Cosmology

Abstract:

Results of the commissioning of the first Gemini Multi-Object Spectrograph (GMOS) are described. GMOS and the Gemini-North telescope act as a complete system to exploit a large 8 m aperture with improved image quality. Key GMOS design features such as the on-instrament wave-front sensor (OIWFS) and active flexure compensation system maintain very high image quality and stability, allowing precision observations of many targets simultaneously while reducing the need for frequent recalibration and reacquisition of targets. In this paper, example observations in imaging, long-slit, and multiobject spectroscopic modes are presented and verified by comparison with data from the literature. The expected high throughput of GMOS is confirmed from standard star observations; it peaks at about 60% when imaging in the r′ and i′ bands, and at 45%-50% in spectroscopic mode at 6300 Å. Deep GMOS photometry in the g′, r′, and i′ filters is compared to data from the literature, and the uniformity of this photometry across the GMOS field is verified. The multiobject spectroscopic mode is demonstrated by observations of the galaxy cluster A383. Centering of objects in the multislit mask was achieved to an rms accuracy of 80 mas across the 5′.5 field, and an optimized setup procedure (now in regular use) improves this to better than 50 mas. Stability during these observations was high, as expected: the average shift between object and slit positions was 5.3 mas hr ^-1, and the wavelength scale drifted by only 0.1 Å hr ^-1 (in a setup with spectral resolution of 6 Å). Finally, the current status of GMOS on Gemini-North is summarized, and future plans are outlined.

Abstract:

The completed 2dF QSO Redshift (2QZ) Survey has been used to search for extreme large-scale cosmological structure (around 200 Mpc) over the redshift range 0100Mpc are in the linear or only weakly non-linear regime and do not represent collapsed non-linear structures. We compare the measurements with the expectation of a standard LCDM model by measuring the variance of counts in cells and find that, provided the distribution of QSOs on large scales exhibits a mild bias with respect to the distribution of dark matter, the observed fluctuations are found to be in good agreement with the model. There is no evidence on such scales for any extreme structures that might require, for example, departures from the assumption of Gaussian initial perturbations. Thus the power-spectrum derived from the 2QZ Survey appears to provide a complete description of the distribution of QSOs. The amount of bias and its redshift dependence that is required is consistent with that found from studying the clustering of 2QZ QSOs on 10 Mpc scales, and may be adequately described by an approximately redshift-invariant power spectrum with normalisation sigma_8=1.0 corresponding to a bias at z=0 of b=1.1 rising to b=2 at the survey's mean redshift z=1.5.

Abstract:

We report on measurements of the cosmological constant, Λ, and the redshift space distortion parameter β= Ωm0.6/b, based on an analysis of the quasi-stellar object (QSO) power spectrum parallel and perpendicular to the observer's line of sight, Ps(k≺, k⊥), from the final catalogue of the Two-Degree Field (2dF) QSO Redshift Survey. We derive a joint Λ - β constraint from the geometric and redshift-space distortions in the power spectrum. By combining this result with a second constraint based on mass clustering evolution, we break this degeneracy and obtain strong constraints on both parameters. Assuming a flat (Ωm + Ωλ = 1) cosmology and a Λ cosmology r(z) function to convert from redshift into comoving distance, we find best-fitting values of Ωλ = 0.71 -0.17+0.09 and βq(z ∼ 1.4) = 0.45 -0.11+0.09 Assuming instead an Einstein-de Sitter cosmology r(z) we find that the best-fitting model obtained, with Ω λ = 0.64+0.16+0.11 and β q(z ∼ 1.4) = 0-40-0.09+0.09, is consistent with the Λ r(z) results, and inconsistent with a Ω λ = 0 flat cosmology at over 95 per cent confidence.

Abstract:

Under ideal MHD conditions the magnetic field strength should be correlated with density in the interstellar medium (ISM). However, observations indicate that this correlation is weak. Ambipolar diffusion can decrease the flux-to-mass ratio in weakly ionized media; however, it is generally thought to be too slow to play a significant role in the ISM except in the densest molecular clouds. Turbulence is often invoked in astrophysical problems to increase transport rates above the (very slow) laminar values predicted by kinetic theory. We describe a series of numerical experiments addressing the problem of turbulent transport of magnetic fields in weakly ionized gases. We show, subject to various geometrical and physical restrictions, that turbulence in a weakly ionized medium rapidly diffuses the magnetic flux to mass ratio through the buildup of appreciable ion-neutral drifts on small scales. These results are applicable to the fieldstrength - density correlation in the ISM, as well as the merging of flux systems such as protostar and accretion disk fields or protostellar jets with ambient matter, and the vertical transport of galactic magnetic fields.