The video - section by section

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Next: Further developments Up: 3D Redshift Space Visualization Previous: Preparing and doing

The video - section by section

In this section the six individual sections of the video will be shortly described and commented.

Furthermore this report is acompanied by six overhead viewgraphs showing one typical frame from each of the six sections. The frame is also shown under each section respectively.

Our system of reference

This first part of the video introduces the viewer to our "system of reference". It consists of a coordinate grid with the three axes marked by the colors red, green and blue where the blue axis points towards the northern celestial pole. A circle is also shown marking the sky's equator placed at a distance of . Also the length of the coordinate axis corresponds to this redshift.

The reason for choosing this distance for dividing the data into "inner" and "outer" is that most of the surveys claim to be more or less complete out to this limit. This is obviously not the case for the samples from out to the most distant objects at .

The full set of data - An overview

To give the viewer and perception af "where we are and where we are going" the second section gives an overview of all the data - zooming in from a great distance, turning the dataset around and moving back and forth inside it.

Here it is most certainly visible that the outermost sectios by no means are complete. Pencil-beam surveys are streching from the innermost parts of the data almost to the outer limit at . This becomes even more obvious as the data at the end of this sequence is shown without the pencil-beam surveys. A far more homogeneous sample emerges.

In this second section the "zone of avoidance" is clearly visible. It is the dark band devoid of galaxies that is due to the obscuration by our own galaxy - the Milky Way.

The innermost section,

In this - the third - section the most interesting subsample of the data is shown. It is the innermost 23000 galaxies within a redshift limit of 0.04 and as it was indicated above these data are to a large extent complete except the "zone of avoidance" mentioned above and some unobserved areas in the southern part of the sky.

In these data a lot of structure is seen. The clusters of galaxies are seen as elongated structures due to their high velocity dispersion ( km/s) which is sometimes referred to as the "finger of God" effect.

A view from Origo

In this fourth section of the video we have placed the viewer in our normal observing point - on the Earth. In this view the projection of the structures onto the celestial sphere makes it if not impossible then at least very difficult to recover the distribution seen in the previous sections of the video. This demonstrates the need for 3D visualization.

The northern CfA slice,

In the fifth section of the video one of the well known CfA slices (Geller & Huchra, 1989) spanning from to in right ascension and from to in declination.

Here the famous "Homunculus" consisting mainly of the Coma cluster is seen and also "The Great Wall" and the Virgo cluster are evident.

An important observation however is that the impression of the structures is strongly dependent of the way you look upon the data. A slight turn can make an "obvious feature" disappear completely!

-slices of the innermost data,

In the last section of the video an animation (as described in section 3.2) is used to show the spongy structure of the data.

The animation is made by creating subsamples of the complete dataset consisting of slices in right ascension each with an increased by with respect to the previous.

In the animation the viewer is placed in a co-moving system moving a bit up and down while the slices are being shown one by one. Because of the speed of the animation one sees a smooth transition of the structures making especially the voids very evident as almost circular areas completely devoid of galaxies.

next up previous contents
Next: Further developments Up: 3D Redshift Space Visualization Previous: Preparing and doing

Bo Milvang-Jensen
Wed Jan 18 05:44:35 MET 1995