Journal Article

The abundance of lensing protoclusters

Anson D'Aloisio, Steven R. Furlanetto and Priyamvada Natarajan

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 394, issue 3, pages 1469-1486
Published in print April 2009 | ISSN: 0035-8711
Published online April 2009 | e-ISSN: 1365-2966 | DOI: http://dx.doi.org/10.1111/j.1365-2966.2009.14400.x
The abundance of lensing protoclusters

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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. Determining the nature of these so-called ‘dark’ lenses is an important step towards understanding the reliability of shear-selection techniques. We develop an analytic model to investigate the claim of Weinberg & Kamionkowski that unvirialized protoclusters account for a significant number of dark lenses. In our model, a protocluster consists of a small virialized region surrounded by infalling matter. We use a simple model for the density profile that assumes the Navarro–Frenk–White form inside of the virial radius and a power law ρ∼r−α outside. 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 (∼1013 M) and large total mass (∼1015 M), with a relatively flat density profile outside of the virial radius (α∼ 0–1). 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 intrinsic ellipticity dispersion σε= 0.3. Their abundance decreases significantly if flat density profiles outside of the virial radius are not common. We conclude that lensing protoclusters with undetectable X-ray luminosities are too rare to account for a significant number of dark lenses.

Keywords: cosmology: theory; large-scale structure of Universe

Journal Article.  11799 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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