Cosmology

Abstract:

We analyse the redshift-space (z-space) distortions of quasi-stellar object (QSO) clustering in the 2-degree field instrument (2dF) QSO Redshift Survey (2QZ). To interpret the z-space correlation function, ξ(σ, π), we require an accurate model for the QSO real-space correlation function, ξ(r). Although a single power-law ξ(r) ξ r-γ model fits the projected correlation function [wp(σ)] at small scales, it implies somewhat too shallow a slope for both wp(σ) and the z-space correlation function, ξ(s), at larger scales (≳20 h-1 Mpc). Motivated by the form for ξ(r) seen in the 2dF Galaxy Redshift Survey (2dFGRS) and in standard A cold dark matter (COM) predictions, we use a double power-law model for ξ(r), which gives a good fit to ξ(s) and w p(σ). The model is parametrized by a slope of γ = 1.45 for 1 < r < 10 h-1 Mpc and γ = 2.30 for 10 < r < 40 h-1 Mpc. As found for the 2dFGRS, the value of β determined from the ratio of ξ(s)/ξ(r) depends sensitively on the form of ξ(r) assumed. With our double power-law form for ξ(r), we measure β(z = 1.4) = 0.32-0.11+0.09. Assuming the same model for ξ(r), we then analyse the z-space distortions in the 2QZ ξ(σ, π) and put constraints on the values of Ωm0 and β(z = 1.4), using an improved version of the method of Hoyle et al. The constraints we derive are Ωm0 = 0.35-0.13+0.19, β(z = 1.4) = 0.50-0.15+0.13 in agreement with our ξ(s)/ξ(r) results at the ∼1σ level. © 2005 RAS.

Abstract:

We have undertaken deep optical imaging observations of three 6.2 < z < 6.5 quasar fields in the i′ and z′ filters. These data are used to search for foreground galaxies that are gravitationally lensing the quasars and distant galaxies physically associated with the quasars. Foreground galaxies are found closer than 5″ to the lines of sight of two of the three quasars. However, the faintness of these galaxies suggests that they have fairly low masses and provide only weak magnifications (μ ≲1.1). No convincing galaxies physically associated with the quasars are found, and the number of i′-band dropouts is consistent with that found in random fields. We consider the expected dark matter halo masses that host these quasars under the assumption that a correlation between black hole mass and dark matter halo mass exists. We show that the steepness of the high-mass tail of the halo mass function at this redshift, combined with realistic amounts of scatter in this correlation, leads to expected halo masses substantially lower than previously believed. This analysis can explain the lack of companion galaxies found here and the low dynamical mass recently published for one of the quasars. © 2005. The American Astronomical Society. All rights reserved.

Abstract:

We address the degree and rapidity of generation of small-scale power over the course of structure formation in cosmologies where the primordial power spectrum is strongly suppressed beyond a given wavenumber. We first summarize the situations where one expects such suppressed power spectra and point out their diversity. We then employ an exponential cut-off, which characterizes warm dark matter (WDM) models, as a template for the shape of the cut-off and focus on damping scales ranging from 106 to 109 h -1 M⊙. Using high-resolution simulations, we show that the suppressed part of the power spectrum is quickly (re)generated and catches up with both the linear and the non-linear evolution of the unsuppressed power spectrum. From z = 2 onwards, a power spectrum with a primordial cut-off at 109 h-1 MŁódź, becomes virtually indistinguishable from an evolved cold dark matter (CDM) power spectrum. An attractor such as that described in Zaldarriaga, Scoccimarro & Hui for power spectra with different spectral indices also emerges in the case of truncated power spectra. Measurements of z ∼ 0 non-linear power spectra at ∼100 h-1 kpc cannot rule out the possibility of linear power spectra damped below ∼109 h-1 M ⊙. Therefore, WDM or scenarios with similar features should be difficult to exclude in this way. © 2005 RAS.