  In spite of recent advances in field delineation methods, bibliometricians
still don't know the extent to which their topic detection algorithms
reconstruct `ground truths', i.e. thematic structures in the scientific
literature. In this paper, we demonstrate a new approach to the delineation of
thematic structures that attempts to match the algorithm to theoretically
derived and empirically observed properties all thematic structures have in
common. We cluster citation links rather than publication nodes, use
predominantly local information and search for communities of links starting
from seed subgraphs in order to allow for pervasive overlaps of topics. We
evaluate sets of links with a new cost function and assume that local minima in
the cost landscape correspond to link communities. Because this cost landscape
has many local minima we define a valid community as the community with the
lowest minimum within a certain range. Since finding all valid communities is
impossible for large networks, we designed a memetic algorithm that combines
probabilistic evolutionary strategies with deterministic local searches. We
apply our approach to a network of about 15,000 Astronomy & Astrophysics papers
published 2010 and their cited sources, and to a network of about 100,000
Astronomy & Astrophysics papers (published 2003--2010) which are linked through
direct citations.
