193,055 research outputs found
Improving the quality of the personalized electronic program guide
As Digital TV subscribers are offered more and more channels, it is becoming increasingly difficult for them to locate the right programme information at the right time. The personalized Electronic Programme Guide (pEPG) is one solution to this problem; it leverages artificial intelligence and user profiling techniques to learn about the viewing preferences of individual users in order to compile personalized viewing guides that fit their individual preferences. Very often the limited availability of profiling information is a key limiting factor in such personalized recommender systems. For example, it is well known that collaborative filtering approaches suffer significantly from the sparsity problem, which exists because the expected item-overlap between profiles is usually very low. In this article we address the sparsity problem in the Digital TV domain. We propose the use of data mining techniques as a way of supplementing meagre ratings-based profile knowledge with additional item-similarity knowledge that can be automatically discovered by mining user profiles. We argue that this new similarity knowledge can significantly enhance the performance of a recommender system in even the sparsest of profile spaces. Moreover, we provide an extensive evaluation of our approach using two large-scale, state-of-the-art online systems—PTVPlus, a personalized TV listings portal and Físchlár, an online digital video library system
Constraining the low-mass end of the Initial Mass Function with Gravitational Lensing
The low-mass end of the stellar Initial Mass Function (IMF) is constrained by
focusing on the baryon-dominated central regions of strong lensing galaxies. We
study in this letter the Einstein Cross (Q2237+0305), a z=0.04 barred galaxy
whose bulge acts as lens on a background quasar. The positions of the four
quasar images constrain the surface mass density on the lens plane, whereas the
surface brightness (H-band NICMOS/HST imaging) along with deep spectroscopy of
the lens (VLT/FORS1) allow us to constrain the stellar mass content, for a
range of IMFs. We find that a classical single power law (Salpeter IMF)
predicts more stellar mass than the observed lensing estimates. This result is
confirmed at the 99% confidence level, and is robust to systematic effects due
to the choice of population synthesis models, the presence of dust, or the
complex disk/bulge population mix. Our non-parametric methodology is more
robust than kinematic estimates, as we do not need to make any assumptions
about the dynamical state of the galaxy or its decomposition into bulge and
disk. Over a range of low-mass power law slopes (with Salpeter being
Gamma=+1.35) we find that at a 90% confidence level, slopes with Gamma>0 are
ruled out.Comment: 5 pages, 6 figures. Accepted for publication in MNRAS Letter
Weak Gravitational Flexion
Flexion is the significant third-order weak gravitational lensing effect
responsible for the weakly skewed and arc-like appearance of lensed galaxies.
Here we demonstrate how flexion measurements can be used to measure galaxy halo
density profiles and large-scale structure on non-linear scales, via
galaxy-galaxy lensing, dark matter mapping and cosmic flexion correlation
functions. We describe the origin of gravitational flexion, and discuss its
four components, two of which are first described here. We also introduce an
efficient complex formalism for all orders of lensing distortion. We proceed to
examine the flexion predictions for galaxy-galaxy lensing, examining isothermal
sphere and Navarro, Frenk & White (NFW) profiles and both circularly symmetric
and elliptical cases. We show that in combination with shear we can precisely
measure galaxy masses and NFW halo concentrations. We also show how flexion
measurements can be used to reconstruct mass maps in 2-D projection on the sky,
and in 3-D in combination with redshift data. Finally, we examine the
predictions for cosmic flexion, including convergence-flexion
cross-correlations, and find that the signal is an effective probe of structure
on non-linear scales.Comment: 17 pages, including 12 figures, submitted to MNRA
The Distribution of Stellar Orbits in the Giant Elliptical Galaxy NGC 2320
We present direct observational constraints on the orbital distribution of
the stars in the giant elliptical NGC 2320. Long-slit spectra along multiple
position angles are used to derive the stellar line-of-sight velocity
distribution within one effective radius. In addition, the rotation curve and
dispersion profile of an ionized gas disk are measured from the [OIII] emission
lines. After correcting for the asymmetric drift, we derive the circular
velocity of the gas, which provides an independent constraint on the
gravitational potential.
To interpret the stellar motions, we build axisymmetric three-integral
dynamical models based on an extension of the Schwarzschild orbit-
superposition technique. We consider two families of gravitational potential,
one in which the mass follows the light (i.e. no dark matter) and one with a
logarithmic gravitational potential. Using chi^2-statistics, we compare our
models to both the stellar and gas data to constrain the value of the V-band
mass-to-light ratio Upsilon-V. We find Upsilon-V = 15.0 \pm 0.6 h75 for the
mass-follows-light models and Upsilon-V = 17.0 \pm 0.7 h75 for the logarithmic
models. For the latter, Upsilon-V is defined within a sphere of 15'' radius.
Models with radially constant Upsilon-V and logarithmic models with dark
matter provide comparably good fits to the data and possess similar dynamical
structure. Across the full range of Upsilon-V permitted by the observational
constraints, the models are radially anisotropic in the equatorial plane over
the radial range of our kinematical data (1'' < r < 40''). Along the true minor
axis, they are more nearly isotropic. (abridged)Comment: 26 pages, 13 figures, accepted for publication in the Astrophysical
Journa
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