Introduction

Beyond three hundred billion light-years, when averaged over 100 Mpc, the visible cosmos can be seen as a gas of galaxies, uniformly distributed. At smaller spatial scales, astronomical observations as well as numerical simulations have shown that the repartition of the luminous matter in the Universe is not so homogeneous. Galaxies cluster within elongated large-scale structures, called filaments, and leave huge voids between those filaments. These filaments, which might only occupy 10% of the volume of the Universe, are organized in a complex three dimensional network often described as leading to a sponge-like or cell-like 3D topology. As shown in figure 1, such a filament is not a single structure with sharp edges, but instead a fuzzy set of points more or less scattered, which makes its detection difficult. Another difficulty in the detection process comes from the difference of spatial scales between sparse and prominent compact features. The gradual disappearance of structures with increasing distance results from the use of a magnitude-limited sample. The apparent luminosity of any object is fainter as distance increases, and only the few galaxies with the highest intrinsic luminosity are then included.

Up to now, there are only a few methods to extract the filamentary structure. The Minimal Spanning Tree (MST) method, firstly formalized by [1] has been then mostly used [2]. Recently, a method based on the ``Candy'' model [3] showed interesting results for the filament detection from a cosmological simulation [4]. In section 4.1, a comparison of our detection result with the MST and ``Candy'' detection results is presented.

As the ``Candy'' model has been successfully adapted to extract cosmic filaments, we proposed to use the ``Quality Candy'' model [5] for the same application. Both models are based on a marked point process approach, whose efficacity has been shown for road network extraction in remote sensing [3,5]. Marked point processes are shortly described in section 2. The proposed detection model is presented in section 3 and tests on 2-D galaxy maps are then shown in section 4.

Figure 1: Three dimensional view of galaxies up to 500 billion light-years, from the two CfA observation cones (credits: Center for Astrophysics, Harvard).

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