The Distribution of Extra-Galactic Nebulae

Abstract

The object of the investigation is to determine the distribution of extra-galactic nebulae to a faint uniform limiting magnitude. The material consists of counts of about 44.000 nebulae on 1283 plates with the 100-inch and 60-inch reflectors, distributed over the three-quarters of the sky north of-30 o Dec. The counts are reduced to the standard conditions of excellent one-hour exposures at the zenith on Eastman 40 plates with the 100-inch reflector. In general no nebulae are found along the Milky Way. The zone of avoidance, representing local obscuration, is irregular and follows the geheral pattern of the known obscuring clouds. It is bordered by partial obscuration, which fades away into the regions of normal distribution where the frequency-curve of log N m closely approximates a Gaussian error-curve. Systematic variations in longitude are appreciable only in the lower latitudes, where obscuration appears to be conspicuously greater in the direction of the galactic center than in the opposite direction. There is a definite variation with latitude, which from the poles to about /S = 15 0 is represented by the cosecant formula log Nm = C-0.15 cosec ß , indicating a total obscuration of 0.5 mag. from pole to pole with no appreciable difference between the two hemispheres. With allowance for the effect of the red shift, the rate of increase of log N with exposure time suggests uniform distribution in depth. The standard conditions represent a threshold of identification for nebulae at about 20.0 pg m. Corrected for red shift, the number of nebulae per square degree to magnitude m is log Nm = o.6m-9.12 , which leads to values for the density of matter in space of log p=-16.4 to-16.8 in nebulae per cubic parsec, or-29.8 to-29.9 gr/cc, depending upon the value adopted for the mean absolute magnitude of nebulae. PART I. RELATIVE DISTRIBUTION OF NEBULAE OVER THE SKY The empirical approach to the problem of the structure of the physical universe consists in extrapolating the observed characteristics of the sample available for inspection. If the sample is fair and the characteristics are well determined, the method may be significant. Investigations have recently emerged from the stellar system and now range through a large volume of space whose inhabitants, the nebulae, are of the same general order as the stellar system itself. There are as yet no indications of a super-system of nebulae analogous to the system of stars. Hence, for the first time, the region now 1 Contributions from the Mount Wilson Observatory, Carnegie Institution of Washing-ton, No. 485. 8