Objective-prism and slit-spectra redshifts in a study of the distribution of galaxies

Abstract

A thorough and detailed investigation into the accuracy, reliability and consistency of the objective-prism redshift technique with changes in plate material, measuring machine and reduction techniques has been undertaken. Providing that the limitations of the procedures involved are appreciated, useful 3-D information, accurate to Z ~±0.01, for large numbers of faint galaxies may be obtained from single UKST fields. A surface brightness correction has enabled the technique to be extended to galaxies as bright as bj ~15.8. The validity of the results were confirmed by external calibration with slit-spectra. The slit-spectra results obtained have more than doubled the size of the calibration sample available and have further vindicated the applicability of the technique. Analysis of the 107 slit-spectra redshifts determined revealed the existence of considerable structure in the field studied, with significant density enhancements and voids being apparent. This structure has been confirmed on larger scales and to greater depths with the subsequent analysis of ~1,500 objective-prism redshifts from COSMOS data of the field. Two large superclusters were discovered. The first centred at Z ~0.05 but extending for at least 150 h⁻¹Mpc, and the second a projected filament of connecting clusters at Z ~0.11 and 23 h⁻¹Mpc long. A selection of 2-D and 3-D statistical analysis techniques have been used to describe the observed large-scale galaxy distribution in this field. A by-product of this work has been the discovery of a new and powerful means of star/galaxy separation based on magnitude comparisons between paired objects from both the direct and objective-prism plates. This technique has revealed ~20% stellar contamination in the star/galaxy separation based on direct plate data alone. In conclusion, objective-prism galaxy redshifts are valuable information for complementing 2-D data in the study of the large-scale distribution of galaxies from COSMOS measures of Large numbers of UKST plates. To this end an automatic technique of objective-prism redshift determination should be developed.