[Paper Review] GaBoDS: The Garching-Bonn Deep Survey -- IX. A sample of 158 shear-selected mass concentration candidates

M. Schirmer, T. Erben+2 more|arXiv (Cornell University)|Jul 3, 2006

Galaxies: Formation, Evolution, Phenomena37 references4 citations

TL;DR

This paper presents a blind, shear-selected sample of 158 mass concentration candidates using weak gravitational lensing on 19 square degrees of deep $R$-band imaging from the GaBoDS survey. By applying an optimized aperture mass filter matching the NFW profile and introducing a new $P$-statistics, the authors identify mass peaks with minimal luminosity bias, finding 72 optical counterparts and showing that 55% of detections are 'dark'—likely due to noise in shallow data or projection effects.

ABSTRACT

The aim of the present work is the construction of a mass-selected galaxy cluster sample based on weak gravitational lensing methods. This sample will be subject to spectroscopic follow-up observations. We apply the mass aperture statistics and a derivative of it to 19 square degrees of high quality, single colour wide field imaging data obtained with the WFI@MPG/ESO 2.2m telescope. For the statistics a family of filter functions is used that approximates the expected tangential radial shear profile and thus allows for the efficient detection of mass concentrations. We identify 158 possible mass concentrations. This is the first time that such a large and blindly selected sample is published. 72 of the detections are associated with concentrations of bright galaxies. For about 22 of those we found spectra in the literature, indicating or proving that the galaxies seen are indeed spatially concentrated. 15 of those were previously known to be clusters or have meanwhile been secured as such. We currently follow-up a larger number of them spectroscopically to obtain deeper insight into their physical properties. The remaining 55% of the possible mass concentrations found are not associated with any optical light, or could not be classified unambiguously. We show that those "dark" detections are to a significant degree due to noise, and appear preferentially in shallow data.

Motivation & Objective

• To construct a mass-selected galaxy cluster sample using weak gravitational lensing to avoid luminosity-based selection bias.
• To develop and apply an optimized aperture mass filter function that matches the expected tangential shear profile of NFW-like mass distributions.
• To identify and classify mass concentration candidates, distinguishing between those associated with luminous galaxies and 'dark' peaks lacking optical counterparts.
• To assess the reliability of shear-selected peaks by analyzing their dependence on data depth and galaxy density, particularly identifying noise-induced false positives.
• To lay the foundation for spectroscopic follow-up of the sample to confirm redshifts and physical properties of the detected mass concentrations.

Proposed method

• Apply the aperture mass statistics ($M_{\mathrm{ap}}$) using a filter function designed to match the tangential shear profile of an NFW dark matter halo.
• Introduce a new $P$-statistics derived from $M_{\mathrm{ap}}$ as a complementary detection method, tested for consistency with $S$-statistics.
• Use 19 square degrees of $R$-band imaging from the WFI@2.2m telescope, drawn from the GaBoDS survey with inhomogeneous depth and high-quality seeing (avg. 0.86" ).
• Perform statistical randomization tests to distinguish real mass peaks from noise-induced fluctuations in the signal-to-noise ($S/N$) distribution.
• Classify detections based on optical counterparts: 72 have bright galaxies, 55% are 'dark' (no clear optical association or ambiguous classification).
• Correlate detection reliability with exposure time and galaxy number density to assess the impact of data depth on false positive rates.

Experimental results

Research questions

• RQ1 Can a filter function optimized for the tangential shear profile of an NFW halo improve the detection of mass concentrations in weak lensing surveys?
• RQ2 How many of the detected mass peaks are associated with luminous galaxies, and how many are 'dark'—i.e., lacking optical counterparts or showing ambiguous associations?
• RQ3 To what extent are 'dark' mass peaks due to noise or projection effects, and how does their prevalence depend on data depth and galaxy density?
• RQ4 How do the $S$- and $P$-statistics compare in detecting mass concentrations, and can $P$-statistics serve as a reliable complement to $S$-statistics?
• RQ5 What fraction of the detected mass concentrations are likely to be real physical structures (e.g., clusters) versus statistical fluctuations or systematics?

Key findings

• The authors identify a total of 158 shear-selected mass concentration candidates across 19 square degrees of deep imaging data.
• Of these, 72 (46%) are associated with concentrations of bright galaxies, and 22 of those have spectra in the literature confirming their redshifts and spatial concentration.
• Fifteen of the 22 optically associated detections were previously known clusters or have since been confirmed as such through follow-up.
• Approximately 55% of the detections are classified as 'dark'—lacking clear optical counterparts or ambiguous associations—indicating a significant noise contribution.
• The prevalence of 'dark' peaks is strongly correlated with shallow exposure times and low galaxy number densities, suggesting that many are due to noise from intrinsic galaxy ellipticities.
• Even in deep fields, a significant fraction of 'dark' peaks remain, but their physical origin is uncertain due to limited statistics and potential bias toward clusters with substructure.