Orientations of galaxies and their distribution in space

Abstract

Distributions of angular momenta of galaxies can be used to test theories of galaxy origin. The observational problems are discussed in detail. Previously, interpretations have been hampered by physical and physiological selection effects that operate on visual measurements. With the COSMOS machine and the deep Schmidt plates many of these problems should be overcome. Before investigating the COSMOS data, two visual sets of measures, were examined. Firstly, Brown's original uncorrected catalogue of position angles was studied. Previous investigators claim remarkable anisotropies exist in this data, and indeed there are some histograms showing peaks significant at the 4б-level. Further examination reveals little reason for considering these excesses to be physical, and, taking into consideration the number of histograms possible (by splitting the data with various parameters), the anisotropies are too insignificant to use in theoretical arguments. Secondly, a set of visual measurements made on a deep UK Schmidt plate recording both position angles and axis ratios was examined. The position angles are consistent with that expected for a random distribution, except for the smallest galaxies where some selection effect may be involved. There is no evidence for a correlation between position angles of adjacent galaxies. When the quantization effects are taken into account the axial ratio distributions are consistent with a random 3-dimensional orientation of a mixture of 83% spirals and 17% ellipticals. A comparison of the distribution of nearest neighbour separations with that expected for a random distribution of galaxies; on the sky shows that there is clustering. When combined with an estimate of the size of clusters from a covariance analysis a mean number of 6.5 galaxies per cluster is obtained. Using COSMOS coarse mode measurements of a portion of plate R1049, galaxy axial ratio distributions have been obtained. The results can not be explained by any mixture of galaxy types. To check COSMOS, the plate was scanned by eye and an assessment of the machine is provided. At the intermediate sizes l00√m< 2a<200√m the identifications are satisfactory, but for larger images there are unexplained errors in the COSMOS data. For both sizes the agreement between the axial ratios from COSMOS and those measured, by eye is disappointing. It is concluded that further work of this nature must be performed only using the fine-mode facilities of the machine.