OBSERVATIONS OF A COMPLETE SAMPLE OF EMISSION-LINE GALAXIES

Abstract

Optical spectroscopic and broad-band B and V imaging observations have been obtained for all 172 emission-line galaxy (ELG) candidates in Lists IV and V of the University of Michigan Curtis Schmidt Objective Prism (UM) Survey. This survey selects galaxies by the presence of emission lines rather than by ultraviolet excess. These data allow for the accurate determination of the completeness limits and selection characteristics of the survey and provide valuable information about the physical nature of the survey constituents. The UM survey is found to be comprehensive in the sense that it includes a wide range of types of active galaxies, from high-luminosity Seyfert galaxies to isolated extragalactic H li regions. Analysis of the data shows that the completeness and selection characteristics of this survey are most accurately parameterized using emission-line flux and equivalent width rather than apparent magnitude. This result leads to the definition of a complete sample of UM galaxies which is used to determine the relative populations of the various types of ELGs found in the survey. In addition to the derivation of parameters such as blue and Hs luminosities, diameters, and colors, important insights into the physical conditions in the emission-line regions of these objects are gained by the use of line-ratio diagnostic diagrams. A previously unrecognized segregation of the galaxies with H ii-region-like spectra by their ELG type is seen in these diagrams. With the aid of published H II region models, this segregation is shown to be due to a continuous change in heavy metal abundance between the different ELG types, and the models are used to derive a metallicityluminosity relationship for the UM ELGs. The most important distinction between the UM survey and other slitless spectroscopic surveys is that the former is much more sensitive to the presence of low-luminosity emission-line objects. This attribute makes the UM survey particularly well suited for the study of the physical properties of dwarf emission-line systems. In addition, the great depth of the survey makes it possible to use these galaxies to study the spatial distribution of emission-line galaxies relative to more normal galaxies. We find that the spatial distribution of the UM galaxies is fairly well correlated with that of the normal galaxies, with the important exception that they avoid galaxy clusters. Additionally, the UM galaxies tend to be located in regions of lower galactic density than the normal galaxies, and at least three low-luminosity ELGs are found to lie in a void in the normal galaxy distribution. These last two points may be relevant for helping to distinguish between various models which attempt to explain the observed large-scale structure in the universe.