164,798 research outputs found
The Extremely Luminous Quasar Survey (ELQS) in the SDSS footprint I.: Infrared Based Candidate Selection
Studies of the most luminous quasars at high redshift directly probe the
evolution of the most massive black holes in the early Universe and their
connection to massive galaxy formation. However, extremely luminous quasars at
high redshift are very rare objects. Only wide area surveys have a chance to
constrain their population. The Sloan Digital Sky Survey (SDSS) has so far
provided the most widely adopted measurements of the quasar luminosity function
(QLF) at . However, a careful re-examination of the SDSS quasar sample
revealed that the SDSS quasar selection is in fact missing a significant
fraction of quasars at the brightest end. We have identified the
purely optical color selection of SDSS, where quasars at these redshifts are
strongly contaminated by late-type dwarfs, and the spectroscopic incompleteness
of the SDSS footprint as the main reasons. Therefore we have designed the
Extremely Luminous Quasar Survey (ELQS), based on a novel near-infrared JKW2
color cut using WISE AllWISE and 2MASS all-sky photometry, to yield high
completeness for very bright () quasars in the redshift
range of . It effectively uses random forest machine-learning
algorithms on SDSS and WISE photometry for quasar-star classification and
photometric redshift estimation. The ELQS will spectroscopically follow-up
new quasar candidates in an area of in the
SDSS footprint, to obtain a well-defined and complete quasars sample for an
accurate measurement of the bright-end quasar luminosity function at . In this paper we present the quasar selection algorithm and the
quasar candidate catalog.Comment: 16 pages, 8 figures, 9 tables; ApJ in pres
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Biomarker discovery and redundancy reduction towards classification using a multi-factorial MALDI-TOF MS T2DM mouse model dataset
Diabetes like many diseases and biological processes is not mono-causal. On the one hand multifactorial studies with complex experimental design are required for its comprehensive analysis. On the other hand, the data from these studies often include a substantial amount of redundancy such as proteins that are typically represented by a multitude of peptides. Coping simultaneously with both complexities (experimental and technological) makes data analysis a challenge for Bioinformatics
How to Find More Supernovae with Less Work: Object Classification Techniques for Difference Imaging
We present the results of applying new object classification techniques to
difference images in the context of the Nearby Supernova Factory supernova
search. Most current supernova searches subtract reference images from new
images, identify objects in these difference images, and apply simple threshold
cuts on parameters such as statistical significance, shape, and motion to
reject objects such as cosmic rays, asteroids, and subtraction artifacts.
Although most static objects subtract cleanly, even a very low false positive
detection rate can lead to hundreds of non-supernova candidates which must be
vetted by human inspection before triggering additional followup. In comparison
to simple threshold cuts, more sophisticated methods such as Boosted Decision
Trees, Random Forests, and Support Vector Machines provide dramatically better
object discrimination. At the Nearby Supernova Factory, we reduced the number
of non-supernova candidates by a factor of 10 while increasing our supernova
identification efficiency. Methods such as these will be crucial for
maintaining a reasonable false positive rate in the automated transient alert
pipelines of upcoming projects such as PanSTARRS and LSST.Comment: 25 pages; 6 figures; submitted to Ap
Support Vector Machine classification of strong gravitational lenses
The imminent advent of very large-scale optical sky surveys, such as Euclid
and LSST, makes it important to find efficient ways of discovering rare objects
such as strong gravitational lens systems, where a background object is
multiply gravitationally imaged by a foreground mass. As well as finding the
lens systems, it is important to reject false positives due to intrinsic
structure in galaxies, and much work is in progress with machine learning
algorithms such as neural networks in order to achieve both these aims. We
present and discuss a Support Vector Machine (SVM) algorithm which makes use of
a Gabor filterbank in order to provide learning criteria for separation of
lenses and non-lenses, and demonstrate using blind challenges that under
certain circumstances it is a particularly efficient algorithm for rejecting
false positives. We compare the SVM engine with a large-scale human examination
of 100000 simulated lenses in a challenge dataset, and also apply the SVM
method to survey images from the Kilo-Degree Survey.Comment: Accepted by MNRA
Near-infrared spectroscopy of AGB star candidates in Fornax, Sculptor and NGC 6822
Context: The Asymptotic Giant Branch (AGB) phase is characterised by
substantial mass loss that is accompanied by the formation of dust. In extreme
cases this will make the star no longer visible in the optical. For a better
understanding of AGB evolution it is important to identify and characterise
these very red AGB stars.
Aims: The first aim of this article is to improve the census of red AGB stars
in three Local Group galaxies, based on near-IR spectroscopic observations of
new candidates with red 2MASS (J-K) colours. The opportunity is taken to
compare the near-IR spectra with those of Milky Way stars.
Methods: We used ISAAC on the ESO VLT to take J and H-band spectra of 36
targets in Fornax, Sculptor and NGC 6822.
Results: Twelve new C-stars are found in Fornax, and one is confirmed in
Sculptor. All C-stars have (J-K) > 1.6, and are brighter than -3.55 in
bolometric magnitude. Ten new oxygen-rich late-type giant stars are identified
in Fornax, but none is extremely red or very luminous. Five luminous O-rich AGB
stars are identified in NGC 6822, of which 3 show water absorption, indicative
of spectral type M. Again, none is as red as Milky Way OH/IR stars, but in this
galaxy the list of candidate AGB stars is biased against very red objects. In
some C-stars with (J-K)>2 an extremely strong 1.53 m absorption band is
found. These stars are probably all Mira variables and the feature is related
to the low temperature, high density chemistry that is a first step towards
dust formation and mass loss.Comment: A&A accepte
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