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In this work we introduce a new method to perform the identification of groups of galaxies and present results of the identification of galaxy groups in the Seventh Data Release of the Sloan Digital Sky Survey (SDSS-DR7). Our methodology follows an approach that resembles the standard friends-of-friends (FoF) method. However, it uses assumptions on the mass of the dark matter halo hosting a group of galaxies to link galaxies in the group using a local linking length. Our method does not assumes any ad-hoc parameter for the identification of groups, nor a linking length or a density threshold. This parameter-free nature of the method, and the robustness of its results, are the most important points of our work. We describe the data used for our study and give details of the implementation of the method. We obtain galaxy groups and halo catalogs for four volume limited samples whose properties are in good agreement with previous works. They reproduces the expected stellar mass functions and follow the expected stellar-halo mass relation. We found that most of the stellar content in groups of galaxies comes from objects with $M_r$ absolute magnitudes larger than -19, meaning that it is important to resolve the low luminosity components of groups of galaxies to acquire detailed information about their properties., Comment: Accepted for publication in MNRAS

We present the implementation of a halo based method for the reconstruction of the cosmic mass density field. The method employs the mass density distribution of dark matter haloes and its environments computed from cosmological N-body simulations and convolves it with a halo catalog to reconstruct the dark matter density field determined by the distribution of haloes. We applied the method to the group catalog of Yang etal (2007) built from the SDSS Data Release 4. As result we obtain reconstructions of the cosmic mass density field that are independent on any explicit assumption of bias. We describe in detail the implementation of the method, present a detailed characterization of the reconstructed density field (mean mass density distribution, correlation function and counts in cells) and the results of the classification of large scale environments (filaments, voids, peaks and sheets) in our reconstruction. Applications of the method include morphological studies of the galaxy population on large scales and the realization of constrained simulations., Comment: Accepted for publication in MNRAS