790 research outputs found
Machine Learning in Astronomy: A Case Study in Quasar-Star Classification
We present the results of various automated classification methods, based on
machine learning (ML), of objects from data releases 6 and 7 (DR6 and DR7) of
the Sloan Digital Sky Survey (SDSS), primarily distinguishing stars from
quasars. We provide a careful scrutiny of approaches available in the
literature and have highlighted the pitfalls in those approaches based on the
nature of data used for the study. The aim is to investigate the
appropriateness of the application of certain ML methods. The manuscript argues
convincingly in favor of the efficacy of asymmetric AdaBoost to classify
photometric data. The paper presents a critical review of existing study and
puts forward an application of asymmetric AdaBoost, as an offspring of that
exercise.Comment: 10 pages, 8 figure
Efficient use of simultaneous multi-band observations for variable star analysis
The luminosity changes of most types of variable stars are correlated in the
different wavelengths, and these correlations may be exploited for several
purposes: for variability detection, for distinction of microvariability from
noise, for period search or for classification. Principal component analysis is
a simple and well-developed statistical tool to analyze correlated data. We
will discuss its use on variable objects of Stripe 82 of the Sloan Digital Sky
Survey, with the aim of identifying new RR Lyrae and SX Phoenicis-type
candidates. The application is not straightforward because of different noise
levels in the different bands, the presence of outliers that can be confused
with real extreme observations, under- or overestimated errors and the
dependence of errors on the magnitudes. These particularities require robust
methods to be applied together with the principal component analysis. The
results show that PCA is a valuable aid in variability analysis with multi-band
data.Comment: 8 pages, 5 figures, Workshop on Astrostatistics and Data Mining in
Astronomical Databases, May 29-June 4 2011, La Palm
Dark Force Detection in Low Energy e-p Collisions
We study the prospects for detecting a light boson X with mass m_X < 100 MeV
at a low energy electron-proton collider. We focus on the case where X
dominantly decays to e+ e- as motivated by recent "dark force" models. In order
to evade direct and indirect constraints, X must have small couplings to the
standard model (alpha_X 10 MeV).
By comparing the signal and background cross sections for the e- p e+ e- final
state, we conclude that dark force detection requires an integrated luminosity
of around 1 inverse attobarn, achievable with a forthcoming JLab proposal.Comment: 38 pages, 19 figures; v2, references adde
Fast, Linear Time Hierarchical Clustering using the Baire Metric
The Baire metric induces an ultrametric on a dataset and is of linear
computational complexity, contrasted with the standard quadratic time
agglomerative hierarchical clustering algorithm. In this work we evaluate
empirically this new approach to hierarchical clustering. We compare
hierarchical clustering based on the Baire metric with (i) agglomerative
hierarchical clustering, in terms of algorithm properties; (ii) generalized
ultrametrics, in terms of definition; and (iii) fast clustering through k-means
partititioning, in terms of quality of results. For the latter, we carry out an
in depth astronomical study. We apply the Baire distance to spectrometric and
photometric redshifts from the Sloan Digital Sky Survey using, in this work,
about half a million astronomical objects. We want to know how well the (more
costly to determine) spectrometric redshifts can predict the (more easily
obtained) photometric redshifts, i.e. we seek to regress the spectrometric on
the photometric redshifts, and we use clusterwise regression for this.Comment: 27 pages, 6 tables, 10 figure
A catalog of Kazarian galaxies
The entire Kazarian galaxies (KG) catalog is presented which combines
extensive new measurements of their optical parameters with a literature and
database search. The measurements were made using images extracted from the
STScI Digitized Sky Survey (DSS) of Jpg(blue), Fpg(red) and Ipg(NIR) band
photographic sky survey plates obtained by the Palomar and UK Schmidt
telescopes. We provide accurate coordinates, morphological type, spectral and
activity classes, blue apparent diameters, axial ratios, position angles, red,
blue and NIR apparent magnitudes, as well as counts of neighboring objects in a
circle of radius 50 kpc from centers of KG. Special attention was paid to the
individual descriptions of the galaxies in the original Kazarian lists, which
clarified many cases of misidentifications of the objects, particularly among
interacting systems. The total number of individual Kazarian objects in the
database is now 706. We also include the redshifts which are now available for
404 galaxies and the 2MASS infrared magnitudes for 598 KG. The database also
includes extensive notes, which summarize information about the membership of
KG in different systems of galaxies, and about revised activity classes and
redshifts. An atlas of several interesting subclasses of KG is also presented.Comment: 15 pages, 5 figures, 1 table, Accepted in Astrophysics, Vol. 53, No.
1, 2010 (English translation of Astrofizika
The dependence of low redshift galaxy properties on environment
We review recent results on the dependence of various galaxy properties on
environment at low redshift. As environmental indicators, we use group mass,
group-centric radius, and the distinction between centrals and satellites;
examined galaxy properties include star formation rate, colour, AGN fraction,
age, metallicity and concentration. In general, satellite galaxies diverge more
markedly from their central counterparts if they reside in more massive haloes.
We show that these results are consistent with starvation being the main
environmental effect, if one takes into account that satellites that reside in
more massive haloes and at smaller halo-centric radii on average have been
accreted a longer time ago. Nevertheless, environmental effects are not fully
understood yet. In particular, it is puzzling that the impact of environment on
a galaxy seems independent of its stellar mass. This may indicate that the
stripping of the extended gas reservoir of satellite galaxies predominantly
occurs via tidal forces rather than ram-pressure.Comment: Invited Review given at the Workshop "Environment and the Formation
of Galaxies: 30 years later" held in Lisbon, 6-7 September 2010. 10 pages, 5
figure
Galaxies appear simpler than expected
Galaxies are complex systems the evolution of which apparently results from
the interplay of dynamics, star formation, chemical enrichment, and feedback
from supernova explosions and supermassive black holes. The hierarchical theory
of galaxy formation holds that galaxies are assembled from smaller pieces,
through numerous mergers of cold dark matter. The properties of an individual
galaxy should be controlled by six independent parameters including mass,
angular-momentum, baryon-fraction, age and size, as well as by the accidents of
its recent haphazard merger history. Here we report that a sample of galaxies
that were first detected through their neutral hydrogen radio-frequency
emission, and are thus free of optical selection effects, shows five
independent correlations among six independent observables, despite having a
wide range of properties. This implies that the structure of these galaxies
must be controlled by a single parameter, although we cannot identify this
parameter from our dataset. Such a degree of organisation appears to be at odds
with hierarchical galaxy formation, a central tenet of the cold dark matter
paradigm in cosmology.Comment: 26 pages, 14 figure
The stellar halo of the Galaxy
Stellar halos may hold some of the best preserved fossils of the formation
history of galaxies. They are a natural product of the merging processes that
probably take place during the assembly of a galaxy, and hence may well be the
most ubiquitous component of galaxies, independently of their Hubble type. This
review focuses on our current understanding of the spatial structure, the
kinematics and chemistry of halo stars in the Milky Way. In recent years, we
have experienced a change in paradigm thanks to the discovery of large amounts
of substructure, especially in the outer halo. I discuss the implications of
the currently available observational constraints and fold them into several
possible formation scenarios. Unraveling the formation of the Galactic halo
will be possible in the near future through a combination of large wide field
photometric and spectroscopic surveys, and especially in the era of Gaia.Comment: 46 pages, 16 figures. References updated and some minor changes.
Full-resolution version available at
http://www.astro.rug.nl/~ahelmi/stellar-halo-review.pd
Search for new phenomena in final states with an energetic jet and large missing transverse momentum in pp collisions at √ s = 8 TeV with the ATLAS detector
Results of a search for new phenomena in final states with an energetic jet and large missing transverse momentum are reported. The search uses 20.3 fb−1 of √ s = 8 TeV data collected in 2012 with the ATLAS detector at the LHC. Events are required to have at least one jet with pT > 120 GeV and no leptons. Nine signal regions are considered with increasing missing transverse momentum requirements between Emiss T > 150 GeV and Emiss T > 700 GeV. Good agreement is observed between the number of events in data and Standard Model expectations. The results are translated into exclusion limits on models with either large extra spatial dimensions, pair production of weakly interacting dark matter candidates, or production of very light gravitinos in a gauge-mediated supersymmetric model. In addition, limits on the production of an invisibly decaying Higgs-like boson leading to similar topologies in the final state are presente
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