arXiv subjects

J. Schneider

Publications and source records attributed to J. Schneider.

80 records · Page 5Linked to original sources

arXiv2000

SDSS-RASS: Next Generation of Cluster-Finding Algorithms

We outline here the next generation of cluster-finding algorithms. We show how advances in Computer Science and Statistics have helped develop robust, fast algorithms for finding clusters of galaxies in large multi-dimensional astronomical databases like the Sloan Digital Sky Survey (SDSS). Specifically, this paper presents four new advances: (1) A new semi-parametric algorithm - nicknamed ``C4'' - for jointly finding clusters of galaxies in the SDSS and ROSAT All-Sky Survey databases; (2) The introduction of the False Discovery Rate into Astronomy; (3) The role of kernel shape in optimizing cluster detection; (4) A new determination of the X-ray Cluster Luminosity Function which has bearing on the existence of a ``deficit'' of high redshift, high luminosity clusters. This research is part of our ``Computational AstroStatistics'' collaboration (see Nichol et al. 2000) and the algorithms and techniques discussed herein will form part of the ``Virtual Observatory'' analysis toolkit.

astro-ph↗

arXiv2000

Fast Algorithms and Efficient Statistics: Density Estimation in Large Astronomical Datasets

In this paper, we outline the use of Mixture Models in density estimation of large astronomical databases. This method of density estimation has been known in Statistics for some time but has not been implemented because of the large computational cost. Herein, we detail an implementation of the Mixture Model density estimation based on multi-resolutional KD-trees which makes this statistical technique into a computationally tractable problem. We provide the theoretical and experimental background for using a mixture model of Gaussians based on the Expectation Maximization (EM) Algorithm. Applying these analyses to simulated data sets we show that the EM algorithm - using the AIC penalized likelihood to score the fit - out-performs the best kernel density estimate of the distribution while requiring no ``fine--tuning'' of the input algorithm parameters. We find that EM can accurately recover the underlying density distribution from point processes thus providing an efficient adaptive smoothing method for astronomical source catalogs. To demonstrate the general application of this statistic to astrophysical problems we consider two cases of density estimation: the clustering of galaxies in redshift space and the clustering of stars in color space. From these data we show that EM provides an adaptive smoothing of the distribution of galaxies in redshift space (describing accurately both the small and large-scale features within the data) and a means of identifying outliers in multi-dimensional color-color space (e.g. for the identification of high redshift QSOs). Automated tools such as those based on the EM algorithm will be needed in the analysis of the next generation of astronomical catalogs (2MASS, FIRST, PLANCK, SDSS) and ultimately in in the development of the National Virtual Observatory.

astro-ph↗

arXiv2000

Computational AstroStatistics: Fast Algorithms and Efficient Statistics for Density Estimation in Large Astronomical Datasets

We present initial results on the use of Mixture Models for density estimation in large astronomical databases. We provide herein both the theoretical and experimental background for using a mixture model of Gaussians based on the Expectation Maximization (EM) Algorithm. Applying these analyses to simulated data sets we show that the EM algorithm - using the both the AIC & BIC penalized likelihood to score the fit - can out-perform the best kernel density estimate of the distribution while requiring no ``fine-tuning'' of the input algorithm parameters. We find that EM can accurately recover the underlying density distribution from point processes thus providing an efficient adaptive smoothing method for astronomical source catalogs. To demonstrate the general application of this statistic to astrophysical problems we consider two cases of density estimation; the clustering of galaxies in redshift space and the clustering of stars in color space. From these data we show that EM provides an adaptive smoothing of the distribution of galaxies in redshift space (describing accurately both the small and large-scale features within the data) and a means of identifying outliers in multi-dimensional color-color space (e.g. for the identification of high redshift QSOs). Automated tools such as those based on the EM algorithm will be needed in the analysis of the next generation of astronomical catalogs (2MASS, FIRST, PLANCK, SDSS) and ultimately in the development of the National Virtual Observatory.

astro-ph↗

arXiv2000

A search for Jovian-mass planets around CM Draconis using eclipse minima timing

For the eclipsing binary system CM Draconis, eclipse minimum times have been monitored with high precision between 1994 and 1999. Periodic deviations of minimum times from a linear ephemeris may indicate the presence of an orbiting third body. Individual measurements of 41 eclipse minimum times result in a standard deviation from linear ephemeris of 5.74 seconds. A power spectral analysis of the residuals reveals only one periodicity with more then 2 seconds amplitude. This feature, with a periodicity between 750 and 1050 days has an amplitude of 2.8+-0.5 seconds, and is also present with similar phases if the power spectral analysis is performed independently for primary and secondary eclipses. It would be compatible with a planet of 1.5-3 Jupiter masses at an orbital distance of 1.1-1.45 AU to the binary barycenter. The assignation of a planet to the CM Dra system can however only be upheld if this periodicity can be followed in future observations for several years. For low-mass eclipsing binary stars, the method of eclipse minimum timing allows one to reach mass limits for the detection of third bodies well below that feasible by radial velocity measurements.

astro-ph↗

arXiv1999

Models of Universe with an inhomogeneous Big Bang singularity. II. CMBR dipole anisotropy as a byproduct of a conic Big Bang singularity

The existence of stars and galaxies reqires cosmological models with an inhomogeneous matter and radiation distribution. But in these models the initial singularity surface t_0(r) is in general homogeneous (independent of r). In this second paper of a series devoted to an inhomogeneous Big Bang singularity, we investigate the cosmic microwave background radiation (CMBR) dipole. A special Tolman-Bondi Universe is used to study the effect of a Big-Bang singularity, depending linearly on r, on the CMBR anisotropy. It is shown that, for an observer located off the ``center'' of this Universe (r=0), the parameters of the model can be tuned so as to reproduce, with a good approximation, the dipole and the quadrupole moments of the CMBR anisotropy observed in recent experiments. If the dipole should prove cosmological, a slight delaying of the Big-Bang over spatial coordinates would thus be a good candidate for its interpretation.

astro-ph↗

arXiv1998

A Solution to the Horizon Problem: A Delayed Big-Bang Singularity

One of the main drawbacks of standard cosmology, known as the horizon problem, was until now thought to be only solvable in an inflationary scenario. A delayed Big-Bang in an inhomogeneous universe is shown to solve this problem while leaving unimpaired the main successful features of the standard model.

astro-ph↗

arXiv1998

Near-Term Detectability of Terrestrial Extrasolar Planets: TEP Network Observations of CM Draconis

Results from a photometric search for extrasolar planetary transits across the eclipsing binary CM Dra are presented. The TEP (Transits of Extrasolar Planets) network has observed this star since 1994, and a lightcurve with 617 hours of coverage has been obtained. The data give a complete phase coverage of the CM Dra system at each of the 3 years of observations, with a noise of less than 5 mmag. New epoch and period values for CM Dra are derived, and a low flare rate of 0.025/hr has been confirmed. The absence of periodic variations in eclipse minimum times excludes the presence of very massive planets with periods of less than a few years. The lightcurve was visually scanned for the presence of unusual events which may be indicative of transits of extrasolar planets with 'massive earth' sizes. Six suspicious events were found which are being followed up for future transits, by planets with sizes between 1.5 and 2.5 RE (Earth Radii). However, none of these events has amplitudes compatible with planets larger than 2.5 RE. Coplanar planets larger than 2.5 RE and with orbital periods of less than 60 days can therefore be ruled out with a confidence of about 80%. Planets smaller than 1.5 RE cannot be detected in the data without a sub-noise detection algorithm. A preliminary signal detection analysis shows that there is a 50% detection confidence for 2 RE planets with a period from 10 to 30 days with the current data. This data-set demonstrates that it is possible to detect terrestrial sized planets with ground based photometry, and that strong constraints on the sizes of planets orbiting in the plane of the CM Dra system can be set.

astro-ph↗

arXiv1997

Mesoscopic Fermi gas in a harmonic trap

We study the thermodynamical properties of a mesoscopic Fermi gas in view of recent possibilities to trap ultracold atoms in a harmonic potential. We focus on the effects of shell closure for finite small atom numbers. The dependence of the chemical potential, the specific heat and the density distribution on particle number and temperature is obtained. Isotropic and anisotropic traps are compared. Possibilities of experimental observations are discussed.

cond-mat.stat-mech↗