25,530 research outputs found
The nature of assembly bias - III. Observational properties
We analyse galaxies in groups in the Sloan Digital Sky Survey (SDSS) and find
a weak but significant assembly-type bias, where old central galaxies have a
higher clustering amplitude (61 9 per cent) at scales > 1 Mpc than young
central galaxies of equal host halo mass (
). The observational sample is volume-limited out to z=0.1 with 5 log. We construct a mock catalogue of galaxies that shows a
similar signal of assembly bias (46 9 per cent) at the same halo mass. We
then adapt the model presented by Lacerna & Padilla (Paper I) to redefine the
overdensity peak height, which traces the assembly bias such that galaxies in
equal density peaks show the same clustering regardless of their stellar age,
but this time using observational features such as a flux limit. The proxy for
peak height, which is proposed as a better alternative than the virial mass,
consists in the total mass given by the mass of neighbour host haloes in
cylinders centred at each central galaxy. The radius of the cylinder is
parametrized as a function of stellar age and virial mass. The best-fitting set
of parameters that make the assembly bias signal lower than 515 per cent for
both SDSS and mock central galaxies are similar. The idea behind the
parametrization is not to minimize the bias, but it is to use this method to
understand the physical features that produce the assembly bias effect. Even
though the tracers of the density field used here differ significantly from
those used in paper I, our analysis of the simulated catalogue indicates that
the different tracers produce correlated proxies, and therefore the reason
behind this assembly bias is the crowding of peaks in both simulations and the
SDSS.Comment: 12 pages, 11 figures. Accepted for publication in MNRA
Laplacian Mixture Modeling for Network Analysis and Unsupervised Learning on Graphs
Laplacian mixture models identify overlapping regions of influence in
unlabeled graph and network data in a scalable and computationally efficient
way, yielding useful low-dimensional representations. By combining Laplacian
eigenspace and finite mixture modeling methods, they provide probabilistic or
fuzzy dimensionality reductions or domain decompositions for a variety of input
data types, including mixture distributions, feature vectors, and graphs or
networks. Provable optimal recovery using the algorithm is analytically shown
for a nontrivial class of cluster graphs. Heuristic approximations for scalable
high-performance implementations are described and empirically tested.
Connections to PageRank and community detection in network analysis demonstrate
the wide applicability of this approach. The origins of fuzzy spectral methods,
beginning with generalized heat or diffusion equations in physics, are reviewed
and summarized. Comparisons to other dimensionality reduction and clustering
methods for challenging unsupervised machine learning problems are also
discussed.Comment: 13 figures, 35 reference
The Spine of the Cosmic Web
We present the SpineWeb framework for the topological analysis of the Cosmic
Web and the identification of its walls, filaments and cluster nodes. Based on
the watershed segmentation of the cosmic density field, the SpineWeb method
invokes the local adjacency properties of the boundaries between the watershed
basins to trace the critical points in the density field and the separatrices
defined by them. The separatrices are classified into walls and the spine, the
network of filaments and nodes in the matter distribution. Testing the method
with a heuristic Voronoi model yields outstanding results. Following the
discussion of the test results, we apply the SpineWeb method to a set of
cosmological N-body simulations. The latter illustrates the potential for
studying the structure and dynamics of the Cosmic Web.Comment: Accepted for publication HIGH-RES version:
http://skysrv.pha.jhu.edu/~miguel/SpineWeb
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