because of the importance of this discovery, deep follow-up
observations with HST within the next two years would be needed to determine
more accurately the kinematics (tangential motions) for these faint blue old
white dwarfs.Comment: Accepted for publication on The Astrophysical Journal, Part 1. 8
pages (AAS Latex macros V4.0), 1 B&W postscript figure, 2 color postscript
figure
The Peculiar Velocity Function of Galaxy Clusters
The peculiar velocity function of clusters of galaxies is determined using an
accurate sample of cluster velocities based on Tully-Fisher distances of Sc
galaxies (Giovanelli et al 1995b). In contrast with previous results based on
samples with considerably larger velocity uncertainties, the observed velocity
function does not exhibit a tail of high velocity clusters. The results
indicate a low probability of \,5\% of finding clusters with
one-dimensional velocities greater than 600 {\kms}. The root-mean-square
one-dimensional cluster velocity is 29328 {\kms}. The observed cluster
velocity function is compared with expectations from different cosmological
models. The absence of a high velocity tail in the observed function is most
consistent with a low mass-density (0.3) CDM model, and is
inconsistent at level with = 1.0 CDM and HDM models.
The root-mean-square one-dimensional cluster velocities in these models
correspond, respectively, to 314, 516, and 632 {\kms} (when convolved with the
observational uncertainties). Comparison with the observed RMS cluster velocity
of 29328 {\kms} further supports the low-density CDM model.Comment: revised version accepted for publication in ApJ Letters, 18 pages,
uuencoded PostScript with 3 figures included; complete paper available
through WWW at http://www.astro.princeton.edu/~library/prep.htm
Catalog of quasars from the Kilo-Degree Survey Data Release 3
We present a catalog of quasars selected from broad-band photometric ugri
data of the Kilo-Degree Survey Data Release 3 (KiDS DR3). The QSOs are
identified by the random forest (RF) supervised machine learning model, trained
on SDSS DR14 spectroscopic data. We first cleaned the input KiDS data from
entries with excessively noisy, missing or otherwise problematic measurements.
Applying a feature importance analysis, we then tune the algorithm and identify
in the KiDS multiband catalog the 17 most useful features for the
classification, namely magnitudes, colors, magnitude ratios, and the stellarity
index. We used the t-SNE algorithm to map the multi-dimensional photometric
data onto 2D planes and compare the coverage of the training and inference
sets. We limited the inference set to r<22 to avoid extrapolation beyond the
feature space covered by training, as the SDSS spectroscopic sample is
considerably shallower than KiDS. This gives 3.4 million objects in the final
inference sample, from which the random forest identified 190,000 quasar
candidates. Accuracy of 97%, purity of 91%, and completeness of 87%, as derived
from a test set extracted from SDSS and not used in the training, are confirmed
by comparison with external spectroscopic and photometric QSO catalogs
overlapping with the KiDS footprint. The robustness of our results is
strengthened by number counts of the quasar candidates in the r band, as well
as by their mid-infrared colors available from WISE. An analysis of parallaxes
and proper motions of our QSO candidates found also in Gaia DR2 suggests that a
probability cut of p(QSO)>0.8 is optimal for purity, whereas p(QSO)>0.7 is
preferable for better completeness. Our study presents the first comprehensive
quasar selection from deep high-quality KiDS data and will serve as the basis
for versatile studies of the QSO population detected by this survey.Comment: Data available from the KiDS website at
http://kids.strw.leidenuniv.nl/DR3/quasarcatalog.php and the source code from
https://github.com/snakoneczny/kids-quasar
Large-scale structure and matter in the universe
This paper summarizes the physical mechanisms that encode the type and
quantity of cosmological matter in the properties of large-scale structure, and
reviews the application of such tests to current datasets. The key lengths of
the horizon size at matter-radiation equality and at last scattering determine
the total matter density and its ratio to the relativistic density; acoustic
oscillations can diagnose whether the matter is collisionless, and small-scale
structure or its absence can limit the mass of any dark-matter relic particle.
The most stringent constraints come from combining data on present-day galaxy
clustering with data on CMB anisotropies. Such an analysis breaks the
degeneracies inherent in either dataset alone, and proves that the universe is
very close to flat. The matter content is accurately consistent with pure Cold
Dark Matter, with about 25% of the critical density, and fluctuations that are
scalar-only, adiabatic and scale-invariant. It is demonstrated that these
conclusions cannot be evaded by adjusting either the equation of state of the
vacuum, or the total relativistic density.Comment: 17 Pages. Review paper from the January 2003 Royal Society Discussion
Meeting, "The search for dark matter and dark energy in the universe
A Simple Method for Computing the Non-Linear Mass Correlation Function with Implications for Stable Clustering
We propose a simple and accurate method for computing analytically the mass
correlation function for cold dark matter and scale-free models that fits
N-body simulations over a range that extends from the linear to the strongly
non-linear regime. The method, based on the dynamical evolution of the pair
conservation equation, relies on a universal relation between the pair-wise
velocity and the smoothed correlation function valid for high and low density
models, as derived empirically from N-body simulations. An intriguing
alternative relation, based on the stable-clustering hypothesis, predicts a
power-law behavior of the mass correlation function that disagrees with N-body
simulations but conforms well to the observed galaxy correlation function if
negligible bias is assumed. The method is a useful tool for rapidly exploring a
wide span of models and, at the same time, raises new questions about large
scale structure formation.Comment: 10 pages, 3 figure
Radio Sources in the 2dF Galaxy Redshift Survey. I. Radio Source Populations
We present the first results from a study of the radio continuum properties
of galaxies in the 2dF Galaxy Redshift Survey, based on thirty 2dF fields
covering a total area of about 100 square degrees. About 1.5% of galaxies with
b(J) < 19.4 mag are detected as radio continuum sources in the NRAO VLA Sky
Survey (NVSS). Of these, roughly 40% are star-forming galaxies and 60% are
active galaxies (mostly low-power radio galaxies and a few Seyferts). The
combination of 2dFGRS and NVSS will eventually yield a homogeneous set of
around 4000 radio-galaxy spectra, which will be a powerful tool for studying
the distriibution and evolution of both AGN and starburst galaxies out to
redshift z=0.3.Comment: 14 pages, 7 figures, accepted for publication in PAS
An Isocurvature CDM Cosmogony. II. Observational Tests
A companion paper presents a worked model for evolution through inflation to
initial conditions for an isocurvature model for structure formation. It is
shown here that the model is consistent with the available observational
constraints that can be applied without the help of numerical simulations. The
model gives an acceptable fit to the second moments of the angular fluctuations
in the thermal background radiation and the second through fourth moments of
the measured large-scale fluctuations in galaxy counts, within the possibly
significant uncertainties in these measurements. The cluster mass function
requires a rather low but observationally acceptable mass density,
0.1\lsim\Omega\lsim 0.2 in a cosmologically flat universe. Galaxies would be
assembled earlier in this model than in the adiabatic version, an arguably good
thing. Aspects of the predicted non-Gaussian character of the anisotropy of the
thermal background radiation in this model are discussed.Comment: 14 pages, 3 postscript figures, uses aas2pp4.st
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