Middle to Late Ordovician age for the Jindalee Group of the Lachlan Fold Belt,New South Wales: Conodont evidence and some tectonic implications

The first definitive evidence for a late Middle to early Late Ordovician age for the Jindalee Group comes from identification of conodonts, including Periodon aculeatus, preserved in chert from an exposure northeast of Cootamundra, New South Wales. In the Grenfell area, the Hoskins Chert, a constituent formation of the Jindalee Group, also yields conodonts of the same general age, although no diagnostic species have been recognised. Conodonts found in the Jindalee Group, along with a distinctive fossil flora of probable cyanobacterial filaments, are similar to those of the Mugincoble Chert in the vicinity of Parkes. Age correlation of the Jindalee Group with the Girilambone Group is confirmed by the newly found conodonts, but at a much more precise level than previously inferred. However, the tectonic settings of the Jindalee and Girilambone Groups might have been quite distinct, with the Jindalee Group forming in an intra‐arc rift and the Girilambone Group depositing in the backarc Wagga Marginal Basin.     

Fast-growing firms as elements of change in Canada’s headquarters city system

This research explores the factors that shape the evolving geographic distribution of business headquarters (HQ) activity. We address an understudied influence on HQ geographies: metropolitan HQ changes driven by the process of small, rapidly expanding businesses growing into mature companies. This investigation focuses on the developmental paths followed by fast-growing firms (FGFs) and the geographic distinctions that can be observed in a FGF tracking study of Canada’s metropolitan regions from 1987 to 2005. Our research findings indicate that geography plays an important role in this development, as FGF tracking records throughout Canada’s metropolitan areas diverge sharply. We find that most FGFs that emerged in Vancouver and Toronto continued as ongoing businesses following their rapid growth phase, while a high proportion of FGFs based in Montréal and Calgary did not. These results contribute to a greater understanding of metropolitan economies, business development, and HQ location in Canada     

Multi-level adaptive particle mesh (MLAPM): a c code for cosmological simulations

We present a computer code written in c that is designed to simulate structure formation from collisionless matter. The code is purely grid-based and uses a recursively refined Cartesian grid to solve Poisson's equation for the potential, rather than obtaining the potential from a Green's function. Refinements can have arbitrary shapes and in practice closely follow the complex morphology of the density field that evolves. The time-step shortens by a factor of 2 with each successive refinement.
Competing approaches to N -body simulation are discussed from the point of view of the basic theory of N -body simulation. It is argued that an appropriate choice of softening length ε is of great importance and that ε should be at all points an appropriate multiple of the local interparticle separation. Unlike tree and P³M codes, multigrid codes automatically satisfy this requirement. We show that at early times and low densities in cosmological simulations, ε needs to be significantly smaller relative to the interparticle separation than in virialized regions. Tests of the ability of the code's Poisson solver to recover the gravitational fields of both virialized haloes and Zel'dovich waves are presented, as are tests of the code's ability to reproduce analytic solutions for plane-wave evolution. The times required to conduct a ΛCDM cosmological simulation for various configurations are compared with the times required to complete the same simulation with the ART, AP³M and GADGET codes. The power spectra, halo mass functions and halo–halo correlation functions of simulations conducted with different codes are compared.
The code is available from http://www-thphys.physics.ox.ac.uk/users/MLAPM .     

A sample of 6C radio sources designed to find objects at redshift z>4– III. Imaging and the radio galaxy K–z relation

In this paper, the third and final of a series, we present complete K -band imaging and some complementary I -band imaging of the filtered 6C* sample. We find no systematic differences between the K – z relation of 6C* radio galaxies and those from complete samples, so the near-infrared properties of luminous radio galaxies are not obviously biased by the additional 6C* radio selection criteria (steep spectral index and small angular size). The 6C* K – z data significantly improve delineation of the K – z relation for radio galaxies at high redshift ( z >2) . Accounting for non-stellar contamination, and for correlations between radio luminosity and stellar mass, we find little support for previous claims that the underlying scatter in the stellar luminosity of radio galaxies increases significantly at z >2 . In a particular spatially flat universe with a cosmological constant (Ω_M=0.3 and Ω_Λ=0.7) , the most luminous radio sources appear to be associated with galaxies with a luminosity distribution with a high mean (≈5  L *), and a low dispersion ( σ ∼0.5 mag) which formed their stars at epochs corresponding to z ≳2.5 . This result is in line with recent submillimetre studies of high-redshift radio galaxies and the inferred ages of extremely red objects from faint radio samples.     

An empirical model for the polarization of pulsar radio emission

We present an empirical model for single pulses of radio emission from pulsars based on Gaussian probability distributions for relevant variables. The radiation at a specific pulse phase is represented as the superposition of radiation in two (approximately) orthogonally polarized modes (OPMs) from one or more subsources in the emission region of the pulsar. For each subsource, the polarization states are drawn randomly from statistical distributions, with the mean and the variance on the Poincaré sphere as free parameters. The intensity of one OPM is chosen from a lognormal distribution, and the intensity of the other OPM is assumed to be partially correlated, with the degree of correlation also chosen from a Gaussian distribution. The model is used to construct simulated data described in the same format as real data: distributions of the polarization of pulses on the Poincaré sphere and histograms of the intensity and other parameters. We concentrate on the interpretation of data for specific phases of PSR B0329+54 for which the OPMs are not orthogonal, with one well defined and the other spread out around an annulus on the Poincaré sphere at some phases. The results support the assumption that the radiation emerges in two OPMs with closely correlated intensities, and that in a statistical fraction of pulses one OPM is invisible.     

Surface glaciochemistry of Taylor Valley,southern Victoria Land,Antarctica and its relationship to stream chemistry

We have analysed a series of surface samples from the alpine glaciers of Taylor Valley, Southern Victoria Land, Antarctica for their geochemical composition. This was done to better establish the role of glaciochemical variation on the chemical composition of meltwater streams emanating from the glaciers. The chemistry of the snow and ice changes with proximity to the ocean and with elevation of the glacier surface. The aeolian transport of dry valley dust, as indicated by both chemical and particulate matter variations, is a major process affecting the overall glacier chemistry and the chemistry of the meltwaters. Streams originating on the western edges of the glaciers are more impacted by the aeolian input as they are in the direct path of the major regional wind direction. Our data strongly suggest that the variation in stream geochemistry in Taylor Valley is a result in part of the differences in chemical composition of glacier melt. Copyright © 2003 John Wiley & Sons, Ltd.     

Can simulations reproduce the observed temperature–mass relation for clusters of galaxies?

It has become increasingly apparent that traditional hydrodynamical simulations of galaxy clusters are unable to reproduce the observed properties of galaxy clusters, in particular overpredicting the mass corresponding to a given cluster temperature. Such overestimation may lead to systematic errors in results using galaxy clusters as cosmological probes, such as constraints on the density perturbation normalization σ ₈. In this paper we demonstrate that inclusion of additional gas physics, namely radiative cooling and a possible pre-heating of gas prior to cluster formation, is able to bring the temperature–mass relation in the innermost parts of clusters into good agreement with recent determinations by Allen, Schmidt & Fabian using Chandra data.