Spectral Classification and Luminosity Function of Galaxies in the Las Campanas Redshift Survey

Abstract

We construct a spectral classification scheme for the galaxies of the Las Campanas Redshift Survey (LCRS) based on a principal component analysis of the measured galaxy spectra. We interpret the physical significance of our six spectral types and conclude that they are sensitive to morphological type and the amount of active star formation. In this first analysis of the LCRS to include spectral classification, we estimate the general luminosity function, expressed as a weighted sum of the type-specific luminosity functions. In the R-band magnitude range of -23 < M <= -16.5, this function exhibits a broad shoulder centered near M = -20, and an increasing faint-end slope which formally converges on an alpha value of about -1.8 in the faint limit. The Schechter parameterization does not provide a good representation in this case, a fact which may partly explain the reported discrepancy between the luminosity functions of the LCRS and other redshift catalogs such as the Century Survey (Geller et al. 1997). The discrepancy may also arise from environmental effects such as the density-morphology relationship for which we see strong evidence in the LCRS galaxies. However, the Schechter parameterization is more effective for the luminosity functions of the individual spectral types. The data show a significant, progressive steepening of the faint-end slope, from alpha = +0.5 for early-type objects, to alpha = -1.8 for the extreme late-type galaxies. The extreme late-type population has a sufficiently high space density that its contribution to the general luminosity function is expected to dominate fainter than M = -16. We conclude that an evaluation of type-dependence is essential to any assessment of the general luminosity function.