Understanding the structure of knowledge communities, and particularly the organization of "epistemic communities", or groups of agents sharing common knowledge concerns, is usually based on either social relationships or semantic similarity. To link social and semantic aspects, a formal framework based on Galois lattices (or concept lattices) categorizes epistemic communities in an automated and hierarchically structured way. The process rebuilds a whole community structure and taxonomy, and notably fields and subfields gathering a certain proportion of agents. It is applied to empirical data to exhibit these alleged structural properties, successfully compared with categories given by domain experts.