1/23The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL

Interactive Visualization of Intercluster Galaxy Structures in the Horologium-Reticulum Supercluster

Matthew C. Fleenor

Roanoke College

Jameson MillerUNC Chapel Hill

Cory W. QuammenUNC Chapel Hill

2/23The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL

Data description• Galaxy positions in RA-

DEC-cz coordinate system> Right-ascension (RA) ~

longitude> Declination (DEC) ~ latitude> cz – radial dimension

(recessional velocity)

• ~2500 galaxy locations• ~30 clusters

3/23The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL

Domain questions• What is the distribution of

intercluster galaxies?• Are there large void regions? How

many?• Does the supercluster have

filaments?• How do clusters fit into the

structure defined by intercluster galaxies?

4/23The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL

Standard 2D plots

5/23The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL

Overvieew

6/23The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL

Prior 3D experience• Collaborators viewed data in

immersive environment• Got lost – no context

7/23The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL

Previous interactive tools

• Cosmic Explorer [Song1993]• SGI Explorer [Christensen1995]• PartiView [Levy2001]• AstroMD [Gheller2002]

8/23The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL

What’s missing• Ability to group galaxies into

structures they define>Voids>Filaments

• Reference axes in all three RA-DEC-cz dimensions

9/23The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL

Data types• Sparse 3D position data• Nominal (categorical) data

>Intercluster galaxies vs. clusters>User-defined groups indicating

structure (filament, void boundary)

InterclusterGalaxy

Cluster

Group 1

Group 2

…

10/23The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL

Nominal encoding• 3D glyphs

> Enough screen real estate

> Distinct shapes encode object type

> Depth queues from perspective and occlusion

• Nominal color encoding> Group membership> Encoded by 10 of 12

colors recommended by Ware2004

11/23The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL

RA-DEC-cz reference axes

• Orientation to dataset

• Colored with just-noticeably-different color than background

• Can turn sides, top, and bottom on or off

12/23The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL

Curved drop lines• Explicit connection

between galaxies and reference axes

• Curved to fit RA-DEC-cz coordinate system

• Allows comparison to standard plots

13/23The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL

Structure perception• Structure-from-

motion> Strongest shape cue

• Torsional rocking> Structure-from-motion

without interaction

• Stereo> Complements other

techniques> User can control eye-

separation parameter

14/23The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL

Confirmation of analysis• Quantitative analysis

shows two separate overdensities

• Overdensities pop out in visualization

• ~95% correspondence between grouping by hand and quantitative grouping

15/23The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL

Positive result• Quick identification

of void regions• Selection of galaxies

along rim defines bounds of void

• Offline sphere-fitting refines estimation

• Six voids identified• Known clusters

reside around voids

16/23The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL

Negative result• Previous 2D plots

identified potential filament structure

• When rotated in 3D, filament is shown to be two separate structures

17/23The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL

Computation vs. visualization

• Voids – could compute• Filaments – maybe could compute• Visualization helps astronomers

know where to focus quantitative analysis

18/23The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL

Effective techniques• User-controlled interaction

> Critical for maintaining viewer orientation> Interactive scaling allows snapping between

overview of data and local features, giving context when zooming in

> Home key moves back to a familiar orientation

• Torsional rocking> Rocking aids void definition when far galaxies peek

out from closer galaxy> View angle optimization

• Stereo> Stereo is helpful for finding galaxies bordering voids> Picking with mouse was hampered by stereo> Collaborators came across campus to use it!

19/23The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL

Software• Called GyVe (Galaxy Viewer)• Built on VTK, Python, and Tkinter• Available at

http://gyve.sourceforge.net/• Runs on Windows and Linux

20/23The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL

Future work• Isosurfaces – initial attempt

unused• Image processing for identifying

voids from density projections• Add interactive statistical tools• Haptic probes to feel around for

structures

21/23The UNIVERSITY of NORTH CAROLINA at CHAPEL HILL

Acknowledgements• NSF Grant AST 04-06443 (Fleenor)• James A. Rose• Russell M. Taylor II