Mean galaxy spectra of the 86 classes

These templates are usually built from a pre-defined classification based on other criteria. We present an unsupervised classification of 702248 spectra of galaxies and quasars with redshifts smaller than 0.25 that were retrieved from the Sloan Digital Sky Survey (SDSS) database, release 7.

The spectra were first corrected for the redshift, then wavelet-filtered to reduce the noise, and finally binned to obtain about 1437 wavelengths per spectrum. Fisher-EM, an unsupervised clustering discriminative latent mixture model algorithm, was applied on these corrected spectra, considering the full set as well as several subsets of 100000 and 300000 spectra. The optimum number of classes given by a penalised likelihood criterion is 86 classes, the 37 most populated ones gathering 99% of the sample.