The Filament-Void Network and the Scale of Homogeneity in the Universe Suketu P. Bhavsar University of Kentucky Graduate Student Seminar, 2005
The Filament-Void Network andthe Scale of Homogeneity in the Universe
Suketu P. Bhavsar
University of Kentucky
Graduate Student Seminar, 2005
Outline
• A brief history of filamentary structureSky surveys and redshift surveysAre the filaments real?
• Analysis of the Las Campanas Redshift SurveyIs there a largest scale for physical filaments?
• Conclusions: Homogeneity - for L > 80Mpc
The Lick counts – southern galactic cap'grey scale' matters for what the eye tells the brain
South Galactic Cap – Seldner et al.
The “stick man” - Slice from the CfA2 redshift survey – a bubbly universe
angular position and radial velocity are plotted for each galaxy
the “wall”CfA2 six slices superposed –
angular position and radial velocity are plotted for each galaxy
What are the scales of the largest real filamentary features in the LCRS?
• Collaborators
–Somnath Bharadwaj (IIT Kh)
–Jatush V. Sheth (IUCAA)
Method Identifying filamentary structure
• Embed a 1 h-1 Mpc x 1 h-1 Mpc rectangular grid on each slice. • Generate “coarse grained” map by filling neighboring cells of occupied cells. This creates larger structure, as the filling factor, FF, increases for a slice. • Use “friends of friends” to define features for at each value of the FF.
“Friends of friends” (Turner & Gott 1977) define clusters
● Shown are 4 levels of smoothing, note how clusters grow (clockwise) with FF
● Colors represent separate clusters
Filamentarity
In 2D the shape of an object can be characterised by: perimeter (L) and area (S).
A dimensionless Shapefinder statistic, filamentarity, F (0 ≤ F ≤ 1), can be constructed out of L and S.
Extremes: F = 0 ...... circleF = 1 ...... a line
•Use Shapefinders to obtain average filamentarity, F2, of
the features as a function of FF.
(Bharadwaj et al. 2000).
Shuffling
–A procedure for randomising structure larger than some scale and keeping it intact below that scale.
Consequences of Shuffling
– Large scale structures that are real, break, and do not re-form when Shuffled
– Large scale structures that are visual, i.e. due to chance, are formed again and again due to statistical chance.
The -3o slice Shuffled at L = 70 and 80 Mpc
● Top: original LCRS slice and a Poisson distribution● Bottom: Shuffled slices
Determining the number of real filaments at various values of L
• Plot F2 versus FF for the original data and the
Shuffled slices for L from 10 Mpc to 100 Mpc • The excess of F2 in the LCRS above its values for Shuffled slices gives the REAL filamentarity through the range of FF for each slice.
Conclusions
The scale of the largest real structures in the LCRS are ~80 h-1 Mpc
The filament void network is statistically repeated on scales > 80-1 Mpc.
This is the scale on which the universe is statistically homogeneous