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Simulations of Weak Lensing from Large-Scale Structures

Very anisotropic large scale structures like (pieces of) walls and filaments are very frequently detected in cosmological N-body simulations, and recent estimates (Colberg, Krughoff and Connolly 2004) suggest that at least 22 % of the dark matter is contained within this kind of structures.
Figure 1.25: Left: Dark matter isodensity contours in a N-body simulation of a region containing few clusters. The latter are the saturated regions, while the contours trace the filament threading the cluster. Right: Shear map obtained from mass-aperture statistics (Schneider 1996). Equal contours are from 1 to 6$\sigma$. The filament is marginally detectable.
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The detection of these structures is however very hard. The simulations themselves show that very few massive halos seem to be associated with filamentary structures (Pimblett et al. 2004). Moreover, most filaments reside in very underdense regions (Antonuccio-Delogu et al. 2002), and the detection of galaxies in these environments is difficult. Weak lensing seems then to be an interesting technique to detect and measure dark matter filaments. Recently, Dietrich et al. (2004) have reported the detection of a dark filamentary structure between A222 and A223
Antonuccio-Delogu and Paulin-Henriksson have made use of a newly developed ray shooting code (Comparato et al., 2004) on the outputs of N-body simulations to explore in a quantitative way the possibilities of this technique. We have found that although it is relatively easy to detect filamentary structures in shear maps, the actual measurement of the density profiles is made very difficult by the presence of intrinsic alignments exceeding a critical level (0.7), and by false detections arising from projection effects. An example of detection of such structures can be see in Figure 1.25, where the dark matter distribution arising from a N.body simulation recently performed with the parallel code of the group (FLY) has been analysed using the ray-shooting code. The shear map on the right depends on many parameters, like the filter function and the distribution of background galaxy images. However, the map shows only the estimated S/N, according to Schneider (1996), and it clearly demonstrates that it could be possible to detect these structures, although the rather low statistical significance level makes the actual measurement of the profile very difficult.


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Next: Laboratory of experimental astrophysics Up: Cosmology and Large-Scale Structure Previous: Galaxy Mass-Luminosity Relationships   Contents   Index
Innocenza Busa' 2005-11-14