arXiv subjects

Simon White

Publications and source records attributed to Simon White.

44 records · Page 3Linked to original sources

arXiv2008

Narrow associated QSO absorbers: clustering, outflows and the line-of-sight proximity effect

Using data from the Sloan Digital Sky Survey data release 3 (SDSS DR3) we investigate how narrow (<700km/s) CIV and MgII quasar absorption line systems are distributed around quasars. The CIV absorbers lie in the redshift range 1.6 < z < 4 and the MgII absorbers in the range 0.4 40% of CIV absorbers within 3,000km/s of the QSO are not a result of large-scale clustering but rather are directly associated with the quasar itself; (iii) that this intrinsic absorber population extends to outflow velocities of order 12,000km/s; (iv) that this outflow component is present in both radio-loud and radio-quiet quasars; and (v) that a small high-velocity outflow component is observed in the MgII population, but any further intrinsic absorber component is undetectable in our clustering analysis. We also find an indication that absorption systems within 3,000km/s are more abundant for radio-loud than for radio-quiet quasars. This suggests either that radio-loud objects live in more massive halos, or that their radio activity generates an additional low-velocity outflow, or both.

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arXiv2000

Populating a cluster of galaxies - I. Results at z=0

We simulate the assembly of a massive rich cluster and the formation of its constituent galaxies in a flat, low-density universe. Our most accurate model follows the collapse, the star-formation history and the orbital motion of all galaxies more luminous than the Fornax dwarf spheroidal, while dark halo structure is tracked consistently throughout the cluster for all galaxies more luminous than the SMC. Within its virial radius this model contains about 2.0e7 dark matter particles and almost 5000 distinct dynamically resolved galaxies. Simulations of this same cluster at a variety of resolutions allow us to check explicitly for numerical convergence both of the dark matter structures produced by our new parallel N-body and substructure identification codes, and of the galaxy populations produced by the phenomenological models we use to follow cooling, star formation, feedback and stellar aging. This baryonic modelling is tuned so that our simulations reproduce the observed properties of isolated spirals outside clusters. Without further parameter adjustment our simulations then produce a luminosity function, a mass-to-light ratio, luminosity, number and velocity dispersion profiles, and a morphology-radius relation which are similar to those observed in real clusters. In particular, since our simulations follow galaxy merging explicitly, we can demonstrate that it accounts quantitatively for the observed cluster population of bulges and elliptical galaxies.

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arXiv1999

Ray Tracing Simulations of Weak Lensing by Large-Scale Structure

We investigate weak lensing by large-scale structure using ray tracing through N-body simulations. Photon trajectories are followed through high resolution simulations of structure formation to make simulated maps of shear and convergence on the sky. Tests with varying numerical parameters are used to calibrate the accuracy of computed lensing statistics on angular scales from about 1 arcminute to a few degrees. Various aspects of the weak lensing approximation are also tested. For fields a few degrees on a side the shear power spectrum is almost entirely in the nonlinear regime and agrees well with nonlinear analytical predictions. Sampling fluctuations in power spectrum estimates are investigated by comparing several ray tracing realizations of a given model. For survey areas smaller than a degree on a side the main source of scatter is nonlinear coupling to modes larger than the survey. We develop a method which uses this effect to estimate the mass density parameter Omega from the scatter in power spectrum estimates for subregions of a larger survey. We show that the power spectrum can be measured accurately from realistically noisy data on scales corresponding to 1-10 Mpc/h. Non-Gaussian features in the one point distribution function of the weak lensing convergence (reconstructed from the shear) are also sensitive to Omega. We suggest several techniques for estimating Omega in the presence of noise and compare their statistical power, robustness and simplicity. With realistic noise Omega can be determined to within 0.1-0.2 from a deep survey of several square degrees.

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arXiv1999

Density and Velocity Fields from the PSCz Survey

We present the results for the predicted density and peculiar velocity fields and the dipole from the PSCz survey of 15,000 IRAS galaxies over 84% of the sky. We find a significant component to the dipole arising between 6000 and 15,000 km/s, but no significant component from greater distances. The misalignment with the CMB is 20 degrees. The most remarkable feature of the PSCz model velocity field is a coherent large-scale flow along the baseline connecting Perseus-Pisces, the Local Supercluster, Great Attractor and the Shapley Concentration. We have measured the parameter beta using the amplitude of the dipole, bulk flow and point by point comparisons between the individual velocities of galaxies in the MarkIII and SFI datasets, and the large-scale clustering distortion in redshift space.All our results are consistent with beta = 0.6 +- 0.1.

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arXiv1999

The PSCz catalogue

We present the catalogue and redshift data for the PSCz survey of 15,000 IRAS galaxies over 84% of the sky. The uniformity, completeness and data quality are assessed, and guidelines and caveats for its use are given.

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arXiv1999

PSCz-1.2 Jy Comparison: A Spherical Harmonics Approach

We perform a detailed comparison of the IRAS PSCz and 1.2-Jy spherical harmonic coefficients of the density and velocity fields in redshift space. The monopole terms predicted from the two surveys show some differences. The mismatch between the velocity monopoles arises from faint sources and disappears when extracting a PSCz subsample of galaxies with fluxes larger than 1.2 Jy. The analysis of PSCz dipole moments confirms the same inconsistencies found by Davis, Nusser and Willick (1996) between the IRAS 1.2-Jy gravity field and MARK III peculiar velocities. We conclude that shot-noise, which is greatly reduced in our PSCz gravity field, cannot be responsible for the observed mismatch.

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arXiv1999

Simulating Weak Lensing by Clusters and Large-Scale Structure

Selected results on estimating cosmological parameters from simulated weak lensing data with noise are presented. Numerical simulations of ray tracing through N-body simulations have been used to generate shear and convergence maps due to lensing by large-scale structure. Noise due to the intrinsic ellipticities of a finite number of galaxies is added. In this contribution we present our main results on estimation of the power spectrum and density parameter Omega from weak lensing data on several degree sized fields. We also show that there are striking morphological differences in the weak lensing maps of clusters of galaxies formed in models with different values of Omega.

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arXiv1998

Numerical Investigations of Weak Lensing by Large-Scale Structure

We use numerical simulations of ray tracing through N-body simulations to investigate weak lensing by large-scale structure. These are needed for testing the analytic predictions of two-point correlators, to set error estimates on them and to investigate nonlinear gravitational effects in the weak lensing maps. On scales larger than 1 degree gaussian statistics suffice and can be used to estimate the sampling, noise and aliasing errors on the measured power spectrum. For this case we describe a minimum variance inversion procedure from the 2-d to 3-d power spectrum and discuss a sparse sampling strategy which optimizes the signal to noise on the power spectrum. On degree scales and smaller the shear and convergence statistics lie in the nonlinear regime and have a non-gaussian distribution. For this regime ray tracing simulations are useful to provide reliable error estimates and calibration of the measurements. We show how the skewness and kurtosis can in principle be used to probe the mean density in the universe, but are sensitive to sampling errors and require large observed areas. The probability distribution function is likely to be more useful as a tool to investigate nonlinear effects. In particular, it shows striking differences between models with different values of the mean density $Ω_m$.

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