Samuel Griffith: The great provincial

In dealing with the contribution of Sir Samuel Griffith to the framing of the Federal Constitution at the 1891 National Australasian Convention whose centenary we are now celebrating I can make no claim to originality. Griffith has already been the subject of Roger Joyce's magisterial biography,1 as well as in two postgraduate theses which have stood the test of time, one by Bishop John Vockler and one by Dr Ross Johnston.2 Nevertheless, Griffith was a complex and many-sided character, subtle and guarded in his lifetime, elusive for his subsequent biographers. We may be too readily misled by the portraits of his mature years, in which the face is framed in the trappings of high judicial office and the mouth shrouded by a generous white beard. He may have seemed cold and even dull; but he wasn't..

The shoals of celebrity

The author recounts his experiences of interacting with Australian politicians, members of the British Royalty and performers while establishing the Australian Studies Centre in London. He narrates incidents which involved his diplomatic skills in humouring celebrities during the process

Non-uniform wordlength delay lines for FIR filters

When FIR filters are designed floating point arithmetic is generally used. However when implemented on hardware such as ASICs, fixed point arithmetic must be used to minimise cost and power requirements. Research to minimise hardware costs has mainly focused on the quantization effects of fixed point wordlengths for the coefficients, multipliers and adders of FIR filters, but with the actual data delays assigned a uniform wordlength and essentially not optimised. This paper proposes that the wordlengths of the delay line can be non-uniform with a minimal increase in quantization noise for parallel implementation of FIR filters where there are differences in the magnitudes of the coefficients. A non-uniform delay line allows hardware savings in terms of delay register wordlengths, delay signal wordlengths and multiplier wordlengths. Results for an FIR design are presented which demonstrate the hardware savingswhen using a non-uniform wordlength delay lin

The nature and evolution of the highly ionized near-zones in the absorption spectra of z~6 quasars

We use state-of-the-art hydrodynamical simulations combined with a 1D radiative transfer code to assess the extent to which the highly ionized regions observed close to z~6 quasars, which we refer to as near-zones, can constrain the ionization state of the surrounding IGM. We find the appearance in Lya absorption of a quasar HII ionization front expanding into a neutral IGM can be very similar to a classical proximity zone, produced by the enhancement in ionizing flux close to a quasar embedded in a highly ionized IGM. The observed sizes of these highly ionized near-zones and their redshift evolution can be reproduced for a wide range of IGM neutral hydrogen fractions for plausible values of the luminosity and lifetime of the quasars. The observed near-zone sizes at the highest observed redshifts are equally consistent with a significantly neutral and a highly ionized surrounding IGM. Stronger constraints on the IGM neutral hydrogen fraction can be obtained by considering the relative size of the near-zones in the Lya and Lyb regions of a quasar spectrum. A large sample of high quality quasar absorption spectra with accurate determinations of near-zone sizes and their redshift evolution in both the Lya and Lyb regions should confirm or exclude the possibility that the Universe is predominantly neutral at the highest observed redshifts. The width of the discrete absorption features in these near-zones will contain important additional information on the ionization state and the previous thermal history of the IGM at these redshifts.Comment: 25 pages, 11 figures, accepted for publication in MNRA

On the rapid demise of Lyman-alpha emitters at z>7 due to the increasing incidence of optically thick absorption systems

A variety of independent observational studies have now reported a significant decline in the fraction of Lyman-break galaxies which exhibit Ly-a emission over the redshift interval z=6-7. In combination with the strong damping wing extending redward of Ly-a 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 HI fractions as large as 40-90 per cent may be required to explain these data assuming there is no intrinsic evolution in the Ly-a 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 Ly-a 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 reionisation is approached. Our simulations suggest these data do not require a large change in the IGM neutral fraction by several tens of per cent from z=6-7, but may instead be indicative of the rapid decrease in the typical mean free path for ionising photons expected during the final stages of reionisation.Comment: 11 pages, 6 figures, accepted to MNRA

The observed ionization rate of the intergalactic medium and the ionizing emissivity at z >5: Evidence for a photon starved and extended epoch of reionization

We use a large set of hydrodynamical simulations, combined with measurements of the Lyman alpha opacity of the IGM taken from the literature, to obtain robust estimates for the photoionization rate per hydrogen atom at z=5 and 6. We find the photoionization rate drops by a factor of two and four, respectively, compared to our recent measurements at z = 2 - 4. The number of ionizing photons emitted by known sources at z=5 and 6, based on an extrapolation of source numbers below the detection limit and standard assumptions for the relationship between the ionizing emissivity and observed luminosity density at 1500 Angstroms, are in reasonable agreement with the photoionization rates inferred from the Lyman alpha forest if the escape fraction of ionizing photons from galaxies is large (>= 20 per cent). Claims to the contrary may be attributed to the adoption of an unduly high value for the clumping factor of ionized hydrogen. Using physically motivated assumptions for the mean free path of ionizing photons our measurements of the photoionization rate can be turned into an estimate of the ionizing emissivity. In comoving units the inferred ionizing emissivity is nearly constant over the redshift range 2-6 and corresponds to 1.5-3 photons emitted per hydrogen atom over a time interval corresponding to the age of the Universe at z=6. This strongly suggests that the epoch of reionization was photon-starved and extended. [Abridged]Comment: 20 pages, 9 figures, accepted for publication in MNRA

A closer look at using quasar near-zones as a probe of neutral hydrogen in the intergalactic medium

We examine a large set of synthetic quasar spectra to realistically assess the potential of using the relative sizes of highly ionized near-zones in the Lya and Lyb forest as a probe of the neutral hydrogen content of the intergalactic medium (IGM) at z>6. The scatter in the relative near-zone size distribution, induced by underlying fluctuations in the baryonic density field and the filtering of ionizing radiation, is considerable even for fixed assumptions about the IGM neutral fraction. As a consequence, the current observational data cannot distinguish between an IGM which is significantly neutral or highly ionized just above z=6. Under standard assumptions for quasar ages and ionizing luminosities, a future sample of several tens of high resolution Lya and Lyb near-zone spectra should be capable of distinguishing between a volume weighted neutral hydrogen fraction in the IGM which is greater or less than 10 per cent.Comment: 6 pages, 3 figures, accepted for publication in MNRAS letter

Bored pile design in stiff clay II:Mechanisms and uncertainty

The soil mechanics related to pile design in clay has been the subject of substantial engineering research. In a companion paper, various codes of practice were reviewed showing the effect on pile capacity of the different global factors of safety that emerge from the various partial factor combinations for the ultimate limit state. Factors of safety are generally specified based on the opinions of experts. In this paper an assessment will be made of various objective procedures that can be used to reduce uncertainty in the design process, especially regarding the adoption of a pile resistance model and the selection of a soil strength profile as part of a ultimate limit state check, and the estimation of pile head settlement in the context of a serviceability limit state check. It is shown that both total stress and effective stress calculation methods are applicable in London Clay. Estimates of settlement using a non-linear soil stress–strain relationship are made and compared with published data. It is shown that the compression of the concrete dominates the settlement of long piles. Given the low settlements observed, recommendations are made for a reduction in standard factors of safety for bored pile design in stiff clays. </jats:p

The Lyman-alpha forest opacity and the metagalactic hydrogen ionization rate at z~2-4

Estimates of the metagalactic hydrogen ionization rate from the Lyman-alpha forest opacity in QSO absorption spectra depend on the complex density distribution of neutral hydrogen along the line-of-sight. We use a large suite of high resolution hydrodynamical simulations to investigate in detail the dependence of such estimates on physical and numerical parameters in the context of Lambda-CDM models. Adopting fiducial values for cosmological parameters together with published values of the temperature of the IGM and the effective optical depth, the metagalactic ionization rates which reproduce the Lyman-alpha effective optical depth at z=[2,3,4] are Gamma_HI=[1.3\pm^0.8_0.5, 0.9\pm0.3, 1.0\pm^0.5_0.3] \times 10^-12 s^-1, respectively. The errors include estimates of uncertainties in the relevant physical parameters and the numerical accuracy of the simulations. We find the errors are dominated by the uncertainty in the temperature of the low-density IGM. The estimated metagalactic hydrogen ionization rate for the neutral hydrogen distribution in the current concordance Lambda-CDM model is more than four times the value inferred for that in an Einstein-de Sitter model of the same r.m.s. density fluctuation amplitude sigma_8. The estimated ionization rate is also more than double that expected from updated estimates of the emissivity of observed QSOs alone. A substantial contribution from galaxies appears to be required at all redshifts.Comment: 13 pages, 7 figures, accepted to MNRAS, minor changes to submitted versio

The Discrete Sources of Cosmic Radio Emission

One of the principal problems in the radio astronomy of the discrete sources is their identification with visible objects. Identification is necessary in . most instances to determine the distance of a source and thus its intrinsic luminosity. It is of assistance in deciding the probable mechanism for the radio emission. Identification of a reasonable sample of radio sources should be made prior to speculations from statistical studies of the radio sources as a class