208,010 research outputs found
Galaxy shear estimation from stacked images
Statistics of the weak lensing of galaxies can be used to constrain cosmology
if the galaxy shear can be estimated accurately. In general this requires
accurate modelling of unlensed galaxy shapes and the point spread function
(PSF). I discuss suboptimal but potentially robust methods for estimating
galaxy shear by stacking images such that the stacked image distribution is
closely Gaussian by the central limit theorem. The shear can then be determined
by radial fitting, requiring only an accurate model of the PSF rather than also
needing to model each galaxy accurately. When noise is significant asymmetric
errors in the centroid must be corrected, but the method may ultimately be able
to give accurate un-biased results when there is a high galaxy density with
constant shear. It provides a useful baseline for more optimal methods, and a
test-case for estimating biases, though the method is not directly applicable
to realistic data. I test stacking methods on the simple toy simulations with
constant PSF and shear provided by the GREAT08 project, on which most other
existing methods perform significantly more poorly, and briefly discuss
generalizations to more realistic cases. In the appendix I discuss a simple
analytic galaxy population model where stacking gives optimal errors in a
perfect ideal case.Comment: 7 pages, 1 figure. Updated to match MNRAS-accepted version; added
appendix describing an analytic galaxy population mode
Theoretical analysis of dynamic chemical imaging with lasers using high-order harmonic generation
We report theoretical investigations of the tomographic procedure suggested
by Itatani {\it et al.} [Nature, {\bf 432} 867 (2004)] for reconstructing
highest occupied molecular orbitals (HOMO) using high-order harmonic generation
(HHG). Using the limited range of harmonics from the plateau region, we found
that under the most favorable assumptions, it is still very difficult to obtain
accurate HOMO wavefunction, but the symmetry of the HOMO and the internuclear
separation between the atoms can be accurately extracted, especially when
lasers of longer wavelengths are used to generate the HHG. We also considered
the possible removal or relaxation of the approximations used in the
tomographic method in actual applications. We suggest that for chemical
imaging, in the future it is better to use an iterative method to locate the
positions of atoms in the molecule such that the resulting HHG best fits the
macroscopic HHG data, rather than by the tomographic method.Comment: 13 pages, 14 figure
Warped discs and the directional stability of jets in Active Galactic Nuclei
Warped accretion discs in Active Galactic Nuclei (AGN) exert a torque on the
black hole that tends to align the rotation axis with the angular momentum of
the outer disc. We compute the magnitude of this torque by solving numerically
for the steady state shape of the warped disc, and verify that the analytic
solution of Scheuer and Feiler (1996) provides an excellent approximation. We
generalise these results for discs with strong warps and arbitrary surface
density profiles, and calculate the timescale on which the black hole becomes
aligned with the angular momentum in the outer disc. For massive holes and
accretion rates of the order of the Eddington limit the alignment timescale is
always short (less than a Myr), so that jets accelerated from the inner disc
region provide a prompt tracer of the angular momentum of gas at large radii in
the disc. Longer timescales are predicted for low luminosity systems, depending
on the degree of anisotropy in the disc's hydrodynamic response to shear and
warp, and for the final decay of modest warps at large radii in the disc that
are potentially observable via VLBI. We discuss the implications of this for
the inferred accretion history of those Active Galactic Nuclei whose jet
directions appear to be stable over long timescales. The large energy
deposition rate at modest disc radii during rapid realignment episodes should
make such objects transiently bright at optical and infrared wavelengths.Comment: MNRAS, in press. Revised to match accepted version, with one new
figure showing alignment timescale as a function of black hole mas
An approach for real world data modelling with the 3D terrestrial laser scanner for built environment
Capturing and modelling 3D information of the built environment is a big challenge. A number of techniques and technologies are now in use. These include EDM, GPS, and photogrammetric application, remote sensing and traditional building surveying applications. However, use of these technologies cannot be practical and efficient in regard to time, cost and accuracy. Furthermore, a multi disciplinary knowledge base, created from the studies and research about the regeneration aspects is fundamental: historical, architectural, archeologically, environmental, social, economic, etc. In order to have an adequate diagnosis of regeneration, it is necessary to describe buildings and surroundings by means of documentation and plans. However, at this point in time the foregoing is considerably far removed from the real situation, since more often than not it is extremely difficult to obtain full documentation and cartography, of an acceptable quality, since the material, constructive pathologies and systems are often insufficient or deficient (flat that simply reflects levels, isolated photographs,..). Sometimes the information in reality exists, but this fact is not known, or it is not easily accessible, leading to the unnecessary duplication of efforts and resources.
In this paper, we discussed 3D laser scanning technology, which can acquire high density point data in an accurate, fast way. Besides, the scanner can digitize all the 3D information concerned with a real world object such as buildings, trees and terrain down to millimetre detail Therefore, it can provide benefits for refurbishment process in regeneration in the Built Environment and it can be the potential solution to overcome the challenges above. The paper introduce an approach for scanning buildings, processing the point cloud raw data, and a modelling approach for CAD extraction and building objects classification by a pattern matching approach in IFC (Industry Foundation Classes) format. The approach presented in this paper from an undertaken research can lead to parametric design and Building Information Modelling (BIM) for existing structures. Two case studies are introduced to demonstrate the use of laser scanner technology in the Built Environment. These case studies are the Jactin House Building in East Manchester and the Peel building in the campus of University Salford. Through these case studies, while use of laser scanners are explained, the integration of it with various technologies and systems are also explored for professionals in Built Environmen
- …