visualisations of 6df data by a.p. fairall using ‘labyrinth’ software developed by carl...

Visualisations of 6dF data

by A.P. FairallUsing ‘Labyrinth’ software developed

byCarl Hultquist and Samesham Perumal

Departments of Astronomy and Computer Science

University of Cape Town

An introduction to Labyrinth

This software allows one to visualise a galaxy database from any chosen position, looking in any chosen direction. One can also interactively fly around the database (although the presentation here uses still frames).

Lets start by looking at some (non-6dF) data with the galaxiesRepresented as white points

The readouts in the lower left corner give direction of viewand position in Cartesian Supergalactic coordinates

Labels can be turned on to identify features

Colour coding can be introduced to represent distance.Nearest galaxies red, distant galaxies blue

This enables a steroscopic view of the distribution usingChromoDepth™ spectacles

But instead of this distracting false colour…

..we change the coding to white (near) to blue (far), which works with or without spectacles

Labyrinth also lets us fade background structures…

Now we see only the nearest galaxies, which can also be shown ..

..as billboards, with images to scale, so givinga realistic visualisation of extragalactic space.

But the main purpose of Labyrinth is to grow “Tully bubbles”around groups and clusters of galaxies

This is 6dF data!

The bubbles can be made completely opaque

The individual galaxies need not be shown

The Software identifiesMinimal SpanningTrees (MSTs) andwraps a surfacearound them.

A minimum number of galaxies per MST can be specified

The MSTs arespecified by apercolationradius (r)At cz = 0

To compensate for the diminishing density of data with increasing redshift, the percolation radius is increased

with incresing cz. In this way the average density of bubblesstays more or less constant with increasing distance

As the bubbles grow, they interconnectto reveal the web of large-scale structures

Now to 6dF! We begin by taking 6dF data with cz < 7500 km/sso to examine very nearby large-scale structures.

The view is looking back from a point at cz = 20000 km/s

in the direction of the North Celestial Pole

Northern Galactic Hemisphere at top

Southern Galactic Hemisphere at bottom

Now to switch on the colour coding

True stereoscopy can be obtained by viewing these imagesWith ChromoDepth spectacles

Individual galaxies

Mimimal spanning trees show the densest regions in the data

The percolation distance r is set at 5 km/s

The minimum number of galaxies per MST is set at 10

As we increase the percolation distance, so the structures grow.Here it is r = 10 km/s

r = 20 km/s

r = 30 km/s

r = 40 km/s

r = 50 km/s

r = 60 km/s

r = 70 km/s

r = 80 km/s

Much moreDetail can be

Seen than was Previouslypossible

r = 90 km/s

r = 100 km/s

r = 120 km/s

r = 140 km/s

r = 160 km/s

r = 180 km/s

r = 200 km/s

But let’s go back ..

.. to r = 100 km/s

Various features can be identified

We can also blur the large-scale structures

And gradually bring back the individual galaxies

Galaxies and Large-scale structures

Now let’s bring in the complete 6dF data

r = 5 km/sOnce again MSTs show the densest regions

r = 10 km/s

r = 20 km/s

r = 30 km/s

r = 40 km/s

r = 50 km/s

r = 60 km/s

r = 70 km/s

r = 80 km/s

r = 90 km/s

r = 100 km/s6dF reveals texture more detailed

than ever before seen

r = 110 km/s

r = 120 km/s

r = 130 km/s

r = 140 km/s

r = 150 km/s

r = 175 km/s

r = 200 km/s

r = 300 km/s

r = 400 km/s

r = 500 km/s

r = 600 km/s

We can also constrain how the percolation radius varies with redshift

100 k/sAnd thereby find groups and clusters(rather than large-scale structures)

75 km/sDecreasing the percolation finds the denser clusters

50 km/s

40 km/s

For example, Labyrinth finds and list just over 100 clusters here.About two-thirds of them are Abell clusters

But work is still in progress!

Thanks to Matthew Colless,

Heath Jones and Lachlan Campbell for access to the 6dF data