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The 400 Prisms of the 2dF

by Duane Dunkerson

The Two degree Field (2dF) Galaxy Redshift Survey uses 400 prisms as part of its apparatus. The apparatus as a whole is employed to map in three dimensions the positions in the Universe of 250,000 galaxies. The 2dF Survey is being conducted with the Anglo-Australian Telescope in New South Wales, Australia.

In astronomical terms a two degree field of study for any project of such exacting precision is of great consequence. Two degrees of sky is approximately four times the width of the full moon as it is seen not close to the horizon. The Survey takes two degree "snapshots" of 400 galaxies at a time. The instrumentation is reset and another 400 galaxies selected. In all, over 3,000 galaxies per night can be added to the Survey.

The prisms are placed into position within 5 microns (one micron is about 0.000039 inch) by means of a robot. The 400 prisms are each positioned on the location of a galaxy. A huge piece of glassware makes this possible by imaging the galaxies. The tiny prisms are connected to optical cables. The cables run to one of two machines. Each machine is responsible for 200 cables. Each machine receives the spectrum of the galaxy that the individual prism helps to disperse onto a photographic image.

The 2dF Survey does not discover these galaxies. Their 2-dimensional position in the sky is known mostly from the Automatic Plate Measuring (APM) survey. The APM provides a galaxy's celestial longitude (right ascension) and celestial latitude (declination). 2dF measures the redshift of the galaxies at these positions.

Of particular interest are the redshifts of the recorded spectra. The APM coordinates of right ascension and declination serve to locate a galaxy in X and Y or left,right and up,down but to find the Z or depth or distance from us the 2dF measures the redshift which equates with how far each galaxy is from us. A map of X,Y, and Z is thus made known for each of the 250,000 galaxies that 2dF is assigned to measure.

Previous galactic distance measurements found galaxies here and there at many distances in every direction. It was in 1929 that Edwin Hubble showed, on the basis of redshifts for 46 galaxies, that the redshift for a galaxy is proportional to its distance from us. The galaxies were receding from us - a general universal expansion had been found. Decades later a search for a "greatness" limit, for how great a structure there could be composed of thousands of galaxies, was begun. No obvious structure is to be found at larger in size than 300 million light years. 2dF reaches to 4 billion light years. Astronomers had been amazed in 1986 to find enormous groupings of galaxies. It had been suspected that more of the same already- to- be- seen clustering of galaxies would be found. Instead another typical grouping of galaxies was found. One of these typical groupings was a particular structure called The Great Wall. 2dF, and other surveys, have revealed giant clusters of galaxies, with long strings of these across enormous distances having areas empty of galaxies. There are no structures bigger than the strings.

2dF is not looking at the entire sky. It covers one twentieth of the sky. It is thought that if any other than the now usual structure is not as expected, then the 2dF Survey would find it. The Survey is in what is taken to be a representative portion of the Universe.

The Survey's results are supportive of the notions that the Universe is forever expanding and, in addition, is undergoing acceleration of that expansion. The acceleration apparently derives from a relatively new concept, dark energy.

Before 2dF the largest redshift survey had dealt with 25,000 galaxies. Still more mapping is to be accomplished by the Sloan Digital Sky Survey. It too will measure redshift distances but for one million galaxies. The Sloan Survey will gather in 25 % of the sky at almost the same depth as the 2dF Survey.






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by Duane Dunkerson

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