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 $\lesssim$\,5\% of finding clusters with one-dimensional velocities greater than $\sim$ 600 {\kms}. The root-mean-square one-dimensional cluster velocity is 293$\pm$28 {\kms}. The observed cluster velocity function is compared with expectations from different cosmological models. The absence of a high velocity tail in the observed function is most consistent with a low mass-density ($\Omega \sim$0.3) CDM model, and is inconsistent at $\gtrsim 3 \sigma$ level with $\Omega$= 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 293$\pm$28 {\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