1,721,013 research outputs found
Simulations for 21 cm radiation lensing at EoR redshifts
Full text linkWe introduce simulations aimed at assessing how well weak gravitational lensing of 21cm radiation from the Epoch of Reionization (z ̃ 8) can be measured by a Square Kilometre Array (SKA)-like radio telescope. A simulation pipeline has been implemented to study the performance of lensing reconstruction techniques. We show how well the lensing signal can be reconstructed using the 3D quadratic lensing estimator in Fourier space assuming different survey strategies. The numerical code introduced in this work is capable of dealing with issues that cannot be treated analytically such as the discreteness of visibility measurements and the inclusion of a realistic model for the antennas distribution. This paves the way for future numerical studies implementing more realistic re-ionization models, foreground subtraction schemes, and testing the performance of lensing estimators that take into account the non-Gaussian distribution of HI after re-ionization. If multiple frequency channels covering z ̃ 7-11.6 are combined, Phase 1 of SKA-Low should be able to obtain good quality images of the lensing potential with a total resolution of ̃1.6 arcmin. The SKA-Low Phase 2 should be capable of providing images with high fidelity even using data from z ̃ 7.7 to 8.3. We perform tests aimed at evaluating the numerical implementation of the mapping reconstruction. We also discuss the possibility of measuring an accurate lensing power spectrum. Combining data from z ̃ 7 to 11.6 using the SKA2-Low telescope model, we find constraints comparable to sample variance in the range L < 1000, even for survey areas as small as 25 deg2
Cosmology from EoR/Cosmic dawn with the SKA
No full textSKA Phase 1 will build upon early detections of the EoR by precursor instruments, such asMWA, PAPER, and LOFAR, and planned instruments, such as HERA, to make the first high signal-tonoise measurements of fluctuations in the 21 cm brightness temperature from both reionization and the cosmic dawn. This will allow both imaging and statistical maps of the 21cm signal at redshifts z = 627 and constrain the underlying cosmology and evolution of the density field. This era includes nearly 60% of the (in principle) observable volume of the Universe and many more linear modes than the CMB, presenting an opportunity for SKA to usher in a new level of precision cosmology. This optimistic picture is complicated by the need to understand and remove the effect of astrophysics, so that systematics rather than statistics will limit constraints. This chapter describes the cosmological, as opposed to astrophysical, information available to SKA Phase 1. Key areas for discussion include: cosmological parameters constraints using 21cm fluctuations as a tracer of the density field; lensing of the 21cm signal, constraints on heating via exotic physics such as decaying or annihilating dark matter; impact of fundamental physics such as non-Gaussianity or warm dark matter on the source population; and constraints on the bulk flows arising from the decoupling of baryons and photons at z = 1000. The chapter explores the path to separating cosmology from 'gastrophysics', for example via velocity space distortions and separation in redshift. We discuss new opportunities for extracting cosmology made possible by the sensitivity of SKA1 and explore the advances achievable with SKA2
Neutrino masses, dark energy and the gravitational lensing of pre-galactic H i
No full textWe study the constraints which the next generation of radio telescopes could place on the mass and number of neutrino species by studying the gravitational lensing of high-redshift 21-cm emission in combination with wide-angle surveys of galaxy lensing. We use simple characterizations of reionization history and of proposed telescope designs to forecast the constraints and detectability threshold for neutrinos. It is found that the degeneracy between neutrino parameters and dark energy parameters is significantly reduced by incorporating 21-cm lensing. The combination of galaxy and 21-cm lensing could constrain the sum of the neutrino masses to within ∼0.04 eV and the number of species to within ∼0.1. This is an improvement of a factor of 2.6 in mass and 1.4 in number over a galaxy lensing survey alone. This includes marginalizing over an 11-parameter cosmological model with a two-parameter model for the dark energy equation of state. If the dark energy equation of state is held fixed at w ≡ p/ρ = -1, the constraints improve to ∼0.025 eV and 0.04. These forecasted errors depend critically on the fraction of sky that can be surveyed in redshifted 21-cm emission (25 per cent is assumed here) and the redshift of reionization (z = 7 is assumed here). It is also found that neutrinos with masses too small to be detected in the data could none the less cause a significant bias in the measured dark energy equation of state. © 2009 The Author. Journal compilation. © 2009 RAS
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
The importance of intergalactic structure to gravitationally lensed quasars
No full textImage flux ratio anomalies have been attributed to substructures within the gravitational lens and to small-mass halos (M ≲ 1010 M ⊙) in intergalactic space. In this paper, analytic calculations are presented that help in the understanding of how intergalactic halos affect magnification ratios. It is found that intergalactic halos can produce anomalies at a level similar to those that are observed. Intergalactic halos with masses < 1010 M⊙ are expected to cause relative deflections between images of order 10 mas, which are then magnified by the primary lens. They will also cause fluctuations in the surface density on the several percent level. The importance of intergalactic halos depends strongly on the radial profile of the halos and the primordial power spectrum at small scales. Strongly lensed quasars provide an opportunity to probe these properties. A strong dependence on the QSO redshift is predicted and can be used to distinguish between intergalactic structure and substructure as the cause of magnification anomalies. This analytic approach also explains why some previous semianalytic estimates disagreed with numerical calculations. © 2005. The American Astronomical Society. All rights reserved
Testing ACDM with gravitational lensing constraints on small-scale structure
No full textThis paper primarily addresses the question of whether recent lensing observations probing the small-scale structure in the universe are consistent with the ACDM model. A conservative approach is taken, in which only the most difficult to explain cases of image flux anomalies in strong lenses are considered. Numerical simulations are performed to compare predictions for the ACDM small-scale mass function with observed flux ratios. By simulating several representative cases, it is found that all the cusp caustic lens anomalies and the disagreements between monochromatic flux ratios and simple lens models can be explained without any substructure in the primary lenses' dark matter halos. Intergalactic ACDM halos are enough to naturally explain these cases. However, thus far, spectroscopic gravitational lensing observations require more small-mass halos (∼106 M⊙) than are expected in the ACDM model © 2005. The American Astronomical Society. All rights reserved
Small-Scale Structure, Missing Galaxies and Gravitational Lensing
No full textThe gravitational lensing constraints on the small mass end of the ΛCDM mass function are discussed. Here a conservative approach is taken where the most difficult to explain image flux anomalies in strong lenses are emphasized. Numerical simulations are performed to compare predictions for the ΛCDM small scale mass function with the observed flux ratios. It is found that the cusp caustic lens anomalies and the disagreements between monochromatic flux ratios and simple lens models can be explained without any substructure in the primary lenses' dark matter halos. Extragalactic ΛCDM halos are enough to naturally explain these cases. This does not mean that substructure within the host lens is not contributing. In fact, it could be dominating the lensing. It should also be noted that this extragalactic population has not been directly observed either
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
- …
