Beecroft Institute for Particle Astrophysics and Cosmology

Abstract:

We calculate the effect that a primordial homogeneous magnetic field, $\B_0$, will have on the different CMB power spectra due to Faraday rotation. Concentrating on the $TB$, $EB$ and $BB$ correlations, we forecast the ability for future CMB polarization experiments to constrain $\B_0$. Our results depend on how well the foregrounds can be subtracted from the CMB maps, but we find a predicted error between $\sigma_{\B_0} = 4 \times 10^{-11}$Gauss (for the QUIET experiment with foregrounds perfectly subtracted) and $3 \times 10^{-10}$Gauss (with the Clover experiment with no foreground subtraction). These constraints are two orders of magnitudes better than the present limits on $\B_0$.

Abstract:

The Lyα forest has opened a new redshift regime for cosmological investigation. At z > 2 it provides a unique probe of cosmic geometry and an independent constraint on dark energy that is not subject to standard candle or ruler assumptions. In Paper I of this series on using the Lya forest observed in pairs of QSOs for a new application of the Alcock-Paczynski test, we present and discuss the results of a campaign to obtain moderate-resolution spectroscopy (FWHM ≃ 2.5 Å) of the Lyα forest in pairs of QSOs with small redshift differences (Δz < 0.25, z > 2.2) and arc-minute separations (θ < 5′). This data set, composed of seven individual QSOs, 35 pairs, and one triplet, is also well suited for future investigations of the coherence of Lya absorbers on ∼1 Mpc transverse scales and the transverse proximity effect. We note seven revisions for previously published QSO identifications and/or redshifts. © 2008. The American Astronomical Society. All rights reserved.

Abstract:

We study numerically the formation of molecular clouds in large-scale colliding flows including self-gravity. The models emphasize the competition between the effects of gravity on global and local scales in an isolated cloud. Global gravity builds up large-scale filaments, while local gravity, triggered by a combination of strong thermal and dynamical instabilities, causes cores to form. The dynamical instabilities give rise to a local focusing of the colliding flows, facilitating the rapid formation of massive protostellar cores of a few hundred M⊙. The forming clouds do not reach an equilibrium state, although the motions within the clouds appear to be comparable to virial. The self-similar core mass distributions derived from models with and without self-gravity indicate that the core mass distribution is set very early on during the cloud formation process, predominantly by a combination of thermal and dynamical instabilities rather than by self-gravity. © 2008. The American Astronomical Society. All rights reserved.

Abstract:

We have developed a semi-empirical simulation of the extragalactic radio continuum sky suitable for aiding the design of next generation radio interferometers such as the Square Kilometre Array (SKA). The emphasis is on modelling the large-scale cosmological distribution of radio sources rather than the internal structure of individual galaxies. Here we provide a description of the simulation to accompany the online release of a catalogue of ≃320 million simulated radio sources. The simulation covers a sky area of 20 × 20 deg2 - a plausible upper limit to the instantaneous field of view attainable with future (e.g. SKA) aperture array technologies - out to a cosmological redshift of z = 20, and down to flux density limits of 10 nJy at 151, 610 MHz, 1.4, 4.86 and 18 GHz. Five distinct source types are included: radio-quiet active galactic nuclei (AGN), radio-loud AGN of the Fanaroff-Riley type I (FR I) and FR II structural classes, and star-forming galaxies, the latter split into populations of quiescent and starbursting galaxies. In our semi-empirical approach, the simulated sources are drawn from observed (or extrapolated) luminosity functions and grafted on to an underlying dark matter density field with biases which reflect their measured large-scale clustering. A numerical Press-Schechter style filtering of the density field is used to identify and populate clusters of galaxies. For economy of output, radio source structures are constructed from point source and elliptical subcomponents, and for FR I and FR II sources an orientation-based unification and beaming model is used to partition flux between the core and extended lobes and hotspots. The extensive simulation output gives users the flexibility to post-process the catalogues to achieve more complete agreement with observational data in the years ahead. The ultimate aim is for the 'idealized skies' generated by this simulation and associated post-processing to be fed to telescope simulators to optimize the design of the SKA itself. © 2008 RAS.

Abstract:

In this paper, we extend the Bayesian model fitting shape measurement method presented in Miller et al., and use the method to estimate the shear from the Shear TEsting Programme simulations (STEP). The method uses a fast model fitting algorithm that uses realistic galaxy profiles and analytically marginalizes over the position and amplitude of the model by doing the model fitting in Fourier space. This is used to find the full posterior probability in ellipticity. The shear is then estimated in a Bayesian way from this posterior probability surface. The Bayesian estimation allows measurement bias arising from the presence of random noise to be removed. In this paper, we introduce an iterative algorithm that can be used to estimate the intrinsic ellipticity prior and show that this is accurate and stable. We present results using the STEP parametrization that relates the input shear γT to the estimated shear γM by introducing a bias m and an offset c: γM - γT = mγT + c. The average number density of galaxies used in the STEP1 analysis was 9 per square arcminute, for STEP2 the number density was 30 per square arcminute. By using the method to estimate the shear from the STEP1 simulations we find the method to have a shear bias of m = 0.006 ± 0.005 and a variation in shear offset with point spread function type of σc = 0.0002. Using the method to estimate the shear from the STEP2 simulations we find that the shear bias and offset are m = 0.002 ± 0.016 and c = -0.0007 ± 0.0006, respectively. In addition, we find that the bias and offset are stable to changes in the magnitude and size of the galaxies. Such biases should yield any cosmological constraints from future weak lensing surveys robust to systematic effects in shape measurement. Finally, we present an alternative to the STEP parametrization by using a quality factor that relates the intrinsic shear variance in a simulation to the variance in shear that is measured and show that the method presented has an average of Q ≳ 100 which is at least a factor of 10 times better than other shape measurement methods. © 2008 RAS.