A taxonomy framework for unsupervised outlier detection techniques for multi-type data sets

The term "outlier" can generally be defined as an observation that is significantly different from the other values in a data set. The outliers may be instances of error or indicate events. The task of outlier detection aims at identifying such outliers in order to improve the analysis of data and further discover interesting and useful knowledge about unusual events within numerous applications domains. In this paper, we report on contemporary unsupervised outlier detection techniques for multiple types of data sets and provide a comprehensive taxonomy framework and two decision trees to select the most suitable technique based on data set. Furthermore, we highlight the advantages, disadvantages and performance issues of each class of outlier detection techniques under this taxonomy framework

A multi-scale filament extraction method: getfilaments

Far-infrared imaging surveys of Galactic star-forming regions with Herschel have shown that a substantial part of the cold interstellar medium appears as a fascinating web of omnipresent filamentary structures. This highly anisotropic ingredient of the interstellar material further complicates the difficult problem of the systematic detection and measurement of dense cores in the strongly variable but (relatively) isotropic backgrounds. Observational evidence that stars form in dense filaments creates severe problems for automated source extraction methods that must reliably distinguish sources not only from fluctuating backgrounds and noise, but also from the filamentary structures. A previous paper presented the multi-scale, multi-wavelength source extraction method getsources based on a fine spatial scale decomposition and filtering of irrelevant scales from images. In this paper, a multi-scale, multi-wavelength filament extraction method getfilaments is presented that solves this problem, substantially improving the robustness of source extraction with getsources in filamentary backgrounds. The main difference is that the filaments extracted by getfilaments are now subtracted by getsources from detection images during source extraction, greatly reducing the chances of contaminating catalogs with spurious sources. The intimate physical relationship between forming stars and filaments seen in Herschel observations demands that accurate filament extraction methods must remove the contribution of sources and that accurate source extraction methods must be able to remove underlying filamentary structures. Source extraction with getsources now provides researchers also with clean images of filaments, free of sources, noise, and isotropic backgrounds.Comment: 15 pages, 19 figures, to be published in Astronomy & Astrophysics; language polished for better readabilit

Advances in Kth nearest-neighbour clutter removal

We consider the problem of feature detection in the presence of clutter in spatial point processes. Classification methods have been developed in previous studies. Among these, Byers and Raftery (1998) models the observed Kth nearest neighbour distances as a mixture distribution and classifies the clutter and feature points consequently. In this paper, we enhance such approach in two manners. First, we propose an automatic procedure for selecting the number of nearest neighbours to consider in the classification method by means of segmented regression models. Secondly, with the aim of applying the procedure multiple times to get a ``better" end result, we propose a stopping criterion that minimizes the overall entropy measure of cluster separation between clutter and feature points. The proposed procedures are suitable for a feature with clutter as two superimposed Poisson processes on any space, including linear networks. We present simulations and two case studies of environmental data to illustrate the method

Cumulative hard X-ray spectrum of local AGN: a link to the cosmic X-ray background

We determine the cumulative spectral energy distribution (SED) of local AGN in the 3-300 keV band and compare it with the spectrum of the cosmic X-ray background (CXB) in order to test the widely accepted paradigm that the CXB is a superposition of AGN and to place constraints on AGN evolution. We performed a stacking analysis of the hard X-ray spectra of AGN detected in two recent all-sky surveys, performed by the IBIS/ISGRI instrument aboard INTEGRAL and by the PCA instrument aboard RXTE, taking into account the space densities of AGN with different luminosities and absorption column densities. We derived the collective SED of local AGN in the 3-300 keV energy band. Those AGN with luminosities below 10^43.5 erg/s (17-60 keV) provide the main contribution to the local volume hard X-ray emissivity, at least 5 times more than more luminous objects. The cumulative spectrum exhibits (although with marginal significance) a cutoff at energies above 100-200 keV and is consistent with the CXB spectrum if AGN evolve over cosmic time in such a way that the SED of their collective high-energy emission has a constant shape and the relative fraction of obscured AGN remains nearly constant, while the AGN luminosity density undergoes strong evolution between z~1 and z=0, a scenario broadly consistent with results from recent deep X-ray surveys. The first direct comparison between the collective hard X-ray SED of local AGN and the CXB spectrum demonstrates that the popular concept of the CXB being a superposition of AGN is generally correct. By repeating this test using improved AGN statistics from current and future hard X-ray surveys, it should be possible to tighten the constraints on the cosmic history of black hole growth.Comment: 12 pages, 9 figures. Revised version accepted for publication in A&

Ultra-deep catalog of X-ray groups in the Extended Chandra Deep Field South

Ultra-deep observations of ECDF-S with Chandra and XMM-Newton enable a search for extended X-ray emission down to an unprecedented flux of $2\times10^{-16}$ ergs s$^{-1}$ cm$^{-2}$. We present the search for the extended emission on spatial scales of 32$^{\prime\prime}$ in both Chandra and XMM data, covering 0.3 square degrees and model the extended emission on scales of arcminutes. We present a catalog of 46 spectroscopically identified groups, reaching a redshift of 1.6. We show that the statistical properties of ECDF-S, such as logN-logS and X-ray luminosity function are broadly consistent with LCDM, with the exception that dn/dz/d$\Omega$ test reveals that a redshift range of $0.2<z<0.5$ in ECDF-S is sparsely populated. The lack of nearby structure, however, makes studies of high-redshift groups particularly easier both in X-rays and lensing, due to a lower level of clustered foreground. We present one and two point statistics of the galaxy groups as well as weak-lensing analysis to show that the detected low-luminosity systems are indeed low-mass systems. We verify the applicability of the scaling relations between the X-ray luminosity and the total mass of the group, derived for the COSMOS survey to lower masses and higher redshifts probed by ECDF-S by means of stacked weak lensing and clustering analysis, constraining any possible departures to be within 30% in mass. Abridged.Comment: 20 pages, 21 figures, 3 tables, to match the journal versio

Comparing Star Formation on Large Scales in the c2d Legacy Clouds: Bolocam 1.1 mm Dust Continuum Surveys of Serpens, Perseus, and Ophiuchus

We have undertaken an unprecedentedly large 1.1 millimeter continuum survey of three nearby star forming clouds using Bolocam at the Caltech Submillimeter Observatory. We mapped the largest areas in each cloud at millimeter or submillimeter wavelengths to date: 7.5 sq. deg in Perseus (Paper I), 10.8 sq. deg in Ophiuchus (Paper II), and 1.5 sq. deg in Serpens with a resolution of 31", detecting 122, 44, and 35 cores, respectively. Here we report on results of the Serpens survey and compare the three clouds. Average measured angular core sizes and their dependence on resolution suggest that many of the observed sources are consistent with power-law density profiles. Tests of the effects of cloud distance reveal that linear resolution strongly affects measured source sizes and densities, but not the shape of the mass distribution. Core mass distribution slopes in Perseus and Ophiuchus (alpha=2.1+/-0.1 and alpha=2.1+/-0.3) are consistent with recent measurements of the stellar IMF, whereas the Serpens distribution is flatter (alpha=1.6+/-0.2). We also compare the relative mass distribution shapes to predictions from turbulent fragmentation simulations. Dense cores constitute less than 10% of the total cloud mass in all three clouds, consistent with other measurements of low star-formation efficiencies. Furthermore, most cores are found at high column densities; more than 75% of 1.1 mm cores are associated with Av>8 mag in Perseus, 15 mag in Serpens, and 20-23 mag in Ophiuchus.Comment: 32 pages, including 18 figures, accepted for publication in Ap

Spitzer Imaging of the Nearby Rich Young Cluster, Cep OB3b

We map the full extent of a rich massive young cluster in the Cep OB3b association with the IRAC and MIPS instruments aboard the {\it Spitzer} Space Telescope and the ACIS instrument aboard the $\it{Chandra}$ X-Ray Observatory. At 700 pc, it is revealed to be the second nearest large ($>1000$ member), young ($< 5$ Myr) cluster known. In contrast to the nearest large cluster, the Orion Nebula Cluster, Cep OB3b is only lightly obscured and is mostly located in a large cavity carved out of the surrounding molecular cloud. Our infrared and X-ray datasets, as well as visible photometry from the literature, are used to take a census of the young stars in Cep OB3b. We find that the young stars within the cluster are concentrated in two sub-clusters; an eastern sub-cluster, near the Cep B molecular clump, and a western sub-cluster, near the Cep F molecular clump. Using our census of young stars, we examine the fraction of young stars with infrared excesses indicative of circumstellar disks. We create a map of the disk fraction throughout the cluster and find that it is spatially variable. Due to these spatial variations, the two sub-clusters exhibit substantially different average disk fractions from each other: $32$ and $50$. We discuss whether the discrepant disk fractions are due to the photodestruction of disks by the high mass members of the cluster or whether they result from differences in the ages of the sub-clusters. We conclude that the discrepant disk fractions are most likely due to differences in the ages.Comment: 48 Pages, 12 figures, 6 table