SPECTRAL CLASSIFICATION OF GALAXIES - AN ORTHOGONAL APPROACH

Classification of galaxy spectral energy distributions in terms of ort hogonal basis functions provides an objective means of estimating the number of significant spectral components that comprise a particular g alaxy type. We apply the Karhunen-Loeve transform to derive a spectral eigensystem from a sample of ten galaxy spectral energy distributions . These spectra cover a wavelength range of 1200 Angstrom to 1 mu m an d galaxy morphologies from elliptical to starburst. We find that the d istribution of spectral types can be fully described by the first two eigenvectors (or eigenspectra). The derived eigenbasis is affected by the normalization of the original spectral energy distributions. We in vestigate different normalization and weighting schemes, including wei ghting to the same bolometric magnitude and weighting by the observed distribution of morphological types. Projecting the spectral energy di stributions onto their eigenspectra, we find that the coefficients def ine a simple spectral classification scheme. The galaxy spectral types can then be described in terms of a one parameter family (the angle i n the plane of the first two eigenvectors). We find a correlation, in the mean, between our spectral classifications and those determined fr om published morphological classifications. (C) 1995 American Astronom ical Society.