Observations of a Complete Sample of Emission-Line Galaxies. II. Properties of the UM Survey Galaxies

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Abstract

We utilize our extensive collection of imaging and spectroscopic data to investigate the properties of the emission-line galaxies (ELGs) found in Lists IV and V of the University of Michigan (UM) objective-prism survey. The majority of the ELGs in this deep sample have apparent magnitudes in the range 16.0 < m_B_ < 19.0. the distribution of B - V colors is nearly identical with that for the Markarian galaxies, indicating that the line-selection technique employed is very efficient at finding the same kinds of activity as found in UV-excess surveys. However, the UM sample differs markedly from complete magnitude-limited samples of galaxies in that it contains a very high proportion of low-luminosity galaxies. The median luminosity is M_B_ = - 18.1, and 25% of the UM galaxies have M_B_ > -16.5. The vast majority are dwarfs, with diameters less than 10 kpc. Each ELG is classified into one of 10 natural groups of active galaxies. Seyfert galaxies make up 11% of the entire sample, a result nearly identical with that for the Markarian survey. The ELG types most commouly found are intermediate- to low-luminosity objects with irregular or unresolved morphologies and a very intense star-formation region which dominates the optical output of the galaxy. Emission-line ratio diagnostic diagrams reveal that the ELGs in the different natural groups tend to have distinctly different line ratios. This suggests that the various ELG types differ from one another in terms of the metal abundance of their ionized gas, and we derive a luminosity-metallicity relationship which is singular to those found previously for disk and irregular galaxies.

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