Spectral Classification of Galaxies: an Orthogonal Approach

Abstract

Classification of galaxy spectral energy distributions in terms of orthogonal\nbasis functions provides an objective means of estimating the number of\nsignificant spectral components that comprise a particular galaxy type. We\napply the Karhunen-Lo\\`{e}ve transform to derive a spectral eigensystem from a\nsample of ten galaxy spectral energy distributions. These spectra cover a\nwavelength range of 1200 \\AA\\ to 1 $\\mu$m and galaxy morphologies from\nelliptical to starburst. We find that the distribution of spectral types can be\nfully described by the first two eigenvectors (or eigenspectra). The derived\neigenbasis is affected by the normalization of the original spectral energy\ndistributions. We investigate different normalization and weighting schemes,\nincluding weighting to the same bolometric magnitude and weighting by the\nobserved distribution of morphological types. Projecting the spectral energy\ndistributions on to their eigenspectra we find that the coefficients define a\nsimple spectral classification scheme. The galaxy spectral types can then be\ndescribed in terms of a one parameter family (the angle in the plane of the\nfirst two eigenvectors). We find a strong correlation in the mean between our\nspectral classifications and those determined from published morphological\nclassifications.\n