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Search Results: 1 - 10 of 297615 matches for " J. Bacon "
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Mapping the 3-D Dark Matter potential with weak shear
D. J. Bacon,A. N. Taylor
Physics , 2002, DOI: 10.1046/j.1365-8711.2003.06922.x
Abstract: We investigate the practical implementation of Taylor's (2002) 3-dimensional gravitational potential reconstruction method using weak gravitational lensing, together with the requisite reconstruction of the lensing potential. This methodology calculates the 3-D gravitational potential given a knowledge of shear estimates and redshifts for a set of galaxies. We analytically estimate the noise expected in the reconstructed gravitational field, taking into account the uncertainties associated with a finite survey, photometric redshift uncertainty, redshift-space distortions, and multiple scattering events. In order to implement this approach for future data analysis, we simulate the lensing distortion fields due to various mass distributions. We create catalogues of galaxies sampling this distortion in three dimensions, with realistic spatial distribution and intrinsic ellipticity for both ground-based and space-based surveys. Using the resulting catalogues of galaxy position and shear, we demonstrate that it is possible to reconstruct the lensing and gravitational potentials with our method. For example, we demonstrate that a typical ground-based shear survey with redshift limit z=1 and photometric redshifts with error Delta z=0.05 is directly able to measure the 3-D gravitational potential for mass concentrations >10^14 M_\odot between 0.1
Impact of local structure on the cosmic radio dipole
Matthias Rubart,David Bacon,Dominik J. Schwarz
Physics , 2014, DOI: 10.1051/0004-6361/201423583
Abstract: We investigate the contribution that a local over- or under-density can have on linear cosmic dipole estimations. We focus here on radio surveys, such as the NRAO VLA Sky Survey (NVSS), and forthcoming surveys such as those with the LOw Frequency ARray (LOFAR), the Australian Square Kilometre Array Pathfinder (ASKAP) and the Square Kilometre Array (SKA). The NVSS has already been used to estimate the cosmic radio dipole; it was shown recently that this radio dipole amplitude is larger than expected from a purely kinematic effect, assuming the velocity inferred from the dipole of the cosmic microwave background. We show here that a significant contribution to this excess could come from a local void or similar structure. In contrast to the kinetic contribution to the radio dipole, the structure dipole depends on the flux threshold of the survey and the wave band, which opens the chance to distinguish the two contributions.
Weak lensing predictions for modified gravities at non-linear scales
Emma Beynon,David J. Bacon,Kazuya Koyama
Physics , 2009, DOI: 10.1111/j.1365-2966.2009.16117.x
Abstract: We present a set of predictions for weak lensing correlation functions in the context of modified gravity models, including a prescription for the impact of the nonlinear power spectrum regime in these models. We consider the DGP and f(R) models, together with dark energy models with the same expansion history. We use the requirement that gravity is close to GR on small scales to estimate the non-linear power for these models. We then calculate weak lensing statistics, showing their behaviour as a function of scale and redshift, and present predictions for measurement accuracy with future lensing surveys, taking into account cosmic variance and galaxy shape noise. We demonstrate the improved discriminatory power of weak lensing for testing modified gravities once the nonlinear power spectrum contribution has been included. We also examine the ability of future lensing surveys to constrain a parameterisation of the non-linear power spectrum, including sensitivity to the growth factor.
Image Simulation with Shapelets
Richard Massey,Alexandre Refregier,Christopher J. Conselice,David J. Bacon
Physics , 2003, DOI: 10.1111/j.1365-2966.2004.07353.x
Abstract: We present a method to simulate deep sky images, including realistic galaxy morphologies and telescope characteristics. To achieve a wide diversity of simulated galaxy morphologies, we first use the shapelets formalism to parametrize the shapes of all objects in the Hubble Deep Fields. We measure this distribution of real galaxy morphologies in shapelet parameter space, then resample it to generate a new population of objects. These simulated galaxies can contain spiral arms, bars, discs, arbitrary radial profiles and even dust lanes or knots. To create a final image, we also model observational effects, including noise, pixellisation, astrometric distortions and a Point-Spread Function. We demonstrate that they are realistic by showing that simulated and real data have consistent distributions of morphology diagnostics: including galaxy size, ellipticity, concentration and asymmetry statistics. Sample images are made available on the world wide web. These simulations are useful to develop and calibrate precision image analysis techniques for photometry, astrometry, and shape measurement. They can also be used to assess the sensitivity of future telescopes and surveys for applications such as supernova searches, microlensing, proper motions, and weak gravitational lensing.
Measuring Dark Matter Substructure with Galaxy-Galaxy Flexion Statistics
D. J. Bacon,A. Amara,J. I. Read
Physics , 2009, DOI: 10.1111/j.1365-2966.2010.17316.x
Abstract: It is of great interest to measure the properties of substructures in dark matter halos at galactic and cluster scales. Here we suggest a method to constrain substructure properties using the variance of weak gravitational flexion in a galaxy-galaxy lensing context. We show the effectiveness of flexion variance in measuring substructures in N-body simulations of dark matter halos, and present the expected galaxy-galaxy lensing signals. We show the insensitivity of the method to the overall galaxy halo mass, and predict the method's signal-to-noise for a space-based all-sky survey, showing that the presence of substructure down to 10^9 M_\odot halos can be reliably detected.
Sub-Arcsecond 2D Photometry and Spectrography of the Nucleus of M31: The Supermassive Black Hole Revisited
R. Bacon,E. Emsellem,G. Monnet,J. L. Nieto
Physics , 1993,
Abstract: Sub-arcsecond imagery (HRCAM, 0".35 - 0".57 FWHM) and two-dimensional spectrography (TIGER, 0".9 FWHM) of the central nucleus of M31 have been obtained at CFHT. The photometric data clearly show the double-peaked nucleus, in excellent agreement with a recent HST image by Lauer et al. 1993. We built deconvolved surface brightness models, using the multi-Gaussian expansion method. We then perform a detailed morphological analysis of the three central photometric components (bulge, nucleus and bright secondary peak) and derive various spatial luminosity models (oblate and triaxial). Stellar velocity and velocity dispersion fields were derived from the TIGER data: the former displays an extremely rapid rotation around the true center of the galaxy, while the latter exhibits a peaked structure offset in the opposite direction of the brightest light peak. Neglecting these offsets,both extended versions of the virial theorem and detailed hydrodynamical models confirm the classical strong central mass concentration, perhaps a supermassive black hole of about 7.10**7 Msun, as well as a large excess of circular motions. These offsets, however, suggest that the nucleus presently undergoes a strong stellar oscillation, with non-stationary, non-axisymmetric dynamics. This too reopens the case for a strong central mass concentration.
Simulating Hamiltonian dynamics using many-qudit Hamiltonians and local unitary control
Michael J. Bremner,Dave Bacon,Michael A. Nielsen
Physics , 2004, DOI: 10.1103/PhysRevA.71.052312
Abstract: When can a quantum system of finite dimension be used to simulate another quantum system of finite dimension? What restricts the capacity of one system to simulate another? In this paper we complete the program of studying what simulations can be done with entangling many-qudit Hamiltonians and local unitary control. By entangling we mean that every qudit is coupled to every other qudit, at least indirectly. We demonstrate that the only class of finite-dimensional entangling Hamiltonians that aren't universal for simulation is the class of entangling Hamiltonians on qubits whose Pauli operator expansion contains only terms coupling an odd number of systems, as identified by Bremner et. al. [Phys. Rev. A, 69, 012313 (2004)]. We show that in all other cases entangling many-qudit Hamiltonians are universal for simulation.
The central black hole masses and Doppler factors of the $γ$-ray loud blazars
J. H. Fan,G. Z. Xie,R. Bacon
Physics , 1999, DOI: 10.1051/aas:1999194
Abstract: In this paper, The central black hole masses and the Doppler factors are derived for PKS 0528+134, PKS 0537-441, 3C279, PKS 1406-074, PKS 1622-297, Q1633+382, Mkn 501, and BL Lacertae. The masses obtained are in the range of (1 -7)$\times 10^{7}M_{\odot}$ and compared with that obtained with the Klein-Nishina cross section considered (Dermer & Gehrels 1995). If we considered only the Thomson cross section, the masses are in the range of 2.6$\times 10^{6}M_{\odot}$ - 2$\times 10^{11}M_{\odot}$. The masses obtained from our method are less sensitive to the flux than those obtained from Dermer & Gehrels (1995) method. The masses obtained from two flares (1991 and 1996 flares) of 3C279 are almost the same. For 3C279 and BL Lacertae, viewing angle, $\theta$, and Lorentz factor, $\Gamma$, are estimated from the derived Doppler factor and the measured superluminal velocity. For 3C279, $\theta = 10^{\circ}.9-15^{\circ}.6$, $\Gamma$ = 2.4-14.4 for $\delta$ = 3.37; $\theta = 8^{\circ}.45-9^{\circ}.7$, $\Gamma$ = 2.95-11.20 for $\delta$ = 4.89; For BL Lacertae, $\theta = 25^{\circ}-29^{\circ}.4$, $\Gamma$ = 2.0-4.0.
Encoded Universality from a Single Physical Interaction
J. Kempe,D. Bacon,D. P. DiVincenzo,K. B. Whaley
Physics , 2001,
Abstract: We present a theoretical analysis of the paradigm of encoded universality, using a Lie algebraic analysis to derive specific conditions under which physical interactions can provide universality. We discuss the significance of the tensor product structure in the quantum circuit model and use this to define the conjoining of encoded qudits. The construction of encoded gates between conjoined qudits is discussed in detail. We illustrate the general procedures with several examples from exchange-only quantum computation. In particular, we extend our earlier results showing universality with the isotropic exchange interaction to the derivation of encoded universality with the anisotropic exchange interaction, i.e., to the XY model. In this case the minimal encoding for universality is into qutrits rather than into qubits as was the case for isotropic (Heisenberg) exchange. We also address issues of fault-tolerance, leakage and correction of encoded qudits.
On the Apparent Differences between Contemporary Pragmatists: Richard Rorty and the New Pragmatism
Michael Bacon
Humanities , 2012, DOI: 10.3390/h1030229
Abstract: Throughout its history pragmatism has been criticised for failing to account for the roles truth and objectivity play in our lives and inquiries. Pragmatists have long sought to guard against this objection, but recently some proponents have identified a form of pragmatism which they think is deficient in the manner identified by its critics. This has led them to claim that pragmatism should be understood as falling into two distinct varieties, and to argue for the superiority of the one over the other. In this paper I argue that behind the apparent differences between contemporary pragmatists lies greater agreement than is commonly thought. Taking Richard Rorty to represent what some find unattractive in their philosophy, I claim that there is little if any substantive difference between pragmatists about the concepts of truth and objectivity. Further, Rorty’s work shows that it is misleading to distinguish pragmatists in terms of whether they highlight the constraints imposed by social practices or whether they seek to free us from such constraint; properly understood, freedom and constraint are a necessary condition of one another.
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