Digital Universe Guide - Hayden Planetarium
Digital Universe Guide - Hayden Planetarium
Digital Universe Guide - Hayden Planetarium
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4.3. EXTRAGALACTIC DATA GROUPS 183<br />
4.3.10 Tully’s Smoothed Density Surfaces<br />
Group Name isoDensity<br />
Reference Tully 8,000 km/s Galaxy Catalog<br />
Prepared by R. Brent Tully (U Hawaii)<br />
Stuart Levy (NCSA/U Illinois)<br />
Labels No<br />
Files d16 100 u02 50k.obj, d16 200 u02 10k.obj<br />
Dependencies none<br />
Accompanying the Tully galaxy catalog (Tully) is a model of the density of the 28,000 galaxies in<br />
that catalog. These appear as 3-D surfaces surrounding the most dense regions of galaxies. Each<br />
surface represents points of constant density. After smoothing the galaxy data, that is, averaging these<br />
data to compensate for random fluctuations, a 3-D mapping of the overall density may be displayed as<br />
surfaces of constant value.<br />
The surfaces represent two values of constant density. The blue, inner surface is a relatively high<br />
density, while the aqua, outer surface represents a lower density. The blue contours mainly show dense<br />
galaxy clusters, like the Virgo Cluster, Fornax Cluster, and most of the Abell clusters in the Tully data<br />
group. These are the regions of highest density in our local <strong>Universe</strong>.<br />
Some of the aqua contours resemble the blue contours in shape, but more often they reveal larger<br />
structures that connect blue contours together. The largest feature appears on the edge of the data<br />
cube and is called the Great Wall. Brighten up the Tully galaxies and, while it’s still the active group, type<br />
see gw. This removes all the galaxies except those in the enormous Great Wall structure, which follow<br />
the density contours closely. Type see all to return all the Tully galaxies to view.<br />
Isolating the Isosurfaces If you desire to see just one of the isosurfaces, then you must change<br />
the color of the surface you wish to remove. In the extragalactic.cf file, the blue and aqua colors<br />
have a color index of 1 and 2, respectively, and are assigned these default red-green-blue colors:<br />
cment 1 0 0 1 (blue)<br />
cment 2 0.0 0.2 0.35 (aqua)<br />
To see the aqua contours alone, change the blue contours to black with the command cment 1 0 0 0.<br />
Then, to emphasize the aqua contours, increase their alpha value using the Alpha Slider. The higher<br />
the value, the more opaque the surfaces become. Set alpha to its maximum value of 1 and the surfaces