arXiv subjects

Anson D'Aloisio

Publications and source records attributed to Anson D'Aloisio.

57 records · Page 4Linked to original sources

arXiv2011

The effects of primordial non-Gaussianity on giant-arc statistics

For over a decade, it has been debated whether the concordance LCDM model is consistent with the observed abundance of giant arcs in clusters. While previous theoretical studies have focused on properties of the lens and source populations, as well as cosmological effects such as dark energy, the impact of initial conditions on the giant-arc abundance is relatively unexplored. Here, we quantify the impact of non-Gaussian initial conditions with the local bispectrum shape on the predicted frequency of giant arcs. Using a path-integral formulation of the excursion set formalism, we extend a semi-analytic model for calculating halo concentrations to the case of primordial non-Gaussianity, which may be useful for applications outside of this work. We find that massive halos tend to collapse earlier in models with positive f_NL, relative to the Gaussian case, leading to enhanced concentration parameters. The converse is true for f_NL < 0. In addition to these effects, which change the lensing cross sections, non-Gaussianity also modifies the abundance of supercritical clusters available for lensing. These combined effects work together to either enhance (f_NL > 0) or suppress (f_NL < 0) the probability of giant-arc formation. Using the best value and 95% confidence levels currently available from the Wilkinson Microwave Anisotropy Probe, we find that the giant-arc optical depth for sources at z_s~2 is enhanced by ~20% and ~45% for f_NL = 32 and 74 respectively. In contrast, we calculate a suppression of ~5% for f_NL = -10. These differences translate to similar relative changes in the predicted all-sky number of giant arcs.

astro-ph.CO↗

arXiv2008

The abundance of lensing protoclusters

Weak gravitational lensing provides a potentially powerful method for the detection of clusters. In addition to cluster candidates, a large number of objects with possibly no optical or X-ray component have been detected in shear-selected samples. We develop an analytic model to investigate the claim of Weinberg & Kamionkowski (2002) that unvirialised protoclusters account for a significant number of these so-called "dark" lenses. In our model, a protocluster consists of a small virialised region surrounded by in-falling matter. We find that, in order for a protocluster to simultaneously escape X-ray detection and create a detectable weak lensing signal, it must have a small virial mass (~10^{13} \Msun) and large total mass (~ 10^{15} \Msun), with a relatively flat density profile outside of the virial radius. Such objects would be characterized by rising tangential shear profiles well beyond the virial radius. We use a semi-analytic approach based on the excursion set formalism to estimate the abundance of lensing protoclusters with a low probability of X-ray detection. We find that they are extremely rare, accounting for less than 0.4 per cent of the total lenses in a survey with background galaxy density n = 30 arcmin^{-2} and an intrinsic ellipticity dispersion of 0.3. We conclude that lensing protoclusters with undetectable X-Ray luminosities are too rare to account for a significant number of dark lenses.

astro-ph↗

arXiv2007

High-redshift voids in the excursion set formalism

Voids are a dominant feature of the low-redshift galaxy distribution. Several recent surveys have found evidence for the existence of large-scale structure at high redshifts as well. We present analytic estimates of galaxy void sizes at redshifts z ~ 5 - 10 using the excursion set formalism. We find that recent narrow-band surveys at z ~ 5 - 6.5 should find voids with characteristic scales of roughly 20 comoving Mpc and maximum diameters approaching 40 Mpc. This is consistent with existing surveys, but a precise comparison is difficult because of the relatively small volumes probed so far. At z ~ 7 - 10, we expect characteristic void scales of ~ 14 - 20 comoving Mpc assuming that all galaxies within dark matter haloes more massive than 10^10 M_sun are observable. We find that these characteristic scales are similar to the sizes of empty regions resulting from purely random fluctuations in the galaxy counts. As a result, true large-scale structure will be difficult to observe at z ~ 7 - 10, unless galaxies in haloes with masses less than ~ 10^9 M_sun are visible. Galaxy surveys must be deep and only the largest voids will provide meaningful information. Our model provides a convenient picture for estimating the "worst-case" effects of cosmic variance on high-redshift galaxy surveys with limited volumes.

astro-ph↗