Anson D'Aloisio

Cosmological constraints from strong gravitational lensing in clusters of galaxies.

Through a Lens Darkly According to recent measurements, 72% of the energy content in the universe is in the form of dark energy, a gravitationally repulsive constituent that is powering the accelerating expansion of the universe but whose nature is unknown. Now, Jullo et al. (p. 924) show how observations of systems of multiple images produced by the strong gravitational lensing effect of a single mass distribution can be used to constrain the properties of dark energy. Applied to the cluster Abel 1689—a galaxy cluster that is known for its lensing properties—and combined with the results of other techniques, this method brings down the overall error in the equation of state parameter of dark energy by 30%. Light from distant sources bends around massive intervening objects and helps to reveal the properties of dark energy. Current efforts in observational cosmology are focused on characterizing the mass-energy content of the universe. We present results from a geometric test based on strong lensing in galaxy clusters. Based on Hubble Space Telescope images and extensive ground-based spectroscopic follow-up of the massive galaxy cluster Abell 1689, we used a parametric model to simultaneously constrain the cluster mass distribution and dark energy equation of state. Combining our cosmological constraints with those from x-ray clusters and the Wilkinson Microwave Anisotropy Probe 5-year data gives Ωm = 0.25 ±0.05 and wx = −0.97 ±0.07, which are consistent with results from other methods. Inclusion of our method with all other available techniques brings down the current 2σ contours on the dark energy equation-of-state parameter wx by ~30%.

Cosmography with cluster strong lenses: the influence of substructure and line-of-sight haloes

We explore the use of strong lensing by galaxy clusters to constrain the dark energy equation of state and its possible time variation. The cores of massive clusters often contain several multiply imaged systems of background galaxies at different redshifts. The locations of lensed images can be used to constrain cosmological parameters due to their dependence on the ratio of angular diameter distances. We employ Monte Carlo simulations of cluster lenses, including the contribution from substructures, to assess the feasibility of this potentially powerful technique. At the present, parametric lens models use well-motivated scaling relations between mass and light to incorporate cluster member galaxies and do not explicitly model line-of-sight structure. Here, we quantify modelling errors due to scatter in the cluster-galaxy scaling relations and unmodelled line-of-sight haloes. These errors are of the order of a few arcseconds on average for clusters located at typical redshifts (z ∼ 0.2-0.3). Using Bayesian Markov chain Monte Carlo techniques, we show that the inclusion of these modelling errors is critical to deriving unbiased constraints on dark energy. However, when the uncertainties are properly quantified, we show that constraints competitive with other methods may be obtained by combining results from a sample of just 10 simulated clusters with 20 families each. Cosmography with a set of well-studied cluster lenses may provide a powerful complementary probe of the dark energy equation of state. Our simulations provide a convenient method of quantifying modelling errors and assessing future strong lensing survey strategies.

On the contribution of active galactic nuclei to the high-redshift metagalactic ionizing background

Motivated by the claimed detection of a large population of faint active galactic nuclei (AGN) at high redshift, recent studies have proposed models in which AGN contribute significantly to the z > 4 H I ionizing background. In some models, AGN are even the chief sources of reionization. If correct, these models would make necessary a complete revision to the standard view that galaxies dominated the high-redshift ionizing background. It has been suggested that AGN-dominated models can better account for two recent observations that appear to be in conflict with the standard view: (1) large opacity variations in the z ~ 5.5 H I Lyman-alpha forest, and (2) slow evolution in the mean opacity of the He II Lyman-alpha forest. Large spatial fluctuations in the ionizing background from the brightness and rarity of AGN may account for the former, while the earlier onset of He II reionization in these models may account for the latter. Here we show that models in which AGN emissions source >~ 50 % of the ionizing background generally provide a better fit to the observed H I Lyman-alpha forest opacity variations compared to standard galaxy-dominated models. However, we argue that these AGN-dominated models are in tension with constraints on the thermal history of the intergalactic medium (IGM). Under standard assumptions about the spectra of AGN, we show that the earlier onset of He II reionization heats up the IGM well above recent temperature measurements. We further argue that the slower evolution of the mean opacity of the He II Lyman-alpha forest relative to simulations may reflect deficiencies in current simulations rather than favor AGN-dominated models as has been suggested.

The effect of large-scale structure on the magnification of high-redshift sources by cluster lenses

Cluster gravitational lensing surveys like the Hubble Space Telescope Frontier Fields survey will detect distant galaxies 10-50 times fainter than any yet discovered. Using these surveys to measure the luminosity function of such faint, distant galaxies, however, requires that magnification maps built from the constraints of strongly-lensed images be accurate. For models that assume the cluster and nearby (correlated) structures are the only significant sources of lensing, a potential source of error in these maps comes from the fact that light rays also suffer weak deflections by uncorrelated large-scale structure along the line-of-sight, i.e. cosmic weak lensing (CWL). To demonstrate the magnitude of this effect, we calculate the magnification change which results when the same cluster-lens is placed along different lines of sight. Using a simple density profile for a cluster-lens at z~0.3-0.5 and the power spectrum of the matter density fluctuations responsible for CWL, we show that the typical magnifications of ~5(10) of sources at z=6-10 can differ by ~10-20(20-30)% from one line-of-sight to another. However, these fluctuations rise to greater than order unity near critical curves, indicating that CWL tends to make its greatest contribution to the most magnified images. We conclude that the neglect of CWL in determining the intrinsic luminosities of highly-magnified galaxies may introduce errors significant enough to warrant further effort to include this contribution in cluster-lens modeling. We suggest that methods of modeling CWL in galaxy-strong-lensing systems should be generalized to cluster-lensing systems.

Primordial non-Gaussianity estimation using 21 cm tomography from the epoch of reionization

Measuring the small primordial nonGaussianity (PNG) predicted by cosmic inflation theories may help diagnose them. The detectability of PNG by its imprint on the 21cm power spectrum from the epoch of reionization is reassessed here in terms of

The observable 21cm signal from reionization may be perturbative

f_{NL}

Cosmic Dawn (CoDa): the first radiation-hydrodynamics simulation of reionization and galaxy formation in the Local Universe

, the local nonlinearity parameter. We find that an optimum, multi-frequency observation by SKA can achieve

High‐redshift voids in the excursion set formalism

\Delta f_{NL} \sim 3

Large fluctuations in the high-redshift metagalactic ionizing background

(comparable to recent Planck CMB limits), while a cosmic-variance-limited array of this size like Omniscope can even detect

Heating of the Intergalactic Medium by Hydrogen Reionization

\Delta f_{NL} \sim 0.2