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Amin M.A.,Kavli Institute for Cosmology and Institute of Astronomy | Lim E.A.,Kings College London | Yang I.-S.,University of Amsterdam
Physical Review Letters | Year: 2013

We derive a closed-form expression for the phase shift experienced by (1+1)-dimensional kinks colliding at ultrarelativistic velocities (γvâ‰1), valid for arbitrary periodic potentials. Our closed-form expression is the leading-order result of a more general scattering theory of solitary waves described in a related paper. This theory relies on a small kinematic parameter 1/(γv)1 rather than a small parameter in the Lagrangian. Our analytic results can be directly extracted from the Lagrangian without solving the equation of motion. Based on our closed-form expression, we prove that kink-kink and kink-antikink collisions have identical phase shifts at leading order. © 2013 American Physical Society. Source


Raskutti S.,University of Melbourne | Bolton J.S.,University of Melbourne | Wyithe J.S.B.,University of Melbourne | Wyithe J.S.B.,University of Sydney | Becker G.D.,Kavli Institute for Cosmology and Institute of Astronomy
Monthly Notices of the Royal Astronomical Society | Year: 2012

Measurements of the intergalactic medium (IGM) temperature provide a potentially powerful constraint on the reionization history due to the thermal imprint left by the photoionization of neutral hydrogen. However, until recently IGM temperature measurements were limited to redshifts 2 ≤z≤ 4.8, restricting the ability of these data to probe the reionization history at z > 6. In this work, we use recent measurements of the IGM temperature in the near-zones of seven quasars at z∼ 5.8-6.4, combined with a semi-numerical model for inhomogeneous reionization, to establish new constraints on the redshift at which hydrogen reionization completed. We calibrate the model to reproduce observational constraints on the electron scattering optical depth and the Hi photoionization rate, and compute the resulting spatially inhomogeneous temperature distribution at z∼ 6 for a variety of reionization scenarios. Under standard assumptions for the ionizing spectra of Population II sources, the near-zone temperature measurements constrain the redshift by which hydrogen reionization was complete to be z r > 7.9 (6.5) at 68 (95) per cent confidence. We conclude that future temperature measurements around other high-redshift quasars will significantly increase the power of this technique, enabling these results to be tightened and generalized. © 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS. Source


Boera E.,Swinburne University of Technology | Murphy M.T.,Swinburne University of Technology | Becker G.D.,Kavli Institute for Cosmology and Institute of Astronomy | Bolton J.S.,University of Nottingham
Monthly Notices of the Royal Astronomical Society | Year: 2014

According to the photoheating model of the intergalactic medium (IGM), He II reionization is expected to affect its thermal evolution. Evidence for additional energy injection into the IGM has been found at 3 ≲ z ≲ 4, though the evidence for the subsequent fall-offbelow z ~ 2.8 is weaker and depends on the slope of the temperature-density relation, γ. Here we present, for the first time, an extension of the IGM temperature measurements down to the atmospheric cut-offof the HI Lyman-α (Lyα) forest at z ≃ 1.5. Applying the curvature method on a sample of 60 Ultraviolet and Visual Echelle Spectrograph (UVES) spectra we investigated the thermal history of the IGM at z < 3 with precision comparable to the higher redshift results. We find that the temperature of the cosmic gas traced by the Lyα forest [T (-δ)] increases for increasing overdensity from T (-δ) ~ 22670 to 33740 K in the redshift range z ~ 2.8-1.6. Under the assumption of two reasonable values for γ, the temperature at the mean density (T0) shows a tendency to flatten at z ≲ 2.8. In the case of γ ~ 1.5, our results are consistent with previous ones which indicate a falling T0 for redshifts z≲2.8. Finally, our T (-δ) values show reasonable agreement with moderate blazar heating models. © 2014 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society. Source


Bolton S.,University of Nottingham | Bolton S.,University of Melbourne | Haehnelt M.G.,Kavli Institute for Cosmology and Institute of Astronomy
Monthly Notices of the Royal Astronomical Society | Year: 2013

A variety of independent observational studies have now reported a significant decline in the fraction of Lyman-break galaxies which exhibit Lya emission over the redshift interval z = 6-7. In combination with the strong damping wing extending redward of Lya in the spectrum of the bright z = 7.085 quasar ULAS 1120+0641, this has strengthened suggestions that the hydrogen in the intergalactic medium (IGM) is still substantially neutral at z ̃ 7. Current theoretical models imply H I fractions as large as 40-90 per cent may be required to explain these data assuming there is no intrinsic evolution in the Lya emitter population. We propose that such large neutral fractions are not necessary. Based on a hydrodynamical simulation which reproduces the absorption spectra of high-redshift (z ̃ 6-7) quasars, we demonstrate that the opacity of the intervening IGM redward of rest-frame Lya can rise rapidly in average regions of the Universe simply because of the increasing incidence of absorption systems which are optically thick to Lyman continuum photons as the tail-end of reionization is approached. Our simulations suggest that these data do not require a large change in the IGM neutral fraction by several tens of per cent from z = 6 to 7, but may instead be indicative of the rapid decrease in the typical mean free path for ionizing photons expected during the final stages of reionization. © 2012 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society. Source


Calverley A.P.,Kavli Institute for Cosmology and Institute of Astronomy | Becker G.D.,Kavli Institute for Cosmology and Institute of Astronomy | Haehnelt M.G.,Kavli Institute for Cosmology and Institute of Astronomy | Bolton J.S.,University of Melbourne
Monthly Notices of the Royal Astronomical Society | Year: 2011

We present measurements of the ionizing ultraviolet background (UVB) at z∼ 5-6 using the quasar proximity effect. The 15 quasars in our sample cover the range 4.6 < zq < 6.4, enabling the first proximity-effect measurements of the UVB at z > 5. The metagalactic hydrogen ionization rate, Γbkg, was determined by modelling the combined ionization field from the quasar and the UVB in the proximity zone on a pixel-by-pixel basis. The optical depths in the spectra were corrected for the expected effect of the quasar until the mean flux in the proximity region equalled that in the average Lyα forest, and from this we make a measurement of Γbkg. A number of systematic effects were tested using synthetic spectra. Noise in the flux was found to be the largest source of bias at z∼ 5, while uncertainties in the mean transmitted Lyα flux are responsible for the largest bias at z∼ 6. The impacts of large-scale overdensities and Lyman limit systems on Γbkgwere also investigated, but found to be small at z > 5. We find a decline in Γbkg with redshift, from log(Γbkg) =-12.15 ± 0.16 at z∼ 5 to log(Γbkg) =-12.84 ± 0.18 at z∼ 6 (1σ errors). Compared to UVB measurements at lower redshifts, our measurements suggest a drop of a factor of 5 in the Hi photoionization rate between z∼ 4 and 6. The decline of Γbkg appears to be gradual, and we find no evidence for a sudden change in the UVB at any redshift that would indicate a rapid change in the attenuation length of ionizing photons. Combined with recent measurements of the evolution of the mean free path of ionizing photons, our results imply a decline in the emissivity of ionizing photons by roughly a factor of 2 from z∼ 5 to 6, albeit with significant uncertainty due to the measurement errors in both Γbkg and the mean free path. © 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS. Source

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