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

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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 to 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 on M ≈ -20 and an increasing faint-end slope that formally converges on α ≈ -1.8 in the faint limit. The Schechter parameterization does not provide a good representation in this case, a fact that may partly explain the reported discrepancy between the luminosity functions of the LCRS and other redshift catalogs such as the Century Survey. 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 α ≈ +0.5 for early-type objects to α ≈ -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 at magnitudes fainter than M = -16. We conclude that an evaluation of type dependence is essential to any assessment of the general luminosity function.