Effects of Red Shifts on the Distribution of Nebulae

Abstract

Three published surveys with large reflectors and two additional, unpublished surveys furnish homogeneous counts of nebulae to five different limiting magnitudes. The results of Mayall's study of the 36-inch Crossley plates at Mount Hamilton,I to m = 19.0, have been, adopted as published, except that latitude corrections for galactic obscuration have been applied to the counts in order to make them strictly comparable with the Mount Wilson data. The hour exposures only have been extracted from the Mount Wilson surveys2 with the 60-inch anA the 100-inch reflectors, and these counts to m = 19.4 and 20.0, respectively, have been reduced separately. The new surveys, using Eastman 40 plates, represent 20-minute exposures with the 60-inch, m = 18.47, and 2-hour exposures with the 100-inch, m = 21.03. The counts were reduced to standard conditions by the same methods as were employed in the earlier surveys. Since the new data represent extreme cases, special attention was devoted to the determination of limiting magnitudes, and the quantities are given to another decimal place. Each survey is confined to the polar caps, B >_ 400, and furnishes N, the number of nebulae per square degree on uniform exposures of excellent quality at the zenith and reduced to the galactic poles. The limiting magnitude, m, represents the threshold of identification which is about 0.5 or 0.6 mag. above the threshold of registration. Comparisons of plates of the same fields made with different exposures confirm the completeness of the counts to the limit m in three of the surveys, and completeness is assumed for Mayall's counts and those on the 2-hour exposures with the 100-inch. The results are summarized in table 1. Since the frequency distribution of log N in each survey approximates a normal error-curve, log N, the sig