Crawling the cosmic network: An exploration of filamentary structure
Abstract
Although large-scale structures are apparent to the eye in redshift surveys, they have so far proven difficult to discriminate with computer algorithms. In this thesis, I present the Smoothed Hessian Major Axis Filament Finder (SHMAFF), an algorithm that uses the eigenvalues and eigenvectors of the Hessian matrix to characterize the overall distribution of structure and identify individual filamentary structures in that distribution. By analyzing the smoothed density field and its Hessian matrix, we can determine the types of structure - walls, filaments, or clumps - that dominate the large-scale distribution of galaxies as a function of scale. I have run the algorithm on mock galaxy distributions in a cosmological N-Body simulation and real galaxy distributions in the Sloan Digital Sky Survey, both of which appear to be filament-dominated on 10-20 h -1 Mpc scales and clump-dominated on 5 h -1 Mpc scales. There is evidence for walls in both distributions, but they are not the dominant structures on scales smaller than ~ 25 Mpc. Individual filaments are traced along the "axis of structure", which is aligned with the Hessian eigenvector corresponding to the largest eigenvalue, and are stopped when the axis orientation changes more rapidly than a preset threshold called the "curvature criterion". In both simulations and SDSS data, the resulting lament length distributions are approximately exponential and are not sensitive to non-linear growth of structure; that is, a gaussian random field with the same power spectrum will have the same filament length distribution. Filament width distributions, however, are much more sensitive to non-linearities and appear to be consistent between the real and mock galaxy distributions. In the simulations, the distribution of dark matter filaments was found to be very similar between z = 3 and z = 0 on smoothing scales as large as 15 h -1 Mpc, suggesting that the outline of filamentary structure is in place long before non-linear growth of structure is normally considered to be important. Overall, SHMAFF appears to be effective at quantifying large-scale structure. The tests run here suggest that the standard cosmological model is consistent with the large-scale distribution of galaxies, but more work is needed to make this comparison more quantitative.
- Publication:
-
Ph.D. Thesis
- Pub Date:
- 2008
- Bibcode:
- 2008PhDT.........1B
- Keywords:
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- Large-scale structure;
- Filament;
- Cosmological simulations;
- Sloan Digital Sky Survey;
- Galaxy distribution;
- Edge-finder