...Groups

Figures 2 and 3 only available in the online version at http://www.edpsciences.com

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...POSS

The images of the SCGGs were obtained using the photographs of the National Geographic Society-Palomar Observatory Sky Survey (NGS-POSS) obtained with the Oschin Telescope on Palomar Mountain. The NGS-POSS was founded by a grant from the National Geographic Society to the California Institute of Technology. The plates were processed into the present compressed digital form with their permission. The Digitized Sky Survey (DSS) was produced at the Space Telescope Science Institute under US Government grant NAG W-2166.

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