165 research outputs found
Insights into Quasar UV Spectra Using Unsupervised Clustering Analysis
Machine learning techniques can provide powerful tools to detect patterns in multidimensional parameter space. We use K-means - a simple yet powerful unsupervised clustering algorithm which picks out structure in unlabelled data - to study a sample of quasar UV spectra from the Quasar Catalog of the 10th Data Release of the Sloan Digital Sky Survey (SDSS-DR10) of Paris et al. Detecting patterns in large data sets helps us gain insights into the physical conditions and processes giving rise to the observed properties of quasars. We use K-means to find clusters in the parameter space of the equivalent width (EW), the blue- and red-half-width at half-maximum (HWHM) of the Mg II 2800 Ă
line, the C IV 1549 Ă
line, and the C III] 1908 Ă
blend in samples of broad absorption line (BAL) and non-BAL quasars at redshift 1.6-2.1. Using this method, we successfully recover correlations well-known in the UV regime such as the anti-correlation between the EW and blueshift of the C IV emission line and the shape of the ionizing spectra energy distribution (SED) probed by the strength of He II and the Si III]/C III] ratio. We find this to be particularly evident when the properties of C III] are used to find the clusters, while those of Mg II proved to be less strongly correlated with the properties of the other lines in the spectra such as the width of C IV or the Si III]/C III] ratio. We conclude that unsupervised clustering methods (such as K-means) are powerful methods for finding `natural\u27 binning boundaries in multidimensional data sets and discuss caveats and future work
Group-theoretical construction of extended baryon operators
The design and implementation of large sets of spatially extended baryon
operators for use in lattice simulations are described. The operators are
constructed to maximize overlaps with the low-lying states of interest, while
minimizing the number of sources needed in computing the required quark
propagators.Comment: 3 pages, 3 tables, talk presented at Lattice2004(spectrum), Fermilab,
June 21-26, 200
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Gas turbine combustion instability
Combustion oscillations are a common problem in development of LPM (lean premix) combustors. Unlike earlier, diffusion style combustors, LPM combustors are especially susceptible to oscillations because acoustic losses are smaller and operation near lean blowoff produces a greater combustion response to disturbances in reactant supply, mixing, etc. In ongoing tests at METC, five instability mechanisms have been identified in subscale and commercial scale nozzle tests. Changes to fuel nozzle geometry showed that it is possible to stabilize combustion by altering the timing of the feedback between acoustic waves and the variation in heat release
Baryonic sources using irreducible representations of the double-covered octahedral group
Irreducible representations (IRs) of the double-covered octahedral group are
used to construct lattice source and sink operators for three-quark baryons.
The goal is to achieve a good coupling to higher spin states as well as ground
states. Complete sets of local and nonlocal straight-link operators are
explicitly shown for isospin 1/2 and 3/2 baryons. The orthogonality relations
of the IR operators are confirmed in a quenched lattice simulation.Comment: Talk presented at Lattice2004(heavy), Fermilab, June 21-26, 2004, 3
page
Baryon Operators and Baryon Spectroscopy
The issues involved in a determination of the baryon resonance spectrum in
lattice QCD are discussed. The variational method is introduced and the need to
construct a sufficient basis of interpolating operators is emphasised. The
construction of baryon operators using group-theory techniques is outlined. We
find that the use both of quark-field smearing and link-field smearing in the
operators is essential firstly to reduce the coupling of operators to
high-frequency modes and secondly to reduce the gauge-field fluctuations in
correlators. We conclude with a status report of our current investigation of
baryon spectroscopy.Comment: Invited talk at Workshop on Computational Hadron Physics, Cyprus,
Sept. 14-17, 200
Hadronic physics with domain-wall valence and improved staggered sea quarks
With the advent of chiral fermion formulations, the simulation of light
valence quarks has finally become realistic for numerical simulations of
lattice QCD. The simulation of light dynamical quarks, however, remains one of
the major challenges and is still an obstacle to realistic simulations. We
attempt to meet this challenge using a hybrid combination of Asqtad sea quarks
and domain-wall valence quarks. Initial results for the proton form factor and
the nucleon axial coupling are presented.Comment: Two Talks presented at Lattice2004(spectrum), LaTex, 6 pages, 6 eps
figure
Towards a New Standard Model for Black Hole Accretion
We briefly review recent developments in black hole accretion disk theory,
emphasizing the vital role played by magnetohydrodynamic (MHD) stresses in
transporting angular momentum. The apparent universality of accretion-related
outflow phenomena is a strong indicator that large-scale MHD torques facilitate
vertical transport of angular momentum. This leads to an enhanced overall rate
of angular momentum transport and allows accretion of matter to proceed at an
interesting rate. Furthermore, we argue that when vertical transport is
important, the radial structure of the accretion disk is modified at small
radii and this affects the disk emission spectrum. We present a simple model
demonstrating how energetic, magnetically-driven outflows modify the emergent
disk emission spectrum with respect to that predicted by standard accretion
disk theory. A comparison of the predicted spectra against observations of
quasar spectral energy distributions suggests that mass accretion rates
inferred using the standard disk model may severely underestimate their true
values.Comment: To appear in the Fifth Stromlo Symposium Proceedings special issue of
ApS
Quasars and their host galaxies
This review attempts to describe developments in the fields of quasar and
quasar host galaxies in the past five. In this time period, the Sloan and 2dF
quasar surveys have added several tens of thousands of quasars, with Sloan
quasars being found to z>6. Obscured, or partially obscured quasars have begun
to be found in significant numbers. Black hole mass estimates for quasars, and
our confidence in them, have improved significantly, allowing a start on
relating quasar properties such as radio jet power to fundamental parameters of
the quasar such as black hole mass and accretion rate. Quasar host galaxy
studies have allowed us to find and characterize the host galaxies of quasars
to z>2. Despite these developments, many questions remain unresolved, in
particular the origin of the close relationship between black hole mass and
galaxy bulge mass/velocity dispersion seen in local galaxies.Comment: Review article, to appear in Astrophysics Update
The High Redshift Integrated Sachs-Wolfe Effect
In this paper we rely on the quasar (QSO) catalog of the Sloan Digital Sky
Survey Data Release Six (SDSS DR6) of about one million photometrically
selected QSOs to compute the Integrated Sachs-Wolfe (ISW) effect at high
redshift, aiming at constraining the behavior of the expansion rate and thus
the behaviour of dark energy at those epochs. This unique sample significantly
extends previous catalogs to higher redshifts while retaining high efficiency
in the selection algorithm. We compute the auto-correlation function (ACF) of
QSO number density from which we extract the bias and the stellar
contamination. We then calculate the cross-correlation function (CCF) between
QSO number density and Cosmic Microwave Background (CMB) temperature
fluctuations in different subsamples: at high z>1.5 and low z<1.5 redshifts and
for two different choices of QSO in a conservative and in a more speculative
analysis. We find an overall evidence for a cross-correlation different from
zero at the 2.7\sigma level, while this evidence drops to 1.5\sigma at z>1.5.
We focus on the capabilities of the ISW to constrain the behaviour of the dark
energy component at high redshift both in the \LambdaCDM and Early Dark Energy
cosmologies, when the dark energy is substantially unconstrained by
observations. At present, the inclusion of the ISW data results in a poor
improvement compared to the obtained constraints from other cosmological
datasets. We study the capabilities of future high-redshift QSO survey and find
that the ISW signal can improve the constraints on the most important
cosmological parameters derived from Planck CMB data, including the high
redshift dark energy abundance, by a factor \sim 1.5.Comment: 20 pages, 18 figures, and 7 table
Constraining Primordial Non-Gaussianity with High-Redshift Probes
We present an analysis of the constraints on the amplitude of primordial
non-Gaussianity of local type described by the dimensionless parameter . These constraints are set by the auto-correlation functions (ACFs) of two
large scale structure probes, the radio sources from NRAO VLA Sky Survey (NVSS)
and the quasar catalogue of Sloan Digital Sky Survey Release Six (SDSS DR6
QSOs), as well as by their cross-correlation functions (CCFs) with the cosmic
microwave background (CMB) temperature map (Integrated Sachs-Wolfe effect).
Several systematic effects that may affect the observational estimates of the
ACFs and of the CCFs are investigated and conservatively accounted for. Our
approach exploits the large-scale scale-dependence of the non-Gaussian halo
bias. The derived constraints on {} coming from the NVSS CCF and
from the QSO ACF and CCF are weaker than those previously obtained from the
NVSS ACF, but still consistent with them. Finally, we obtain the constraints on
() and () from
NVSS data and SDSS DR6 QSO data, respectively.Comment: 16 pages, 8 figures, 1 table, Accepted for publication on JCA
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