Friday, July 5, 2013

1307.1232 (Elmo Tempel et al.)

Orientation of cosmic web filaments with respect to the underlying velocity field    [PDF]

Elmo Tempel, Noam I. Libeskind, Yehuda Hoffman, Lauri J. Liivamägi, Antti Tamm
The large scale structure of the Universe is characterised by a web-like structure made of voids, sheets, filaments and knots. It has been suggested that the structure of this so-called cosmic web is dictated by the local velocity shear tensor, and in particular that the local direction of a filament should be strongly aligned with e3, the eigenvector associated with the smallest eigenvalue of the shear tensor. The aim of the paper is to test that conjecture on the basis of a high resolution cosmological simulation. The cosmic web delineated by the dark matter (DM) haloes distribution is probed by a marked point process with interactions (the Bisous model) in order to detect filaments directly from the dark halo distribution (the P-web). It is shown that the detected P-web filaments are strongly aligned with the local e3 eigenvector: the alignment is within 30 degree for ~80% of the detected elements. This result indicates that the large-scale filaments defined purely from the number density distribution of haloes (or galaxies) carry more than just morphological information, although the Bisous model does not make any prior assumption on the underlying shear tensor, nor does it make any use of it. The P-web filaments are also compared to the large scale structure revealed directly from an analysis of the velocity shear tensor (the V-web). In the densest regions, the P- and V-web filaments overlap well (90%), whereas in lower density regions, the P-web filaments preferentially mark regions which are collapsing in one dimension, defined to be sheets in the V-web. Further work is needed to inspect the causes and implications of the latter result.
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