845 research outputs found
Author response: India and China in Africa: a comparative perspective of the oil industry by Raj Verma
No full textEarlier this month Ian Taylor reviewed India and China in Africa, a new book about Asian engagement in the West African oil industry. Here, the book’s author Raj Verma responds to Taylor’s comments, outlining the rationale and evidence for the framework used in the study. India and China in Africa: A comparative perspective of the oil industry. Raj Verma. London: Routledge. 2017
Strong gravitational lenses in the era of wide-field surveys
No full textGravitational lensing is the deflection of a light beam due to the distortion of space-time caused by a massive object. Strong gravitational lensing occurs when this deflection is sufficient to produce multiple images of the same background source as viewed by an observer. The applications of such strong lens systems are widespread, from studying the initial mass function to measurements of the Hubble Constant. In the coming years, strong lens science will undergo a revolution with key data releases from the Legacy Survey of Space and Time (LSST) and Euclid Wide Survey (EWS) which are each expected to identify ∼100 000 lensed systems, increasing the number of known candidates by two orders of magnitude. In this thesis, I describe the opportunities that this data will bring. Typical searches for strong lenses have to date focussed on visible, sub-millimetre and radio wavelengths. However, the advent of large format, sensitive, Near-Infrared (NIR) detectors as seen in Euclid and the James Webb Space Telescope (JWST) will enable searches in high-resolution NIR surveys. I demonstrate that JWST will identify lensed galaxies at higher redshift than ever seen before and that Euclid will allow the NIR lens population to be studied at scale. The vast data volume from wide-field surveys such as Euclid and LSST presents a false positive problem, whereby high-scoring samples from lens classifiers are dominated by false positives. I show that the current performance of strong lens classifiers can be improved by combining these into an ensemble. Moreover, I demonstrate that even the state-of-the-art classifiers will still produce heavily contaminated or incomplete samples of lenses when applied to wide-field surveys. Given the vast majority of the lenses identified in such surveys will not receive spectroscopic confirmation, analysis of the complete lens candidate datasets will require the possibility of contamination to be accounted for. I demonstrate that cosmological parameter inference can still be conducted even in the presence of such contamination. To do so requires accepting a known contamination rate and knowledge of the probability that each system is a lens but, as I demonstrate, such data can be obtained from current lens classifiers. The coming years will be a truly exciting time for strong lens science; this thesis aims to confront some of the challenges we will face
Exploring galaxy evolution with luminosity functions across cosmic time
No full textIn this thesis I investigate galaxy evolution by measuring the luminosity functions of galaxies across a wide range in redshift. I measure the abundances of high redshift galaxies in deep HST imaging of the GOODS-North field from the CANDELS survey. I follow this evolution to lower redshift by measuring the luminosity functions of galaxies in ground-based imaging of the XMM-LSS field from the VIDEO survey with optical data from the CFHTLS.
First, at high-redshift, I identify 22 candidate z ≈ 7 and 6 candidate z ≈ 8 â 9 galaxies. By comparing the number of candidate galaxies with those found in the GOODS-South field, I determine that cosmic variance is not the dominant source of uncertainty on the number counts. I constrain the Schechter parameters for the UV luminosity function at z ≈ 7 and z ≈ 8 â 9, finding evidence for evolution in the number density of high redshift galaxies.
Next, I present the Ks-band luminosity functions in the 1 degree2 and 4.5 degree2 overlaps between the VIDEO-XMM field and the CFHTLS-D1 and CFHTLS-W1 fields. I measure the luminosity functions with the 1/Vmax method over the range 0.2 < z < 3 in VIDEO-CFHTLS-D1, and over the range 0.2 < z < 1.5 in the shallower VIDEO-CFHTLS-W1 field. I find the evolution of these luminosity functions is best described by luminosity dependent density evolution, where the characteristic magnitude has dimmed at a constant rate since z = 3, while the density has increased since z = 3, first rapidly from z = 3 to z ≈ 1.5 and then more slowly from z ≈ 1.5 to z = 0.2. I measure a significant upturn at the faint end of the luminosity function at low redshift.
Finally, I compare the VIDEO-CFHTLS-D1 and VIDEO-CFHTLS-W1 luminosity functions with predicted K-band luminosity functions from the Horizon-AGN simulation. I find both an over-prediction in the numbers of faint galaxies and an under-prediction in the numbers of bright galaxies, implying that the feedback from supernovae is insufficient while the feedback from AGN is over-sufficient.</p
Sweeping has no effect on renormalized turbulent viscosity
No full textWe perform renormalization group analysis (RG) of the Navier-Stokes equation in the presence of constant mean velocity field , and show that the renormalized viscosity is unaffected by , thus negating the ``sweeping effect", proposed by Kraichnan [Phys. Fluids {\bf 7}, 1723 (1964)] using random Galilean invariance. Using direct numerical simulation, we show that the correlation functions for and differ from each other, but the renormalized viscosity for the two cases are the same. Our numerical results are consistent with the RG calculations
A Unified Shell model for Buoyancy-Driven Turbulence
No full textWe construct a unified shell model for stably stratified and convective turbulence. Shell model simulation of stably stratified flow in turbulent regime exhibit Bolgiano-Obukhbov (BO) scaling in which the kinetic energy spectrum varies as . However, simulation of convective turbulence shows Kolmogorov's spectrum. These results are consistent with the direct numerical simulations of Kumar {\em et al.} [Phys. Rev. E {\bf 90}, 023016 (2014)]. We also observe a dual scaling ( and ) for a limited range of parameters in stably stratified flow
Energy transfers in small-scale and large-scale dynamos
No full textWe study energy transfers during magnetic energy growth in small-scale and large-scale dynamos. We perform direct numerical simulations for magnetic Prandtl number Pm =20 and 0.2 in a periodic box on 1024^3 grid. Energy fluxes and shell-to-shell energy transfers indicate that in small-scale dynamo for Pm =20, the magnetic energy growth takes place due to a non-local energy transfer from large-scale velocity field to small-scale magnetic field. On the other hand, in large-scale dynamo for Pm =0.2, local energy transfers from large-scale velocity field to large-scale magnetic field takes place
Role of the strain-rate tensor in turbulent scalar-transport modeling
No full textWe examine the geometric orientation of the subfilter-scale scalar-flux vector in homogeneous isotropic turbulence. Vector orientation is determined using the eigenframe of the resolved strain-rate tensor. The Schmidt number is kept sufficiently large so as to leave the velocity field, and hence, the strain-rate tensor, unaltered by filtering in the viscous-convective subrange. Strong preferential alignment is observed for the case of Gaussian and box filters, whereas the sharp-spectral filter leads to close to a random orientation. The orientation angle obtained with the Gaussian and box filters is largely independent of the filter-width and the Schmidt number. It is shown that the alignment direction observed numerically using these two filters is predicted very well by the tensor-diffusivity model. Further a-priori tests indicate poor alignment of the Smagorinsky and stretched vortex model predictions with the exact subfilter flux
Systems of Differential Operators and Generalized Verma Modules
No full textIn this paper we close the cases that were left open in our earlier works on the study of conformally invariant systems of second-order differential operators for degenerate principal series. More precisely, for these cases, we find the special values of the systems of differential operators, and determine the standardness of the homomorphisms between the generalized Verma modules, that come from the conformally invariant systems.The author was supported by the Global COE program at the Graduate School of Mathematical
Sciences, the University of Tokyo, Japan. He would like to be thankful for the referees for their
careful reading and invaluable comments
Twisted de Rham Complex on Line and Singular Vectors in sl₂ˆ Verma Modules
No full textWe consider two complexes. The first complex is the twisted de Rham complex of scalar meromorphic differential forms on the projective line, holomorphic on the complement to a finite set of points. The second complex is the chain complex of the Lie algebra of sl₂-valued algebraic functions on the same complement, with coefficients in a tensor product of contragradient Verma modules over the affine Lie algebra sl₂ˆ. In [Schechtman V., Varchenko A., Mosc. Math. J. 17 (2017), 787-802] a construction of a monomorphism of the first complex to the second was suggested, and it was indicated that under this monomorphism, the existence of singular vectors in the Verma modules (the Malikov-Feigin-Fuchs singular vectors) is reflected in the relations between the cohomology classes of the de Rham complex. In this paper, we prove these results.The authors thank V. Schechtman for useful discussions. The second author was supported in part by NSF grant DMS-1665239
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